Pohang, South Korea
Pohang, South Korea

Pohang University of Science and Technology or POSTECH is a private university located in Pohang, South Korea dedicated to research and education in science and technology. In 2012 and 2013, the Times Higher Education ranked POSTECH 1st in its "100 Under 50 Young Universities" rankings. Wikipedia.

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Kinaci A.,Texas A&M University | Sevincli H.,Leibniz Institute of Polymer Research | Cuniberti G.,Leibniz Institute of Polymer Research | Cuniberti G.,Pohang University of Science and Technology
ACS Nano | Year: 2011

The influence of the structural detail and defects on the thermal and electronic transport properties of graphene nanoribbons (GNRs) is explored by molecular dynamics and nonequilibrium Green's function methods. A variety of randomly oriented and distributed defects, single and double vacancies, Stone-Wales defects, as well as two types of edge form (armchair and zigzag) and different edge roughnesses are studied for model systems similar in sizes to experiments (>100 nm long and >15 nm wide). We observe substantial reduction in thermal conductivity due to all forms of defects, whereas electrical conductance reveals a peculiar defect-type-dependent response. We find that a 0.1% single vacancy concentration and a 0.23% double vacancy or Stone-Wales concentration lead to a drastic reduction in thermal conductivity of GNRs, namely, an 80% reduction from the pristine one of the same width. Edge roughness with an rms value of 7.28 Å leads to a similar reduction in thermal conductivity. Randomly distributed bulk vacancies are also found to strongly suppress the ballistic nature of electrons and reduce the conductance by 2 orders of magnitude. However, we have identified that defects close to the edges and relatively small values of edge roughness preserve the quasi-ballistic nature of electronic transport. This presents a route of independently controlling electrical and thermal transport by judicious engineering of the defect distribution; we discuss the implications of this for thermoelectric performance. © 2011 American Chemical Society.

Tiwari J.N.,Pohang University of Science and Technology | Tiwari R.N.,Toyota Technological Institute | Singh G.,National Chiao Tung University | Kim K.S.,Pohang University of Science and Technology
Nano Energy | Year: 2013

Continuous growth in global energy demand has sparked concerns about energy security and environmental sustainability. In the past two decades, attempts have been made in the development of innovative energy technologies. The direct methanol fuel cell (DMFC) is among the most promising alternative energy sources for the near future. Simple construction, compact design, high energy density and relatively high energy-conversion efficiency give the DMFC an advantage over other promising power sources in terms of portability. However, the translation of DMFCs into commercially successful products is precluded due to poor performance. In addition, low activity, poor durability and reliability and an expensive anode and cathode further discourage the application of DMFCs. In this regard, the present review article focuses on recent progress in the development of anode and cathode catalysts for DMFCs. The first part of the review discusses the recent developments in the synthesis of single-, double-, and multiple-component catalysts and new catalyst supports for anode electrodes. The section is followed by the chemical approaches employed to make alloys and composite catalysts, aiming to enhance their activity, reliability and durability for the methanol oxidation reaction. Finally, exciting new research that pushes the development of single-, double-, and multiple-component catalysts and new catalyst supports for cathode electrodes is introduced. In addition, size-, shape- and composition-dependent electrocatalysts that are advocated for methanol oxidation at the anode and oxygen reduction at the cathode are highlighted to illustrate the potential of the newly developed electrocatalysts for DMFC applications. Moreover, this article provides a comprehensive review of the experimental work that is devoted to understanding the fundamental problems and recent progress in the development of anode and cathode catalysts for DMFCs.© 2013 Elsevier Ltd.

Lim J.,Pohang University of Science and Technology | Hong D.,Sogang University
IEEE Signal Processing Letters | Year: 2013

We propose Gaussian particle filtering (PF) approach for estimating carrier frequency offset (CFO) in OFDM systems. PF is more powerful especially for nonlinear problems where classical approaches (e.g., maximum likelihood estimators) may not show optimal performance. Standard PF undergoes the particle impoverishment (PI) problem resulting from resampling process for this static parameter (i.e., CFO) estimation. Gaussian PF (GPF) avoids the PI problem because resampling process is not needed in the algorithm. We show that GPF outperforms current approaches in this nonlinear estimation problem. © 1994-2012 IEEE.

Kang H.-W.,Wake Forest Institute for Regenerative Medicine | Cho D.-W.,Pohang University of Science and Technology
Tissue Engineering - Part C: Methods | Year: 2012

Tissue engineering, which is the study of generating biological substitutes to restore or replace tissues or organs, has the potential to meet current needs for organ transplantation and medical interventions. Various approaches have been attempted to apply three-dimensional (3D) solid freeform fabrication technologies to tissue engineering for scaffold fabrication. Among these, the stereolithography (SL) technology not only has the highest resolution, but also offers quick fabrication. However, a lack of suitable biomaterials is a barrier to applying the SL technology to tissue engineering. In this study, an indirect SL method that combines the SL technology and a sacrificial molding process was developed to address this challenge. A sacrificial mold with an inverse porous shape was fabricated from an alkali-soluble photopolymer by the SL technology. A sacrificial molding process was then developed for scaffold construction using a variety of biomaterials. The results indicated a wide range of biomaterial selectivity and a high resolution. Achievable minimum pore and strut sizes were as large as 50 and 65μm, respectively. This technology can also be used to fabricate three-dimensional organ shapes, and combined with traditional fabrication methods to construct a new type of scaffold with a dual-pore size. Cytotoxicity tests, as well as nuclear magnetic resonance and gel permeation chromatography analyses, showed that this technology has great potential for tissue engineering applications. © Copyright 2012, Mary Ann Liebert, Inc.

Wang H.,Pohang University of Science and Technology | Agoulmine N.,University of Évry Val d'Essonne | Ma M.,Nanyang Technological University | Jin Y.,Shanghai University
IEEE Journal on Selected Areas in Communications | Year: 2010

Network lifetime (NL) is a critical metric in the design of energy-constrained wireless sensor networks (WSNs). In this paper, we investigate a joint optimal design of the physical, medium access control (MAC) and routing layers to maximize NL of a multiple-sources and single-sink (MSSS) WSN with energy constraints. The problem of NL maximization (NLM) can be formulated as a mixed integer-convex optimization problem with adoption of time division multiple access (TDMA) technique. When the integer constraints are relaxed to take real values, the problem can be transformed into a convex problem and the solution achieves the upper bounds. We provide an analytical framework for the relaxed NLM problem of a WSN in general planar topology. We first restrict the topologies to the planar networks on a small scale, including triangle and regular quadrangle topologies. In this special case, we employ the Karush-Kuhn-Tucker (KKT) optimality conditions to derive analytical expressions of the globally optimal NL, which take the influence of data rate, link access and routing into account. To handle larger scale planar networks, an iterative algorithm is proposed using the D&C approach. Numerical results illustrate that the proposed algorithm can be extended to the large planar case and its performance is close to globally optimal performance. © 2010 IEEE.

Kim J.K.,Sungkyunkwan University | Moon J.H.,Sogang University | Lee T.-W.,Pohang University of Science and Technology | Park J.H.,Sungkyunkwan University
Chemical Communications | Year: 2012

Inverse opal structured tungsten trioxide (WO3) films with mesoporous morphology were prepared using self-assembled polystyrene (PS) colloid as an organic template and by addition of polyethyleneglycol (PEG) to a precursor as an organic surfactant. The photoelectrochemical properties of these films were investigated. By adjusting the tungsten to PEG weight ratio in the precursor, an inverse opaline WO3 film with a nano-particular skeleton and increased specific surface area was constructed. The unique morphology of this film led to enhanced photoelectrochemical catalytic responses under UV-Visible 1-sun illumination. This journal is © The Royal Society of Chemistry.

Yoon J.W.,Swinburne University of Technology | Yoon J.W.,University of Aveiro | Dick R.E.,Alcoa | Barlat F.,Pohang University of Science and Technology
International Journal of Plasticity | Year: 2011

Commercial canmaking processes include drawing, redrawing and several ironing operations. It is experimentally observed that during the drawing and redrawing processes earing develops, but during the ironing processes earing is reduced. It is essential to understand the earing mechanism during drawing and ironing for an advanced material modeling. A new analytical approach that relates the earing profile to r-value and yield stress directionalities is presented in this work. The analytical formula is based on the exact integration of the logarithmic strain. The derivation is for a cylindrical cup under the plane stress condition based on rigid perfect plasticity while force equilibrium is not considered. The earing profile is obtained solely from anisotropic plastic properties in simple tension. The earing mechanism is explained from the present theory with explicit formulae. It has been proved that earing is the combination of the contributions from r-value and yield stress directionalities. From a directionality (y-axis) vs. angle from the rolling (x-axis) plot, the earing profile is generated to be a scaled mirror image of the r-value directionality with respect to 90° (x = 90) and also a scaled mirror image of the yield stress directionality with respect to the reference yield stress (y = 1). Three different materials (Al-5% Mg alloy, AA 2090-T3 and AA 3104 RPDT control coil) are considered for verification purposes. This approach provides a fundamental basis for understanding the earing mechanism. In practice, the present theory is also very useful for the prediction of the earing profile of a drawn and iron cup and its related convolute cut-edge design for an earless cup. © 2011 Elsevier Ltd. All rights reserved.

Gwak B.,Sogang University | Lee B.-H.,Sogang University | Lee B.-H.,Asia Pacific Center for Theoretical Physics | Lee B.-H.,Pohang University of Science and Technology
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2016

We have shown that changes occur in a (2. +. 1)-dimensional charged black hole by adding a charged probe. The particle increases the entropy of the black hole and guarantees the second law of thermodynamics. The first law of thermodynamics is derived from the change in the black hole mass. Using the particle absorption, we test the extremal black hole and find out that the mass of the extremal black hole increases more than the electric charge. Therefore, the outer horizon of the black hole still exists. However, the extremal condition becomes non-extremal. © 2016 The Authors.

Lim S.-R.,University of California at Davis | Park J.M.,Pohang University of Science and Technology
Industrial and Engineering Chemistry Research | Year: 2010

Water network synthesis is to optimize a water supply system by connecting water sources to sinks in order to maximize water reuse. This synthesis has been applied within a factory to reduce freshwater consumption. This study illustrates (i) the synthesis and design of an interfactory and intrafactory water network system for an eco-industrial park (EIPWNS) utilizing more opportunities for water reuse within an industrial park, and (ii) an environmental and economic feasibility study to demonstrate benefits from industrial symbiosis. An EIPWNS is synthesized, designed, and compared to a conventional water system (CWS). The feasibility study using life cycle assessment and life cycle costing shows that the total environmental impacts of the EIPWNS are 7.5% to 16.0% less than those of the CWS, and the life cycle cost of the EIPWNS is 15.6% less. Therefore, the EIPWNS can be employed in remodeling a conventional industrial park to an eco-industrial park. This study provides a good example for disseminating EIPWNSs. © 2010 American Chemical Society.

Kole G.K.,National University of Singapore | Kojima T.,Pohang University of Science and Technology | Kawano M.,Pohang University of Science and Technology | Vittal J.J.,National University of Singapore
Angewandte Chemie - International Edition | Year: 2014

A [2+2] cycloaddition reaction has been observed in a number of solids. The cyclobutane ring in a photodimerized material can be cleaved into olefins by UV light and heat. The high thermal stability of the metal-organic salt K 2SDC (H2SDC=4,4'-stilbenedicarboxylic acid) has been successfully utilized to investigate the reversible cleavage of a cyclobutane ring. The two polymorphs of K2SDC undergo reversible cyclobutane formation by UV light and cleavage by heat in cycles. Of these, one polymorph retains its single-crystal nature during the reversible processes. Polymorphs are known to show different physical properties and chemical reactivities. This work reveals that the retention of single-crystal nature is strongly associated with the packing of molecules, which is controlled by kinetics and thermodynamics. The photoemissive nature of the products makes this as a promising material for photoswitches and optical data storage devices. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Park C.,Sogang University | Park C.,Asia Pacific Center for Theoretical Physics | Park C.,Pohang University of Science and Technology
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

In a d-dimensional conformal field theory, it has been known that a relevant deformation operator with the conformal dimension, Δ=d+22, generates a logarithmic correction to the entanglement entropy. In the large 't Hooft coupling limit, we can investigate such a logarithmic correction holographically by deforming an AdS space with a massive scalar field dual to the operator with Δ=d+22. There are two sources generating the logarithmic correction. One is the metric deformation and the other is the minimal surface deformation. In this work, we investigate the change of the entanglement entropy caused by the minimal surface deformation and find that the second order minimal surface deformation leads to an additional logarithmic correction. © 2015 American Physical Society.

Tang H.,University of Victoria | Fuentealba D.,University of Victoria | Ko Y.H.,Pohang University of Science and Technology | Selvapalam N.,Pohang University of Science and Technology | And 2 more authors.
Journal of the American Chemical Society | Year: 2011

The binding dynamics of R-(+)-2-naphthyl-1-ethylammonium cation (NpH +) with cucurbit[7]uril (CB[7]) was investigated. Competitive binding with Na + or H 3O + cations enabled the reaction to be slowed down sufficiently for the kinetics to be studied by fluorescence stopped-flow experiments. The binding of two Na + cations to CB[7], i.e., CB[7]•Na + (K 01 = 130 ± 10 M -1) and Na +•CB[7]•Na + (K 02 = 21 ± 2 M -1), was derived from the analysis of binding isotherms and the kinetic studies. NpH + binds only to free CB[7] ((1.06 ± 0.05) × 10 7 M -1), and the association rate constant of (6.3 ± 0.3) × 10 8 M -1 s -1 is 1 order of magnitude lower than that for a diffusion-controlled process and much higher than the association rate constant previously determined for other CB[n] systems. The high equilibrium constant for the NpH +@CB[7] complex is a consequence of the slow dissociation rate constant of 55 s -1. The kinetics results showed that formation of a complex between a positively charged guest with CB[n] can occur at a rate close to the diffusion-controlled limit with no detection of a stable exclusion complex. © 2011 American Chemical Society.

Park S.K.,San | Park J.M.,Pohang University of Science and Technology
Water Research | Year: 2014

News Article | December 7, 2016
Site: www.nature.com

News Article | January 12, 2016
Site: www.cemag.us

A unique filtering technology that combines light and sound waves on a single chip is expected to better detect radar and communications frequencies. “We have developed a powerful signal filtering technology that could revolutionize signal processing systems that rely solely on conventional electronics,” says Patrick Chu, manager of applied photonic microsystems for Sandia National Laboratories. The radio frequency (RF) filters, which promise both high bandwidth and wide functional flexibility, would form the basis for spectrometers that would let users “see” energies placed in various frequency bands across a wide spectral range. The novel, very thin filter structures are in the laboratory stage. A system demonstration — complete with lasers, modulators, detectors, and battery — should be a bit larger than a computer hard drive, weigh only a few pounds and become available within three to five years. The filter uses a relatively new concept called photon/phonon coupling. This technique lets the hybrid device temporarily change RF signals propagating as photons (light) into phonons (sound), enabling efficient analog manipulation of those slower-moving signals. With this hybrid approach, also known as nano-optomechanical coupling, the researchers were able to combine the high bandwidth offered by light — demonstrated at frequencies up to 20 gigahertz and easily extended to 100 gigahertz — with the linearity and sharp resonances provided by phononic filters. The energy cost of this photon-to-phonon conversion is offset by the high-resolution filter responses that exhibit very little signal distortion over a wide frequency range, says Charles Reinke, who leads the Sandia effort. A simple analogy for the photon-phonon information transfer is the tin can telephone: two cans connected by a string that transmits sound between a speaker and listener. The speaker’s cup is like the emitter waveguide; it converts audible sound to vibration in the string. The cup by the ear is the receiver waveguide, which converts the vibration back into sound. The string, representing an engineered material called a phononic crystal, not only carries the message but changes its tone by filtering out high-pitch sounds, a kind of signal processing. Creating a phononic crystal requires taking a thin film of material, in this case silicon nitride and modifying its mechanical properties by creating patterns on it. The resultant crystal exhibits propagation properties not normally found in nature for mechanical waves like sound, and is dependent on the geometry and pattern of the film. For the filtering system, two materials are key: silicon nitride to form membranes in which the acoustic signals propagate, and silicon to create waveguides that confine the optical signals. The dual system optimizes the acoustic and optical properties of the device independent of each other. The photonic-phononic devices also could be incorporated with on-chip photodetectors and other electronics. A paper published in March in Nature Communications, “Control of coherent information via on-chip photonic-phononic emitter-receivers,” describes the work, initiated by Sandia’s Laboratory Directed Research and Development office and currently funded for almost $5 million by the Defense Advanced Research Projects Agency (DARPA). Lead researcher Peter Rakich, now a professor at Yale University, began the work at Sandia, and the collaboration continues to evolve with research partners at the University of Texas at Austin and the industrial firm Rockwell Collins. First author Heedeuk Shin, who was a postdoctoral employee at Sandia under Rakich’s mentorship, is a professor at Pohang University of Science and Technology in South Korea. News Article | October 6, 2016 Site: www.cemag.us A team led by Cory Dean, assistant professor of physics at Columbia University, Avik Ghosh, professor of electrical and computer engineering at the University of Virginia, and James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, has directly observed — for the first time — negative refraction for electrons passing across a boundary between two regions in a conducting material. First predicted in 2007, this effect has been difficult to confirm experimentally. The researchers were able to observe the effect in graphene, demonstrating that electrons in the atomically thin material behave like light rays, which can be manipulated by such optical devices as lenses and prisms. The findings, which are published in the September 30 edition of Science, could lead to the development of new types of electron switches, based on the principles of optics rather than electronics. “The ability to manipulate electrons in a conducting material like light rays opens up entirely new ways of thinking about electronics,” says Dean. “For example, the switches that make up computer chips operate by turning the entire device on or off, and this consumes significant power. Using lensing to steer an electron ‘beam’ between electrodes could be dramatically more efficient, solving one of the critical bottlenecks to achieving faster and more energy efficient electronics.” Dean adds, “These findings could also enable new experimental probes. For example, electron lensing could enable on-chip versions of an electron microscope, with the ability to perform atomic scale imaging and diagnostics. Other components inspired by optics, such as beam splitters and interferometers, could additionally enable new studies of the quantum nature of electrons in the solid state.” While graphene has been widely explored for supporting high electron speed, it is notoriously hard to turn off the electrons without hurting their mobility. Ghosh says, “The natural follow-up is to see if we can achieve a strong current turn-off in graphene with multiple angled junctions. If that works to our satisfaction, we’ll have on our hands a low-power, ultra-high-speed switching device for both analog (RF) and digital (CMOS) electronics, potentially mitigating many of the challenges we face with the high energy cost and thermal budget of present day electronics.” Light changes direction — or refracts — when passing from one material to another, a process that allows us to use lenses and prisms to focus and steer light. A quantity known as the index of refraction determines the degree of bending at the boundary, and is positive for conventional materials such as glass. However, through clever engineering, it is also possible to create optical “metamaterials” with a negative index, in which the angle of refraction is also negative. “This can have unusual and dramatic consequences,” Hone notes. “Optical metamaterials are enabling exotic and important new technologies such as super lenses, which can focus beyond the diffraction limit, and optical cloaks, which make objects invisible by bending light around them.” Electrons travelling through very pure conductors can travel in straight lines like light rays, enabling optics-like phenomena to emerge. In materials, the electron density plays a similar role to the index of refraction, and electrons refract when they pass from a region of one density to another. Moreover, current carriers in materials can either behave like they are negatively charged (electrons) or positively charged (holes), depending on whether they inhabit the conduction or the valence band. In fact, boundaries between hole-type and electron-type conductors, known as p-n junctions (“p” positive, “n” negative), form the building blocks of electrical devices such as diodes and transistors. “Unlike in optical materials,” says Hone, “where creating a negative index metamaterial is a significant engineering challenge, negative electron refraction occurs naturally in solid state materials at any p-n junction.” The development of two-dimensional conducting layers in high-purity semiconductors such as GaAs (Gallium arsenide) in the 1980s and 1990s allowed researchers to first demonstrate electron optics including the effects of both refraction and lensing. However, in these materials, electrons travel without scattering only at very low temperatures, limiting technological applications. Furthermore, the presence of an energy gap between the conduction and valence band scatters electrons at interfaces and prevents observation of negative refraction in semiconductor p-n junctions. In this study, the researchers’ use of graphene, a 2D material with unsurpassed performance at room temperature and no energy gap, overcame both of these limitations. The possibility of negative refraction at graphene p-n junctions was first proposed in 2007 by theorists working at both the University of Lancaster and Columbia University. However, observation of this effect requires extremely clean devices, such that the electrons can travel ballistically, without scattering, over long distances. Over the past decade, a multidisciplinary team at Columbia - including Hone and Dean, along with Kenneth Shepard, Lau Family Professor of Electrical Engineering and professor of biomedical engineering, Abhay Pasupathy, associate professor of physics, and Philip Kim, professor of physic at the time (now at Harvard) — has worked to develop new techniques to construct extremely clean graphene devices. This effort culminated in the 2013 demonstration of ballistic transport over a length scale in excess of 20 microns. Since then, they have been attempting to develop a Veselago lens, which focuses electrons to a single point using negative refraction. But they were unable to observe such an effect and found their results puzzling. In 2015, a group at Pohang University of Science and Technology in South Korea reported the first evidence focusing in a Veselago-type device. However, the response was weak, appearing in the signal derivative. The Columbia team decided that to fully understand why the effect was so elusive, they needed to isolate and map the flow of electrons across the junction. They utilized a well-developed technique called “magnetic focusing” to inject electrons onto the p-n junction. By measuring transmission between electrodes on opposite sides of the junction as a function of carrier density they could map the trajectory of electrons on both sides of the p-n junction as the incident angle was changed by tuning the magnetic field. Crucial to the Columbia effort was the theoretical support provided by Ghosh’s group at the University of Virginia, who developed detailed simulation techniques to model the Columbia team’s measured response. This involved calculating the flow of electrons in graphene under the various electric and magnetic fields, accounting for multiple bounces at edges, and quantum mechanical tunneling at the junction. The theoretical analysis also shed light on why it has been so difficult to measure the predicted Veselago lensing in a robust way, and the group is developing new multi-junction device architectures based on this study. Together the experimental data and theoretical simulation gave the researchers a visual map of the refraction, and enabled them to be the first to quantitatively confirm the relationship between the incident and refracted angles (known as Snell’s Law in optics), as well as confirmation of the magnitude of the transmitted intensity as a function of angle (known as the Fresnel coefficients in optics). “In many ways, this intensity of transmission is a more crucial parameter,” says Ghosh, “since it determines the probability that electrons actually make it past the barrier, rather than just their refracted angles. The transmission ultimately determines many of the performance metrics for devices based on these effects, such as the on-off ratio in a switch, for example.” The Science study was supported by Semiconductor Research Corporation’s NRI Center for Institute for Nanoelectronics Discovery and Exploration (INDEX). The collaboration was made possible through the support of the NRI program, which has brought together some of the country’s best groups in the study of 2D materials to focus on novel device architectures that may outperform conventional silicon-based technologies. News Article | October 6, 2016 Site: www.nanotech-now.com Home > Press > Electrons in graphene behave like light, only better: Researchers discover that electrons mimic light in graphene, confirming a 2007 prediction  their finding may enable new low power electronics and lead to new experimental probes Abstract: A team led by Cory Dean, assistant professor of physics at Columbia University, Avik Ghosh, professor of electrical and computer engineering at the University of Virginia, and James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, has directly observedfor the first timenegative refraction for electrons passing across a boundary between two regions in a conducting material. First predicted in 2007, this effect has been difficult to confirm experimentally. The researchers were able to observe the effect in graphene, demonstrating that electrons in the atomically thin material behave like light rays, which can be manipulated by such optical devices as lenses and prisms. The findings, which are published in the September 30 edition of Science, could lead to the development of new types of electron switches, based on the principles of optics rather than electronics. The ability to manipulate electrons in a conducting material like light rays opens up entirely new ways of thinking about electronics, says Dean. For example, the switches that make up computer chips operate by turning the entire device on or off, and this consumes significant power. Using lensing to steer an electron beam between electrodes could be dramatically more efficient, solving one of the critical bottlenecks to achieving faster and more energy efficient electronics. Dean adds, These findings could also enable new experimental probes. For example, electron lensing could enable on-chip versions of an electron microscope, with the ability to perform atomic scale imageing and diagnostics. Other components inspired by optics, such as beam splitters and interferometers, could additionally enable new studies of the quantum nature of electrons in the solid state. While graphene has been widely explored for supporting high electron speed, it is notoriously hard to turn off the electrons without hurting their mobility. Ghosh says, The natural follow-up is to see if we can achieve a strong current turn-off in graphene with multiple angled junctions. If that works to our satisfaction, well have on our hands a low-power, ultra-high-speed switching device for both analog (RF) and digital (CMOS) electronics, potentially mitigating many of the challenges we face with the high energy cost and thermal budget of present day electronics. Light changes direction  or refracts - when passing from one material to another, a process that allows us to use lenses and prisms to focus and steer light. A quantity known as the index of refraction determines the degree of bending at the boundary, and is positive for conventional materials such as glass. However, through clever engineering, it is also possible to create optical metamaterials with a negative index, in which the angle of refraction is also negative. This can have unusual and dramatic consequences, Hone notes. Optical metamaterials are enabling exotic and important new technologies such as super lenses, which can focus beyond the diffraction limit, and optical cloaks, which make objects invisible by bending light around them. Electrons travelling through very pure conductors can travel in straight lines like light rays, enabling optics-like phenomena to emerge. In materials, the electron density plays a similar role to the index of refraction, and electrons refract when they pass from a region of one density to another. Moreover, current carriers in materials can either behave like they are negatively charged (electrons) or positively charged (holes), depending on whether they inhabit the conduction or the valence band. In fact, boundaries between hole-type and electron-type conductors, known as p-n junctions (p positive, n negative), form the building blocks of electrical devices such as diodes and transistors. Unlike in optical materials, says Hone, where creating a negative index metamaterial is a significant engineering challenge, negative electron refraction occurs naturally in solid state materials at any p-n junction. The development of two-dimensional conducting layers in high-purity semiconductors such as GaAs (Gallium arsenide) in the 1980s and 1990s allowed researchers to first demonstrate electron optics including the effects of both refraction and lensing. However, in these materials, electrons travel without scattering only at very low temperatures, limiting technological applications. Furthermore, the presence of an energy gap between the conduction and valence band scatters electrons at interfaces and prevents observation of negative refraction in semiconductor p-n junctions. In this study, the researchers use of graphene, a 2D material with unsurpassed performance at room temperature and no energy gap, overcame both of these limitations. The possibility of negative refraction at graphene p-n junctions was first proposed in 2007 by theorists working at both the University of Lancaster and Columbia University. However, observation of this effect requires extremely clean devices, such that the electrons can travel ballistically, without scattering, over long distances. Over the past decade, a multidisciplinary team at Columbia - including Hone and Dean, along with Kenneth Shepard, Lau Family Professor of Electrical Engineering and professor of biomedical engineering, Abhay Pasupathy, associate professor of physics, and Philip Kim, professor of physic at the time (now at Harvard)  has worked to develop new techniques to construct extremely clean graphene devices. This effort culminated in the 2013 demonstration of ballistic transport over a length scale in excess of 20 microns. Since then, they have been attempting to develop a Veselago lens, which focuses electrons to a single point using negative refraction. But they were unable to observe such an effect and found their results puzzling. In 2015, a group at Pohang University of Science and Technology in South Korea reported the first evidence focusing in a Veselago-type device. However, the response was weak, appearing in the signal derivative. The Columbia team decided that to fully understand why the effect was so elusive, they needed to isolate and map the flow of electrons across the junction. They utilized a well-developed technique called magnetic focusing to inject electrons onto the p-n junction. By measuring transmission between electrodes on opposite sides of the junction as a function of carrier density they could map the trajectory of electrons on both sides of the p-n junction as the incident angle was changed by tuning the magnetic field. Crucial to the Columbia effort was the theoretical support provided by Ghoshs group at the University of Virginia, who developed detailed simulation techniques to model the Columbia teams measured response. This involved calculating the flow of electrons in graphene under the various electric and magnetic fields, accounting for multiple bounces at edges, and quantum mechanical tunneling at the junction. The theoretical analysis also shed light on why it has been so difficult to measure the predicted Veselago lensing in a robust way, and the group is developing new multi-junction device architectures based on this study. Together the experimental data and theoretical simulation gave the researchers a visual map of the refraction, and enabled them to be the first to quantitatively confirm the relationship between the incident and refracted angles (known as Snells Law in optics), as well as confirmation of the magnitude of the transmitted intensity as a function of angle (known as the Fresnel coefficients in optics). In many ways, this intensity of transmission is a more crucial parameter, says Ghosh, since it determines the probability that electrons actually make it past the barrier, rather than just their refracted angles. The transmission ultimately determines many of the performance metrics for devices based on these effects, such as the on-off ratio in a switch, for example. The Science study was supported by Semiconductor Research Corporations NRI Center for Institute for Nanoelectronics Discovery and Exploration (INDEX). The collaboration was made possible through the support of the NRI program, which has brought together some of the countrys best groups in the study of 2D materials to focus on novel device architectures that may outperform conventional silicon-based technologies. For more information, please click Contacts: Holly Evarts Director Strategic Communications and Media Relations Columbia Engineering 212-854-3206 (o) 347-453-7408 (c) Elizabeth Thiel Mather Director of Communications University of Virginia School of Engineering and Applied Science 434-924-1381 (o) 757-319-3664 (c) If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content. Lee Y.K.,Kongju National University | Kim Y.H.,Chonnam National University | Heo J.,Pohang University of Science and Technology | Im W.B.,Chonnam National University | Chung W.J.,Kongju National University Optics Letters | Year: 2014 Phosphor-in-glass (PiG) color converters for LED applications were fabricated with a mixture of phosphors, Y3Al5O 12:Ce3+ (yellow) and CaAlSiN3:Eu2+ (red). The low sintering temperature (550°C) of SiO2-Na 2O-RO (R = Ba, Zn) glass powder enabled the inclusion of CaAlSiN 3:Eu2+ (red) phosphor which cannot be embedded with conventional glass powders for PiGs. By simply varying the mixing ratio of glass to phosphors as well as the ratio of yellow to red phosphors, the facile control of the CIE chromaticity coordinates and correlated color temperature of the LED following the Planckian locus has been achieved. Phosphors were well distributed within the glass matrix without noticeable reactions, preserving the enhanced thermal quenching property of the PiG compared to those with silicone resins, for LEDs. © 2014 Optical Society of America. Jeon T.H.,Kyungpook National University | Choi W.,Pohang University of Science and Technology | Park H.,Kyungpook National University Physical Chemistry Chemical Physics | Year: 2011 BiVO4 semiconductor electrodes were coupled with cobalt-phosphate complexes (CoPi) to enhance the photoelectrochemical (PEC) performance for water oxidation reaction. CoPi was deposited on a 550 nm-thick BiVO4 film via electrodeposition (ED) and photodeposition (PD) methods for comparison of their effects. The CoPi on BiVO4 exhibited Co:P atomic ratios of approximately 1:7 for the electrodeposited sample and approximately 1:18 for the photodeposited sample, and Co2+ and Co3+ co-existed in both samples. Optimized CoPi ED resulted in a CoPi overlayer of approximately 850 nm thick, which showed an electrochromic-like behavior that was likely due to limited access of phosphate into BiVO 4 across the CoPi layer. Optimized CoPi PD, however, had very thin and rather uniform CoPi dispersion and did not show electrochromic-like behavior. Despite the lesser amount of CoPi, the PEC performance of BiVO 4/CoPi (PD) was comparable to that of BiVO4/CoPi (ED). Real-time measurements of the headspace molecular oxygen that evolved from water oxidation indicated that CoPi enhances O2 production and photocurrent generation at BiVO4 by a factor of around 15 and a maximum of 20, respectively, at 0.576 VSCE (equivalent to 1.23 V RHE) under air mass 1.5 irradiation (400 mW cm-2). Prolonged irradiation of BiVO4/CoPi (ED) resulted in a reduced Co:P ratio to 1:1.77 without changing the mixed valency of Co(ii/iii). This finding indicates that incorporation of phosphate into the CoPi was kinetically slower than water oxidation. The primary role of CoPi has been suggested as a hole-conducting electrocatalyst making the photogenerated electrons more mobile and, consequently, increasing conductivity and boosting the PEC water oxidation performance of BiVO4. © 2011 the Owner Societies. Jang H.-G.,Chonnam National University | Min H.-K.,Pohang University of Science and Technology | Hong S.B.,Pohang University of Science and Technology | Seo G.,Chonnam National University Journal of Catalysis | Year: 2013 The reaction intermediates of methanol-to-olefin (MTO) conversion over phosphorous-modified HZSM-5 (P-MFI) catalysts were investigated by electron spin resonance (ESR) and gas chromatography-mass spectroscopy (GC-MS). Phosphorous modification partially neutralized the strong acid sites in HZSM-5. The phosphorous compounds suppress the migration of polymethylbenzenium radical cations, lowering spin-spin interactions and thus preserved their hyperfine splitting. The ESR spectra of the organic species formed in P-MFI zeolites during MTO conversion were similar to those of the cation species generated from 1,2,4,5-tetramethylbenzene in mordenite. GC-MS of organic extracts revealed the predominant formation of polymethylbenzenes with 4-6 methyl groups. The correlation between conversion and the number of spins of tetramethylbenzenium radical cations exhibited that these radical cations might be major active intermediates of MTO conversion over P-MFI catalysts. © 2012 Elsevier Inc. All rights reserved. Hwang H.J.,Pohang University of Science and Technology | Velazquez J.J.L.,Complutense University of Madrid Archive for Rational Mechanics and Analysis | Year: 2010 In this paper we prove global existence for solutions of the Vlasov-Poisson system in convex bounded domains with specular boundary conditions and with a prescribed outward electrical field at the boundary. © Springer-Verlag 2009. Moon P.-G.,Kyungpook National University | You S.,Pohang University of Science and Technology | Lee J.-E.,Kyungpook National University | Hwang D.,Pohang University of Science and Technology | Baek M.-C.,Kyungpook National University Mass Spectrometry Reviews | Year: 2011 A number of highly abundant proteins in urine have been identified through proteomics approaches, and some have been considered as disease-biomarker candidates. These molecules might be clinically useful in diagnosis of various diseases. However, none has proven to be specifically indicative of perturbations of cellular processes in cells associated with urogenital diseases. Exosomes could be released into urine which flows through the kidney, ureter, bladder and urethra, with a process of filtration and reabsorption. Urinary exosomes have been recently suggested as alternative materials that offer new opportunities to identify useful biomarkers, because these exosomes secreted from epithelial cells lining the urinary track might reflect the cellular processes associated with the pathogenesis of diseases in their donor cells. Proteomic analysis of such urinary exosomes assists the search of urinary biomarkers reflecting pathogenesis of various diseases and also helps understanding the function of urinary exosomes in urinary systems. Thus, it has been recently suggested that urinary exosomes are one of the most valuable targets for biomarker development and to understand pathophysiology of relevant diseases. © 2011 Wiley Periodicals, Inc. Hobza P.,Czech Institute of Organic Chemistry And Biochemistry | Hobza P.,Palacky University | Hobza P.,Pohang University of Science and Technology Accounts of Chemical Research | Year: 2012 Although covalent interactions determine the primary structure of a molecule, the noncovalent interactions are responsible for the tertiary and quaternary structure of a molecule and create the fascinating world of the 3D architectures of biomacromolecules. For example, the double helical structure of DNA is of fundamental importance for the function of DNA: it allows it to store and transfer genetic information. To fulfill this role, the structure is rigid to maintain the double helix with a proper positioning of the complementary base, and floppy to allow for its opening. Very strong covalent interactions cannot fulfill both of these criteria, but noncovalent interactions, which are about 2 orders of magnitude weaker, can. This Account highlights the recent advances in the field of the design of novel wave function theory (WFT) methods applicable to noncovalent complexes ranging in size from less than 100 atoms, for which highly accurate ab initio methods are available, up to extended ones (several thousands atoms), which are the domain of semiempirical QM (SQM) methods.Accurate interaction energies for noncovalent complexes are generated by the coupled-cluster technique, taking single- and double-electron excitations iteratively and triple-electron excitation perturbatively with a complete basis set description (CCSD(T)/CBS). The procedure provides interaction energies with high accuracy (error less than 1 kcal/mol). Because the method is computationally demanding, its application is limited to complexes smaller than 30 atoms. But researchers would also like to use computational methods to determine these interaction energies accurately for larger biological and nanoscale structures. Standard QM methods such as MP2, MP3, CCSD, or DFT fail to describe various types of noncovalent systems (H-bonded, stacked, dispersion-controlled, etc.) with comparable accuracy. Therefore, novel methods are needed that have been parametrized toward noncovalent interactions, and existing benchmark data sets represent an important tool for the development of new methods providing reliable characteristics of noncovalent clusters.Our laboratory developed the first suitable data set of CCSD(T)/CBS interaction energies and geometries of various noncovalent complexes, called S22. Since its publication in 2006, it has frequently been applied in parametrization and/or verification of various wave function and density functional techniques. During the intense use of this data set, several inconsistencies emerged, such as the insufficient accuracy of the CCSD(T) correction term or its unbalanced character, which has triggered the introduction of a new, broader, and more accurate data set called the S66 data set. It contains not only 66 CCSD(T)/CBS interaction energies determined in the equilibrium geometries but also 1056 interaction energies calculated at the same level for nonequilibrium geometries. The S22 and S66 data sets have been used for the verification of various WFT methods, and the lowest RMSE (S66, in kcal/mol) was found for the recently introduced SCS-MI-CCSD/CBS (0.08), MP2.5/CBS (0.16), MP2.X/6-31G* (0.27), and SCS-MI-MP2/CBS (0.38) methods. Because of their computational economy, the MP2.5 and MP2.X/6-31G* methods can be recommended for highly accurate calculations of large complexes with up to 100 atoms.The evaluation of SQM methods was based only on the S22 data set, and because some of these methods have been parametrized toward the same data set, the respective results should be taken with caution. For really extended complexes such as protein-ligand systems, only the SMQ methods are applicable. After adding the corrections to the dispersion energy and H-bonding, several methods exhibit surprisingly low RMSE (even below 0.5 kcal/mol). Among the various SMQ methods, the PM6-DH2 can be recommended because of its computational efficiency and it can be used for optimization (which is not the case for other SQM methods). The PM6-DH2 is the base of our novel scoring function used in in silico drug design. © 2012 American Chemical Society. Zhao J.,University of Wollongong | Jiang Z.,University of Wollongong | Lee C.S.,Pohang University of Science and Technology Materials and Design | Year: 2013 In the present work, the effects of hot forging and post-forging heat treatment on the impact fracture toughness and tensile properties of a microalloyed cast steel were investigated. Mechanical tests were used to evaluate the room temperature impact fracture toughness and tensile properties of the steel. The resulting microstructures were analysed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The extraction replica technology was used to investigate the characterisation of complex precipitates formed during heat treatment. The obtained results showed that the coarse-grained microstructure of the forged specimen was significantly refined after post-forging heat treatment. Presence of complex precipitates had a favourable effect on the formation of refined austenite grains, and consequently refined final microstructure. Hot forging was beneficial to enhance the impact fracture toughness and tensile properties of the microalloyed cast steel. After 920. °C-treatment followed by air cooling, the impact energy of the forged specimen was significantly increased from 19.3 to 208.3. J, and further enhancement in tensile properties was obtained. The enhanced impact fracture toughness and tensile properties of the microalloyed cast steel after hot forging and post-forging heat treatment were closely related to the refined and homogenised ferritic-pearlitic microstructure. © 2012 Elsevier Ltd. Park Y.-S.,Kookmin University | Lee J.-S.,Pohang University of Science and Technology Advanced Materials | Year: 2015 A fully feasible and versatile way to fabricate highly reliable organic-transistor memory devices is made possible by a novel design of the charge-trappling layer. Gold@silica (core-shell)-structured nano particles are synthesized and used as the charge-trapping layer. Superior electrical reliability is obtained because the silica shell acts as a built-in tunnel potential barrier. (Figure Presented). © 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Rahman M.R.,University of Illinois at Urbana - Champaign | Boutaba R.,University of Waterloo | Boutaba R.,Pohang University of Science and Technology IEEE Transactions on Network and Service Management | Year: 2013 Network virtualization can offer more flexibility and better manageability for the future Internet by allowing multiple heterogeneous virtual networks (VN) to coexist on a shared infrastructure provider (InP) network. A major challenge in this respect is the VN embedding problem that deals with the efficient mapping of virtual resources on InP network resources. Previous research focused on heuristic algorithms for the VN embedding problem assuming that the InP network remains operational at all times. In this paper, we remove this assumption by formulating the survivable virtual network embedding (SVNE) problem. We then develop a pro-active, and a hybrid policy heuristic to solve it, and a baseline policy heuristic to compare to. The hybrid policy is based on a fast re-routing strategy and utilizes a pre-reserved quota for backup on each physical link. Our evaluation results show that our proposed heuristics for SVNE outperform the baseline heuristic in terms of long term business profit for the InP, acceptance ratio, bandwidth efficiency, and response time. © 2004-2012 IEEE. Huh S.,Pohang University of Science and Technology | Park J.,Pohang University of Science and Technology | Kim K.S.,Pohang University of Science and Technology | Hong B.H.,Advanced Institute of Nanotechnology SAINT | And 2 more authors. ACS Nano | Year: 2011 Selective n-type doping of graphene is developed by utilizing patternable gold nanoparticles functionalized with photoreactive cinnamate moieties. The gold nanoparticles can be regularly patterned on the graphene by UV-induced cross-linking of cinnamate, which provides a convenient method to control the optical and electrical properties of graphene site-specifically. The strong n-type doping of graphene covered with the patterned gold nanoparticles was confirmed by Raman, X-ray photoelectron spectroscopy, and electron transport measurements. We believe that our method would find numerous applications in the area of graphene-based optoelectronics including light-emitting devices, solar cells, and optical sensors. © 2011 American Chemical Society. Jeong I.,Pohang University of Science and Technology | Hyon B.J.,Pohang University of Science and Technology | Hyon B.J.,Hyundai Mobis | Nam K.,Pohang University of Science and Technology IEEE Transactions on Power Electronics | Year: 2013 A dynamic model for a faulted surface-mount permanent magnet synchronous motor (SPMSM) is derived using a deformed flux model. In reflecting the internal turn short into the dynamics, the variations in inductance and back EMF term were considered. Then, the faulted model was transformed into the two synchronous$dq\$-models: one for the positive sequence and the other for the negative sequence. Also, a torque equation, which shows the relation between the ripple and the negative sequence current, is derived. The negative sequence current should be suppressed to eliminate the torque ripple. The dual current controller is utilized for this purpose: in the dual controller, the positive and negative sequences are controlled separately in their own synchronous frames. Notch filters are utilized in each synchronous frame to extract positive or negative sequence component. Experiments were performed with an SPMSM specially designed to make an internal turn short artificially. The experimental results coincide well with the corresponding simulation results, and exhibit a strength of the dual current controller in suppressing the negative sequence current. © 2012 IEEE.

Kolaski M.,Pohang University of Science and Technology | Kolaski M.,University of Silesia | Arunkumar C.R.,Pohang University of Science and Technology | Kim K.S.,Pohang University of Science and Technology
Journal of Chemical Theory and Computation | Year: 2013

Excited dimers (excimers) formed by aromatic molecules are important in biological systems as well as in chemical sensing. The structure of many biological systems is governed by excimer formation. Since theoretical studies of such systems provide important information about mutual arrangement of aromatic molecules in structural biology, we carried out extensive calculations on the benzene excimer using EOM-CCSD, RI-CC2, CASPT2, and TD-DFT approaches. For the benzene excimer, we evaluate the reliability of the TD-DFT method based on the B3LYP, PBE, PBE0, and ωPBEh functionals. We extended the calculations to naphthalene, anthracene, and pyrene excimers. We find that nearly parallel stacked forms are the minimum energy structure. On the basis of the benzene to pyrene excimers, we might roughly estimate the equilibrium layer-to-layer distance for bilayer-long arenes in the first singlet excited state, which is predicted to be bound. © 2012 American Chemical Society.

Bak A.,Kyungpook National University | Choi W.,Pohang University of Science and Technology | Park H.,Kyungpook National University
Applied Catalysis B: Environmental | Year: 2011

Photoelectrochemical (PEC) water oxidation using hematite (α-Fe2O3) is of great interest in terms of solar fuels and artificial photosynthesis. In this study, Cd-incorporated nanocrystalline hematite films (Cd-Fe2O3) supported on conducting glass have been prepared via co-electrodeposition of aqueous Fe(III) and Cd(II) with varying Cd:Fe atomic ratios (up to 3.2at.%) and optimized for their PEC performances under a simulated solar light (AM 1.5-irradiation). Surface analysis indicates that the Cd co-deposition increases the hematite particle size from ca. 50nm to 70-100nm due to interparticle agglomeration and decreases the overall UV-Vis absorbance of hematite. X-ray photoelectron spectroscopic study also indicates that Cd incorporation shifts the binding energy of oxygen atoms to lower energy direction whereas it does not affect the binding energy of Fe 3d. This suggests that Cd exists mainly as CdO and/or Cd(OH)2 in the hematite surface. When an optimal level of Cd content (∼1at.%) is electrodeposited, the photocurrent of hematite film is significantly enhanced by a factor of ca. four at E=1.23VRHE under AM 1.5-irradiation and the photoactive spectral region is red-shifted. Electrochemical impedance spectroscopic analysis further reveals that the flat band potential of hematite is shifted by ca. -30mV to negative potential direction and the charge transfer resistance (Rct) is significantly reduced by Cd incorporation. Detailed surface analyses, optimization for preparation condition of hematite films, and discussion for PEC behaviors were described. © 2011 Elsevier B.V.

Lee S.-J.,Pohang University of Science and Technology | Kim J.,Pohang University of Science and Technology | Kane S.N.,Devi Ahilya University | Cooman B.C.D.,Pohang University of Science and Technology
Acta Materialia | Year: 2011

Room temperature dynamic strain aging (DSA) is often observed from the beginning of plastic deformation in high Mn Fe-18% Mn-0.6% C and Fe-22% Mn-0.6% C twinning-induced plasticitysteels. Although the phenomenon is in many cases very pronounced, there have up to now been no attempts to explain the phenomenon of room temperature DSA in TWIP steel containing solute C. It is proposed that DSA occurs by a single diffusive jump of the C atom of the point defect complex in the stacking fault region. DSA is only observed when the C atom reorientation time is smaller than the residence time of the stacking fault at the location of the point defect complex. The latter interaction can explain the DSA-suppressing effect of Al, which increases the stacking fault energy. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Georgakilas V.,Greek National Center For Scientific Research | Otyepka M.,Palacky University | Bourlinos A.B.,Palacky University | Chandra V.,Pohang University of Science and Technology | And 7 more authors.
Chemical Reviews | Year: 2012

Graphene, the two-dimensional sp2-hybridized carbon, is currently, without any doubt, the most intensively studied material. This single-atom-thick sheet of carbon atoms arrayed in a honeycomb pattern is the world's thinnest, strongest, and stiffest material, as well as being an excellent conductor of both heat and electricity. Despite the great application potential, it is worth mentioning that graphene itself possesses zero band gap as well as inertness to reaction, which weakens the competitive strength of grapheme in the field of semiconductors and sensors. The functionalization of pristine graphene sheets with organic functional groups has been developed for several purposes. The main purpose is the dispersibility of graphene in common organic solvents that is usually obtained after attachment of certain organic groups. The dispersion of graphene sheets in organic solvents is a crucial move toward the formation of nanocomposite materials with graphene.

Park H.,Kyungpook National University | Park Y.,Pohang University of Science and Technology | Kim W.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology
Journal of Photochemistry and Photobiology C: Photochemistry Reviews | Year: 2013

This paper reviews recent studies on the semiconductor photocatalysis based on surface-modified TiO2 of which application is mainly focused on environmental remediation. TiO2 photocatalysis that is based on the photoinduced interfacial charge transfer has been extensively studied over the past four decades. A great number of modification methods of semiconductor photocatalysts have been developed and investigated to accelerate the photoconversion, to enable the absorption of visible light, or to alter the reaction mechanism to control the products and intermediates. In this regard, various modification methods of TiO2 are classified according to the kind of surface modifiers (metal-loading, impurity doping, inorganic adsorbates, polymer coating, dye-sensitization, charge transfer complexation) and their effects on photocatalytic reaction mechanism and kinetics are discussed in detail. Modifying TiO2 in various ways not only changes the mechanism and kinetics under UV irradiation but also introduces visible light activity that is absent with pure TiO2. Each modification method influences the photocatalytic activity and mechanism in a way different from others and the observed modification effects are often different depending on the test substrates and conditions even for the same modification method. Better understanding of the modification effects on TiO2 photocatalysis is necessary to obtain reliable results, to assess the photoconversion efficiency more quantitatively, and to further improve the modification methods. © 2012 Elsevier B.V.

Kamal T.,Kyungpook National University | Shin T.J.,Pohang University of Science and Technology | Park S.-Y.,Kyungpook National University
Macromolecules | Year: 2012

The structural evolution of poly(ε-caprolactone) (PCL) during uniaxial tensile deformation at 25 °C was examined using small- and wide-angle X-ray scatterings (SAXS and WAXS) techniques with simultaneous stress and strain (S-S) curves. A high-energy X-ray beam at the recently upgraded Pohang synchrotron radiation source revealed the complete lamellar deformation behavior of PCL. Slope-based division of the S-S curves indicated three distinct regions of elastic (region I), yielding (region II) and plastic deformations (region III). In region I, which showed elastic deformation, the WAXS patterns were isotropic, whereas the SAXS patterns became oblate due to elongation of the amorphous chains along the draw direction. In region II, which showed yielding deformation, the WAXS patterns showed a slight orientation, whereas the SAXS patterns exhibited a change from oblate to four-point and to six-point patterns due to the simultaneous fragmentation and melting of the chain-folded lamellae (leading to the four-point pattern) and the subsequent formation of chain-extended lamellae (adding another two maxima along the meridian). In region III, the WAXS patterns revealed the development of the orientation of PCL crystals, whereas SAXS patterns exhibited a two-point pattern. The newly formed chain-extended lamellae in regions II and III might produce network junctions that can transfer an applied force to the PCL crystals for increased orientation. The six-point pattern in region II for PCL was not observed or reported in the past during the uniaxial tensile deformation experiment. This might be due to fast acquisition of the X-ray patterns during mechanical drawing using synchrotron radiation. © 2012 American Chemical Society.

Park D.,Kyungpook National University | Yun Y.-S.,Chonbuk National University | Park J.M.,Pohang University of Science and Technology
Biotechnology and Bioprocess Engineering | Year: 2010

The discovery and further development of biosorption phenomena provide a basis for a whole new technology aimed at the removal of various pollutants or the recovery of valuable resources from aqueous systems. Today, biosorption is one of the main components of environmental and bioresource technology. Since the status of scientific development of a technology can be reflected through analyses of the literatures pertaining to it, in this review, we qualitatively examine almost all aspects of biosorption research. A range of subjects are covered, including the initial history, raw materials, mechanisms, instrumental tools, process factors, modification and immobilization methods, recovery and regeneration, continuous processes, commercial application, and modeling studies of biosorption. Finally, we summarized the important considerations of the current research on biosorption, as well as the suggestions for its future directions. We believe that this review will prove to be useful for scientists and engineers in the performance of their research into biosorption. © KSBB.

Choi S.K.,Kyungpook National University | Choi W.,Pohang University of Science and Technology | Park H.,Kyungpook National University
Physical Chemistry Chemical Physics | Year: 2013

A naturally abundant nickel-borate (Ni-Bi) complex is demonstrated to successfully catalyze the photoelectrochemical (PEC) water oxidation of BiVO4 electrodes at 1.23 VRHE with nearly 100% faradaic efficiency for oxygen evolution. Ni-Bi is electrodeposited (ED) and photodeposited (PD) for varying times on BiVO4 electrodes in the 0.1 M borate electrolyte with 1 mM Ni2+ at pH 9.2. Surprisingly, optimally deposited Ni-Bi films (ED-10 s and PD-30 min) display the same layer thickness of ca. 40 nm. Both Ni-Bi films enhance the photocurrent generation of BiVO4 at 1.23 VRHE by a factor of 3-4 under AM 1.5-light irradiation (100 mW cm-2) along with ca. 250% increase in the incident and absorbed photon-to-current efficiencies. Impedance analysis further reveals that the charge transfer resistance at BiVO4 is markedly decreased by Ni-Bi deposits. The primary role of Ni-Bi has been suggested to be a hole-conductor making photogenerated electrons more mobile and catalyzing a four-hole transfer to water through cyclic changes between the lower and higher Ni oxidation states. However, thick Ni-Bi films (>∼40 nm) significantly reduce the PEC performance of BiVO4 due to the kinetic bottleneck and charge recombination. Under identical PEC conditions (0.1 M, pH 9.2), the borate electrolyte (good proton acceptor) is found to be better than nitrate (poor proton acceptor), indicative of a proton-coupled electron transfer pathway in PEC water oxidation. © 2013 the Owner Societies.

Lee Y.K.,Kongju National University | Lee J.S.,Chonnam National University | Heo J.,Pohang University of Science and Technology | Im W.B.,Chonnam National University | Chung W.J.,Kongju National University
Optics Letters | Year: 2012

Phosphor-in-glass (PiG) typed robust color converters were fabricated using Pb-free silicate glasses for high-power white LED applications. SiO 2-B2O3-RO(R = Ba, Zn) glass powder showed good sintering behavior and high visible transparency under the sintering condition of 750 °C for 30 min without noticeable interaction with phosphors. By simply changing the thickness of the PiG plate or mixing ratio of glass to Y3Al5O12:Ce3+ phosphor, CIE chromaticity coordinates of the LED can be easily controlled. Enhanced thermal quenching property of PiG compared to phosphor with conventional silicone resin suggests its prominent feasibility for high-power/high-brightness white LEDs. © 2012 Optical Society of America.

Ham Y.-G.,Chonnam National University | Kug J.-S.,Pohang University of Science and Technology
Climate Dynamics | Year: 2014

In this study, the El Nino-Southern Oscillation (ENSO) phase-locking to the boreal winter in CMIP3 and CMIP5 models is examined. It is found that the models that are poor at simulating the winter ENSO peak tend to simulate colder seasonal-mean sea-surface temperature (SST) during the boreal summer and associated shallower thermocline depth over the eastern Pacific. These models tend to amplify zonal advection and thermocline depth feedback during boreal summer. In addition, the colder eastern Pacific SST in the model can reduce the summertime mean local convective activity, which tends to weaken the atmospheric response to the ENSO SST forcing. It is also revealed that these models have more serious climatological biases over the tropical Pacific, implying that a realistic simulation of the climatological fields may help to simulate winter ENSO peak better. The models that are poor at simulating ENSO peak in winter also show excessive anomalous SST warming over the western Pacific during boreal winter of the El Nino events, which leads to strong local convective anomalies. This prevents the southward shift of El Nino-related westerly during boreal winter season. Therefore, equatorial westerly is prevailed over the western Pacific to further development of ENSO-related SST during boreal winter. This bias in the SST anomaly is partly due to the climatological dry biases over the central Pacific, which confines ENSO-related precipitation and westerly responses over the western Pacific. © 2014 Springer-Verlag Berlin Heidelberg.

Park M.B.,Pohang University of Science and Technology | Cho S.J.,Chonnam National University | Hong S.B.,Pohang University of Science and Technology
Journal of the American Chemical Society | Year: 2011

Aluminosilicate and gallosilicate zeolite syntheses via a charge density mismatch (CDM) approach are compared at intermediate-silica compositions (Si/Me = 5-16, where Me is Al or Ga). With a variation of the crystallization temperature and of the type and/or concentration of alkali metal ions added as a crystallization structure-directing agent (SDA) to tetraethylammonium- tetramethylammonium, tetraethylammonium-hexamethonium, and strontium-choline mixed-SDA systems, we were able to obtain 11 different zeolite structures. However, only 5 out of a total 40 pairs of aluminosilicate and gallosilicate synthesis runs at otherwise identical chemical compositions were found to give the same zeolite product with no detectable impurities, suggesting that the structure-directing ability of Ga is quite different from that of Al even in intermediate-silica synthesis conditions. The CDM approach to offretite synthesis led to hexagonal plate-like crystals with aspect ratios lower than 0.3, and UZM-22 exhibited no significant preference of Al substitution for particular tetrahedral sites, especially for site T1, unlike its framework type material ZSM-18. More interestingly, the EU-1 zeolite obtained from an aluminosilicate synthesis mixture containing Li + as an inorganic crystallization SDA in the tetraethylammonium-hexamethonium double-organic additive system has been characterized to locate about half of its Li + ions in the framework, while the Li distribution over the 10 topologically different tetrahedral sites is nonrandom in nature. © 2011 American Chemical Society.

Doh J.,Pohang University of Science and Technology | Kim M.,Pohang University of Science and Technology | Krummel M.F.,University of California at San Francisco
Biomaterials | Year: 2010

Cell-cell cooperativity in populations of motile and transiently interacting immune cells has been difficult to assess in the absence of tools to control proximity and communication. Here, we describe the generation of cell-laden microwells that can precisely control contact-mediated interactions and multicellular 'quorum' decisions in lymphocytes. Different types of fate decisions for activating T cells can be shown to variously obey 'binary' or 'density' outcomes, correlated with cell-cell contact, using this new platform. © 2010 Elsevier Ltd. All rights reserved.

Chowdhury M.,University of California at Berkeley | Rahman M.R.,University of Illinois at Urbana - Champaign | Boutaba R.,University of Waterloo | Boutaba R.,Pohang University of Science and Technology
IEEE/ACM Transactions on Networking | Year: 2012

Network virtualization allows multiple heterogeneous virtual networks (VNs) to coexist on a shared infrastructure. Efficient mapping of virtual nodes and virtual links of a VN request onto substrate network resources, also known as the VN embedding problem, is the first step toward enabling such multiplicity. Since this problem is known to be NP-hard, previous research focused on designing heuristic-based algorithms that had clear separation between the node mapping and the link mapping phases. In this paper, we present ViNEYard-a collection of VN embedding algorithms that leverage better coordination between the two phases. We formulate the VN embedding problem as a mixed integer program through substrate network augmentation.We then relax the integer constraints to obtain a linear program and devise two online VN embedding algorithms D-ViNE and R-ViNE using deterministic and randomized rounding techniques, respectively. We also present a generalized window-based VN embedding algorithm (WiNE) to evaluate the effect of lookahead on VN embedding. Our simulation experiments on a large mix of VN requests show that the proposed algorithms increase the acceptance ratio and the revenue while decreasing the cost incurred by the substrate network in the long run. © 2011 IEEE.

Park H.,Kyungpook National University | Kim Y.K.,Kyungpook National University | Choi W.,Pohang University of Science and Technology
Journal of Physical Chemistry C | Year: 2011

A facile synthesis of high efficiency semiconductor photocatalyst hybrids is of great importance in making the photocatalytic systems more viable and applicable. This study presents that simply reversing chemical precipitation order of CdS results in significantly different photocatalytic activity in terms of hydrogen production from water under visible light when hybridized with platinized TiO2 particles (Pt-TiO2). It has been found that CdS obtained via dropping an aqueous cadmium cation in aqueous sulfide solution (i.e., Pt-TiO2 suspension with S2-) with equal molar ratios (hereafter CdSR) has a maximum >10-fold greater amount of hydrogen than that obtained by simply reversing the dropping order (i.e., dropping S2- to Pt-TiO2 suspension with Cd 2+; hereafter CdRS). Such a high activity of CdS R, however, is very sensitive to photocatalytic running conditions, in particular, kind and concentration of electron donor (Na2S and/or Na2SO3) which largely changes the hydrogen production ratio (RH) of CdSR to CdRS. Detailed surface analyses indicate that physicochemical properties of CdSR are very different from those of CdRS including larger and red-shifted onset light absorption and altered photoluminescence, S/Cd atomic ratios >1, and hexagonal crystallinity (vs cubic-CdRS), the differences of which were attributed to the primary reasons for higher activity of CdSR. Finally, the photocatalytic hydrogen production mechanism was proposed based on the experimental results. © 2011 American Chemical Society.

Jeon T.H.,Kyungpook National University | Choi W.,Pohang University of Science and Technology | Park H.,Kyungpook National University
Journal of Physical Chemistry C | Year: 2011

Nanocrystalline hematite particles (α-Fe2O3) were electrodeposited on the TiO2 nanotube (TiNT) arrays that were fabricated via anodization of Ti foils. The short precontact time (1 h) of aqueous ferric ions (Fe3+) on TiNT resulted in formation of hematite particles selectively on the mouth surface of TiNT (hematite@1 h/TiNT), whereas the long precontact time (24 h) resulted in complete filling of the TiNT inside and an even full-covering of the TiNT top surface with the hematite particles (hematite@24 h/TiNT). For comparison, hematite particles were also electrodeposited on TiO2-nanoparticulate films obtained via oxidative annealing of Ti foil resulting in hematite fully covered TiO2 nanoparticles (hematite/TiNP). Photoelectrochemical (PEC) study with AM 1.5 light (UV + Vis) indicated that the PEC activity of TiNT decreased by ca. 40% and almost completely vanished when hematite covered the full surface of TiNT (hematite@ 24 h/TiNT) and loaded on the mouth surface of TiNT (hematite@1 h/TiNT), respectively. The relatively higher PEC activity of hematite@24 h/TiNT was further observed under varying visible light conditions (400 nm < < 500 nm). Hematite/TiNP also has ca. 40%-reduced PEC activity as compared to TiNP under AM 1.5 light, the tendency of which is similar to hematite@24 h/TiNT. Photocatalytic (PC) activities of TiNT and hematite/TiNT for degradation of aqueous phenol under AM 1.5-light were also compared, which indicates that the PC activity of TiNT vanishes almost completely with hematite@1 h/TiNT, whereas it is recovered at a moderate level with hematite@24 h/TiNT. All of these PEC and PC behaviors of TiNT and hematite/TiNT were discussed in terms of hematite-induced charge recombination due to an energy level mismatch between TiO2 and hematite, as well as surface-specific photoactivity of TiNT (i.e., mouth surface vs interwall and/or underlying base layer). Various surface analysis techniques (XRD, XPS, TEM, UV-vis diffuse reflectance) were employed to understand the surface states of TiNT and hematite/TiNT. Finally, more detailed charge transfer mechanism was proposed. © 2011 American Chemical Society.

News Article | December 8, 2016

Seungbae Park, professor of Division of General Studies at UNIST has recently published his new work, "Great Debates in Philosophy", which presents important current issues in philosophy in the form of a debate. This book, published by Korean Studies Information Co., Ltd., provides the public an opportunity to gain new insights into issues in general philosophy of science. The book consists of seven chapters that cover the following topics: philosophy of religion, ethics, philosophy of mind, epistemology, evolution, as well as personal identity. Each chapter in this book is developed from a series of lectures, presented over the last 15 years at Arizona university, University of Maryland, as well as UNIST. After a general introduction to philosophy and logic and an explanation of argument analysis, the book explores contemporary issues in philosophy, stimulating debate and critical thinking among readers of all levels. His book not only introduces students to great philosophical thinkers, it also teaches them the essential skills of rigorous critical thinking. Professor Seungbae Park joined the faculty of UNIST in 2009, as an Associate Professor with research and teaching focus in the philosophical and ethical aspects of science and technology. Prior to joining UNIST, Professor Park has also held several faculty positions at the University of Arizona, the University of Maryland-College Park, and Pohang University of Science and Technology. Professor Park received his Ph.D in Philosophy of Science from the University of Arizona (2001), under the guidance of Dr. Richard A. Healey. He has published more than 20 publications that are regarded as the highly cited papers and some of them belong to the top 5% in their field, according to Scimago Journal & Country Ranking (SJR).

News Article | December 16, 2015
Site: www.nanotech-now.com

Home > Press > Spintronics, low-energy electricity take a step closer: A new class of topological insulators discovered Abstract: Topological insulators are materials that let electric current flow across their surface while keeping it from passing it through their bulk. This exotic property makes topological insulators very promising for electricity with less energy loss, spintronics, and perhaps even quantum computing. EPFL scientists have now identified a new class of topological insulators, and have discovered its first representative material, which could propel topological insulators into applications. The work, which was carried out within the framework of the EPFL-led NCCR Marvel project, is published in Nature Materials. The technological promise of topological insulators has led to an intense search for optimal natural and man-made materials with such properties. Such research combines theoretical work that predicts what properties the structure of a particular material would have. The "candidate" materials that are identified with computer simulations are then passed for experimental examination to see if their topological insulating properties match the theoretical predictions. This is what the lab of Oleg Yazyev at EPFL's Institute of Theoretical Physics has accomplished, working with experimentalist colleagues from around the world. By theoretically testing potential candidates from the database of previously described materials, the team has identified a material, described as a "crystalline phase" of bismuth iodide, as the first of a new class of topological insulators. What makes this material particularly exciting is the fact that its atomic structure does not resemble any other topological insulator known to date, which makes its properties very different as well. One clear advantage of bismuth iodide is that its structure is more ordered than that of previously known topological insulators, and with fewer natural defects. In order to have an insulating interior, a material must have as few defects in its structure as possible. "What we want is to pass current across the surface but not the interior," explains Oleg Yazyev. "In theory, this sounds like an easy task, but in practice you'll always have defects. So you need to find a new material with as few of them as possible." The study shows that even these early samples of bismuth iodide appear to be very clean with very small concentration of structural imperfections. After characterizing bismuth iodide with theoretical tools, the scientists tested it experimentally with an array of methods. The main evidence came from a direct experimental technique called "angle-resolved photoemission spectroscopy". This method allows researchers to "see" electronic states on the surface of a solid material, and has become a key technique for proving the topological nature of electronic states at the surface. The measurements, carried out at the Lawrence Berkeley National Lab, proved to be fully consistent with the theoretical predictions made by Gabriel Autès, a postdoc at Yazyev's lab and lead author of the study. The actual electron structure calculations were performed at the Swiss National Supercomputing Centre, while data analysis included a number of scientists from EPFL and other institutions. "This study began as theory and went through the entire chain of experimental verification," says Yazyev. "For us is a very important collaborative effort." His lab is now exploring further the properties of bismuth iodide, as well materials with similar structures. Meanwhile, other labs are joining the effort to support the theory behind the new class of topological insulators and propagate the experimental efforts. ### This study was carried out within the framework of NCCR Marvel, a research effort on Computational Design and Discovery of Novel Materials, created by the Swiss National Science Foundation and led by EPFL. It currently includes 33 labs across 11 Swiss institutions. The work presented here involved a collaboration of EPFL's Institute of Theoretical Physics and Institute of Condensed Matter Physics with TU Dresden; the Lawrence Berkeley National Laboratory; the University of California, Berkeley; Lomonosov Moscow State University; Ulm University; Yonsei University; Pohang University of Science and Technology; and the Institute for Basic Science, Pohang. The study was funded by the Swiss National Science Foundation, the ERC, NCCR-MARVEL, the Deutsche Forschungsgemeinschaft, the U.S. Department of Energy, and the Carl-Zeiss Foundation. Reference Autès G, Isaeva A, Moreschini L, Johannsen JC, Pisoni A, Mori R, Zhang W, Filatova TG, Kuznetsov AN, Forró L, Van den Broek W, Kim Y, Kim KS, Lanzara A, Denlinger JD, Rotenberg E, Bostwick A, Grioni M, Yazyev OV. A Novel Quasi-One-Dimensional Topological Insulator in Bismuth Iodide β-Bi4I4. Nature Materials 14 December 2015. DOI: 10.1038/nmat4488 For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

News Article | March 28, 2016
Site: phys.org

Majority of human Src homology 2 domains not only bind to proteins, but also interact with membrane lipids with high affinity and specificity. The SH2 domain-containing proteins play important roles in various physiological processes and are involved in cancer development. This study reveals how lipids control SH2 domain-mediated cellular protein interaction networks and suggests a new strategy for the therapeutic modulation of pY-signaling pathways. Prof. You-Me Kim and her student Dajung Jung at Pohang University of Science and Technology (POSTECH), in collaboration with Prof. Wonhwa Cho's group at the University of Illinois at Chicago, have identified that the majority of human Src homology 2 (SH2) domains not only bind to proteins, but also interact with membrane lipids with high affinity and specificity. Their research was published in the online edition of Molecular Cell on March 24th. The SH2 domain interacts with proteins and participates in intracellular signaling by binding to phosphortyrosine (pY) residues of partner proteins. Their mode of interaction with other proteins has been well characterized for a long time. Prof. Kim and her team found that the newly identified lipid binding by the SH2 domain is evolutionarily conserved, suggesting that the interaction serves as an important function for controlling intracellular signal transmission. The SH2 domain-containing proteins play important roles in various physiological processes and are involved in cancer development. This study reveals how lipids control SH2 domain-mediated cellular protein interaction networks and suggests a new strategy for the therapeutic modulation of pY-signaling pathways. Specific inhibitors blocking the SH2 domain-lipid interaction can potentially be developed as an anti-cancer drug. Explore further: Chemists get grip on slippery lipids

News Article | October 5, 2016
Site: www.rdmag.com

A team led by Cory Dean, assistant professor of physics at Columbia University, Avik Ghosh, professor of electrical and computer engineering at the University of Virginia, and James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, has directly observed--for the first time--negative refraction for electrons passing across a boundary between two regions in a conducting material. First predicted in 2007, this effect has been difficult to confirm experimentally. The researchers were able to observe the effect in graphene, demonstrating that electrons in the atomically thin material behave like light rays, which can be manipulated by such optical devices as lenses and prisms. The findings, which are published in the September 30 edition of Science, could lead to the development of new types of electron switches, based on the principles of optics rather than electronics. "The ability to manipulate electrons in a conducting material like light rays opens up entirely new ways of thinking about electronics," says Dean. "For example, the switches that make up computer chips operate by turning the entire device on or off, and this consumes significant power. Using lensing to steer an electron 'beam' between electrodes could be dramatically more efficient, solving one of the critical bottlenecks to achieving faster and more energy efficient electronics." Dean adds, "These findings could also enable new experimental probes. For example, electron lensing could enable on-chip versions of an electron microscope, with the ability to perform atomic scale imageing and diagnostics. Other components inspired by optics, such as beam splitters and interferometers, could additionally enable new studies of the quantum nature of electrons in the solid state." While graphene has been widely explored for supporting high electron speed, it is notoriously hard to turn off the electrons without hurting their mobility. Ghosh says, "The natural follow-up is to see if we can achieve a strong current turn-off in graphene with multiple angled junctions. If that works to our satisfaction, we'll have on our hands a low-power, ultra-high-speed switching device for both analog (RF) and digital (CMOS) electronics, potentially mitigating many of the challenges we face with the high energy cost and thermal budget of present day electronics." Light changes direction - or refracts - when passing from one material to another, a process that allows us to use lenses and prisms to focus and steer light. A quantity known as the index of refraction determines the degree of bending at the boundary, and is positive for conventional materials such as glass. However, through clever engineering, it is also possible to create optical "metamaterials" with a negative index, in which the angle of refraction is also negative. "This can have unusual and dramatic consequences," Hone notes. "Optical metamaterials are enabling exotic and important new technologies such as super lenses, which can focus beyond the diffraction limit, and optical cloaks, which make objects invisible by bending light around them." Electrons travelling through very pure conductors can travel in straight lines like light rays, enabling optics-like phenomena to emerge. In materials, the electron density plays a similar role to the index of refraction, and electrons refract when they pass from a region of one density to another. Moreover, current carriers in materials can either behave like they are negatively charged (electrons) or positively charged (holes), depending on whether they inhabit the conduction or the valence band. In fact, boundaries between hole-type and electron-type conductors, known as p-n junctions ("p" positive, "n" negative), form the building blocks of electrical devices such as diodes and transistors. "Unlike in optical materials", says Hone, "where creating a negative index metamaterial is a significant engineering challenge, negative electron refraction occurs naturally in solid state materials at any p-n junction." The development of two-dimensional conducting layers in high-purity semiconductors such as GaAs (Gallium arsenide) in the 1980s and 1990s allowed researchers to first demonstrate electron optics including the effects of both refraction and lensing. However, in these materials, electrons travel without scattering only at very low temperatures, limiting technological applications. Furthermore, the presence of an energy gap between the conduction and valence band scatters electrons at interfaces and prevents observation of negative refraction in semiconductor p-n junctions. In this study, the researchers' use of graphene, a 2D material with unsurpassed performance at room temperature and no energy gap, overcame both of these limitations. The possibility of negative refraction at graphene p-n junctions was first proposed in 2007 by theorists working at both the University of Lancaster and Columbia University. However, observation of this effect requires extremely clean devices, such that the electrons can travel ballistically, without scattering, over long distances. Over the past decade, a multidisciplinary team at Columbia - including Hone and Dean, along with Kenneth Shepard, Lau Family Professor of Electrical Engineering and professor of biomedical engineering, Abhay Pasupathy, associate professor of physics, and Philip Kim, professor of physic at the time (now at Harvard) - has worked to develop new techniques to construct extremely clean graphene devices. This effort culminated in the 2013 demonstration of ballistic transport over a length scale in excess of 20 microns. Since then, they have been attempting to develop a Veselago lens, which focuses electrons to a single point using negative refraction. But they were unable to observe such an effect and found their results puzzling. In 2015, a group at Pohang University of Science and Technology in South Korea reported the first evidence focusing in a Veselago-type device. However, the response was weak, appearing in the signal derivative. The Columbia team decided that to fully understand why the effect was so elusive, they needed to isolate and map the flow of electrons across the junction. They utilized a well-developed technique called "magnetic focusing" to inject electrons onto the p-n junction. By measuring transmission between electrodes on opposite sides of the junction as a function of carrier density they could map the trajectory of electrons on both sides of the p-n junction as the incident angle was changed by tuning the magnetic field. Crucial to the Columbia effort was the theoretical support provided by Ghosh's group at the University of Virginia, who developed detailed simulation techniques to model the Columbia team's measured response. This involved calculating the flow of electrons in graphene under the various electric and magnetic fields, accounting for multiple bounces at edges, and quantum mechanical tunneling at the junction. The theoretical analysis also shed light on why it has been so difficult to measure the predicted Veselago lensing in a robust way, and the group is developing new multi-junction device architectures based on this study. Together the experimental data and theoretical simulation gave the researchers a visual map of the refraction, and enabled them to be the first to quantitatively confirm the relationship between the incident and refracted angles (known as Snell's Law in optics), as well as confirmation of the magnitude of the transmitted intensity as a function of angle (known as the Fresnel coefficients in optics). "In many ways, this intensity of transmission is a more crucial parameter," says Ghosh, "since it determines the probability that electrons actually make it past the barrier, rather than just their refracted angles. The transmission ultimately determines many of the performance metrics for devices based on these effects, such as the on-off ratio in a switch, for example."

Chandra V.,Pohang University of Science and Technology | Park J.,Pohang University of Science and Technology | Chun Y.,Pohang University of Science and Technology | Lee J.W.,Pohang University of Science and Technology | And 2 more authors.
ACS Nano | Year: 2010

Magnetite - graphene hybrids have been synthesized via a chemical reaction with a magnetite particle size of ∼10 nm. The composites are superparamagnetic at room temperature and can be separated by an external magnetic field. As compared to bare magnetite particles, the hybrids show a high binding capacity for As(III) and As(V), whose presence in the drinking water in wide areas of South Asia has been a huge problem. Their high binding capacity is due to the increased adsorption sites in the M - RGO composite which occurs by reducing the aggregation of bare magnetite. Since the composites show near complete (over 99.9%) arsenic removal within 1 ppb, they are practically usable for arsenic separation from water. © 2010 American Chemical Society.

Jin K.-H.,Pohang University of Science and Technology | Choi S.-M.,Pohang University of Science and Technology | Jhi S.-H.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The adsorption of alkali metals (AMs) on single layer graphene is studied using first principles methods. We observe a common trend in the binding distance, the charge transfer, and the work function (W) at certain coverage of AMs with increase in the proportion ρ (adatom/C atom) of the graphene covered by the AM. A dip in these properties occurs at ρ≈0.04 for all AMs except Li, for which it occurs at ρ≈0.08. This behavior is due to a transition of adsorbed metals from individual atoms to two-dimensional metallic sheets that exert a depolarization effect. W of graphene exhibits asymmetric dependence on ρ: a dip in the adatom layer side but saturation on the graphene side, which is in contrast to the case of bulk graphite. © 2010 The American Physical Society.

Jang J.-W.,Pohang University of Science and Technology | Kwon J.,Korea Atomic Energy Research Institute | Lee B.-J.,Pohang University of Science and Technology
Scripta Materialia | Year: 2010

The effect of stress on self-diffusion in body-centered cubic Fe has been investigated using a molecular dynamics simulation. The diffusivities under hydrostatic, uniaxial and shear stresses are calculated and analyzed to clarify the governing factors that affect diffusion under these stresses. The diffusivity is retarded by compressive pressures, enhanced by shear stresses, and shows an intermediate behavior under uniaxial stresses. The mechanism for these stress dependencies is discussed in terms of the effect of stress on vacancy formation enthalpy and migration energy. © 2010 Acta Materialia Inc.

Bokare A.D.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology
Journal of Hazardous Materials | Year: 2014

Iron-catalyzed hydrogen peroxide decomposition for in situ generation of hydroxyl radicals (HO•) has been extensively developed as advanced oxidation processes (AOPs) for environmental applications. A variety of catalytic iron species constituting metal salts (in Fe2+ or Fe3+ form), metal oxides (e.g., Fe2O3, Fe3O4), and zero-valent metal (Fe0) have been exploited for chemical (classical Fenton), photochemical (photo-Fenton) and electrochemical (electro-Fenton) degradation pathways. However, the requirement of strict acidic conditions to prevent iron precipitation still remains the bottleneck for iron-based AOPs. In this article, we present a thorough review of alternative non-iron Fenton catalysts and their reactivity towards hydrogen peroxide activation. Elements with multiple redox states (like chromium, cerium, copper, cobalt, manganese and ruthenium) all directly decompose H2O2 into HO• through conventional Fenton-like pathways. The in situ formation of H2O2 and decomposition into HO• can be also achieved using electron transfer mechanism in zero-valent aluminum/O2 system. Although these Fenton systems (except aluminum) work efficiently even at neutral pH, the H2O2 activation mechanism is very specific to the nature of the catalyst and critically depends on its composition. This review describes in detail the complex mechanisms and emphasizes on practical limitations influencing their environmental applications. © 2014 Elsevier B.V.

Ahn H.,Pohang University of Science and Technology | Park M.J.,Pohang University of Science and Technology
Macromolecular Rapid Communications | Year: 2011

A highly versatile approach to fabricate functional gold nanoparticle (AuNP)-polymer hybrids is demonstrated by employing sulfonated block copolymers. The 3-5 nm sized ionic domain of the sulfonated poly(styrene-block- methylbutylene) (S nMB m) copolymers can be utilized as a nanoreactor where the Au ions can be selectively sequestered and reduced to AuNPs using a simple photochemical method. The size of the AuNPs can be adjusted in fine-steps from 2.0 ± 0.3 to 3.9 ± 0.5 nm by changing the sulfonation levels of the S nMB m copolymers. Remarkably, significantly improved methanol oxidation properties are achieved with the hybrid materials owing to the ion conducting-SO 3H groups and the interconnected network of AuNPs confined within the self-assembled microstructures, which provides electronic conductivity. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Rahman G.,University of Ulsan | Rahman G.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Using first-principles calculations within generalized gradient approximation, the electronic and magnetic properties of zinc blende (zb) CrP/MnP superlattice are investigated. The equilibrium lattice constant is calculated to be 5.33 Å. The stability of ferromagnetic zb CrP/MnP superlattice against antiferromagnetism is considered and it is found that the ferromagnetic CrP/MnP superlattice is more stable than the antiferromagnetic one. It is shown that at the equilibrium lattice constant the CrP/MnP superlattice does not show any half metallicity mainly due to the minority t2g states of Cr and Mn. However, if strain is imposed on the CrP/MnP superlattice then the minority t2g electrons shift to higher energies and the proposed superlattice becomes a half-metal ferromagnet. The effect of tetragonal and orthorhombic distortions on the half metallicity of zb CrP/MnP superlattice is also discussed. It is also shown that InP-CrP/MnP/InP is a true half-metal ferromagnet. The half metallicity and magnetization of these superlattices are robust against tetragonal/orthorhombic deformation. © 2010 The American Physical Society.

Kim S.H.,Pohang University of Science and Technology | Jang M.,Inha University | Yang H.,Inha University | Anthony J.E.,University of Kentucky | Park C.E.,Pohang University of Science and Technology
Advanced Functional Materials | Year: 2011

A chemically coupled polymer layer is introduced onto inorganic oxide dielectrics from a dilute chlorosilane-terminated polystyrene (PS) solution. As a result of this surface modification, hydrophilic-oxide dielectrics gain hydrophobic, physicochemically stable properties. On such PS-coupled SiO 2 or AlOx dielectrics, various vacuum- and solution-processable organic semiconductors can develop highly ordered crystalline structures that provide higher field-effect mobilities (μFETs) than other surface-modified systems, and negligible hysteresis in organic field-effect transistors (OFETs). In particular, the use of PS-coupled AlOx nanodielectrics enables a solution-processable triethylsilylethynyl anthradithiophene OFET to operate with μFET ∼ 1.26 cm2 V-1 s-1 at a gate voltage below -1 V. In addition, a complementary metal-oxide semiconductor-like organic inverter with a high voltage gain of approximately 32 was successfully fabricated on a PS-coupled SiO2 dielectric. Ultrathin polymer layers were chemically coupled on oxide dielectrics with chlorosilane-terminated polystyrene (PS). The PS-brush layer was physicochemically stable and provided a smooth, hydrophobic surface to induce highly ordered crystalline structures of vacuum- and solution-processable organic semiconductors. In particular, PS-coupled AlOx nanodielectrics allowed solution-processable organic field-effect transistors to operate at a gate voltage of approximately -1 V, with a field-effect mobility of ∼1.26 cm2 V-1 s -1. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Park Y.S.,Pohang University of Science and Technology | Han S.H.,Pohang University of Science and Technology
International Journal of Industrial Ergonomics | Year: 2010

This study investigated effects of touch key sizes and locations on one-handed thumb input on a mobile phone. Three different touch key sizes (i.e. square shape with 4 mm, 7 mm, and 10 mm wide) and twenty-five locations were examined in an experiment. A total of thirty subjects participated in the experiment in which they preformed a task of pressing a single target on a small touch screen. Two time-related measures (first transition time and task completion time), number of errors, and subjective satisfaction (pressing convenience) were collected in the experiment. The results revealed that the touch key size of 7 mm and 10 mm provided the best performance for time-related measures, while the touch key size of 10 mm only provided the best results for the other measures. In addition, the usability of touch key locations was statistically analyzed. Touch key locations providing good usability ('good regions') were also identified for each measure. Recommendations were proposed for designing a touch user interface on a mobile phone based on the results of this study. Relevance to industry: The touch user interface is in the limelight of the handset industry. This study conducted basic research to investigate the effects of touch key sizes and touch key locations for one-handed interaction. The results of this study could be used for designing a touch user interface to enhance the usability of mobile phones and other small devices with a touch screen as well. © 2009 Elsevier B.V. All rights reserved.

Cho S.,Pohang University of Science and Technology | Lee K.-H.,Pohang University of Science and Technology
Journal of Alloys and Compounds | Year: 2011

We report a method for synthesizing zinc aluminum layered double hydroxide (ZnAl:LDH) nanostructures at room temperature. The ZnAl:LDH nanoplates could be converted into zinc aluminum mixed metal oxide (MMO) nanostructures by calcination in air. The crystalline nature and morphology of the MMO nanostructures could be tuned by varying the calcination temperature. At low calcination temperatures (450-650 °C), nanostructures were composed of crystalline ZnO regions and amorphous regions. The crystalline orientations of the ZnO crystal grains were almost identical throughout the nanostructure. At calcination temperatures above 750 °C, ZnAl2O4 crystal grains appeared and amorphous regions could not be found in MMO nanostructures. As the calcination temperature increased, the crystal grain size and surface roughness of MMO nanostructures increased. Calcination at 950 °C resulted in the formation of MMO nanoparticles. The optical properties of the MMO nanostructures were probed by UV-vis diffuse reflectance spectroscopy. The spectra varied depending on their dimensions and crystalline natures. © 2011 Elsevier B.V. All rights reserved.

Jang J.S.,Pohang University of Science and Technology | Kim H.G.,Korea Basic Science Institute | Lee J.S.,Pohang University of Science and Technology
Catalysis Today | Year: 2012

Heterojunction semiconductors are discussed as a strategy to develop efficient visible light photocatalysts for water splitting. The concept has been demonstrated in photovoltaic cells and optoelectronic devices, for which junction-type semiconductors show greatly enhanced efficiency compared to the devices consisting of a single semiconductor. We applied this proven concept to fabricate photocatalysts of inorganic semiconductors. Thus heterojunction structures of Shottky junctions, p-n junctions (or p-n diode), p-n junctions with Ohmic layer, and bulk heterojunctions were fabricated and their photoactivity was tested for reduction or oxidation of water under visible light. The formation of heterojunctions results in the efficient separation of electron-hole pairs to minimize the energy-wasteful electron-hole recombination, which leads to the high photocatalytic activity. As the complexity and sophistication of the photocatalyst fabrication increased, the photoactivity also increased. Modern nanomaterial synthetic techniques were employed to bring into reality the highly engineered material configurations. © 2011 Elsevier B.V. All rights reserved.

Hwang A.B.,Pohang University of Science and Technology | Lee S.-J.,Pohang University of Science and Technology
Aging | Year: 2011

A mild reduction in mitochondrial respiration extends the life span of many species, including C. elegans. We recently showed that hypoxia-inducible factor 1 (HIF-1) is required for the acquisition of a long life span by mutants with reduced respiration in C. elegans. We suggested that increased levels of reactive oxygen species (ROS) produced in the respiration mutants increase HIF-1 activity and lead to this longevity. In this research perspective, we discuss our findings and recent advances regarding the roles of ROS and HIF-1 in aging, focusing on the longevity caused by reduced respiration. © Hwang.

Xu W.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Journal of Materials Chemistry | Year: 2011

The effects of electrical leakage and capacitance density were investigated in the low-voltage operated organic field-effect transistors (OFETs) by using a gate dielectric with a bi-layer structure of atomic layer deposited alumina (ALD-Al2O3) and high-k polymeric cyanoethylated pullulan (CEP) layer. A significant improvement in the device performance was achieved by compromising the two effects, suppressing the leakage current with ALD-Al 2O3 dielectric and maintaining the high capacitance with high-k polymeric layer. With the optimized thickness of ∼5 nm alumina (Ci,CEP/Al2O3 ∼ 85 nF cm-2), a high mobility of ∼5 cm2 V-1 s-1 and sharp subthreshold slope (SS) of 0.066 V dec-1 were obtained. The smoother surface of the polymeric dielectric surface enhanced the 2-dimensional vertical molecular layer growth and contributed to the better device performance. © 2011 The Royal Society of Chemistry.

Oh J.K.,Concordia University at Montréal | Park J.M.,Pohang University of Science and Technology
Progress in Polymer Science (Oxford) | Year: 2011

Recent advances in the development and biological applications of polymeric nanomaterials embedded with superparamagnetic iron oxide nanoparticles (SIONPs) are summarized. Novel SIONP-polymer hybrid nanoparticles are prepared by various methods, including direct modification with polymers, surface-initiated controlled polymerization, inorganic silica/polymer hybridization, self-assembly, self-association, and various heterogeneous polymerization methods. They have potential for various biomedical applications, including magnetic resonance imaging (MRI) contrast enhancement, targeted drug delivery, hyperthermia, biological separation, protein immobilization, and biosensors. © 2010 Elsevier Ltd All rights reserved.

Kim S.Y.,Pohang University of Science and Technology | Yoon E.,Pohang University of Science and Technology | Joo T.,Pohang University of Science and Technology | Park M.J.,Pohang University of Science and Technology
Macromolecules | Year: 2011

We have explored the link between morphologies and conductivities for ionic liquids (ILs) incorporated block copolymer electrolytes by combining small-angle X-ray scattering, transmission electron microscopy, and impedance spectroscopy. The block copolymer electrolytes investigated in present study are a series of partially sulfonated poly(styrenesulfonate-b-methylbutylene) (SnMBm) copolymers with different molecular weights and sulfonation levels (SLs). Imidazolium-based ILs are selectively doped into hydrophilic domains of SnMBm copolymers, and various morphologies have been observed as a function of the amount of absorbed ILs and SLs in SnMBm copolymers. We have demonstrated that the morphologies of ILs impregnated SnMBm copolymers are sensitive function of kinds of counteranions in IL, yielding remarkable discrepancy in conductivities. When the morphology of sample is appeared to be a lamellar structure, significant reduction in through-plane conductivity value was detected due to the nonrandom orientation of microdomains. In contrast, hexagonally perforated lamellar forming samples exhibit the highest conductivities in both through-plane and in-plane directions on account of the better connectivity of ionic domains along the perforated hydrophilic phases. Once the morphology effects were vanished by employing highly sulfonated S nMBm copolymers, it has been revealed that the conductivities of ILs incorporated copolymers are closely related to the polarity of ILs as confirmed by solvation dynamics study. © 2011 American Chemical Society.

Moon H.C.,Pohang University of Science and Technology | Anthonysamy A.,Pohang University of Science and Technology | Kim J.K.,Pohang University of Science and Technology | Hirao A.,Tokyo Institute of Technology
Macromolecules | Year: 2011

We have demonstrated a facile synthetic route for well-defined poly(3-hexylthiophene)-block-poly(methyl methacrylate) copolymer (P3HT-b-PMMA) by anionic coupling reaction. For successful coupling reaction, newly generated anions should be more stable (less reactive) than initial anions of reactants. In this study, we chose α-phenyl acrylate (PA)-capped P3HTs for successful coupling with living PMMA anions because the anions at PA group are more stable than living PMMA anions. We found that all of the PA groups located at the end of P3HT were completely coupled with living PMMA anions having slightly excess amount (1.5 equiv relative to PA-capped P3HT). The unreactive PMMA homopolymers in crude product were completely removed by using column chromatography, resulting in narrow molecular weight distribution of pure P3HT-b-PMMA. The optical property and thin film morphology of the P3HT-b-PMMA were investigated by using UV-vis spectra and atomic force microscopy, respectively. © 2011 American Chemical Society.

Karthikeyan S.,Pohang University of Science and Technology | Lee H.M.,Pohang University of Science and Technology | Kim K.S.,Pohang University of Science and Technology
Journal of Chemical Theory and Computation | Year: 2010

There are no clear conclusions over the structures of the acetylene clusters. In this regard, we have carried out high-level calculations for acetylene clusters (C2H2)2-5 using dispersion-corrected density functional theory (DFT-D), Møller-Plesset second-order perturbation theory (MP2); and coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] at the complete basis set limit. The lowest energy structure of the acetylene dimer has a T-shaped structure of C2v symmetry, but it is nearly isoenergetic to the displaced stacked structure of C2h symmetry. We find that the structure shows the quantum statistical distribution for configurations between the T-shaped and displaced stacked structures for which the average angle (|θ|) between two acetylene molecules would be 53-78°, close to the T-shaped structure. The trimer has a triangular structure of C3h symmetry. The tetramer has two lowest energy isomers of S4 and C 2h symmetry in zero-point energy (ZPE)-uncorrected energy (ΔEe), but one lowest energy isomer of C2v symmetry in ZPE-corrected energy (ΔE0). For the pentamer, the global minimum structure is C1 symmetry with eight sets of T-type π-H interactions and a set of π-π interactions. Our high-level ab initio calculations are consistent with available experimental data. © 2010 American Chemical Society.

Jung K.-H.,Pohang University of Science and Technology | Lee D.,University of Ulsan | Lee J.,Pohang University of Science and Technology
Pattern Recognition | Year: 2010

In many support vector-based clustering algorithms, a key computational bottleneck is the cluster labeling time of each data point which restricts the scalability of the method. In this paper, we review a general framework of support vector-based clustering using dynamical system and propose a novel method to speed up labeling time which is log-linear to the size of data. We also give theoretical background of the proposed method. Various large-scale benchmark results are provided to show the effectiveness and efficiency of the proposed method. © 2009 Elsevier Ltd. All rights reserved.

Chung J.-H.,Pohang University of Science and Technology | Yang K.,Pohang University of Science and Technology
IEEE Transactions on Information Theory | Year: 2010

A frequency-hopping sequence (FHS) of length v and frequency set size M is called a (v,M,λ)-FHS if its maximum out-of-phase Hamming autocorrelation is λ. Three new classes of optimal FHSs with respect to the LempelGreenberger bound are presented in this paper. First, new optimal (p,M,f)-FHSs are constructed when p=Mf+1 is an odd prime such that f is even and p≡ 3 mod 4. And then, a construction for optimal (kp,p,k)-FHSs is given for any odd prime p and a positive integer k

Jung Y.J.,Nanogea Corporation | Albrecht J.A.,National Genetics Institute | Kwak J.-W.,Pohang University of Science and Technology | Park J.W.,Pohang University of Science and Technology
Nucleic Acids Research | Year: 2012

Force-based atomic force microscopy (AFM) was used to detect HCV (hepatitis C virus) RNA directly and to quantitatively analyse it without the need for reverse transcription or amplification. Capture and detection DNA probes were designed. The former was spotted onto a substrate with a conventional microarrayer, and the latter was immobilized on an AFM probe. To control the spacing between the immobilized DNAs on the surface, dendron selfassembly was employed. Force-distance curves showed that the mean force of the specific unbinding events was 32 ± 5 pN, and the hydrodynamic distance of the captured RNA was 30-60 nm. Adhesion force maps were generated with criteria including the mean force value, probability of obtaining the specific curves and hydrodynamic distance. The maps for the samples whose concentrations ranged from 0.76 fM to 6.0 fM showed that cluster number has a linear relationship with RNA concentration, while the difference between the observed number and the calculated one increased at low concentrations. Because the detection limit is expected to be enhanced by a factor of 10 000 when a spot of 1 micron diameter is employed, it is believed that HCV RNA of a few copy numbers can be detected by the use of AFM. © 2012 The Author(s).

Park Y.,Pohang University of Science and Technology | Kang S.-H.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology
Physical Chemistry Chemical Physics | Year: 2011

The hybrid of graphite oxide (GO)/TiO2 was prepared through the spontaneous exfoliation of bulky graphite oxide and reorganization with TiO 2 nanoparticles as a solar conversion and hydrogen-generating photocatalyst. GO/TiO2 showed enhanced activities for both photocurrent generation (in an electrode form) and hydrogen production (in a slurry form) than those of bare TiO2 under UV light irradiation. The enhanced photocatalytic activity of GO/TiO2 is ascribed to the ability of graphitic layers in accepting and transporting electrons from excited TiO2, promoting the charge separation. When GO was hybridized with platinized TiO2 (Pt/TiO2), it showed a marked synergistic effect for the photocatalytic hydrogen production compared with GO/TiO 2 and Pt/TiO2. This indicates that the cheap and abundant carbon material can be a good candidate for an electron attracting reservoir and an auxiliary co-catalyst for the photocatalytic hydrogen production. © 2011 the Owner Societies.

Kim K.I.,Pohang University of Science and Technology | Lin Z.,Yangzhou University | Zhang Q.,Yangzhou University
Nonlinear Analysis: Real World Applications | Year: 2013

An SIR epidemic model with free boundary is investigated. This model describes the transmission of diseases. The behavior of positive solutions to a reaction-diffusion system in a radially symmetric domain is investigated. The existence and uniqueness of the global solution are given by the contraction mapping theorem. Sufficient conditions for the disease vanishing or spreading are given. Our result shows that the disease will not spread to the whole area if the basic reproduction number R0<1 or the initial infected radius h0 is sufficiently small even that R0>1. Moreover, we prove that the disease will spread to the whole area if R 0>1 and the initial infected radius h0 is suitably large. © 2013 Elsevier Ltd. All rights reserved.

Kim J.-M.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Journal of the Electrochemical Society | Year: 2012

To study the effect of stainless steel (SS) surface morphology as a counter electrode and a flexible substrate in dye sensitized solar cells (DSCs), SS surface was treated with chemical mechanical polishing process or etched with hydrochloric acid for various times. Pt catalytic layer was deposited either with sputtering at room temperature or thermal decomposition of precursor solution (H 2PtCl 6). From the electrochemical impedance measurement on the symmetric cell, the charge transfer resistance at the electrolytecounter electrode interface and the internal resistance were obtained. When Pt was deposited on polished SS with sputtering, fill factor (FF) and efficiency were decreased due to the decrease in surface area. On the other hand, the catalytic activity was improved with etching due to the increase in the surface area leading to the improvement in FF and efficiency (71.0 and 7.7). When the Pt layer was formed with thermal decomposition on the SS substrate, FF and efficiency were significantly decreased with poor catalytic activity regardless of the surface area, due to the oxidation of the SS surface during the thermal decomposition process at high temperatures. © 2011 The Electrochemical Society.

Kim S.-G.,Pohang University of Science and Technology | Lee S.-J.,Pohang University of Science and Technology
International Journal of Hydrogen Energy | Year: 2012

Synchrotron X-ray micro-computed tomography (X-ray μCT) is employed to measure the volume variation of gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). In the present study, 3D structures are reconstructed by merging orthogonal-plane images. Using the 3D reconstruction, the variation of structural parameters such as the porosity in GDL is investigated under freeze-thaw cycles. The freez-thaw cycles are established using cryo system and light source, respectively. As a result, a structural transformation is observed at the interface between GDL and micro porous layer (MPL). In addition, the porosity is critically changed with irreversible transition under freeze-thaw cycles. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Jeon S.,Pohang University of Science and Technology | Yong K.,Pohang University of Science and Technology
Journal of Materials Chemistry | Year: 2010

A facile route to control the morphology of tungsten oxide hierarchical hollow structures was demonstrated. The morphology of the tungsten oxide nanostructures could be tuned from size-controlled hollow urchins to nanowires by adjusting the concentration of the tungsten precursor, with no need for catalysts, surfactants, or templates. The tungsten oxide nanostructures were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and nitrogen adsorption-desorption measurements. The as-prepared tungsten oxide hierarchical hollow nanostructures showed a very high specific surface area and demonstrated an excellent ability to remove organic pollutants. The adsorption capability of our tungsten oxide hierarchical hollow nanostructure was much higher than those of previously reported transition metal oxide nanostructures including Mn3O 4, Fe2O3, and MnO2 as well as other alternative adsorbents such as MCM-22, fly ash, and red mud. This study provides a simple strategy for template/surfactant-free synthesis of hierarchical hollow nanostructures. Also, the as-prepared products are expected to be new promising materials for environmental remediation. © 2010 The Royal Society of Chemistry.

Han J.-H.,Pohang University of Science and Technology | Lee I.-B.,Pohang University of Science and Technology
Industrial and Engineering Chemistry Research | Year: 2012

A multiperiod stochastic programming model is developed for planning a carbon capture and storage (CCS) infrastructure including CO 2 utilization and disposal in an uncertain environment and with a time-varying investment environment. An inexact two-stage stochastic programming approach is used to analyze the effect of possible uncertainties in product prices, operating costs, and CO 2 emissions. The proposed model determines where and how much CO 2 to capture, store, transport, utilize, or sequester for the purpose of maximizing the total profit of handling the uncertainty while meeting the CO 2 mitigation target during each time period of a given planning interval. The capability of the proposed model to provide correct decisions despite a changing uncertain environment is tested by applying it to designing and operating the future CCS infrastructure on the eastern coast of Korea over a 20-year planning interval (2011-2030). © 2012 American Chemical Society.

Nam W.,StradVision Inc. | Dollar P.,Microsoft | Han J.H.,Pohang University of Science and Technology
Advances in Neural Information Processing Systems | Year: 2014

Even with the advent of more sophisticated, data-hungry methods, boosted decision trees remain extraordinarily successful for fast rigid object detection, achieving top accuracy on numerous datasets. While effective, most boosted detectors use decision trees with orthogonal (single feature) splits, and the topology of the resulting decision boundary may not be well matched to the natural topology of the data. Given highly correlated data, decision trees with oblique (multiple feature) splits can be effective. Use of oblique splits, however, comes at considerable computational expense. Inspired by recent work on discriminative decorrelation of HOG features, we instead propose an efficient feature transform that removes correlations in local neighborhoods. The result is an overcomplete but locally decorrelated representation ideally suited for use with orthogonal decision trees. In fact, orthogonal trees with our locally decorrelated features outperform oblique trees trained over the original features at a fraction of the computational cost. The overall improvement in accuracy is dramatic: on the Caltech Pedestrian Dataset, we reduce false positives nearly tenfold over the previous state-of-the-art.

Zhang G.,Pohang University of Science and Technology | Song I.Y.,Pohang University of Science and Technology | Ahn K.H.,Pohang University of Science and Technology | Park T.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology
Macromolecules | Year: 2011

Min H.-K.,Pohang University of Science and Technology | Cha S.H.,Pohang University of Science and Technology | Hong S.B.,Pohang University of Science and Technology
ACS Catalysis | Year: 2012

The mechanisms of m-xylene isomerization and disproportionation over 13 medium-pore zeolites and three large-pore ones are investigated. While H-TNU-10 and H-ZSM-57 with intersecting 10- and 8-ring channels were found to show considerably higher p/o ratios than H-ZSM-5, a commercial m-xylene isomerization catalyst, the GC-MS results from used zeolite catalysts demonstrate the intrazeolitic build-up of tri- and tetramethylated diphenylmethane species, whose existence during the m-xylene transformation over any acidic catalyst has not been experimentally verified until now. These dicyclic aromatic compounds were ascertained to serve as reaction intermediates of bimolecular m-xylene isomerization within the micropores not only of large-pore zeolites but also of medium-pore materials at temperatures lower than 523 K or so, once there are internal void spaces larger than 10-rings. Flushing experiments with used zeolites followed by GC-MS analyses strongly suggest that the high p-xylene selectivity found in some medium-pore zeolites is largely due to product shape selectivity rather than to transition state one. More importantly, the overall GC-MS results of our work demonstrate that transition state and product shape selectivities are experimentally distinguishable from each other. © 2012 American Chemical Society.

Ahn H.S.,Incheon National University | Kim M.H.,Pohang University of Science and Technology
International Journal of Heat and Mass Transfer | Year: 2013

This study examined saturated water/alumina nanofluid pool boiling on a 10-mm-diameter, upward-facing, plain copper heater under atmospheric pressure. In order to investigate the effect of a nanoparticle coating on the boiling phenomenon near critical heat flux, we compared the boiling characteristics of water boiling before, alumina nanofluid boiling, and water boiling after alumina nanoparticles. The unusual boiling characteristics (bending point in the boiling curve at a high heat flux), CHF enhancement, and gradual increase of wall temperature just after the CHF (instead of the rapid increase of wall temperature) were analyzed based on high-speed visualizations. A thicker macrolayer resulting from the nanoparticle coating is estimated to influence the CHF enhancement, which played a role similar to a porous medium. Thus, we revealed that the nanoparticle coating induced the transition boiling with the nucleate boiling and the partial dry-patch (vapor film in the large vapor mushroom) at a high heat flux as the role of porous medium, which explained the bending point in the boiling curve and the gradual increase of wall temperature. © 2013 Elsevier B.V. All rights reserved.

Jang H.M.,Pohang University of Science and Technology | Park S.K.,Pohang University of Science and Technology | Ha J.H.,Pohang University of Science and Technology | Park J.M.,Pohang University of Science and Technology
Bioresource Technology | Year: 2013

An effective two-stage sewage sludge digestion process, consisting of thermophilic aerobic digestion (TAD) followed by mesophilic anaerobic digestion (MAD), was developed for efficient sludge reduction and methane production. Using TAD as a biological pretreatment, the total volatile suspended solid reduction (VSSR) and methane production rate (MPR) in the MAD reactor were significantly improved. According to denaturing gradient gel electrophoresis (DGGE) analysis, the results indicated that the dominant bacteria species such as Ureibacillus thermophiles and Bacterium thermus in TAD were major routes for enhancing soluble organic matter. TAD pretreatment using a relatively short SRT of 1. day showed highly increased soluble organic products and positively affected an increment of bacteria populations which performed interrelated microbial metabolisms with methanogenic species in the MAD; consequently, a quantitative real-time PCR indicated greatly increased Methanosarcinales (acetate-utilizing methanogens) in the MAD, resulting in enhanced methane production. © 2013 Elsevier Ltd.

Ahn H.S.,Pohang University of Science and Technology | Kim M.H.,Pohang University of Science and Technology
International Journal of Multiphase Flow | Year: 2012

We examined the influence of the macrolayer under a large vapor mushroom on critical heat flux (CHF) during pool boiling with a small plate heater. Evidence of a macrolayer was provided, and its measured thickness was compared and well agreed with values reported in the literature. The classical CHF models of pool boiling do not consider the effect of the heater size. For a small and horizontal heater, a hydrodynamic liquid inflow increases the CHF beyond the predictions of most models, which are based on an infinite and horizontal heater. Using high speed visualization of CHF, we proposed a new CHF triggering mechanism for a small heater. Because the hydrodynamic liquid inflow supplies liquid to the edge of the heater, nucleate boiling is maintained in this region, even when a large dry patch beneath the large mushroom is generated at the center of the heater. The CHF occurs when the macrolayer at the edge of the heater dries out (i.e., becomes coated with a vapor film) and meets the large dry patch at the center of the heater. Finally, we proposed the shape of macrolayer beneath the large mushroom in order to explain the CHF triggering mechanism on the small heater. © 2011 Elsevier Ltd.

Jung K.A.,Pohang University of Science and Technology | Lim S.-R.,Kangwon National University | Kim Y.,Pohang University of Science and Technology | Park J.M.,Pohang University of Science and Technology
Bioresource Technology | Year: 2013

Macroalgae, so-called seaweeds, have recently attracted attention as a possible feedstock for biorefinery. Since macroalgae contain various carbohydrates (which are distinctively different from those of terrestrial biomasses), thorough assessments of macroalgae-based refinery are essential to determine whether applying terrestrial-based technologies to macroalgae or developing completely new technologies is feasible. This comprehensive review was performed to show the potentials of macroalgae as biorefinery feedstocks. Their basic background information was introduced: taxonomical classification, habitat environment, and carbon reserve capacity. Their global production status showed that macroalgae can be mass-cultivated with currently available farming technology. Their various carbohydrate compositions implied that new microorganisms are needed to effectively saccharify macroalgal biomass. Up-to-date macroalgae conversion technologies for biochemicals and biofuels showed that molecular bioengineering would contribute to the success of macroalgae-based biorefinery. It was concluded that more research is required for the utilization of macroalgae as a new promising biomass for low-carbon economy. © 2012 Elsevier Ltd.

Moon J.,Pohang University of Science and Technology | Kim J.,Pohang University of Science and Technology | Kim B.,Pohang University of Science and Technology
IEEE Transactions on Microwave Theory and Techniques | Year: 2010

This paper presents the operation principle of Class-J power amplifiers (PAs) with linear and nonlinear output capacitors ( Couts). The efficiency of a Class-J amplifier is enhanced by the nonlinear capacitance because of the harmonic generation from the nonlinear Cout, especially the second-harmonic voltage component. This harmonic voltage allows the reduction of the phase difference between the fundamental voltage and current components from 45° to less than 45° while maintaining a half-sinusoidal shape. Therefore, a Class-J amplifier with the nonlinear C out can deliver larger output power and higher efficiency than with a linear Cout. As a further optimized structure of the Class-J amplifier, the saturated PA, a recently-reported amplifier in our group, is presented. The phase difference of the proposed PA is zero. Like the Class-J amplifier, the PA uses a nonlinear Cout to shape the voltage waveform with a purely resistive fundamental load impedance at the current source, which enhances the output power and efficiency. The PA is favorably compared to the Class-J amplifier in terms of the waveform, load impedance, output power, and efficiency. These operations are described using both the ideal and real models of the transistor in Agilent Advanced Design System. A highly efficient amplifier based on the saturated PA is designed by using a Cree GaN HEMT CGH40010 device at 2.14 GHz. It provides a power-added efficiency of 77.3% at a saturated power of 40.6 dBm (11.5 W). © 2006 IEEE.

Kim H.K.,Pohang University of Science and Technology | Lee S.J.,Pohang University of Science and Technology
New Phytologist | Year: 2010

Summary: A comprehensive understanding of the sap flow dynamics and xylem hydraulic properties is essential to unravel the functional features of water transport from roots to shoots in vascular plants. To evaluate quantitatively the safety and efficiency of this system, nondestructive methods to assess the interactions between sap ascent kinetics and xylem structure are required. In this study, synchrotron X-ray microscopy was employed to observe anatomical structures and sap flow dynamics in rice (Oryza sativa) xylem simultaneously. The phase-contrast imaging technique allowed nondestructive observation of the xylem structural characteristics and the air-water interfaces generated by dehydration-rehydration cycles in excised leaves. This X-ray microimaging method provided a unique tool to characterize the perforated end walls of vessel elements and to evaluate their influence on hydraulic resistance during the refilling of embolized vessels. The real-time monitoring of the axial and radial sap flow under various environmental conditions highlighted the important role of perforation plates..In summary, we report a new methodology to study the sap flow dynamics and xylem hydraulic properties with μm spatial and ms temporal resolution using X-ray microscopy. The experimental procedure described herein provides a useful handle to understand key sap transport phenomena in xylem. © The Authors (2010). Journal compilation © New Phytologist Trust (2010).

Cho K.-H.,Korea Advanced Institute of Science and Technology | Kim M.-H.,Pohang University of Science and Technology
International Journal of Heat and Mass Transfer | Year: 2012

Here we develop new vascular designs for the volumetric bathing of smart structures under time-varying conditions. The three flow configurations described in this paper are the first, second, and third constructal structures with optimized hydraulic diameters (D 1 and D 2) and non-optimized hydraulic diameter (D) for one system size, 20 × 20. The main objective was to determine the longest permissible time delay so that the maximum temperatures do not exceed the maximum allowable limit. It is the best to cool with the first construct in the optimized constructal configurations when pressure drop number (Be) is lower than 1 × 10 11 and the best structure is the second constructal structure when pressure drop number (Be) is greater than 2 × 10 11, whereas the best structure in the non-optimized constructal configurations is the third construct. It is also shown that the most attractive configurations have larger allowable delay times: in both the optimized and non-optimized constructal structures, the best configuration is the second construct where the pressure drop is fixed at about 1 kPa. © 2012 Elsevier Ltd. All rights reserved.

Lu Q.,University of Alberta | Hwang D.S.,Pohang University of Science and Technology | Liu Y.,University of Alberta | Zeng H.,University of Alberta
Biomaterials | Year: 2012

Protective coating of the byssus of mussels (Mytilus sp.) has been suggested as a new paradigm of medical coating due to its high extensibility and hardness co-existence without their mutual detriment. The only known biomacromolecule in the extensible and tough coating on the byssus is mussel foot protein-1 (mfp-1), which is made up with positively charged residues (~20 mol%) and lack of negatively charged residues. Here, adhesion and molecular interaction mechanisms of Mytilus californianus foot protein-1 (mcfp-1) from California blue mussel were investigated using a surface forces apparatus (SFA) in buffer solutions of different ionic concentrations (0.2-0.7 M) and pHs (3.0-5.5). Strong and reversible cohesion between opposed positively charged mcfp-1 films was measured in 0.1 M sodium acetate buffer with 0.1 M KNO 3. Cohesion of mcfp-1 was gradually reduced with increasing the ionic strength, but was not changed with pH variations. Oxidation of 3,4-dihydroxyphenylalanine (DOPA) residues of mcfp-1, a key residue for adhesive and coating proteins of mussel, didn't change the cohesion strength of mcfp-1 films, but the addition of chemicals with aromatic groups (i.e., aspirin and 4-methylcatechol) increased the cohesion. These results suggest that the cohesion of mcfp-1 films is mainly mediated by cation-π interactions between the positively charged residues and benzene rings of DOPA and other aromatic amino acids (~20 mol% of total amino acids of mcfp-1), and π-π interactions between the phenyl groups in mcfp-1. The adhesion mechanism obtained for the mcfp-1 proteins provides important insight into the design and development of functional biomaterials and coatings mimicking the extensible and robust mussel cuticle coating. © 2011 Elsevier Ltd.

Lim J.H.,Pohang University of Science and Technology | Seo S.W.,Pohang University of Science and Technology | Kim S.Y.,Pohang University of Science and Technology | Jung G.Y.,Pohang University of Science and Technology
Bioresource Technology | Year: 2013

In this study, the native redox cofactor regeneration system in Escherichia coli was engineered for the production of butyric acid. The synthetic butyrate pathway, which regenerates NAD+ from NADH using butyrate as the only final electron acceptor, enabled high-yield production of butyric acid from glucose (83.4% of the molar theoretical yield). The high selectivity for butyrate, with a butyrate/acetate ratio of 41, suggests dramatically improved industrial potential for the production of butyric acid from nonnative hosts compared to the native producers (Clostridium species). Furthermore, this strategy could be broadly utilized for the production of various other useful chemicals in the fields of metabolic engineering and synthetic biology. © 2012 Elsevier Ltd.

Wy J.,Pohang University of Science and Technology | Kim B.-I.,Pohang University of Science and Technology
Computers and Operations Research | Year: 2013

This paper discusses the rollon-rolloff vehicle routing problem, a sanitation routing problem in which large containers are left at customer locations such as construction sites and shopping centers. Customers dump their garbage into large waste containers and request for waste treatment services. Tractors then transport a container at a time between customer locations, disposal facility, and depot. The objective of the problem is to determine routes that minimize the number of required tractors and their deadhead time to serve all given customer demands. We propose a hybrid metaheuristic approach that consists of a large neighborhood search and various improvement methods to solve the problem. The effectiveness of the proposed approach is demonstrated by computational experiments using benchmark data. New best-known solutions are found for 17 problems out of 20 benchmark instances. © 2013 Elsevier Ltd.

Kim E.,Pohang University of Science and Technology | Baek J.,Pohang University of Science and Technology
Journal of Non-Newtonian Fluid Mechanics | Year: 2012

The impact dynamics of a droplet onto a solid surface are important in a variety of applications, such as inkjet printing and spray coating. Many fluids encountered in practical industrial applications exhibit non-Newtonian behavior, and therefore more research associated with non-Newtonian fluids is necessary. This paper reports on a numerical study of the impact dynamics of yield-stress fluid droplets. The numerical simulation is performed using a computational fluid dynamics package, Fluent 6.3, with a volume of fluid model. The numerical simulation models the presence of yield-stress and shear-rate dependent viscosity using the Herschel-Bulkley rheological model. The numerical results are found to be in qualitative agreement with experimental data in the literature. By performing extensive numerical simulations varying the impact velocity, rheological parameters, and surface tension, the influence of these parameters on the impact dynamics are evaluated, and the dominant effects that govern the spreading and relaxation phases are determined. The results show that while the spreading behavior is determined by the power-law index n, the non-Newtonian Reynolds number Re n, and the Weber number We, the retraction behavior is determined by the non-Newtonian capillary Ca n and the Bingham-capillary number B̂. In addition, the scaling law that predicts the maximum spreading diameter is presented. © 2012 Elsevier B.V.

Lim Y.T.,Pohang University of Science and Technology | Son J.Y.,Kyung Hee University | Rhee J.-S.,Kyung Hee University
Ceramics International | Year: 2013

We report a vertical ZnO nanorod array as a highly sensitive hydrogen gas sensor. The vertical ZnO nanorod array on an Nb/Si substrate was fabricated using an anodized aluminum oxide nanotemplate and an atomic layer deposition method. The vertical ZnO nanorod array hydrogen gas sensor exhibited a high sensitivity for hydrogen in a wide concentration range. © 2012 Elsevier Ltd and Techna Group S.r.l.

Lim J.H.,Pohang University of Science and Technology | Seo S.W.,Pohang University of Science and Technology | Kim S.Y.,Pohang University of Science and Technology | Jung G.Y.,Pohang University of Science and Technology
Metabolic Engineering | Year: 2013

The intracellular redox state plays an important role in the cellular physiology that determines the efficiency of chemical and biofuel production by microbial cell factories. However, it is difficult to achieve optimal redox rebalancing of synthetic pathways owing to the sensitive responses of cellular physiology according as the intracellular redox state changes. Here, we demonstrate optimal rebalancing of the intracellular redox state by model-driven control of expression using n-butanol production in Escherichia coli as a model system. The synthetic n-butanol production pathway was constructed by implementing synthetic constitutive promoters and designing synthetic 5'-untranslated regions (5'-UTR) for each gene. Redox rebalancing was achieved by anaerobically activating the pyruvate dehydrogenase (PDH) complex and additionally tuning the expression level of NAD+-dependent formate dehydrogenase (fdh1 from Saccharomyces cerevisiae) through rational UTR engineering. Interestingly, efficient production of n-butanol required different amounts of reducing equivalents depending on whether the substrate was glucose or galactose. One intriguing implication of this work is that additional strain improvement can be achieved, even within given genetic components, through rebalancing intracellular redox state according to target products and substrates. © 2013 Elsevier Inc.

Jun B.,Pohang University of Science and Technology | Kim T.,Pohang University of Science and Technology | Kim D.,Pohang University of Science and Technology
Pattern Recognition | Year: 2011

We propose a compact LBP using the maximization of mutual information (MMI). The CLBP shows better classification performance with smaller number of codes. We show CLBP effectiveness in face recognition and facial expression recognition. © 2010 Elsevier Ltd. All rights reserved.

Kwon S.G.,Pohang University of Science and Technology | Kang M.H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2014

We identify the atomic structure of the Au/Si(111)-(5×2) surface by using density functional theory calculations. With seven Au atoms per unit cell, our model forms a bona fide (5×2) atomic structure, which is energetically favored over the leading model of Erwin et al. [Phys. Rev. B 80, 155409 (2009)PRBMDO1098-012110.1103/PhysRevB.80.155409], and well reproduces the Y-shaped and V-shaped (5×2) STM images. This surface is metallic with a prominent half filled band of surface states, mostly localized around the Au-chain area. The correct identification of the atomic and band structure of the clean surface further clarifies the adsorption structure of Si adatoms and the physical origin of the intriguing metal-to-insulator transition driven by Si adatoms. © 2014 American Physical Society.

Cho Y.-W.,Pohang University of Science and Technology | Park K.-K.,Pohang University of Science and Technology | Lee J.-C.,Pohang University of Science and Technology | Kim Y.-H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2014

The nonclassical photon pair, generated via a parametric process, is naturally endowed with a specific form of frequency-time quantum correlations. Here, we report complete control of frequency-time quantum correlations of narrow-band biphotons generated via spontaneous four-wave mixing in a cold atomic ensemble. We have experimentally confirmed the generation of frequency-anticorrelated, frequency-correlated, and frequency-uncorrelated narrow-band biphoton states, as well as verifying the strong nonclassicality of the correlations. Our work opens up new possibilities for engineering narrow-band entangled photons for various quantum optical and quantum information applications. © 2014 American Physical Society.

Weon B.M.,Pohang University of Science and Technology | Je J.H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2013

Colloidal particles suspended in a fluid usually inhibit complete wetting of the fluid on a solid surface and cause pinning of the contact line, known as self-pinning. We show differences in spreading and drying behaviors of pure and colloidal droplets using optical and confocal imaging methods. These differences come from spreading inhibition by colloids confined at a contact line. We propose a self-pinning mechanism based on spreading inhibition by colloids. We find a good agreement between the mechanism and the experimental result taken by directly tracking individual colloids near the contact lines of evaporating colloidal droplets. © 2013 American Physical Society.

Chung J.-H.,Pohang University of Science and Technology | Yang K.,Pohang University of Science and Technology
IEEE Transactions on Information Theory | Year: 2011

For an integer k ≥ 1, let qi, 1 ≤ I ≤ k, be prime powers such that qi=Mi f + 1 for some integers M i and f. In this paper, the k -fold cyclotomy of double-struck F signq1 × ⋯ × double-struck F signqk as a nontrivial generalization of the conventional cyclotomy ( k=1 case) and its application to frequency-hopping sequences (FHSs) are presented, where double-struck F signq is the finite field with q elements. First, the definitions of k -fold cyclotomic classes and k-fold cyclotomic numbers are given. And then, their basic properties including k -fold diagonal sums are derived. Based on them, new optimal FHS sets of length N and frequency set size M or M+1 with respect to the Peng-Fan bound are constructed for a product N of distinct odd primes and a divisor M of N-1. Furthermore, new optimal FHSs of length N and frequency set size M with respect to the LempelGreenberger bound are constructed when N has at least one prime factor which is 3 modulo 4 and (N-1)/M is an even integer. Our constructions give several new optimal parameters not covered in the literature, which are summarized in Table I. © 2011 IEEE.

Kang E.,Pohang University of Science and Technology | Jeon G.,Pohang University of Science and Technology | Kim J.K.,Pohang University of Science and Technology
Chemical Communications | Year: 2013

The mesoporous carbon nanofiber arrays that stand on carbon-gold double-layer current collectors are synthesized by self-assembly of a PS-b-PEO copolymer and resol in AAO templates for a high-power micro-supercapacitor at high current densities. © 2013 The Royal Society of Chemistry.

Hazra D.K.,Asia Pacific Center for Theoretical Physics | Shafieloo A.,Asia Pacific Center for Theoretical Physics | Shafieloo A.,Pohang University of Science and Technology | Souradeep T.,Inter-University Center for Astronomy and Astrophysics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

Constraints on the main cosmological parameters using cosmic microwave background (CMB) or large scale structure data are usually based on the power-law assumption of the primordial power spectrum (PPS). However, in the absence of a preferred model for the early Universe, this raises a concern that current cosmological parameter estimates are strongly prejudiced by the assumed power-law form of PPS. In this paper, for the first time, we perform cosmological parameter estimation allowing the free form of the primordial spectrum. This is in fact the most general approach to estimate cosmological parameters without assuming any particular form for the primordial spectrum. We use a direct reconstruction of the PPS for any point in the cosmological parameter space using the recently modified Richardson-Lucy algorithm; however, other alternative reconstruction methods could be used for this purpose as well. We use WMAP 9 year data in our analysis considering the CMB lensing effect, and we report, for the first time, that the flat spatial universe with no cosmological constant is ruled out by more than a 4σ confidence limit without assuming any particular form of the primordial spectrum. This would be probably the most robust indication for dark energy using CMB data alone. Our results on the estimated cosmological parameters show that higher values of the baryonic and matter density and a lower value of the Hubble parameter (in comparison to the estimated values by assuming power-law PPS) is preferred by the data. However, the estimated cosmological parameters by assuming a free form of the PPS have an overlap at 1σ confidence level with the estimated values assuming the power-law form of PPS. © 2013 American Physical Society.

Min H.-K.,Pohang University of Science and Technology | Cha S.H.,Pohang University of Science and Technology | Hong S.B.,Pohang University of Science and Technology
Chemical Communications | Year: 2013

The nitrogen-substituted delaminated ITQ-2 zeolite was found to be a highly active and recyclable catalyst for the Knoevenagel condensation of bulky aromatic aldehydes with ethyl cyanoacetate which cannot take place over the base sites placed within the micropores of conventional zeolites due to spatial limitations. © 2013 The Royal Society of Chemistry.

Kang B.,Pohang University of Science and Technology | Lim S.,Pohang University of Science and Technology | Lee W.H.,Konkuk University | Jo S.B.,Pohang University of Science and Technology | Cho K.,Pohang University of Science and Technology

RGO electrodes with work functions that can be widely tuned using direct surface functionalization are demonstrated by self-assembled monolayers anchored onto the surfaces of the RGO electrodes, which can remarkably enhance the device performance of organic field-effect transistors. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kim K.T.,Pohang University of Science and Technology | Kim B.H.,Pohang University of Science and Technology
Chemical Communications | Year: 2013

A 6-mer oligonucleotide containing a fluorescent BodU moiety has been used as a novel fluorescent probe for the 3′-overhang of telomeric DNA based on competition between non-fluorescent tetramolecular and fluorescent (3+1) intermolecular G-quadruplexes. This journal is © The Royal Society of Chemistry 2013.

Zhou W.,Northwestern University | Zhou W.,Harvard University | Dridi M.,Northwestern University | Suh J.Y.,Northwestern University | And 6 more authors.
Nature Nanotechnology | Year: 2013

Periodic dielectric structures are typically integrated with a planar waveguide to create photonic band-edge modes for feedback in one-dimensional distributed feedback lasers and two-dimensional photonic-crystal lasers. Although photonic band-edge lasers are widely used in optics and biological applications, drawbacks include low modulation speeds and diffraction-limited mode confinement. In contrast, plasmonic nanolasers can support ultrafast dynamics and ultrasmall mode volumes. However, because of the large momentum mismatch between their nanolocalized lasing fields and free-space light, they suffer from large radiative losses and lack beam directionality. Here, we report lasing action from band-edge lattice plasmons in arrays of plasmonic nanocavities in a homogeneous dielectric environment. We find that optically pumped, two-dimensional arrays of plasmonic Au or Ag nanoparticles surrounded by an organic gain medium show directional beam emission (divergence angle <1.5and linewidth <1.3 nm) characteristic of lasing action in the far-field, and behave as arrays of nanoscale light sources in the near-field. Using a semi-quantum electromagnetic approach to simulate the active optical responses, we show that lasing is achieved through stimulated energy transfer from the gain to the band-edge lattice plasmons in the deep subwavelength vicinity of the individual nanoparticles. Using femtosecond-transient absorption spectroscopy, we verified that lattice plasmons in plasmonic nanoparticle arrays could reach a 200-fold enhancement of the spontaneous emission rate of the dye because of their large local density of optical states. © 2013 Macmillan Publishers Limited. All rights reserved.

Kang S.-J.,Pohang University of Science and Technology | Yoo S.,Pohang University of Science and Technology | Kim Y.H.,Pohang University of Science and Technology
IEEE Transactions on Circuits and Systems for Video Technology | Year: 2010

In this letter, we present a new motion estimation algorithm for frame rate up-conversion. The proposed dual motion estimation algorithm enhances the estimation accuracy of motion vectors by using the unidirectional and bidirectional matching ratios of blocks in the previous and current frames. In addition, the proposed motion estimation approach uses motion vector validity to evaluate the accuracy of motion vectors thereby avoiding false motion vectors. In experiments using benchmark image sequences, the proposed motion estimation algorithm improved the average peak signal-to-noise ratio of interpolated frames by up to 2.272 dB, when compared to conventional motion estimation algorithms. For the comparison of the perceptual image quality using the structural similarity, the average value of the proposed dual motion estimation was by up to 0.062 higher than those of the conventional algorithms. © 2006 IEEE.

Bhavani A.G.,Pohang University of Science and Technology | Kim W.Y.,Pohang University of Science and Technology | Lee J.S.,Ulsan National Institute of Science and Technology
ACS Catalysis | Year: 2013

Barium substituted lanthanum manganite La1-xBa xMnO3 (x = 0.10-0.50) of a single phase perovskite structure was used as catalysts for CO2 reforming of CH4 for the first time. The optimal level of Ba substitution (Ba/Mn = 0.10, 0.15) produced La1-xBaxMnO3 of high surface area, uniform particle dispersion, and highly ordered pores. The optimally substituted perovskite catalysts showed much improved reducibility of Mn 3+/Mn4+ to Mn2+ to provide oxygen vacancies and rapid migration of lattice oxygen from the bulk toward the surface. The ability of donating lattice oxygen to the catalytic cycle seems responsible for the facilitated decomposition and dissociation of CH4 and CO2, which led to high conversions, excellent syngas selectivity, and stability with little coke formation. The addition of oxygen to the dry reforming reaction showed improved conversion and selectivity to syngas by making catalysts less prone to coke formation. To the best of our knowledge, the results represent the first example of Mn-based reforming catalysts that perform better than more common Ni-based catalysts of the same structure. © 2013 American Chemical Society.

Ha H.,Pohang University of Science and Technology | Lee S.-J.,Pohang University of Science and Technology
Microvascular Research | Year: 2013

Platelet aggregation has been known to be closely influenced by the surrounding hemodynamic environments. Especially, platelet activation, aggregation, and thrombus formation frequently occur at the locally stenosed blood vessel where recirculation and stagnation flow regions are developed. However, the relationship between hemodynamic feature and platelet aggregation is not fully understood yet. The main objective of this study is to investigate the hemodynamic characteristics of blood flow in a stenosis channel and their effects on platelet aggregation. Whole blood was injected into a stenosed microchannel with 85% severity at various flow rates, ranging from 10 to 50mLhr-1. The velocity vector field of the blood flow in the stenosed microchannel was measured using newly developed LED (light emitting diode)-illumination microparticle image velocimetry (micro-PIV). The blood flow is highly disturbed by the micro-stenosis, and a recirculation flow region is formed at the post-stenosis region. The occurring site and the shape of the platelet aggregation are highly influenced by the hemodynamic characteristics of blood flow around the stenosis. Especially, the platelet aggregation is found to occur at the interface where the downward momentum of the central jet at the post-stenosis region and the upward momentum of the recirculation flow are balanced. These experimental results would be helpful to understand the platelet aggregation under disturbed blood flow conditions. © 2013 Elsevier Inc.

Park J.H.,Pohang University of Science and Technology | Park J.M.,Pohang University of Science and Technology
Surface and Coatings Technology | Year: 2014

In the present work, electrophoretic deposition (EPD) was used to deposit graphene oxide (GO) onto carbon steel from a GO water suspension. The GO coating was used as a barrier layer for the corrosion protection of carbon steel. The suspension of GO was characterized by measuring the zeta potential of the solution using a dispersion optical analyzer. The morphology of the GO and the surface morphology of the EPD layers were investigated using scanning electron microscopy (SEM), and Raman spectroscopy was performed to determine the effects of EPD on the electrochemical reduction of GO. GO was successfully synthesized using the modified Hummers method, and a GO layer was efficiently deposited onto mild steel using EPD. EPD and the partial reduction of the GO occurred simultaneously under the applied electrical fields. Moreover, the anti-corrosion performance of the GO-EPD layers was characterized using electrochemical impedance spectroscopy (EIS), the potentiodynamic polarization method, and a cyclic corrosion test (CCT). The applications of GO-EPD as an underlying layer improved the anti-corrosion performance of organic top-coated steel due to the superior barrier property of GO. © 2014 Elsevier B.V.

Kim K.-W.,Pohang University of Science and Technology | Lee H.-W.,Pohang University of Science and Technology | Lee K.-J.,Korea University | Stiles M.D.,U.S. National Institute of Standards and Technology
Physical Review Letters | Year: 2013

As nanomagnetic devices scale to smaller sizes, spin-orbit coupling due to the broken structural inversion symmetry at interfaces becomes increasingly important. Here, we study interfacial spin-orbit coupling effects in magnetic bilayers using a simple Rashba model. The spin-orbit coupling introduces chirality into the behavior of the electrons and through them into the energetics of the magnetization. In the derived form of the magnetization dynamics, all of the contributions that are linear in the spin-orbit coupling follow from this chirality, considerably simplifying the analysis. For these systems, an important consequence is a correlation between the Dzyaloshinskii-Moriya interaction and the spin-orbit torque. We use this correlation to analyze recent experiments. © 2013 American Physical Society.

Kim H.,Pohang University of Science and Technology | Namgung R.,Pohang University of Science and Technology | Singha K.,Pohang University of Science and Technology | Oh I.-K.,Korea Advanced Institute of Science and Technology | Kim W.J.,Pohang University of Science and Technology
Bioconjugate Chemistry | Year: 2011

Graphene oxide (GO) has attracted an increasing amount of interest because of its potential applications in biomedical fields such as biological imaging, molecular imaging, drug/gene delivery, and cancer therapy. Moreover, GO could be fabricated by modifying its functional groups to impart specific functional or structural attributes. This study demonstrated the development of a GO-based efficient gene delivery carrier through installation of polyethylenimine, a cationic polymer, which has been widely used as a nonviral gene delivery vector. It was revealed that a hybrid gene carrier fabricated by conjugation of low-molecular weight branched polyethylenimine (BPEI) to GO increased the effective molecular weight of BPEI and consequently improved DNA binding and condensation and transfection efficiency. Furthermore, this hybrid material facilitated sensing and bioimaging because of its tunable and intrinsic electrical and optical properties. Considering the extremely high transfection efficiency comparable to that of high-molecular weight BPEI, high cell viability, and its application as a bioimaging agent, the BPEI-GO hybrid material could be extended to siRNA delivery and photothermal therapy. © 2011 American Chemical Society.

Das L.K.,University of Calcutta | Park S.-W.,Pohang University of Science and Technology | Cho S.J.,Chosun University | Ghosh A.,University of Calcutta
Dalton Transactions | Year: 2012

Two new trinuclear hetero-metallic copper(ii)-zinc(ii) complexes [(CuL)2Zn(N3)2] (1A and 1B) have been synthesized using [CuL] as a so-called "metalloligand" (where H 2L = N,N′-bis(salicylidene)-1,3-propanediamine) and structurally characterized. Complexes 1A and 1B have the same molecular formula but crystallize in different crystal systems (triclinic for 1A and monoclinic for 1B) with space group P1 for 1A and P21/c for 1B. 1A is an angular trinuclear species, in which two terminal four-coordinate square planar "metalloligand" [CuL] are coordinated to a central Zn(ii) through double phenoxido bridges. The Zn(ii) is in a six-coordinate distorted octahedral environment being bonded additionally to two mutually cis nitrogen atoms of terminal azide ions. In complex 1B, in addition to the double phenoxido bridge, the two terminal Cu(ii) ions are linked to the central Zn(ii) via a μ-l,l azido bridge giving rise to a square pyramidal environment around the Cu(ii) ions and consequently the structure becomes linear. These two species can be considered as "linear-bent" isomers. EPR spectra and ESI mass spectra show that the two isomers are identical in solution. The DFT calculation reveals that the energy of 1A is 7.06 kcal mol-1 higher than that of 1B. The existence of both isomers in the solid state suggests that crystal packing interactions in 1A are more efficient and probably compensate for the difference in energy. © 2012 The Royal Society of Chemistry.

Soare S.,Technical University of Cluj Napoca | Barlat F.,Pohang University of Science and Technology
Journal of the Mechanics and Physics of Solids | Year: 2010

It is shown that some of the recently proposed orthotropic yield functions obtained through the linear transformation method are homogeneous polynomials. This simple observation has the potential to simplify considerably their implementation into finite element codes. It also leads to a general method for designing convex polynomial yield functions with powerful modeling capabilities. Convex parameterizations are given for the fourth, sixth and eighth order plane stress orthotropic homogeneous polynomials. Illustrations are shown for the modeling of biaxial and directional yielding properties of steel and aluminum alloy sheets. The parametrization method can be easily extended to general, 3D stress states. © 2010 Elsevier Ltd. All rights reserved.

Lim H.-T.,Pohang University of Science and Technology | Kim Y.-S.,Pohang University of Science and Technology | Ra Y.-S.,Pohang University of Science and Technology | Bae J.,Korea Institute for Advanced Study | Kim Y.-H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2011

We report the first experimental realization of an approximate partial transpose for photonic two-qubit systems. The proposed scheme is based on the local operation on single copies of quantum states and classical communication, and therefore can be easily applied for other quantum information tasks within current technologies. Direct detection of entanglement, i.e., without performing quantum state tomography, using the partial transpose operation, is also demonstrated. © 2011 American Physical Society.

Kim K.S.,Pohang University of Science and Technology | Kim K.S.,Lawrence Berkeley National Laboratory | Yeom H.W.,Pohang University of Science and Technology
Physical Review Letters | Year: 2011

The electronic band structure of a liquid metal was investigated by measuring precisely the evolution of angle-resolved photoelectron spectra during the melting of a Pb monolayer on a Si(111) surface. We found that the liquid monolayer exhibits a free-electron-like band and it undergoes a coherent radial scattering, imposed by the radial correlation of constituent atoms, to form a characteristic secondary hole band. These unique double-radial bands and their gradual evolution during melting can be quantitatively reproduced, including detailed spectral intensity profiles, with our radial scattering model based on a theoretical prediction of 1962. Our result establishes the radial band structure as a key concept for describing the nature of electrons in strongly disordered states of matter. © 2011 American Physical Society.

Lee J.-H.,Pohang University of Science and Technology | Jeong Y.K.,Pohang University of Science and Technology | Park J.H.,Pohang University of Science and Technology | Oak M.-A.,Pohang University of Science and Technology | And 3 more authors.
Physical Review Letters | Year: 2011

SmFeO3, a family of centrosymmetric rare-earth orthoferrites, is known to be nonferroelectric. However, we have found that SmFeO3 is surprisingly ferroelectric at room temperature with a small polarization along the b axis of Pbnm. First-principles calculations indicate that the canted antiferromagnetic ordering with two nonequivalent spin pairs is responsible for this extraordinary polarization and that the reverse Dzyaloshinskii-Moriya interaction dominates over the exchange-striction mechanism in the manifestation of the improper ferroelectricity. SmFeO3 further exhibits an interesting phenomenon of spontaneous magnetization reversal at cryogenic temperatures. This reversal is attributed to the activation of the Sm-spin moment which is antiparallel to the Fe-spin moment below ∼5K. © 2011 American Physical Society.

Kim H.-K.,Pohang University of Science and Technology | Kim R.-R.,Pohang University of Science and Technology | Oh J.-H.,California Institute of Technology | Cho H.,Pohang University of Science and Technology | And 2 more authors.
Cell | Year: 2014

The Arg/N-end rule pathway targets for degradation proteins that bear specific unacetylated N-terminal residues while the Ac/N-end rule pathway targets proteins through their Nα-terminally acetylated (Nt-acetylated) residues. Here, we show that Ubr1, the ubiquitin ligase of the Arg/N-end rule pathway, recognizes unacetylated N-terminal methionine if it is followed by a hydrophobic residue. This capability of Ubr1 expands the range of substrates that can be targeted for degradation by the Arg/N-end rule pathway because virtually all nascent cellular proteins bear N-terminal methionine. We identified Msn4, Sry1, Arl3, and Pre5 as examples of normal or misfolded proteins that can be destroyed through the recognition of their unacetylated N-terminal methionine. Inasmuch as proteins bearing the Nt-acetylated N-terminal methionine residue are substrates of the Ac/N-end rule pathway, the resulting complementarity of the Arg/N-end rule and Ac/N-end rule pathways enables the elimination of protein substrates regardless of acetylation state of N-terminal methionine in these substrates. © 2014 Elsevier Inc.

Chun J.,Pohang University of Science and Technology | Lee J.,Pohang University of Science and Technology
European Journal of Inorganic Chemistry | Year: 2010

In this microreview, the recent progress on various synthetic methods for 1D semiconductor nanowires is summarized. The colloidal synthetic method has been popularly employed to prepare various semiconductor nanorods/nanowires such as ZnS and TiO2. The vapor-liquid-solid (VLS) synthetic method has been used to fabricate Si and ZnO nanowires. For the growth of semiconductor nanowires by the VLS method, metals that can form a eutectic mixture with a target material have been used as catalysts. After the first report on the synthesis of crystalline III-V semiconductor nanowires bythe solution-liquid-solid (SLS) method, various kinds of III-V and II-VI semiconductor nanowires have been synthesized by the SLS method. Various types of templates, including anodic aluminum oxide (AAO), sacrificial nanowire templates, and self-assembled surfactants, have been employed to fabricate 1D semiconductor nanowires that resemble the shape of the template employed. Using 1D semiconductor nanowires, high-performance photovoltaic cells can be fabricated due to facile electron transport within nanowires. © 2010 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim.

Nam S.-G.,Pohang University of Science and Technology | Hwang E.H.,Sungkyunkwan University | Lee H.-J.,Pohang University of Science and Technology
Physical Review Letters | Year: 2013

We report measurements of heat transport along the edge conducting channels in monolayer graphene in the integer quantum Hall regime. Hot charge carriers are injected to the edge channels, and the thermoelectric voltage is measured at a distance along the edge from the heat injection point. We confirm that heat transport in graphene in the quantum Hall regime is chiral and the thermoelectric signal is correlated with the charge conductance of ballistic transport, following the Mott relation. The thermoelectric signal decays with distance from the heater, indicating that carriers are partially thermalized during edge transmission. © 2013 American Physical Society.

Choi I.,Pohang University of Science and Technology | Ahn H.,Pohang University of Science and Technology | Park M.J.,Pohang University of Science and Technology
Macromolecules | Year: 2011

We have explored new Li-polymer batteries composed of surface functionalized Si nanoparticles (SiNPs) as anode active materials and nanostructured block copolymers as solid electrolytes. Surface protection of SiNPs with poly(ethylene oxide) chains successfully prevents aggregation of SiNPs during cycling and also helps fast Li+ transport to the active centers in the anodes. The self-assembly nature of block copolymer electrolytes in ca. 50 nm periodicity is aimed to restrain the formation of macroscopic ionic clusters during Li-insertion/desertion. To decouple the electrical and mechanical properties of polymer electrolytes, two different nonvolatile additives (ionic liquid and non ionic plasticizer) were incorporated and remarkably different cycle performances have been observed. The incorporation of ionic liquid yields the utmost ionic conductivity and distinctly large first lithium insertion capacity of 2380 mA h/g was seen. However, the formation of solid electrolyte interphase (SEI) was responsible for highly irreversible lithium desertion capacity and the system indicate fast capacity fading during cycling. With the use of non ionic plasticizer, in contrast, the SiNPs anode can store lithium up to a reversible capacity of ∼1850 mA h/g under aggressive test profiles of 80 °C and voltage window between 0-4.5 V. The focused ion beam technique was successfully used to obtain ex-situ transmission electron microscopy images of cycled polymer electrolytes and anode materials to underpin the origin of capacity retention or fading upon cycling. The results suggest that the structural retention of both polymer electrolytes and SiNPs during cycling attributes to the improved battery performance. © 2011 American Chemical Society.

Lee D.,University of Ulsan | Lee J.,Pohang University of Science and Technology
IEEE Transactions on Knowledge and Data Engineering | Year: 2010

Clustering methods utilizing support estimates of a data distribution have recently attracted much attention because of their ability to generate cluster boundaries of arbitrary shape and to deal with outliers efficiently. In this paper, we propose a novel dissimilarity measure based on a dynamical system associated with support estimating functions. Theoretical foundations of the proposed measure are developed and applied to construct a clustering method that can effectively partition the whole data space. Simulation results demonstrate that clustering based on the proposed dissimilarity measure is robust to the choice of kernel parameters and able to control the number of clusters efficiently. © 2006 IEEE.

Kim J.,Ohio State University | Somers D.E.,Ohio State University | Somers D.E.,Pohang University of Science and Technology
Plant Physiology | Year: 2010

Rapid assessment of the effect of reduced levels of gene products is often a bottleneck in determining how to proceed with an interesting gene candidate. Additionally, gene families with closely related members can confound determination of the role of even a single one of the group. We describe here an in vivo method to rapidly determine gene function using transient expression of artificial microRNAs (amiRNAs) in Arabidopsis (Arabidopsis thaliana) mesophyll protoplasts. We use a luciferasebased reporter of circadian clock activity to optimize and validate this system. Protoplasts transiently cotransfected with promoter-luciferase and gene-specific amiRNA plasmids sustain free-running rhythms of bioluminescence for more than 6 d. Using both amiRNA plasmids available through the Arabidopsis Biological Resource Center, as well as custom design of constructs using the Weigel amiRNA design algorithm, we show that transient knockdown of known clock genes recapitulates the same circadian phenotypes reported in the literature for loss-of-function mutant plants. We additionally show that amiRNA designed to knock down expression of the casein kinase II β-subunit gene family lengthens period, consistent with previous reports of a short period in casein kinase II β-subunit overexpressors. Our results demonstrate that this system can facilitate a much more rapid analysis of gene function by obviating the need to initially establish stably transformed transgenics to assess the phenotype of gene knockdowns. This approach will be useful in a wide range of plant disciplines when an endogenous cell-based phenotype is observable or can be devised, as done here using a luciferase reporter. © 2010 American Society of Plant Biologists.

Oh Y.S.,Rutgers Center for Emergent Materials | Yang J.J.,Pohang University of Science and Technology | Horibe Y.,Rutgers Center for Emergent Materials | Cheong S.-W.,Rutgers Center for Emergent Materials | Cheong S.-W.,Pohang University of Science and Technology
Physical Review Letters | Year: 2013

Selenium substitution drastically increases the transition temperature of iridium ditelluride (IrTe2) to a diamagnetic superstructure from 278 to 560 K. Transmission electron microscopy experiments revealed that this enhancement is accompanied by the evolution of nonsinusoidal structure modulations from q=1/5(101̄) to q=1/6(101̄) types. These comprehensive results are consistent with the concept of the destabilization of polymeric Te-Te bonds at the transition, the temperature of which is increased by chemical and hydrostatic pressure and by the substitution of Te with the more electronegative Se. This temperature-induced depolymerization transition in IrTe2 is unique in crystalline inorganic solids. © 2013 American Physical Society.

Choi D.-S.,Pohang University of Science and Technology | Kim D.-K.,Pohang University of Science and Technology | Kim Y.-K.,Pohang University of Science and Technology | Gho Y.S.,Pohang University of Science and Technology
Proteomics | Year: 2013

Mammalian cells secrete two types of extracellular vesicles either constitutively or in a regulated manner: exosomes (50-100 nm in diameter) released from the intracellular compartment and ectosomes (also called microvesicles, 100-1000 nm in diameter) shed directly from the plasma membrane. Extracellular vesicles are bilayered proteolipids enriched with proteins, mRNAs, microRNAs, and lipids. In recent years, much data have been collected regarding the specific components of extracellular vesicles from various cell types and body fluids using proteomic, transcriptomic, and lipidomic methods. These studies have revealed that extracellular vesicles harbor specific types of proteins, mRNAs, miRNAs, and lipids rather than random cellular components. These results provide valuable information on the molecular mechanisms involved in vesicular cargo-sorting and biogenesis. Furthermore, studies of these complex extracellular organelles have facilitated conceptual advancements in the field of intercellular communication under physiological and pathological conditions as well as for disease-specific biomarker discovery. This review focuses on the proteomic, transcriptomic, and lipidomic profiles of extracellular vesicles, and will briefly summarize recent advances in the biology, function, and diagnostic potential of vesicle-specific components. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Gong J.-O.,Asia Pacific Center for Theoretical Physics | Gong J.-O.,Pohang University of Science and Technology
Journal of Cosmology and Astroparticle Physics | Year: 2014

We examine the possibility of positive spectral index of the power spectrum of the primordial tensor perturbation produced during inflation in the light of the detection of the B-mode polarization by the BICEP2 collaboration. We find a blue tilt is in general possible when the slow-roll parameter decays rapidly. We present two known examples in which a positive spectral index for the tensor power spectrum can be obtained. We also briefly discuss other consistency tests for further studies on inflationary dynamics. © 2014 IOP Publishing Ltd and Sissa Medialab srl .

Seo K.W.,Pohang University of Science and Technology | Byeon H.J.,Pohang University of Science and Technology | Lee S.J.,Pohang University of Science and Technology
Optics Letters | Year: 2014

The light scattering properties of a horizontally and vertically oriented spheroidal particle under laser illumination are experimentally investigated using digital inline holography. The reconstructed wave field shows the bright singular points as a result of the condensed beam formed by a transparent spheroidal particle acting as a lens. The in-plane (θ) and out-of-plane (ω) rotating angles of an arbitrarily oriented spheroidal particle are measured by using these scattering properties. As a feasibility test, the 3D orientation of a transparent spheroidal particle suspended in a microscale pipe flow is successfully reconstructed by adapting the proposed method. © 2014 Optical Society of America.

Panwar A.,Pohang University of Science and Technology | Ryu C.-M.,Pohang University of Science and Technology
Physics of Plasmas | Year: 2014

The modulational instability and associated rogue structures of a slow magnetosonic wave are investigated for a Hall magnetohydrodynamic plasma. Nonlinear Schrodinger equation is obtained by using the multiple scale method, which shows a modulationally unstable slow magnetosonic mode evolving into bright wavepackets. The dispersive effects induced by the Hall electron current increase with the increase in plasma β and become weaker as the angle of propagation increases. The growth rate of the modulational instability also increases with the increase in plasma β. The growth rate is greatest for the parallel propagation and drops to zero for perpendicular propagation. The envelope wavepacket of a slow magnetosonic is widened with less oscillations as plasma β increases. But the wavepacket becomes slightly narrower and more oscillatory as the angle of propagation increases. Further a non-stationary envelope solution of the Peregrine soliton is analyzed for rogue waves. The Peregrine soliton contracts temporally and expands spatially with increase in plasma β. However, the width of a slow magnetosonic Peregrine soliton decreases both temporally and spatially with increase of the propagation angle. © 2014 AIP Publishing LLC.

Purich A.,CSIRO | Cowan T.,CSIRO | Min S.-K.,Pohang University of Science and Technology | Cai W.,CSIRO
Journal of Climate | Year: 2013

In recent decades, Southern Hemisphere midlatitude regions such as southern Africa, southeastern Australia, and southern Chile have experienced a reduction in austral autumn precipitation; the cause of which is poorly understood. This study focuses on the ability of global climate models that form part of the Coupled Model Intercomparison Project phase 5 to simulate these trends, their relationship with extratropical and subtropical processes, and implications for future precipitation changes. Models underestimate both the historical autumn poleward expansion of the subtropical dry zone and the positive southern annular mode (SAM) trend. The multimodel ensemble (MME) is also unable to capture the spatial pattern of observed precipitation trends across semiarid midlatitude regions. However, in temperate regions that are located farther poleward such as southern Chile, the MME simulates observed precipitation declines. The MME shows a strong consensus in twenty-first-century declines in autumn precipitation across southern Chile in both the medium-low and high representative concentration pathway (RCP) scenarios and across southern Africa in the high RCP scenario, but little change across southeastern Australia. Projecting a strong positive SAM trend and continued subtropical dry-zone expansion, the models converge on large SAM and dry-zoneexpansion-induced precipitation declines across southern midlatitudes. In these regions, the strength of future precipitation trends is proportional to the strength of modeled trends in these phenomena, suggesting that unabated greenhouse gas-induced climate change will have a large impact on austral autumn precipitation in such midlatitude regions.

Lee J.,POSCO | Lee J.S.,Pohang University of Science and Technology
Computers and Industrial Engineering | Year: 2010

For scheduling flexible manufacturing systems efficiently, we propose new heuristic functions for A algorithm that is based on the T-timed Petri net. In minimizing makespan, the proposed heuristic functions are usually more efficient than the previous functions in the required number of states and computation time. We prove that these heuristic functions are all admissible and one of them is more informed than that using resource cost reachability matrix. We also propose improved versions of these heuristic functions that find a first near-optimal solution faster. In addition, we modify the heuristic function of Yu, Reyes, Cang, and Lloyd (2003b) and propose an admissible version in all states. The experimental results using a random problem generator show that the proposed heuristic functions perform better as we expected. © 2010 Elsevier Ltd. All rights reserved.

Seo K.W.,Pohang University of Science and Technology | Kang Y.J.,Chosun University | Lee S.J.,Pohang University of Science and Technology
Physics of Fluids | Year: 2014

The lateral migration of microspheres across streamlines induced by elasticity and inertia in a square microchannel flow of viscoelastic fluids is investigated using a holographicmicroscopy technique.We experimentally demonstrate the exact particle positions driven by the elasticity of fluid in the channel cross-section. The effects of the blockage ratio, flow rate, and shear-thinning property of the viscoelastic fluids on particle migration are evaluated. In particular, the focusing patterns ofmicrospheres in three-dimensional volume are analyzed under different conditions, namely, dominant inertia, dominant elasticity, and the combined effects of inertia and elasticity. © 2014 AIP Publishing LLC.

Lee J.,Pohang University of Science and Technology | Lee J.-H.,Pohang University of Science and Technology | Park J.,Pohang University of Science and Technology | Kim J.S.,Pohang University of Science and Technology | Lee H.-J.,Pohang University of Science and Technology
Physical Review X | Year: 2014

The topologically protected conducting state is expected to exist on the entire surface of threedimensional topological insulators (TIs). Using concurrent measurements of the local and nonlocal conduction, we provide experimental evidence for the topological robustness of the surface-conducting states of bulk-insulating Bi1.5Sb0.5Te1.7Se1.3 crystalline flakes. The detailed investigation of local and nonlocal charge conductance on the top surfaces, combining with the comprehensive numerical simulation, reveals that the charge current is widely distributed over the entire surface of a TI. Our findings show evidence of the presence of the topologically protected conducting state at the side wall with irregularly stacked edges between the top and bottom surfaces. This study provides a reliable means of accurately characterizing the topological surface states with inherent nonlocal surface-dominant conducting channels in a TI.

Ko W.-S.,Pohang University of Science and Technology | Lee B.-J.,Pohang University of Science and Technology
Philosophical Magazine | Year: 2014

Atomistic simulations based on interatomic potentials have frequently failed to correctly reproduce the brittle fracture of materials, showing an unrealistic blunting. We analyse the origin of the unrealistic blunting during atomistic simulations by modified embedded-atom method (MEAM) potentials for experimentally well-known brittle materials such as bcc tungsten and diamond silicon. The radial cut-off which has been thought to give no influence on MEAM calculations is found to have a decisive effect on the crack propagation behaviour. Extending both cut-off distance and truncation range can prevent the unrealistic blunting, reproducing many well-known fracture behaviour which have been difficult to reproduce. The result provides a guideline for future atomistic simulations that focus on various fracture-related phenomena including the failure of metallic-covalent bonding material systems using MEAM potentials. © 2014 Taylor & Francis.

Ghosh S.,Pohang University of Science and Technology
Materials Chemistry and Physics | Year: 2010

Nb alloyed ferritic stainless steel is an attractive material to be used in automobile exhaust systems. Recently, in some published experimental work it was reported that coarsening rate of Laves phase (Fe2Nb) can be higher than NbC in Nb alloyed ferritic stainless steels during aging at 700 °C. This observation was attributed to the fact that NbC has a more coherent interface with ferrite than has Laves phase. We explore this conclusion and find that the real reason for the smaller coarsening rate of NbC is the incredibly low solubility of carbon in ferrite. © 2010 Elsevier B.V. All rights reserved.

Kim G.J.,Pohang University of Science and Technology | Kim W.,Pohang University of Science and Technology | Kim K.T.,Pohang University of Science and Technology | Lee J.K.,Pohang University of Science and Technology
Applied Physics Letters | Year: 2010

Nonthermal plasma is known to induce animal cell death but the mechanism is not yet clear. Here, cellular and biochemical regulation of cell apoptosis is demonstrated for plasma treated cells. Surface type nonthermal air plasma triggered apoptosis of B16F10 mouse melanoma cancer cells causing DNA damage and mitochondria dysfunction. Plasma treatment activated caspase-3, apoptosis executioner. The plasma treated cells also accumulated gamma-H2A.X, marker for DNA double strand breaks, and p53 tumor suppressor gene as a response to DNA damage. Interestingly, cytochrome C was released from mitochondria and its membrane potential was changed significantly. © 2010 American Institute of Physics.

Gong Y.F.,Pohang University of Science and Technology | De Cooman B.C.,Pohang University of Science and Technology
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2010

The selective oxidation of high Mn austenitic steel was investigated by transmission electron microscopy. The annealing resulted in a MnO surface layer and a Mn-depleted ferritic layer. At the MnO/steel interface, voids were formed between the a-xMnO.SiO2 and c-MnO.Al2O3 layers. This is the first time that void formation is observed during selective oxidation of steel. The Kirkendall voids grow and develop characteristic void surfaces because of the fast diffusion of MnO on the void surface. © 2010 The Minerals, Metals & Materials Society and ASM International.

Hong D.K.,Pusan National University | Hong D.K.,Pohang University of Science and Technology
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2011

We consider fermionic dense matter under a magnetic field, where fermions couple minimally to gauge fields, and calculate anomalous currents at one loop. We find anomalous currents are spontaneously generated along the magnetic field but fermions only in the lowest Landau level contribute to anomalous currents. We then show that there are no more corrections to the anomalous currents from two or higher loops. © 2011 Elsevier B.V.

Kim K.-S.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We show that introduction of vertex corrections in the fully self-consistent ladder approximation does not modify dynamics of spinons and gauge fluctuations in the U(1) gauge theory with Fermi surface. © 2010 The American Physical Society.

Kang M.H.,Pohang University of Science and Technology | Jung S.C.,Pohang University of Science and Technology | Park J.W.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We have studied the effect of Au intercalation on the atomic and electronic structure of the graphene/Ni(111) surface by using density functional theory calculations. Our calculations demonstrate that (1) Au atoms energetically favor interface intercalation over surface adsorption, (2) Au intercalation drastically changes the electronic structure of graphene/Ni(111) so that the graphene π bands almost recover the Dirac cone of ideal free-standing graphene, and (3) the Fermi edge locates closely at the Dirac point, indicating that the underlying Au/Ni(111) substrate is inert. The present theory confirms a recent experimental claim that graphene grown on Ni(111) and intercalated by one monolayer Au can be regarded as quasifree standing. © 2010 The American Physical Society.

Ghosh S.,Pohang University of Science and Technology
Scripta Materialia | Year: 2010

Refining cementite with Si and Mn is of technological interest due to the low cost and recyclability. Recently, some experimental data were published which permit a discussion of the operative mechanism. A kinetic study led by simulation indicates that enrichment of ferrite with Si is time consuming; dissolution of Mn from cementite is the key to the slow coarsening rate of the carbide when Si is substituted by Mn in Fe-0.6C wt.% steels during tempering. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Takimoto T.,Pohang University of Science and Technology
Journal of the Physical Society of Japan | Year: 2011

Three-dimensional topological insulators represent a new quantum state. They were originally proposed by a theoretical approach, but subsequently observed experimentally, e.g., in Bi1-xSbx and Bi 2-xYxX3 with X = Se and Y = Ca as well as X = Te and Y = Sn. A characteristic feature is the existence of metallic surface states with an odd number of Dirac points, which are protected by the time reversal symmetry. Furthermore, it was also suggested that some of the so-called Kondo insulators might be topological insulators. Here, we predict on the basis of calculations of topological indices and of spectrum of surface states of a detailed model, that SmB6 should indeed be a strong topological insulator. We suggest that the metallic surface states of this strong topological insulator are hitherto unexplained in-gap states. Note that, to distiguish it from the systems mentioned above, SmB6 is a stoichiometric compound. © 2011 The Physical Society of Japan.

Son J.H.,Pohang University of Science and Technology | Lee J.-L.,Pohang University of Science and Technology
Applied Physics Letters | Year: 2010

The effects of piezoelectric polarization on efficiency droop in InGaN/GaN light-emitting diodes (LEDs) have been investigated using numerical analysis. The simulation results showed that the severe band bending in InGaN quantum-well was improved as the piezoelectric polarization is reduced, resulting in the improved overlap of electron and hole wave functions. As a results, the internal quantum efficiency increases and efficiency droop significantly reduces. The reduction in piezoelectric polarization could be derived by applying a tensile stress to relax compressive stress in GaN epilayer, improving the efficiency droop of vertical-structure LEDs, agree well with simulation ones. © 2010 American Institute of Physics.

Kim B.H.,Pohang University of Science and Technology | Min B.I.,Pohang University of Science and Technology
New Journal of Physics | Year: 2011

Employing the microscopic superexchange model incorporating the effect of spin-orbit interaction, we have investigated the Dzyaloshinsky-Moriya (DM) interaction in perovskite transition-metal (TM) oxides and explored the interplay between the DM interaction and the TM-3d orbital symmetry. For d 3 and d5 systems with isotropic orbital symmetry, the DM vectors are well described by a simple symmetry analysis considering only the bond geometry. In contrast, the DM interaction for d4 systems with anisotropic orbital symmetry shows slightly different behavior, which does not obey simple symmetry analysis. The direction as well as the strength of the DM vector varies depending on the occupied orbital shape. We have understood this behavior based on the orbital symmetry induced by local crystal field variation. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Kim J.,Pohang University of Science and Technology | Jhi S.-H.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We report a discovery, through first-principles calculations, that crystalline Ge-Sb-Te (GST) phase-change materials exhibit the topological insulating property. Our calculations show that the materials become topological insulator or develop conducting surfacelike interface states depending on the layer stacking sequence. It is shown that the conducting interface states originate from topological insulating Sb2 Te3 layers in GSTs and can be crucial to the electronic property of the compounds. These interface states are found to be quite resilient to atomic disorders but sensitive to the uniaxial strains. We presented the mechanisms that destroy the topological insulating order in GSTs and investigated the role of Ge migration that is believed to be responsible for the amorphorization of GSTs. © 2010 The American Physical Society.

Kim K.-S.,Pohang University of Science and Technology | Pepin C.,CEA Saclay Nuclear Research Center | Pepin C.,Federal University of Rio Grande do Norte
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We present a series of arguments showing that the Seebeck coefficient can be used as a decisive experiment to characterize the nature of the quantum-critical point (QCP) in heavy fermion compounds. Being reactive almost exclusively to the presence of delocalized entropic carriers, the Seebeck coefficient shows a drastic collapse at the Kondo breakdown QCP, as the reconstruction of the Fermi surface takes place. In contrast, around a spin-density-wave QCP, the Seebeck coefficient is broadly symmetric. We discuss the possibility of a change of sign at the QCP, the characteristic variation in | S/T | with temperature and external parameter, as well as the capacity of the Seebeck coefficient to distinguish between localized and itinerant antiferromagnetism. Suggestions of experiments are given in the case of four nonconventional compounds: YbRh2 Si2, Ce (Mn) In 5, CeCu6-x Aux, and URu2 Si 2. © 2010 The American Physical Society.

Park S.H.,Pohang University of Science and Technology | Hong S.-G.,Korea Research Institute of Standards and Science | Lee C.S.,Pohang University of Science and Technology
Scripta Materialia | Year: 2010

Initial {1 0 -1 2} twin influenced the plastic deformation mechanism activated during fatigue deformation, especially during compressive loading, by enhancing the activity of the detwinning mechanism. With the aid of initial {1 0 -1 2} twin, compressive deformation could be fully accommodated by detwinning alone. This change in the predominant plastic deformation mechanism during compressive loading significantly decreased compressive flow stress, causing an increase in mean stress, and thus deteriorated the fatigue resistance of the material. © 2010 Acta Materialia Inc.

Choi Y.-S.,Pohang University of Science and Technology | Lee S.-J.,Pohang University of Science and Technology
Microfluidics and Nanofluidics | Year: 2010

The inertial migration of neutrally buoyant spherical particles suspended in a micro-scale pipe flow was investigated in a Reynolds number range of 1.6 ≤ Re ≤ 77.4. A microtube, 350 μm in diameter, was used for the micro-scale pipe flow, and the ratios of the tube diameter (D) to the particle diameter (d) were D/d = 50, 23, and 12. The three-dimensional positions of the particles were measured using a digital holography technique, and the detailed structures of the Segré-Silberberg annulus were visualized. By analyzing the probability density distributions of the particles, the quantitative data of the equilibrium particle positions were obtained and compared to those of previous experimental and numerical studies. Several characteristics of the inertial migration in a microscale pipe flow, including the effects of Re and D/d, were analyzed. The results were found to be similar to those obtained in macro-scale flows. The degree of inertial migration was also quantified using the obtained probability density function. Based on these results, simple criteria were suggested on the entry lengths required in the design of inertial microfluidic devices. © Springer-Verlag 2010.

Hong S.J.,Pohang University of Science and Technology | Jun H.,Pohang University of Science and Technology | Lee J.S.,Pohang University of Science and Technology
Scripta Materialia | Year: 2010

Highly photoactive, nanocrystalline WO3 films were fabricated by an economic solution process based on polymer-assisted direct deposition. The optimized film exhibited a photocurrent of 2.3 mA cm-2 (at 1.23 V vs. reversible hydrogen electrode) under simulated solar illumination (air mass 1.5 global) and a maximum incident photon to current efficiency of 71% at an incident wavelength of 340 nm. The observed performance represents one of the highest values reported for WO3. High crystallinity, porosity and optimum thickness of nanocrystalline films were identified as the critical variables that induce the high photoactivity. © 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Rahman G.,Pohang University of Science and Technology | Garcia-Suarez V.M.,University of Oviedo | Garcia-Suarez V.M.,Lancaster University
Applied Physics Letters | Year: 2010

The magnetism of C-doped SnO2 (001) surfaces is studied using first-principles calculations. It is found that carbon does not induce magnetism in bulk SnO2 when located at the oxygen site but shows a large magnetic moment at the SnO2 (001) surface. The magnetic moment is mainly contributed by the carbon atoms due to empty minority spins of p orbitals and is localized at the surface and subsurface atoms. No magnetism is observed when the carbon atom is located at the subsurface oxygen sites. The origin of magnetism is discussed in the context of surface bonding. © 2010 American Institute of Physics.

Son J.Y.,Pohang University of Science and Technology | Shin Y.-H.,University of Ulsan | Kim H.,Yonsei University | Jang H.M.,Pohang University of Science and Technology
ACS Nano | Year: 2010

In this study, a NiO RRAM nanocapacitor array was fabricated on a graphene sheet, which was on a Nb-doped SrTiO3 substrate containing terraces with a regular interval of about 100 nm and an atomically smooth surface. For the formation of the NiO RRAM nanocapacitor (Pt/NiO/graphene capacitor) array, an anodic aluminum oxide (AAO) nanotemplate with a pore diameter of about 30 nm and an interpore distance of about 100 nm was used. NiO and Pt were subsequently deposited on the graphene sheet. The NiO RRAM nanocapacitor had a diameter of about 30 ± 2 nm and a thickness of about 33 ± 3 nm. Typical unipolar switching characteristics of the NiO RRAM nanocapacitor array were confirmed. The NiO RRAM nanocapacitor array on graphene exhibited lower SET and RESET voltages than that on a bare surface of Nb-doped SrTiO3. © 2010 American Chemical Society.

Ha S.,Pohang University of Science and Technology | Kim K.,Pohang University of Science and Technology
International Journal of Mechanical Sciences | Year: 2010

In this study, we investigate deformation behavior of f.c.c. single crystals containing microvoids by using three-dimensional finite element methods. The unit cell analysis has been conducted to study the effect of stress triaxialities, crystallographic orientations and initial void volume fractions on the growth and coalescence of voids in f.c.c. single crystals. The locally homogeneous constitutive model for the rate-dependent single crystal plasticity is implemented into a finite element program (ABAQUS) by means of the user-defined subroutine (UMAT). To identify the effect of stress triaxiality and the crystallographic orientation on the void evolution, the stress triaxiality was kept constant during deformation. The numerical results showed that the stress triaxiality and the deformation mode specified by the crystallographic orientation have a competitive effect on the evolution of voids. For the low level of stress triaxiality, the deformation mode is mainly determined by the crystallographic orientation. For high stress triaxiality, however, the deviation from the specific deformation mode is large even for incipient void growth and the void growth rate is mainly determined by stress triaxiality and the initial void volume fraction. For the small initial void volume fraction, the growth rate of a void is rapid compared with the large one and the effect of the initial crystallographic orientation is significant. © 2010 Elsevier Ltd. All rights reserved.

Kim H.,Kyung Hee University | Ahn H.S.,Pohang University of Science and Technology | Kim M.H.,Pohang University of Science and Technology
Journal of Heat Transfer | Year: 2010

The pool boiling characteristics of water-based nanofluids with alumina and titania nanoparticles of 0.01 vol % were investigated on a thermally heated disk heater at saturated temperature and atmospheric pressure. The results confirmed the findings of previous studies that nanofluids can significantly enhance the critical heat flux (CHF), resulting in a large increase in the wall superheat. It was found that some nanoparticles deposit on the heater surface during nucleate boiling, and the surface modification due to the deposition results in the same magnitude of CHF enhancement in pure water as for nanofluids. Subsequent to the boiling experiments, the interfacial properties of the heater surfaces were examined using dynamic wetting of an evaporating water droplet. As the surface temperature increased, the evaporating meniscus on the clean surface suddenly receded toward the liquid due to the evaporation recoil force on the liquid-vapor interface, but the nanoparticle-fouled surface exhibited stable wetting of the liquid meniscus even at a remarkably higher wall superheat. The heat flux gain attainable due to the improved wetting of the evaporating meniscus on the fouled surface showed good agreement with the CHF enhancement during nanofluid boiling. It is supposed that the nanoparticle layer increases the stability of the evaporating microlayer underneath a bubble growing on a heated surface and thus the irreversible growth of a hot/dry spot is inhibited even at a high wall superheat, resulting in the CHF enhancement observed when boiling nanofluids. © 2010 by ASME.

Xu W.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Organic Electronics: physics, materials, applications | Year: 2010

Low-voltage operable organic field-effect transistors (OFETs) were fabricated with a high-k polymer gate insulator, consisting of cyanoethylated pullulan (CEP) and poly(methylated melamine-co-formaldehyde) (PMMF) as a cross-linker. Effect of the cross-linker amount on the dielectric properties of the film was studied and transistor performance was evaluated. At the optimum PMMF contents, field-effect mobility as high as 2.16 cm2/V s, on/off current ratio of ∼3 × 105, low hysteresis (ΔVth ∼ 0.01 V) and a steep inverse subthreshold slope of 0.066 V/dec were obtained. A utilization of stainless steel as a gate metal and substrate markedly improved the device performance under a low-voltage operation (∼1 V) due to the positively shifted threshold voltage from the work function change. The devices showed very little degradation in electrical properties with bending. © 2010 Elsevier B.V. All rights reserved.

Lim J.,Pohang Light Source | Kim H.,GIST Inc | Park S.Y.,Pohang University of Science and Technology
Journal of Synchrotron Radiation | Year: 2014

The synchrotron-based hard X-ray nanotomography beamline, named 7C X-ray Nano Imaging (XNI), was recently established at Pohang Light Source II. This beamline was constructed primarily for full-field imaging of the inner structures of biological and material samples. The beamline normally provides 46 nm resolution for still images and 100 nm resolution for tomographic images, with a 40 μm field of view. Additionally, for large-scale application, it is capable of a 110 μm field of view with an intermediate resolution. © 2014 International Union of Crystallography.

Han B.,Pohang University of Science and Technology | Davis L.S.,University of Maryland University College
IEEE Transactions on Pattern Analysis and Machine Intelligence | Year: 2012

Background modeling and subtraction is a natural technique for object detection in videos captured by a static camera, and also a critical preprocessing step in various high-level computer vision applications. However, there have not been many studies concerning useful features and binary segmentation algorithms for this problem. We propose a pixelwise background modeling and subtraction technique using multiple features, where generative and discriminative techniques are combined for classification. In our algorithm, color, gradient, and Haar-like features are integrated to handle spatio-temporal variations for each pixel. A pixelwise generative background model is obtained for each feature efficiently and effectively by Kernel Density Approximation (KDA). Background subtraction is performed in a discriminative manner using a Support Vector Machine (SVM) over background likelihood vectors for a set of features. The proposed algorithm is robust to shadow, illumination changes, spatial variations of background. We compare the performance of the algorithm with other density-based methods using several different feature combinations and modeling techniques, both quantitatively and qualitatively. © 2012 IEEE.

Kim I.,Pohang University of Science and Technology | Kim B.,Pohang University of Science and Technology
IEEE MTT-S International Microwave Symposium Digest | Year: 2010

We demonstrates an optimized design of a highly efficient 3'stage Doherty PA for the 802.16e Mobile world inter operability for microwave access(WiMAX) application at 2.655- GHz. The '3'stage' Doherty PA is the most efficient architecture among the various Doherty PAs for a high peak to average power ratio(PAPR) signal. However, due to the low quiescent bias point of the peaking PA, it has an improper load modulation problem, which causes over saturated operation of the carrier PA and insufficient peak power. We have solved the problem using a gate envelope tracking(ET) technique. Using the ADS and Matlab simulations, the behavior of the 3-stage Doherty PA employing the ET technique is presented. For the WiMAX signal with a 7.8 ∼ 8.5 dB PAPR and 20 MHz instantaneous bandwidth, the measured efficiency of the proposed 3'stage Doherty PA has been maintained 56.7 % at an average output power of 43.3 dBm, a 7.2 dB backed off output power from peak output power level. These experimental results clearly show that the 3'stage Doherty with ET technique has a superior efficiency with high peak power. The PAs accompanied with the DPD are very good candidates for the highly efficient and linear base station transmitter. © 2010 IEEE.

Jee D.-W.,Pohang University of Science and Technology | Seo Y.-H.,Pohang University of Science and Technology | Park H.-J.,Pohang University of Science and Technology | Sim J.-Y.,Pohang University of Science and Technology
IEEE Journal of Solid-State Circuits | Year: 2012

This paper presents a low-power noise-shaping ΔΣ time-to-digital converter (TDC) and its application to a fractional-N digital PLL. With a simple structure of single-delay-stage Δ modulator followed by a charge pump based Σ modulator, a wide range of TDC input is converted to ΔΣ modulated single bit stream without loss of signal information. The ΔΣ architecture of TDC effectively improves the conversion performance of linearity and resolution while handling a large input range due to the operation of the dual-modulus divider. In addition, with a downscaling of the amount of the single delay in Δ modulator, the signal and noise transfer characteristics of TDC can be profiled to suppress the out-band noises at the input to the loop filter, resulting in easy filtering without any extra noise cancelling scheme. The DPLL is fabricated with a 0.13 μm CMOS technology. With a loop bandwidth of 1 MHz, DPLL shows an in-band phase noise of-107 dBc/Hz at 500 kHz offset and an out-of-band phase noise of-118.5 dBc/Hz at 3 MHz offset. The TDC consumes 1 mA. © 2012 IEEE.

Lee S.,Pohang University of Science and Technology | Jeong J.,Pohang University of Science and Technology | Kwak Y.,Pohang University of Science and Technology | Park S.K.,Pohang University of Science and Technology
Molecular Brain | Year: 2010

Extensive studies have led to a variety of hypotheses for the molecular basis of depression and related mood disorders, but a definite pathogenic mechanism has yet to be defined. The monoamine hypothesis, in conjunction with the efficacy of antidepressants targeting monoamine systems, has long been the central topic of depression research. While it is widely embraced that the initiation of antidepressant efficacy may involve acute changes in monoamine systems, apparently, the focus of current research is moving toward molecular mechanisms that underlie long-lasting downstream changes in the brain after chronic antidepressant treatment, thereby reaching for a detailed view of the pathophysiology of depression and related mood disorders. In this minireview, we briefly summarize major themes in current approaches to understanding mood disorders focusing on molecular views of depression and antidepressant action. © 2010 Lee et al; licensee BioMed Central Ltd.

Park S.H.,Pohang University of Science and Technology | Hong S.-G.,Korea Research Institute of Standards and Science | Lee C.S.,Pohang University of Science and Technology
Scripta Materialia | Year: 2010

The {1 0 -1 2} extension twinning characteristics of magnesium alloys were found to be significantly influenced by the activation mode: tension parallel to the c-axis of the hexagonal close-packed unit cell or compression perpendicular to the c-axis. The Schmid factor criterion was successfully used to explain the activation of {1 0 -1 2} extension twin variants and its related twin morphology and texture evolution for each activation mode. © 2009 Acta Materialia Inc.

Kang Y.-B.,Pohang University of Science and Technology
Calphad: Computer Coupling of Phase Diagrams and Thermochemistry | Year: 2015

Abstract A new approach for calculation of surface tension of solutions is presented. Based on the proposal of Butler (Proc. R. Soc. Lond. A: Math. Phys. Eng. Sci. 135 (1932) 348 [3]), a surface phase was assumed to be in equilibrium with a corresponding bulk phase. Following the proposal of Pajarre et al. (Calphad 30 (2006) 196 [7]), a surface area element (X) was introduced, only in the surface phase. The surface tension can be obtained by Constrained Gibbs Energy Minimization (CGEM) under a constraint of constant surface. The equilibrium chemical potential of the surface element, X, was interpreted to be proportional to the surface tension. According to the present author (Calphad 50 (2015) 23 [9]), the surface tension calculation by the CGEM can be interpreted as if the bulk phase and the surface phase are in equilibrium with each other, and the chemical potential of the area element is obtained as a result of the equilibrium. It was shown that a geometrical relationship among the molar Gibbs energies of the bulk phase and the surface phase, the surface tension, and the surface concentration is found. In the present study, further improvement was made in such a way that the chemical potential of the area element can be simply obtained by combining the CGEM with Zero Phase Fraction (ZPF) line in Type II phase diagram, where the chemical potential of the area element is used as an axis variable. Such phase diagram can be easily calculated using conventional CALPHAD softwares. The proposed approach was validated for a number of binary and ternary systems. © 2015 Elsevier Ltd.

Kang Y.-B.,Pohang University of Science and Technology
Calphad: Computer Coupling of Phase Diagrams and Thermochemistry | Year: 2015

In the context of a boundary phase model, surface tension (σ) of a solution can be regarded as a system property of an equilibrium between a bulk phase and a surface phase. In the present article, a geometric relationship is shown among molar Gibbs energy of the bulk phase (g), that of the surface phase (gs), and corresponding surface tension of the system. The geometric relationship is based on a phase equilibrium between the bulk phase and the surface phase, under a constraint: constant surface area (A). The relationship is consistent with the proposal of Butler, Proc. R. Soc. Lond. A: Math. Phys. Eng. Sci., 135 (1932) 348 [1], and is mathematically equivalent to the Constrained Gibbs Energy Minimization (CGEM) for the surface tension calculation by Pajarre et al., Calphad 30 (2006) 196 [7]. The geometric relationship can be simply utilized by available CALPHAD type code, in order to calculate surface tension of a solution composed of any number of components. Role of various properties (surface tension (σi°) and molar surface area (Ai°) of pure components, excess Gibbs energy of the bulk phase and that of the surface phase) in the surface tension and surface concentration is examined using the CGEM. © 2015 Elsevier B.V. All rights reserved.

Park J.,Pohang University of Science and Technology | Seo J.,Pohang University of Science and Technology | Park S.,Pohang University of Science and Technology
Conference Record of the Annual ACM Symposium on Principles of Programming Languages | Year: 2013

While separation logic is acknowledged as an enabling technology for large-scale program verification, most of the existing verification tools use only a fragment of separation logic that excludes separating implication. As the first step towards a verification tool using full separation logic, we develop a nested sequent calculus for Boolean BI (Bunched Implications), the underlying theory of separation logic, as well as a theorem prover based on it. A salient feature of our nested sequent calculus is that its sequent may have not only smaller child sequents but also multiple parent sequents, thus producing a graph structure of sequents instead of a tree structure. Our theorem prover is based on backward search in a refinement of the nested sequent calculus in which weakening and contraction are built into all the inference rules. We explain the details of designing our theorem prover and provide empirical evidence of its practicality. © 2013 ACM.

Lee I.,Pohang University of Science and Technology | Choi S.,Pohang University of Science and Technology
2013 World Haptics Conference, WHC 2013 | Year: 2013

We introduce a learning system for the sight reading skill of simple drum sequences. Sight reading is a cognitive-motor skill that requires reading of music symbols and actions of multiple limbs for playing the music. The system provides knowledge of results (KR) pertaining to the learner's performance by color-coding music symbols, and guides the learner by indicating the corresponding action for a given music symbol using additional auditory or vibrotactile cues. To evaluate the effects of KR and guidance cues, three learning methods were experimentally compared: KR only, KR with auditory cues, and KR with vibrotactile cues. The task was to play random 16-note-long drum sequence displayed on a screen. Thirty university students learned the task using one of the learning methods in a between-subjects design. The experimental results did not show statistically significant differences between the methods in terms of task accuracy and completion time. This suggests that KR can be dominant for learning the task and the role of guidance cues can be subsidiary. © 2013 IEEE.

Kim J.,Pohang University of Science and Technology | Kim S.-K.,Pohang University of Science and Technology | Yu H.,Pohang University of Science and Technology
Proceedings - International Conference on Data Engineering | Year: 2013

As social network services connect people across the world, influence maximization, i.e., finding the most influential nodes (or individuals) in the network, is being actively researched with applications to viral marketing. One crucial challenge in scalable influence maximization processing is evaluating influence, which is #P-hard and thus hard to solve in polynomial time. We propose a scalable influence approximation algorithm, Independent Path Algorithm (IPA) for Independent Cascade (IC) diffusion model. IPA efficiently approximates influence by considering an independent influence path as an influence evaluation unit. IPA are also easily parallelized by simply adding a few lines of OpenMP meta-programming expressions. Also, overhead of maintaining influence paths in memory is relieved by safely throwing away insignificant influence paths. Extensive experiments conducted on large-scale real social networks show that IPA is an order of magnitude faster and uses less memory than the state of the art algorithms. Our experimental results also show that parallel versions of IPA speeds up further as the number of CPU cores increases, and more speed-up is achieved for larger datasets. The algorithms have been implemented in our demo application for influence maximization (available at http://dm.postech.ac.kr/ipa demo), which efficiently finds the most influential nodes in a social network. © 2013 IEEE.

Rho Y.,Pohang University of Science and Technology | Ahn B.,Pohang University of Science and Technology | Yoon J.,Dong - A University | Ree M.,Pohang University of Science and Technology
Journal of Applied Crystallography | Year: 2013

A complete grazing-incidence X-ray scattering (GIXS) formula has been derived for nanopores buried in a polymer dielectric thin film supported by a substrate. Using the full power of the scattering formula, GIXS data from nanoporous polymethylsilsesquioxane dielectric thin films, a model nanoporous system, have successfully been analysed. The nanopores were found to be spherical and to have a certain degree of size distribution but were randomly dispersed in the film. In the film, GIXS was confirmed to arise predominantly via the first scattering process in which the incident X-ray beam scatters without reflection; the other scattering processes and their contributions were significantly dependent on the grazing angle. This study also confirmed that GIXS scattering can be analysed using only independent scattering terms, but this simple approach can only provide structural parameters. The cross terms were found to make a relatively small contribution to the intensity of the overall scattering but were required for the complete characterization of the measured two-dimensional scattering data, in particular the extracted out-of-plane scattering data, and their inclusion in the analysis enabled film properties such as film thickness, critical angle (i.e. electron density), refractive index and the absorption term to be determined. Copyright © International Union of Crystallography 2013.

Zhang G.,Pohang University of Science and Technology | Kim G.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology
Energy and Environmental Science | Year: 2014

Visible light harvesting or utilization through semiconductor photocatalysis is a key technology for solar chemical conversion processes. Although titania nanoparticles are popular as a base material of photocatalysis, the lack of visible light activity needs to be overcome. This mini-review is focused on an uncommon approach to visible light activation of titania: the ligand-to-metal charge transfer (LMCT) that takes place between TiO2 nanoparticles and surface adsorbates under visible light irradiation. We discuss a basic concept of photoinduced LMCT and the recent advances in LMCT-mediated visible light photocatalysis which has been applied in environmental remediation and solar energy conversion. Although the LMCT processes have been less investigated and limited in photocatalytic applications compared with other popular visible light activation methods such as impurity doping and dye sensitization, they provide lots of possibilities and flexibility in that a wide variety of organic or inorganic compounds can form surface complexes with TiO2 and introduce a new absorption band in the visible light region. The LMCT complexes may serve as a visible light sensitizer that initiates the photocatalytic conversion of various substrates or the self-degradation of the ligand complexes (usually pollutants) themselves. We summarized and discussed various LMCT photocatalytic systems and their characteristics. The LMCT-mediated activation of titania and other wide bandgap semiconductors has great potential to be developed as a more general method of solar energy utilization in photocatalytic systems. More systematic design and utilization of LMCT complexes on semiconductors are warranted to advance the solar-driven chemical conversion processes. © 2014 The Royal Society of Chemistry.

Lee J.,Pohang University of Science and Technology | Kim D.H.,Pohang University of Science and Technology | Hwang I.,Pohang University of Science and Technology
Frontiers in Plant Science | Year: 2014

Chloroplasts and mitochondria are endosymbiotic organelles thought to be derived from endosymbiotic bacteria. In present-day eukaryotic cells, these two organelles play pivotal roles in photosynthesis and ATP production. In addition to these major activities, numerous reactions, and cellular processes that are crucial for normal cellular functions occur in chloroplasts and mitochondria. To function properly, these organelles constantly communicate with the surrounding cellular compartments. This communication includes the import of proteins, the exchange of metabolites and ions, and interactions with other organelles, all of which heavily depend on membrane proteins localized to the outer envelope membranes. Therefore, correct and efficient targeting of these membrane proteins, which are encoded by the nuclear genome and translated in the cytosol, is critically important for organellar function. In this review, we summarize the current knowledge of the mechanisms of protein targeting to the outer membranes of mitochondria and chloroplasts in two different directions, as well as targeting signals and cytosolic factors. © 2014 Lee, Kim and Hwang.

Lee W.H.,Konkuk University | Choi H.H.,Pohang University of Science and Technology | Kim D.H.,Soongsil University | Cho K.,Pohang University of Science and Technology

Recent studies of the bias-stress-driven electrical instability of organic field-effect transistors (OFETs) are reviewed. OFETs are operated under continuous gate and source/drain biases and these bias stresses degrade device performance. The principles underlying this bias instability are discussed, particularly the mechanisms of charge trapping. There are three main charge-trapping sites: the semiconductor, the dielectric, and the semiconductor-dielectric interface. The charge-trapping phenomena in these three regions are analyzed with special attention to the microstructural dependence of bias instability. Finally, possibilities for future research in this field are presented. This critical review aims to enhance our insight into bias-stress-induced charge trapping in OFETs with the aim of minimizing operational instability. Recent studies of the bias-stress-driven electrical instability of organic field-effect transistors (OFETs) are reviewed. The principles underlying this bias instability are discussed, particularly the mechanisms of charge trapping. The charge-trapping phenomena in the semiconductor, the dielectric, and the semiconductor-dielectric interface are analyzed with special attention to the microstructural dependence of bias instability. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Wenk P.,Jacobs University Bremen | Kettemann S.,Jacobs University Bremen | Kettemann S.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The quantum correction to the conductivity in disordered quantum wires with linear Rashba spin-orbit coupling is obtained. For quantum wires with spin-conserving boundary conditions, we find a crossover from weak antilocalization to weak localization as the wire width W is reduced using exact diagonalization of the Cooperon equation. This crossover is due to the dimensional dependence of the spin relaxation rate of conduction electrons, which becomes diminished, when the wire width W is smaller than the bulk spin precession length LSO. We thus confirm previous results for small wire width, W/ Lso≲1 [S. Kettemann, Phys. Rev. Lett.98, 176808 (2007)] where only the transverse 0 modes of the Cooperon equation had been taken into account. We find that spin helix solutions become stable for arbitrary ratios of linear Rashba and Dresselhaus coupling in narrow wires. For wider wires, the spin relaxation rate is found to be not monotonous as function of wire width W: it becomes first enhanced for W on the order of the bulk spin precession length LSO before it becomes diminished for smaller wire widths. In addition, we find that the spin relaxation is smallest at the edge of the wire for wide wires. The effect of the Zeeman coupling to the magnetic field perpendicular to the 2D electron system (2DES) is studied and found to result in a modification of the magnetoconductivity: it shifts the crossover from weak antilocalization to weak localization to larger wire widths Wc. When the transverse confinement potential of the quantum wire is smooth, the boundary conditions become rather adiabatic. Then, the spin relaxation rate is found to be enhanced as the wire width W is reduced. We find that only a spin-polarized state retains a finite spin relaxation rate in such narrow wires. Thus, we conclude that the injection of polarized spins into nonmagnetic quantum wires should be favorable in wires with smooth confinement potential. Finally, in wires with tubular shape, corresponding to transverse periodic boundary conditions, we find no reduction of the spin relaxation rate. © 2010 The American Physical Society.

Kim W.-G.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Microelectronic Engineering | Year: 2010

Effect of the top electrode (TE) metal on the resistive switching of (TE)/TiO2/Pt structure was investigated. It was confirmed that the potential barrier height between the metal and TiO2 is an important factor on the resistive switching characteristics. When high Schottky barrier was formed with the TiO2 film, using Pt or Au as a top electrode, both stable URS (unipolar) and BRS (bipolar resistive switching) characteristics were observed depending on the current compliance level. In the case of Ag, which forms a relatively low Schottky barrier, only BRS characteristics were observed, regardless of the current compliance level. In the case of Ni and Al, which have similar work function as Ag, unstable URS and BRS at very low current compliance levels were observed due to a chemical reaction at the interface. For the Ti electrode, resistive switching was not observed, because the work function of Ti is lower than that of TiO2 and TiO phase was formed at the interface (Ti/TiOx contact is ohmic). © 2009 Elsevier B.V. All rights reserved.

Hoang A.T.,Pohang University of Science and Technology | Hoang A.T.,Institute of Physics
Journal of Physics Condensed Matter | Year: 2010

We study electronic phase transitions in the half-filled ionic Hubbard model with an on-site Coulomb repulsion U and an ionic energy Δ by using the coherent potential approximation. For a fixed and finite Δ two transitions from the band insulator via a metallic state to a Mott insulator are found with increasing U. The values of the critical correlation-driven metal-insulator transitions Uc1(Δ) and Uc2(Δ) are estimated. Our results are in reasonable agreement with the ones obtained by single-site dynamical mean-field theory and determinant quantum Monte Carlo simulation. © 2010 IOP Publishing Ltd.

Choi S.-M.,Pohang University of Science and Technology | Jhi S.-H.,Pohang University of Science and Technology | Son Y.-W.,Korea Institute for Advanced Study
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We present first-principles calculations of electronic properties of graphene under uniaxial and isotropic strains, respectively. The semimetallic nature is shown to persist up to a very large uniaxial strain of 30% except a very narrow strain range where a tiny energy gap opens. As the uniaxial strain increases along a certain direction, the Fermi velocity parallel to it decreases quickly and vanishes eventually, whereas the Fermi velocity perpendicular to it increases by as much as 25%. Thus, the low energy properties with small uniaxial strains can be described by the generalized Weyl's equation while massless and massive electrons coexist with large ones. The work function is also predicted to increase substantially as both the uniaxial and isotropic strain increases. Hence, the homogeneous strain in graphene can be regarded as the effective electronic scalar potential. © 2010 The American Physical Society.

Jung S.C.,Pohang University of Science and Technology | Kang M.H.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

Molecular and dissociative adsorption of a single water molecule on the Fe(100) surface has been studied by using density-functional theory calculations. We found that there exists a locally stable molecular adsorption state with an adsorption energy of 0.39 eV, where the H2 O molecule adsorbs on top of a surface Fe atom in a flat-lying molecular configuration. This molecular configuration is found to well reproduce the water-induced vibrational frequencies measured in a low-temperature electron-energy-loss spectroscopy (EELS) study. The H2 O molecular state is subject to a dissociation into H+OH species with an activation barrier of 0.35 eV. A further dissociation of the OH group into H+O species requires a higher activation energy of 0.79 eV. The prediction of the H2 O molecular precursor and the energy diagram for its dissociation is in good accordance with the adsorption picture which was suggested in a previous EELS study but has been incompatible with a previous density-functional study predicting a barrierless H+OH dissociation of water molecule. © 2010 The American Physical Society.

Lee J.Y.,Pohang University of Science and Technology | Lee S.J.,Pohang University of Science and Technology
Microfluidics and Nanofluidics | Year: 2010

Murray's law which is related to the bifurcations of vascular blood vessels states that the cube of a parent vessel's diameter equals the sum of the cubes of the daughter vessels' diameters D03= D1 3 + D23, alpha = D0 3 {D13+ D23 \right) = 1, where D 0, D 1, and D 2 are the diameters of the parent and two daughter vessels, respectively and α is the ratio). The structural characteristics of the vessels are crucial in the development of the cardiovascular system as well as for the proper functioning of an organism. In order to understand the vascular circulation system, it is essential to understand the design rules or scaling laws of the system under a homeostatic condition. In this study, Murray's law in the extraembryonic arterial bifurcations and its relationship with the bifurcation angle (θ) using 3-day-old chicken embryos in vivo has been investigated. Bifurcation is an important geometric factor in biological systems, having a significant influence on the circulation in the vascular system. Parameters such as diameter and bifurcation angle of all the 140 vessels tested were measured using image analysis softwares. The experimental results for α (= 1.053 ± 0.188) showed a good agreement with the ratio of 1 for Murray's law. Furthermore, the diameter relation α approached the theoretical value of 1 as the diameter of parent vessel D 0 decreased below 100 μm. The bifurcation angle θ decreased as D 0 increased and vice versa. For the arterial bifurcations of chicken embryos tested in this study, the bifurcation pattern appears to be symmetric (D 1 = D 2). The bifurcation angle exhibited a nearly constant value of 77°, close to the theoretical value of 75° for a symmetric bifurcation. © 2009 Springer-Verlag.

Lee C.,Pohang University of Science and Technology | Lee C.,North Carolina State University | Hong J.,Pohang University of Science and Technology | Whangbo M.-H.,North Carolina State University | Shim J.H.,Pohang University of Science and Technology
Chemistry of Materials | Year: 2013

We explored how to improve the thermoelectric properties of the layered transition-metal dichalcogenides 2H-MQ2 (M = Mo, W; Q = S, Se, Te) by comparing the thermoelectric properties of hypothetical mixed-layer systems 2H-MQ2/2H-MQ'2, in which two different layers 2H-MQ 2 and 2H-MQ'2 (Q, Q′ = S, Se, Te) alternate, with those of their pure components on the basis of density functional calculations. Our study predicts that the mixed-layer compounds MS2/MTe2 (M = Mo, W) strongly enhance the thermoelectric properties as a consequence of reducing the band gap and the interlayer van der Waals interactions. The layer-mixing is predicted to be a promising way of improving the thermoelectric properties of 2H-MQ2. © 2013 American Chemical Society.

Yoo S.,Pohang University of Science and Technology | Kang B.,Pohang University of Science and Technology
Nanotechnology | Year: 2013

Nanosized LiFePO4 particles easily show a fast electrochemical response that can be achieved via a non-equilibrium pathway. To understand this intriguing phase transition behavior in nanosized LiFePO4 particles, the metastable solid-solution phase was prepared by thermal treatment with a chemically delithiated nanosized Li0.5FePO4 sample. Thermal treatment makes all the nanosized particles transform easily to the metastable solid-solution phase because of the large thermal energy while an electrochemical reaction does not. The phase separation behavior of the metastable solid-solution sample (Li0.5FePO4) was investigated under various kinetic conditions to understand critical factors affecting the phase separation behavior of nanosized LiFePO4 particles. The main findings in this study are as follows. The first finding is that the depressed phase separation behavior of the metastable phase may originate from the nanoparticle effect, in which the formation of a second phase inside a nanosized particle is not energetically favored because of the large interfacial energy. Therefore, phase separation in nanosized particles occurs between particles rather than inside a particle. If there was no over-potential, such as in the relaxed pellet experiment or in the relaxed electrode experiment in the electrolyte, the metastable phase was quite stable showing no phase separation behavior even though efficient pathways for lithium ions and electrons were well developed. The second finding is that the phase separation behavior of the metastable phase actually depends on the over-potential. Under open circuit voltage (OCV) conditions, the metastable phase started to exhibit a slight structural change during a long relaxation time, about ten days. The slow change of the metastable phase may be due to the low driving force, less than 10 mV, which comes from the energetic difference between the two-phase state and the metastable phase. This indicates that the phase separation behavior may require a large over-potential. When a large over-potential was applied using an external current, phase separation of the metastable phase was achieved, indicating that the phase separation behavior may be related to activation processes. Furthermore, the requirement for a large over-potential indirectly shows that the spinodal decomposition is depressed in nanosized particles. Considering that phase separation in nanosized particles occurs between particles, the surface charge transfer reaction can be a limited reaction for achieving phase separation because it is an activated process and governed by the over-potential. Considering the understanding obtained from the phase separation behavior of the metastable phase, the phase transition behavior of nanosized LiFePO4 particles during charging/discharging can proceed via the metastable phase because there is no spinodal decomposition behavior in nanosized particles and the metastable phase is quite stable. © 2013 IOP Publishing Ltd.

Kim H.K.,International Maize and Wheat Improvement Center | Park J.,Pohang University of Science and Technology | Hwang I.,Pohang University of Science and Technology
Journal of Experimental Botany | Year: 2014

Our understanding of physical and physiological mechanisms depends on the development of advanced technologies and tools to prove or re-evaluate established theories, and test new hypotheses. Water flow in land plants is a fascinating phenomenon, a vital component of the water cycle, and essential for life on Earth. The cohesion-tension theory (CTT), formulated more than a century ago and based on the physical properties of water, laid the foundation for our understanding of water transport in vascular plants. Numerous experimental tools have since been developed to evaluate various aspects of the CTT, such as the existence of negative hydrostatic pressure. This review focuses on the evolution of the experimental methods used to study water transport in plants, and summarizes the different ways to investigate the diversity of the xylem network structure and sap flow dynamics in various species. As water transport is documented at different scales, from the level of single conduits to entire plants, it is critical that new results be subjected to systematic cross-validation and that findings based on different organs be integrated at the whole-plant level. We also discuss the functional trade-offs between optimizing hydraulic efficiency and maintaining the safety of the entire transport system. Furthermore, we evaluate future directions in sap flow research and highlight the importance of integrating the combined effects of various levels of hydraulic regulation. © 2013 The Author.

Lee S.-K.,Pohang University of Science and Technology | Lee S.-K.,Samsung | Lee S.-H.,Pohang University of Science and Technology | Sylvester D.,University of Michigan | And 2 more authors.
Digest of Technical Papers - IEEE International Solid-State Circuits Conference | Year: 2013

Data communication between local system blocks through on-chip global interconnects presents significant design challenges in scaled VLSI systems. The goal of this research is to reduce the energy consumed per bit transmitted, while achieving Gb/s data rates over interconnect lengths up to 10mm. Voltage-mode signaling with capacitive boosting [1-2] has been proposed for low-power on-chip interconnects. To increase the data rate over RC-limited interconnect, aggressive equalization schemes should be used in receivers [1-3] and transmitters [1-2] at the cost of significant power consumption. As an alternative to voltage-mode signaling, current-mode signaling has been considered. It was originally used for fast bitline sensing in memory [4-5] to take inherent advantage of a reduced RC time constant. However, prior work on current-mode transceivers for on-chip interconnect shows worse energy efficiency than their voltage-mode counterparts due to large static power dissipation by current-sensing circuit [6-7]. This paper presents a 95fJ/b current-mode transceiver for on-chip global interconnect. The transceiver is implemented in 65nm CMOS and achieves a data rate of up to 4Gb/s over a 10mm link with a BER of less than 10-12. © 2013 IEEE.

Seo K.W.,Pohang University of Science and Technology | Choi Y.S.,Pohang University of Science and Technology | Seo E.S.,Pohang University of Science and Technology | Lee S.J.,Pohang University of Science and Technology
Optics Letters | Year: 2012

In this study, we proposed a simple and fast numerical approach to compensate for aberrations induced by objective phase curvature. This method is based on the extraction of virtual background phase from reconstructed phase values using a line profile, followed by subtraction of the virtual background phase from the reconstructed phase image. The performance and feasibility of the method were demonstrated by applying it to the phase imaging of polystyrene microspheres and red blood cells. © 2012 Optical Society of America.

Wagoner R.H.,Ohio State University | Lim H.,Sandia National Laboratories | Lee M.-G.,Pohang University of Science and Technology
International Journal of Plasticity | Year: 2013

For purposes of this review, springback is the elastically driven change of shape of a metal sheet during unloading and following forming. Scientific advances related to this topic have accelerated dramatically over roughly the last decade, since the publication of two reviews in the 2004-2006 timeframe (Wagoner, 2004; Wagoner et al., 2006). The current review focuses on the period following those publications, and on work in the first author's laboratory. Much of this recent work can be categorized into five main topics. (1) Plastic constitutive equations (2) Variable Young's modulus (3) Through-thickness integration of stress (4) Magnesium (5) Advanced high strength steels (AHSS) The first two subjects are related to accurate material representation, the third to numerical procedures, and the last two to particular classes of sheet materials. The principal contributions in these areas were summarized and put into context. © 2012 Elsevier Ltd. All rights reserved.

Lee J.,Pohang University of Science and Technology | Lee J.-Y.,Pohang University of Science and Technology | Barlat F.,Pohang University of Science and Technology | Wagoner R.H.,Ohio State University | And 2 more authors.
International Journal of Plasticity | Year: 2013

Materials modeling and numerical formulations were conducted to describe the complex material behavior upon strain path change in order to enhance the prediction accuracy of springback in advanced high strength steels (AHSS). An approach without kinematic hardening rule, or the homogeneous anisotropic hardening (HAH) model, was incorporated to the newly conceived quasi-plastic-elastic strain (QPE) formulations. The HAH model is able to capture complex plastic flow behavior of sheet metals such as the Bauschinger effect, transient behavior, work-hardening stagnation and permanent softening. The QPE approach was developed to reproduce the nonlinear elastic behavior during unloading and reloading. The two models were independently validated for predicting springback, with better performance than conventional constitutive models. In this study, the two models are combined and extended to enhance the prediction capability of springback in AHSS. For this purpose, fully implicit numerical algorithms were re-formulated to link the two modeling approaches using general anisotropic yield function and hardening for shell element. The original model was only valid for continuum isotropic element with analytical stress integration procedure. Simulations of 2D draw bending test were performed to validate the developed approach for two AHSS, DP780 and TRIP780, sheets. The springback prediction was significantly improved if most of the complex material behavior relating to elasticity and plasticity were taken into account in the finite element simulations. © 2013 Elsevier Ltd. All rights reserved.

Seol M.,Pohang University of Science and Technology | Jang J.-W.,Pohang University of Science and Technology | Cho S.,Pohang University of Science and Technology | Lee J.S.,Pohang University of Science and Technology | Yong K.,Pohang University of Science and Technology
Chemistry of Materials | Year: 2013

Although cadmium chalcogenide quantum dot-sensitized photoanode can utilize the whole visible region of the solar spectrum, its poor photochemical stability owing to hole-induced anodic corrosion remains a major problem for the application in photoelectrochemical hydrogen generation systems. Here, modification with IrOx·nH2O, a well-known water-oxidation catalyst substantially improves the photochemical stability of the quantum dot-sensitized photoanode. Moreover, it induces an increased photocurrent and a cathodic shift of the onset potential. This is the first example that an oxygen-evolution catalyst is employed on a quantum dot-sensitized electrode system, and it shows 13.9 mA cm-2 (at 0.6 V) and -0.277 V vs the reversible hydrogen electrode (RHE), which are the highest photocurrent density and the lowest onset potential attained with a ZnO-based electrode, respectively. An average hydrogen evolution rate of 240 μmol h-1 cm-2 at 0.6 V vs RHE has been achieved on a IrO x·nH2O modified electrode, with almost 100% of faradaic efficiency. © 2012 American Chemical Society.

Cho S.,Pohang University of Science and Technology | Kwag J.,Pohang University of Science and Technology | Jeong S.,Pohang University of Science and Technology | Baek Y.,Pohang University of Science and Technology | Kim S.,Pohang University of Science and Technology
Chemistry of Materials | Year: 2013

We report a designed strategy for a synthesis of highly luminescent and photostable composites by incorporating quantum dots (QDs) into layered double hydroxide (LDH) matrices without deterioration of a photoluminescence (PL) efficiency of the fluorophores during the entire processes of composite formations. The QDs synthesized in an organic solvent are encapsulated by polymers, poly(maleic acid-alt-octadecene) to transfer them into water without altering the initial surface ligands. The polymer-encapsulated QDs with negative zeta potentials (-29.5 ± 2.2 mV) were electrostatically assembled with positively charged (24.9 ± 0.6 mV) LDH nanosheets to form QD-polymer-LDH composites (PL quantum yield: 74.1%). QD-polymer-LDH composite films are fabricated by a drop-casting of the solution on substrates. The PL properties of the films preserve those of the organic QD solutions. We also demonstrate that the formation of the QD-polymer-LDH composites affords enhanced photostabilities through multiple protections of QD surface by polymers and LDH nanosheets from the environment. © 2013 American Chemical Society.

Zhang G.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology | Kim S.H.,Pohang University of Science and Technology | Hong S.B.,Pohang University of Science and Technology
Journal of Hazardous Materials | Year: 2011

The selective photocatalytic degradation of charged pollutants in water was achieved on titania encapsulated into FAU-type zeolites. The electrostatic attraction of cationic substrates and repulsion of anionic substrates by the negatively charged zeolite framework facilitated the selective photocatalytic degradation of charged substrates. The hybrid zeolite-titania photocatalysts were prepared through the ion-exchange method. The titania clusters were mainly well distributed within the cavities of FAU-type zeolites whereas no TiO2 nanoparticles aggregates were observed on the external surface of zeolite crystals. The hybrid zeolite-titania photocatalysts were characterized by diffuse reflectance UV-visible spectroscopy, transmission electron microscopy, energy-dispersive X-ray analysis and X-ray photoelectron spectroscopy. The selective degradation of charged pollutants was investigated by employing three pairs of oppositely charged substrates. The comparison between the cationic and anionic substrates clearly showed that the degradation rates for the cationic substrates on the hybrid photocatalysts are markedly higher than those for the anionic substrates. Among the cationic substrates, the smaller cations such as tetramethylammoniums were preferentially degraded. This enabled the selective removal of cationic substrates among the mixture. Such a selective photocatalytic degradation of water pollutants may provide a useful strategy for the development of economical photocatalytic process by targeting only the most recalcitrant pollutant. © 2011 Elsevier B.V.

Kwon W.,Pohang University of Science and Technology | Do S.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology

Highly luminescent nearly monodisperse carbon quantum dots (CQDs) are synthesized by facile emulsion-templated carbonization of low cost and non toxic carbohydrates excluding the size selection procedure. The present method is further combined with in situ nitric acid treatment to offer high quantum yields up to 53% which, to our best knowledge, is unprecedented in the past. © 2012 The Royal Society of Chemistry.

Oh Y.J.,Pohang University of Science and Technology | Hwang I.,Pohang University of Science and Technology
Cell Calcium | Year: 2015

Chloroplasts produce carbohydrates, hormones, vitamins, amino acids, pigments, nucleotides, ATP, and secondary metabolites. Channels and transporters are required for the movement of molecules across the two chloroplast envelope membranes. These transporters and channel proteins are grouped into two different types, including β-barrel proteins and transmembrane-domain (TMD) containing proteins. Most β-barrel proteins are localized at the outer chloroplast membrane, and TMD-containing proteins are localized at the inner chloroplast membrane. Many of these transporters and channels are encoded by nuclear genes; therefore, they have to be imported into chloroplasts after translation on cytosolic ribosomes. These proteins should have specific targeting signals for their final destination in the chloroplast membrane and for assembly into specific complexes. In this review, we summarize recent progress in the identification, functional characterization, and biogenesis of transporters and channels at the chloroplast envelope membranes, and discuss outstanding questions regarding transporter and channel protein biogenesis. © 2014 Elsevier Ltd.

Kim J.H.,Pohang University of Science and Technology | Lee J.,Pohang University of Science and Technology | Park J.,Pohang University of Science and Technology | Gho Y.S.,Pohang University of Science and Technology
Seminars in Cell and Developmental Biology | Year: 2015

Like mammalian cells, Gram-negative and Gram-positive bacteria release nano-sized membrane vesicles into the extracellular environment either in a constitutive manner or in a regulated manner. These bacterial extracellular vesicles are spherical bilayered proteolipids enriched with bioactive proteins, lipids, nucleic acids, and virulence factors. Recent progress in this field supports the critical pathophysiological functions of these vesicles in both bacteria-bacteria and bacteria-host interactions. This review provides an overview of the current understanding on Gram-negative and Gram-positive bacterial extracellular vesicles, especially regarding the biogenesis, components, and functions in poly-species communities. We hope that this review will stimulate additional research in this emerging field of bacterial extracellular vesicles and contribute to the development of extracellular vesicle-based diagnostic tools and effective vaccines against pathogenic Gram-negative and Gram-positive bacteria. © 2015 Elsevier Ltd.

Kim J.,Pohang University of Science and Technology | Song I.Y.,Pohang University of Science and Technology | Park T.,Pohang University of Science and Technology
Chemical Communications | Year: 2011

The diblock copolymer, BP26, assembled into polymeric vesicles with double layers that formed a hydrophobic crystalline interior and a hydrophilic amorphous exterior in methanol, a selective solvent for the PEGT block. The vesicles were demonstrated to encapsulate a hydrophobic guest polyfluorene (PF). © The Royal Society of Chemistry 2011.

Kyu An T.,Pohang University of Science and Technology | Eon Park C.,Pohang University of Science and Technology | Sung Chung D.,Chung - Ang University
Applied Physics Letters | Year: 2013

We demonstrate an approach to enhancing the photoresponsivity of a polymer photodetector (PPD). Both conventional bulk heterojunction (BHJ) and planar heterojunction (PHJ) PPDs were fabricated considering that the interface between a CdSe nanocrystal and a polymer can create photoconductive gain. A systematic study of the illumination wavelength and light power dependence of the photocurrent gain, combined with the charge carrier transport analysis, suggested that the PHJ-PPD could yield a higher hole mobility than could be achieved in a BHJ-PPD without compromising on the selective electron trapping effects. The optimized PHJ-PPD led to a photoconductive detectivity of 1.3 × 1010 cm Hz1/2/W. © 2013 AIP Publishing LLC.

Cho W.J.,Pohang University of Science and Technology | Cho Y.,Pohang University of Science and Technology | Min S.K.,Pohang University of Science and Technology | Kim W.Y.,KAIST | Kim K.S.,Pohang University of Science and Technology
Journal of the American Chemical Society | Year: 2011

Spintronic devices are very important for futuristic information technology. Suitable materials for such devices should have half-metallic properties so that only one spin passes through the device. In particular, organic half metals have the advantage that they may be used for flexible devices and have a long spin-coherence length. We predict that the one-dimensional infinite chromium porphyrin array, which we call Cr-PA ∞, shows half-metallic behavior when the spins on the chromium atoms are in a parallel alignment. Since the chromium atoms are separated by a large distance (>8 Å), the coupling between spins is small and thus their directions can be readily controlled by an external magnetic field. In the ferromagnetic state, the band gap for major spin electrons is 0.30 eV, while there is no band gap for the minor spin electrons, thus reflecting the half-metallic property. This unique property originates from the high spin state of Cr which results in the spin asymmetry of the conduction band in Cr-PA ∞. Electron transport of Cr-PA1,2,3 is calculated with the nonequilibrium Green function technique in the presence of Au electrodes. It turned out that the spin-filtering ability appears from the dimeric Cr-PA2. Thus, a new organometallic framework for designing a spin filter is proposed. Though many others have designed novel spintronic devices, none of them are realized due to the lack of a practical fabrication method at present. However, the porphyrin-based spintronic device provides a synthesizable framework. © 2011 American Chemical Society.

Ryu J.,Pohang University of Science and Technology | Lee K.-J.,Korea University | Lee H.-W.,Pohang University of Science and Technology
Applied Physics Letters | Year: 2013

We theoretically study the current-driven domain wall motion in the presence of both the spin Hall effect and an extrinsic pinning potential. The spin Hall effect mainly affects the damping ratio of the domain wall precession in the pinning potential. When the pinning potential is not too strong, this results in a significant reduction of a threshold current density for the depinning of a domain wall with certain polarity. We also propose one way to distinguish the spin Hall effect induced spin-transfer torque from the one induced by the Rashba spin-orbit coupling experimentally. © 2013 AIP Publishing LLC.

Song C.-I.,Pohang University of Science and Technology | Rhee Y.M.,Pohang University of Science and Technology
Journal of the American Chemical Society | Year: 2011

Dynamics of the firefly luciferase-oxyluciferin complex in its electronic ground and excited states are studied using various theoretical approaches. By mimicking the physiological conditions with realistic models of the chromophore oxyluciferin, the enzyme luciferase, and solvating water molecules and by performing real time simulations with a molecular dynamics technique on the model surfaces, we reveal that the local chromophore-surrounding interaction patterns differ rather severely in the two states. Because of the presence of protein, the solvation dynamics of water around the chromophore is also peculiar and shows widely different time scales on the two terminal oxygen atoms. In addition, simulations of the emission with the quantum-mechanics/molecular- mechanics approach show a close relationship between the emission color variation and the environmental dynamics, mostly through electrostatic effects from the chromophore-surrounding interaction. We also discuss the importance of considering the time scales of the luminescence and the dynamics of the interaction. © 2011 American Chemical Society.

Lim W.,Pohang University of Science and Technology | Kim J.,Pohang University of Science and Technology | Rhee Y.H.,Pohang University of Science and Technology
Journal of the American Chemical Society | Year: 2014

A ligand-directed metal-catalyzed asymmetric intermolecular hydroalkoxylation of alkoxyallene is reported. Combined with ring-closing-metathesis, this reaction offers a new atom-efficient synthetic method toward various cyclic acetals with elaborate anomeric control. Synthetic utility of the reaction was demonstrated by the atom-efficient and stereodivergent access to various mono- and disaccharides. © 2014 American Chemical Society.

Kim G.,Ulsan National Institute of Science and Technology | Kang S.-J.,Dongguk University | Dutta G.K.,Ulsan National Institute of Science and Technology | Han Y.-K.,Dongguk University | And 3 more authors.
Journal of the American Chemical Society | Year: 2014

By considering the qualitative benefits associated with solution rheology and mechanical properties of polymer semiconductors, it is expected that polymer-based electronic devices will soon enter our daily lives as indispensable elements in a myriad of flexible and ultra low-cost flat panel displays. Despite more than a decade of research focused on designing and synthesizing state-of-the-art polymer semiconductors for improving charge transport characteristics, the current mobility values are still not sufficient for many practical applications. The confident mobility in excess of ∼10 cm2/V·s is the most important requirement for enabling the realization of the aforementioned near-future products. We report on an easily attainable donor-acceptor (D-A) polymer semiconductor: poly(thienoisoindigo-alt- naphthalene) (PTIIG-Np). An unprecedented mobility of 14.4 cm 2/V·s, by using PTIIG-Np with a high-k gate dielectric poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)), is achieved from a simple coating processing, which is of a magnitude that is very difficult to obtain with conventional TFTs by means of molecular engineering. This work, therefore, represents a major step toward truly viable plastic electronics. © 2014 American Chemical Society.

Hwang J.,Pohang University of Science and Technology | Jo C.,Pohang University of Science and Technology | Hur K.,Korea Institute of Science and Technology | Lim J.,Pohang Light Source | And 2 more authors.
Journal of the American Chemical Society | Year: 2014

Hierarchically porous oxide materials have immense potential for applications in catalysis, separation, and energy devices, but the synthesis of these materials is hampered by the need to use multiple templates and the associated complicated steps and uncontrollable mixing behavior. Here we report a simple one-pot strategy for the synthesis of inorganic oxide materials with multiscale porosity. The inorganic precursor and block copolymer are coassembled into an ordered mesostructure (microphase separation), while the in situ-polymerized organic precursor forms organic-rich macrodomains (macrophase separation) around which the mesostructure grows. Calcination generates hierarchical meso/macroporous SiO2 and TiO2 with three-dimensionally interconnected pore networks. The continuous 3D macrostructures were clearly visualized by nanoscale X-ray computed tomography. The resulting TiO2 was used as the anode in a lithium ion battery and showed excellent rate capability compared with mesoporous TiO2. This work is of particular importance because it (i) expands the base of BCP self-assembly from mesostructures to complex porous structures, (ii) shows that the interplay of micro- and macrophase separation can be fully exploited for the design of hierarchically porous inorganic materials, and therefore (iii) provides strategies for researchers in materials science and polymer science. © 2014 American Chemical Society.

Ponomareva V.G.,RAS Institute of Solid State Chemistry and Mechanochemistry | Kovalenko K.A.,RAS Nikolaev Institute of Inorganic Chemistry | Chupakhin A.P.,Novosibirsk State University | Dybtsev D.N.,Pohang University of Science and Technology | And 2 more authors.
Journal of the American Chemical Society | Year: 2012

The extensive implementation of hydrogen-powered technology today is limited by a number of fundamental problems related to materials research. Fuel-cell hydrogen conversion technology requires proton-conducting materials with high conductivity at intermediate temperatures up to 120 °C. The development of such materials remains challenging because the proton transport of many promising candidates is based on extended microstructures of water molecules, which deteriorate at temperatures above the boiling point. Here we show the impregnation of the mesoporous metal-organic framework (MOF) MIL-101 by nonvolatile acids H 2SO 4 and H 3PO 4. Such a simple approach affords solid materials with potent proton-conducting properties at moderate temperatures, which is critically important for the proper function of on-board automobile fuel cells. The proton conductivities of the H 2SO 4@MIL-101 and H 3PO 4@MIL-101 at T = 150 °C and low humidity outperform those of any other MOF-based materials and could be compared with the best proton conductors, such as Nafion. © 2012 American Chemical Society.

Kim M.,Pohang University of Science and Technology
Nature Physics | Year: 2016

Ever since the discovery of graphene, valley symmetry and its control in the material have been a focus of continued studies in relation to valleytronics. Carrier-guiding quasi-one-dimensional (1D) graphene nanoribbons (GNRs) with quantized energy subbands preserving the intrinsic Dirac nature have provided an ideal system to that end. Here, by guiding carriers through dual-gate operation in high-mobility monolayer graphene, we report the realization of quantized conductance in steps of 4e2/h in zero magnetic field, which arises from the full symmetry conservation of quasi-1D ballistic GNRs with effective zigzag-edge conduction. A tight-binding model calculation confirms conductance quantization corresponding to zigzag-edge conduction even for arbitrary GNR orientation. Valley-symmetry conservation is further confirmed by intrinsic conductance interference with a preserved Berry phase of π in a graphene-based Aharonov–Bohm (AB) ring prepared by similar dual gating. This top-down approach for gate-defined carrier guiding in ballistic graphene is of particular relevance in the efforts towards efficient and promising valleytronic applications. © 2016 Nature Publishing Group

Kim D.,Pohang University of Science and Technology | Ryu H.G.,Pohang University of Science and Technology | Ahn K.H.,Pohang University of Science and Technology
Organic and Biomolecular Chemistry | Year: 2014

Fluorescent probes are essential tools for studying biological systems. The last decade has witnessed particular interest in the development of two-photon excitable probes, owing to their advantageous features in tissue imaging compared to the corresponding one-photon probes. This review summarizes various types of two-photon probes that have been applied to bioimaging, categorized by the principles in the probe design and the target analytes, which would provide a basis for the future development of novel two-photon probes for tackling important biological issues. This journal is © 2014 the Partner Organisations.

Seo J.,Pohang University of Science and Technology | Huh K.Y.,Pohang University of Science and Technology
Proceedings of the Combustion Institute | Year: 2013

3D DNS is performed for n-heptane sprays that go through ignition and combustion in different injection conditions. Conditional statistics are obtained for multiple Lagrangian groups of sequentially evaporating fuel in the group and the collective combustion regime. Ignition occurs close to but leaner than the most reactive mixture fraction for fuel of the longest residence time and low scalar dissipation rates. Combustion propagates with strong interaction among neighboring groups after ignition of the preceding flame group in the given conditions. Multiple flame groups are required for accurate description of combustion of a spray over long injection duration or multiply injected sprays. Reasonable agreement is shown between DNS and model predictions of conditional evaporation and scalar dissipation rates in the range of meaningful probabilities. Budgets are checked for all component terms in the balance equation of conditional sensible enthalpy with evaporation source terms. © 2012 The Combustion Institute.

Kwon W.,System on ip Chemical Process Research Center | Do S.,System on ip Chemical Process Research Center | Lee J.,System on ip Chemical Process Research Center | Hwang S.,Pohang University of Science and Technology | And 2 more authors.
Chemistry of Materials | Year: 2013

In this work, nitrogen-rich carbon nanodots (CNDs) are prepared by the emulsion-templated carbonization of polyacrylamide. The formation mechanism and chemical structure are investigated by infrared, nuclear magnetic resonance, and X-ray photoelectron spectroscopies. Transmission electron microscopy also reveals that the obtained CNDs have well-developed graphitic structure and narrow size distribution without any size selection procedure. We vary the molecular weight of the polymer to control the size of the CNDs and finally obtain the CNDs rendering bright visible light under UV illumination with a high quantum yield of 40%. Given that the CNDs are worth utilizing in phosphor applications, we fabricate large-scale (20 × 20 cm) freestanding luminescent films of the CNDs based on a poly(methyl methacrylate) matrix. The polymer matrix can not only provide mechanical support but also disperse the CNDs to prevent solid-state quenching. For practical application, we demonstrate white LEDs consisting of the films as color-converting phosphors and InGaN blue LEDs as illuminators. Such white LEDs exhibit no temporal degradation in the emission spectrum under practical operation conditions. This study would suggest a promising way to exploit the luminescence from solid-state CNDs and offer strong potential for future CND-based solid-state lighting systems. © 2013 American Chemical Society.

Zare R.N.,Stanford University | Kim S.,Pohang University of Science and Technology
Annual Review of Biomedical Engineering | Year: 2010

Microfluidics, the study and control of the fluidic behavior in microstructures, has emerged as an important enabling tool for single-cell chemical analysis. The complex procedures for chemical cytometry experiments can be integrated into a single microfabricated device. The capability of handling a volume of liquid as small as picoliters can be utilized to manipulate cells, perform controlled cell lysis and chemical reactions, and efficiently minimize sample dilution after lysis. The separation modalities such as chromatography and electrophoresis within microchannels are incorporated to analyze various types of intracellular components quantitatively. The microfluidic approach offers a rapid, accurate, and cost-effective tool for single-cell biology. We present an overview of the recent developments in microfluidic technology for chemical-content analysis of individual cells. © 2010 by Annual Reviews. All rights reserved.

Son S.,Pohang University of Science and Technology | Singha K.,Pohang University of Science and Technology | Kim W.J.,Pohang University of Science and Technology
Biomaterials | Year: 2010

The work demonstrated development of multifunctional gene carrier which has incorporated reducible moiety, tumor targeting ligands as well as PEG to achieve efficient release of pDNA, enhanced tumor-specificity and long circulation, respectively. In our successful one-pot synthesis of multifunctional polymer, low molecular weight branched polyethylenimine (BPEI) was thiolated with propylene sulfide, and mixed with α-Maleimide-ω-N-hydroxysuccinimide ester polyethylene glycol (MAL-PEG-NHS, MW: 5000), and cyclic NGR peptide. The structural elucidation of the cNGR conjugated reducible BPEI containing disulfide bond (BPEI-SS-PEG-cNGR), was done by NMR and GPC study. Complex formation as well as reducible property of the polymer was confirmed by gel retardation assay. In order to achieve efficient tumor targeting, we have used cNGR peptide which is known to bind to CD13 overexpressed in neovasculature endothelial cells. Tumor target-specificity of polymer was established by carrying out competitive inhibition assay with free cNGR peptide. Cellular uptake of polymers was evaluated by confocal laser scanning microscope (CLSM). Finally, addition of free cNGR and buthionine sulfoximine (BSO) reduced transfection efficiency synergistically, which implied that multifunctional polymer-mediated gene transfection took place tumor-specifically and via GSH-dependent pathway. © 2010 Elsevier Ltd.

Kim K.-S.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Recently, enhancement of the longitudinal magnetoelectrical conductivity (LMEC) has been observed in Bi1-xSbxaround x∼3% under E-B (E, external electric field and B, external magnetic field) [Phys. Rev. Lett. 111, 246603 (2013)PRLTAO0031-900710.1103/PhysRevLett.111.246603], where an enhancement factor proportional to B2 is suggested to result from the E·B term. In the present study, we show that this B2 enhancement is not limited on the LMEC, where both the Seebeck and thermal conductivities in the longitudinal setup (Eâ¥ B) are predicted to show essentially the same enhancement proportional to B2. In particular, the B2 enhancement factor of the LMEC turns out to differ from that of the longitudinal thermal conductivity, responsible for the breakdown of the Wiedemann-Franz (WF) law, which means that anomalous currents flowing through the dissipationless channel differ from each other. Since the breakdown of the WF law appears in spite of the existence of electron quasiparticles, regarded to be a purely topological character (chiral anomaly), the Weyl metallic state cannot be identified with the Landau Fermi-liquid fixed point. We propose the violation of the WF law as another hallmark of the Weyl metallic phase, which originates from axion electrodynamics. © 2014 American Physical Society.

Kim K.-S.,Pohang University of Science and Technology | Kim H.-J.,Daegu University | Sasaki M.,Yamagata University
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Weyl metal is regarded as a platform toward interacting topological states of matter, where its topological structure gives rise to anomalous transport phenomena, referred to as chiral magnetic effect and â€œ negativeâ€ magnetoresistivity, the origin of which is chiral anomaly. Recently, the negative magnetoresistivity has been observed with the signature of weak antilocalization at x=3-4% in Bi1a xSbx, where a magnetic field is applied in parallel with an electric field (Eâ¥B). Based on the Boltzmann equation approach, we find the negative magnetoresistivity in the presence of weak antilocalization. An essential ingredient is to introduce the topological structure of chiral anomaly into the Boltzmann equation approach, resorting to semiclassical equations of motion with Berry curvature. © 2014 American Physical Society.

Kim B.,Pohang University of Science and Technology | Min B.I.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We have investigated electronic and magnetic properties of ultrathin SrRuO3 (SRO) films grown on (111) SrTiO3 substrates using the ab initio electronic structure calculations. Ru-terminated SRO (111) films suffer from strong surface atomic relaxations, while SrO3-terminated ones preserve the surface structure of ideal perovskites. Both Ru- and SrO3-terminated SRO (111) films show unexpected interlayer antiferromagnetic (AFM) structures at the surface, but with different characters and mechanisms. The AFM structure for the former results from the large surface atomic relaxation, whereas that for the latter results from the truncated film effect. Interestingly, for the SrO3-termination case, the half-metallic nature emerges despite the interlayer AFM structure. Upon reducing the thickness, the collapsing behavior of magnetic anisotropy from out-of-plane to in-plane easy axis is found to occur for the Ru-termination case, which, however, does not pertain to the SrO3-termination case. © 2014 American Physical Society.

Park J.-H.,Pohang University of Science and Technology | Lee H.-J.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Out-of-plane spin-injection and detection through naturally stacked graphene layers were investigated in ferromagnet/graphene/ferromagnet (FGF) junctions. We obtained a maximum magnetoresistance (MR) of 4.6% at T=4.2 K in the junction of a four-layer graphene insertion, having a very small area-junction-resistance product of 0.2 Ωμm2. According to resistance-temperature and current-voltage characteristics, the graphene layer in the FGF junction acted as a metal-like insertion rather than as an insulating barrier. A lower value for the interfacial spin asymmetry coefficient (γ=0.25±0.05) obtained from the fitting of variations with interfacial resistance implies that the spin-injection efficiency along the out-of-plane direction was reduced by spin-flip scattering at graphene/ferromagnet interfaces. Our results showed that highly transparent graphene/ferromagnet interfaces with crystalline ferromagnet (FM) electrodes are required to achieve higher spin-injection efficiency through the graphene layer in a FGF junction along the out-of-plane direction. © 2014 American Physical Society.

Kwon S.G.,Pohang University of Science and Technology | Kang M.H.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Density functional calculations predict that the In/Si(111)-(2×2) surface, a key intermediate phase leading to the complex In phase development on Si(111), consists of an intriguing one-atom-thick In overlayer. In atoms of 1 monolayer (ML) (referred to as one In atom per surface Si) there forms a quasihoneycomb network of two In units, a monomer and a triangular trimer, which well accounts for the measured microscopy images and In 4d core-level shifts. This In single layer is, however, semiconducting with a band gap of 0.47 eV and thus could not represent the long-sought two-dimensional (2D) limit of metallic In overlayers. But its In coverage of 1 ML sets a definite lower bound on the coverage for such a 2D-metallic one-atom-thick In overlayer. © 2014 American Physical Society.

Kim M.,Pohang University of Science and Technology | Min B.I.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We have investigated the temperature (T)-dependent evolution of orbital states in a typical spin-orbital-lattice-coupled 2p electron Mott system KO2, based on the electronic structures obtained by the dynamical mean-field theory as well as the density functional theory. We have shown that KO2 exhibits the orbital fluctuation feature at high T due to degenerate πg* orbitals. Upon cooling, the orbital fluctuation is suppressed by the Jahn-Teller-type crystal field that becomes stronger with the lowering of structural symmetry, and then the ferro-orbital (FO) ordering emerges at low T. This FO ordering feature distinguishes KO2 from RbO2 and CsO2 in that the latter two seem to have antiferro-orbital orderings at low T, indicating that the underlying physics is different between them. We propose that the suppression of the orbital fluctuation in KO2 can be observed by thermal-conductivity measurement, as observed in spin-orbital-lattice-coupled 3d transition-metal oxides such as LaVO3. © 2014 American Physical Society.

Kim B.H.,Pohang University of Science and Technology | Khaliullin G.,Max Planck Institute for Solid State Research | Min B.I.,Pohang University of Science and Technology
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We have investigated the excitation spectra of jeff=12 Mott insulator Na2IrO3. Taking into account a relativistic multiplet structure of Ir ions, we have calculated the optical conductivity σ(ω) and resonant inelastic x-ray scattering (RIXS) spectra, which manifest different features from those of a canonical jeff=12 system Sr2IrO4. Distinctly from the two-peak structure in Sr2IrO4, σ(ω) in Na2IrO3 has a broad single peak dominated by interband transitions from jeff=32 to 12. RIXS spectra exhibit the spin-orbit (SO) exciton that has a two-peak structure arising from the crystal-field effect, and the magnon peak at energies much lower than in Sr2IrO4. In addition, a small peak near the optical-absorption edge is found in RIXS spectra, originating from the coupling between the electron-hole (e-h) excitation and the SO exciton. Our findings corroborate the validity of the relativistic electronic structure and importance of both itinerant and local features in Na2IrO3. © 2014 American Physical Society.

Lee S.,Pohang University of Science and Technology | De Cooman B.C.,Pohang University of Science and Technology
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2014

The exceptional elongation obtained during tensile testing of intercritically annealed 10 pct Mn steel, with a two phase ferrite-austenite microstructure at room temperature, was investigated. The austenite phase exhibited deformation-twinning and strain-induced transformation to martensite. These two plasticity-enhancing mechanisms occurred in succession, resulting in a high rate of work hardening and a total elongation of 65 pct for a tensile strength of 1443 MPa. A constitutive model for the tensile behavior of the 10 pct Mn steel was developed using the Kocks-Mecking hardening model.© The Minerals, Metals &Materials Society and ASM International 2013.

Lee S.,Pohang University of Science and Technology | Estrin Y.,Pohang University of Science and Technology | De Cooman B.C.,Monash University
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2014

The strain-rate dependence of the plasticity-enhancing mechanisms in Fe-12 pct Mn-0.6 pct C-0.06 pct N steel was investigated. At low strain rates, deformation-induced e-martensite was formed. At high strain rate, the strain-induced formation of e-martensite was inhibited, and mechanical twinning was the dominant plasticity-enhancing deformation mechanism. This transition was associated with an increased work hardening rate and a higher total elongation. Dynamic strain aging (DSA) took place at all strain rates. While propagating type C Portevin-Le Chatelier (PLC) bands were observed at low strain rates, isolated propagating type A PLC bands were observed at high strain rates. The critical strain for the occurrence of DSA had an anomalous negative strain-rate dependence at low strain rates and a normal positive dependence at high strain rates. The transition from negative-to-positive strain-rate dependence was associated with a sharp change in the strain-rate sensitivity of the flow stress. Transmission electron microscopy was used to analyze the relationship between the stacking fault energy (SFE), the strain rate, and the plasticity-enhancing mechanisms. The SFE and critical resolved shear stress for the onset of the twinning and the e-martensite transformation were calculated and compared with experimental results.© The Minerals, Metals &Materials Society and ASM International 2013.

Kwon W.,Pohang University of Science and Technology | Kim J.-M.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Electrochimica Acta | Year: 2012

In this research, a new equivalent circuit for porous carbon electrodes is proposed. In order to analyze the validity of the new model, electrochemical impedance spectra of carbon black (CB) electrodes in a symmetric cell configuration are examined by varying the CB particle size and the electrode thickness. This model decouples and identifies the following elements: (i) the electron transport resistance and trap capacitance in the CB layer, (ii) the charge transfer resistance and the double layer capacitance, and (iii) the Nernst diffusion impedance at the electrode/electrolyte interface. The fit quality is quantified by the chi-square test, and the fit data show consistency with the measured conductivity, surface area, and thickness of the CB electrode. © 2012 Elsevier Ltd. All rights reserved.

Zhu Y.,Pusan National University | Chandra P.,Pusan National University | Song K.-M.,Pohang University of Science and Technology | Ban C.,Pohang University of Science and Technology | Shim Y.-B.,Pusan National University
Biosensors and Bioelectronics | Year: 2012

Highly sensitive label-free detection of kanamycin is achieved with an aptamer sensor based on a conducting polymer/gold self-assembled nanocomposite. The sensor probe is fabricated by covalently immobilizing an in vitro selected DNA aptamer for kanamycin onto gold nanoparticle (AuNP)-comprised conducting polymer, poly-[2, 5-di-(2-thienyl)-1. H-pyrrole-1-(p-benzoic acid)] (poly-DPB). The self-assembling of DPB on AuNP is investigated by TEM and UV-vis spectroscopy and the modification of the aptamer sensor is characterized using XPS and electrochemical impedance spectroscopy. The probe is applied to detect kanamycin by using voltammetric techniques. The sensor shows a pair of redox peaks around 0.26/ 0.08. V (vs. Ag/AgCl) for kanamycin captured by the aptamer-immobilized probe. The parameters that can affect the response, such as aptamer concentration, incubation time, temperature, and pH are optimized. The calibration plot shows a linear range from 0.05 μM to 9.0 μM kanamycin with a detection limit of 9.4±0.4. nM. The proposed aptamer sensor is examined with a real sample. © 2012 Elsevier B.V.

Kim D.,Pohang University of Science and Technology | Yamamoto K.,Tohoku University | Ahn K.H.,Pohang University of Science and Technology
Tetrahedron | Year: 2012

A simple BODIPY derivative is demonstrated to fluorescently sense Hg 2+ in a ratiometric manner. The probe, an 8-methylthio-BODIPY, undergoes Hg 2+-promoted hydrolysis to produce the corresponding 8-hydroxy-BODIPY, which conversion is accompanied with a large emission wavelength change. The probe can selectively detect Hg 2+ over various other metal cations, with a detection limit of 1 ppb. © 2012 Published by Elsevier Ltd.

Kim C.S.,Pohang University of Science and Technology | Seo J.H.,Pohang University of Science and Technology | Cha H.J.,Pohang University of Science and Technology
Analytical Chemistry | Year: 2012

The development of analytical tools is important for understanding the infection mechanisms of pathogenic bacteria or viruses. In the present work, a functional carbohydrate microarray combined with a fluorescence immunoassay was developed to analyze the interactions of Vibrio cholerae toxin (ctx) proteins and GM1-related carbohydrates. Ctx proteins were loaded onto the surface-immobilized GM1 pentasaccharide and six related carbohydrates, and their binding affinities were detected immunologically. The analysis of the ctx-carbohydrate interactions revealed that the intrinsic selectivity of ctx was GM1 pentasaccharide ≫ GM2 tetrasaccharide > asialo GM1 tetrasaccharide ≥ GM3trisaccharide, indicating that a two-finger grip formation and the terminal monosaccharides play important roles in the ctx-GM1 interaction. In addition, whole cholera toxin (ctxAB5) had a stricter substrate specificity and a stronger binding affinity than only the cholera toxin B subunit (ctxB). On the basis of the quantitative analysis, the carbohydrate microarray showed the sensitivity of detection of the ctxAB5-GM1 interaction with a limit-of-detection (LOD) of 2 ng mL-1 (23 pM), which is comparable to other reported high sensitivity assay tools. In addition, the carbohydrate microarray successfully detected the actual toxin directly secreted from V. cholerae, without showing cross-reactivity to other bacteria. Collectively, these results demonstrate that the functional carbohydrate microarray is suitable for analyzing toxin protein-carbohydrate interactions and can be applied as a biosensor for toxin detection. © 2012 American Chemical Society.

Kim S.J.,Pohang University of Science and Technology | Jung H.G.,POSCO | Kim K.Y.,Pohang University of Science and Technology
Electrochimica Acta | Year: 2012

The new hydrogen permeation technique developed for the electrochemical permeation method makes it possible to evaluate the effect of applied tensile stress in both elastic and plastic range on hydrogen permeation behavior in high-strength steels. This study suggests that the iron sulfide film acts as sites for hydrogen reduction reaction and that the applied elastic stress weakens the stability of the sulfide film due to lots of cracks forming in the local sulfur deficient region, resulting in increase in both the anodic dissolution and hydrogen reduction reaction. In addition, the effect of applied stress in plastic range on hydrogen permeation behavior was clearly evaluated by excluding other side effects such as the reduced rate of hydrogen oxidation caused by the disruption of the Pd layer under stress. Furthermore, by evaluating hydrogen permeation behavior in the transient range from elastic to plastic stress, the effect of newly generated dislocation on the permeation current was accurately investigated without any side effect. © 2012 Elsevier Ltd.

Kim J.-M.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Electrochimica Acta | Year: 2012

Electrochemical properties of carbon black (CB) porous layer as a counter electrode in dye-sensitized solar cells (DSC) are studied. CB electrode for triiodide (I 3 -) ion reduction is coated with spray coating method on the fluorine-doped tin oxide glass at 120 °C. The CB particle size is varied from 20 nm to 90 nm and the CB electrode thickness is controlled from 1 μm to 9 μm by controlling the spraying time. The electrochemical impedance spectroscopy is analyzed for a symmetric cell and a new circuit model is applied to identify electrochemical parameters. As the CB particle size is decreased, the catalytic activity is improved because of the increase in the surface area and the conductivity of the CB layer. Increased CB electrode thickness also improves the catalytic activity and leads to the low charge transfer resistance at the electrolyte/CB electrode interface. The CB counter electrode with the particle size of 20 nm and the thickness of 9 μm for DSC shows the energy conversion efficiency of 7.2% with the highest fill factor (FF) of 65.6%, which is similar to the Pt counter electrode with FF of 65.8% and the efficiency of 7.6%. © 2012 Elsevier Ltd. All rights reserved.

Kim J.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology
Energy and Environmental Science | Year: 2010

The production of hydrogen accompanied by the simultaneous degradation of organic pollutants in water was achieved using surface-modified TiO2 photocatalysts working under solar irradiation conditions. © 2010 The Royal Society of Chemistry.

Kim H.,Pohang University of Science and Technology | Kim J.,Pohang University of Science and Technology
Microfluidics and Nanofluidics | Year: 2014

Bhadeshia H.K.D.H.,Pohang University of Science and Technology | Bhadeshia H.K.D.H.,University of Cambridge
Scripta Materialia | Year: 2014

The creation and use of computational models is seminal to the design of steels and associated processes, and many such models have now become of generic value. We illustrate here a few examples that explain the vitality of the subject and how the methodology is leading to benefits for commerce and academia alike. There are some breathtaking developments, which are critically assessed. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Ryu J.C.,Pohang University of Science and Technology | Park H.J.,Pohang University of Science and Technology | Park J.K.,Pohang University of Science and Technology | Kang K.H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2010

We report a new type of electrohydrodynamic (EHD) flow generated around a circular cylinder and a spherical particle in a dielectric liquid under dc and ac electric fields. The EHD flow is observed for various combinations of dielectric liquids and polar additives. We suggest that the EHD flow is caused by a gradient of electrical conductivity produced by a nonuniform electric field and subsequent generation of free charge in the bulk liquid. Analytical and numerical analyses which are based on the leaky-dielectric model show good agreement with experimental results. © 2010 The American Physical Society.

Lee B.-M.,Pohang University of Science and Technology | Lee B.-J.,Pohang University of Science and Technology
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2014

A comparative study on hydrogen diffusion in amorphous and simple crystalline structures has been carried out using molecular dynamics simulations. The Cu-Zr bulk metallic glass (BMG) system is selected as the model material and a modified embedded-atom method (MEAM) interatomic potential for the Cu-Zr-H ternary system is developed for the atomistic simulation. It is found that the diffusivity of hydrogen in amorphous alloys is lower than that in open structured crystals but higher than that in close-packed crystals. The hydrogen diffusion in amorphous alloys is strongly hydrogen concentration dependent compared to crystals, increasing as the hydrogen content increases, and the Arrhenius plot of hydrogen diffusion in amorphous alloys shows an upward curvature. The reasons to rationalize all the findings are discussed based on the variety of energy state and migration energy barrier for interstitial sites in amorphous alloys. © 2014 The Minerals, Metals , Materials Society and ASM International.

Kim K.S.,Pohang University of Science and Technology | Jung S.C.,Pohang University of Science and Technology | Kang M.H.,Pohang University of Science and Technology | Yeom H.W.,Pohang University of Science and Technology
Physical Review Letters | Year: 2010

We demonstrate the realization of nearly massless electrons in the most widely used device material, silicon, at the interface with a metal film. Using angle-resolved photoemission, we found that the surface band of a monolayer lead film drives a hole band of the Si inversion layer formed at the interface with the film to have a nearly linear dispersion with an effective mass about 20 times lighter than bulk Si and comparable to graphene. The reduction of mass can be accounted for by a repulsive interaction between neighboring bands of the metal film and Si substrate. Our result suggests a promising way to take advantage of massless carriers in silicon-based thin-film devices, which can also be applied to various other semiconductor devices. © 2010 The American Physical Society.

Suh B.-C.,Pohang University of Science and Technology | Shim M.-S.,Pohang University of Science and Technology | Shin K.S.,Seoul National University of Science and Technology | Kim N.J.,Pohang University of Science and Technology
Scripta Materialia | Year: 2014

Critical issues relating to the strength and formability of Mg sheet alloys are assessed. It is shown that Mg alloys in general have slow aging kinetics and even the most formable Mg alloys have less than optimum orthotropic texture. It is suggested that future research and development efforts should be concentrated on identifying the alloying elements and optimizing the related processes that can accelerate the age-hardening response and modify the texture to be weak and symmetric in the sheet plane. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Lee D.,Pohang University of Science and Technology | Huh K.Y.,Pohang University of Science and Technology
Combustion and Flame | Year: 2012

A general expression is derived for the turbulent burning velocity, S T, from the continuum form of the c̄ transport equation and shown to be valid in all turbulent premixed combustion regimes. It involves the inverse length scale, 1/. L w, for which new analytical relationships are proposed in the laminar flamelet and the distributed reaction regime. They are combined to give new predictive relationships for the S T in the two limiting regimes and extended to be applicable in the intermediate regime as well. They involve flamelet thickness, mean curvature, molecular and turbulent diffusivities at the leading edge without any tuning constants. The proposed relationships are shown to be consistent with measurements in literature at varying pressures, laminar flame speeds, turbulent intensities and mixture compositions. Convincing agreement is achieved for S T and 1/. L w for different turbulence and laminar flame properties of stagnating compressible flames and in parametric study with respect to turbulent intensity, laminar flamelet thickness and integral length scale for freely propagating incompressible flames. There is no gravity and the Lewis number is assumed unity for simplification. © 2011 The Combustion Institute.

Son J.H.,Pohang University of Science and Technology | Lee J.-L.,Pohang University of Science and Technology
Optics Express | Year: 2010

We present a method of increasing Tight output power and suppressing efficiency droop in vertical-structure InGaN/GaN MQW LEDs without modifying their epitaxial layers. These improvements are achieved by reducing the quantum-confined Stark effect (QCSE) by reducing piezoelectric polarization that results from compressive stress in the GaN epilayer. This compressive stress is relaxed due to the external stress induced by an electro-plated Ni metal substrate. In simulations, the severe band bending in the InGaN quantum well is reduced and subsequently internal quantum efficiency increases as the piezoelectric polarization is reduced. © 2010 Optical Society of America.

Jung S.-M.,Pohang University of Science and Technology
ISIJ International | Year: 2013

A study was conducted to evaluate the fractional reduction of raw iron ore and char composite in terms of TGA data. The fractional reduction evaluated by thermogravimetric analysis (TGA) was validated by the chemical analysis of the oxygen combined with Fe in the reduced titanomagnetite (TTM). Pulverized TTM and char were homogeneously mixed with a stoichiometric ratio using a ball mill. The pellets were formed into cylindrical shape weighing 1.5 g by a hydraulic press under the pressure of 50 MPa for 1 min with distilled water added to ensure complete compactness. The pellets were air dried in an oven at 423 K before use and placed into a cylindrical Al2O3 crucible, which was suspended and connected using a Pt wire to the balance part of a TGA apparatus. The isothermal reduction experiments were performed rapidly by heating up the TGA furnace at 100 K/min to 1 473 K.

Park J.,Pohang University of Science and Technology | Lee S.,Samsung | Lee J.,Pohang University of Science and Technology | Yong K.,Pohang University of Science and Technology

A light incident angle selectivity of a memory device is demonstrated. As a model system, the ZnO resistive switching device has been selected. Electrical signal is reversibly switched between memristor and resistor behaviors by modulating the light incident angle on the device. Moreover, a liquid passivation layer is introduced to achieve stable and reversible exchange between the memristor and WORM behaviors. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Lee S.G.,Pohang University of Science and Technology | Kim H.,Pohang University of Science and Technology | Choi H.H.,Pohang University of Science and Technology | Bong H.,Pohang University of Science and Technology | And 3 more authors.

The evaporation-induced self-alignment of semiconductor nanowires is achieved using wrinkled elastomeric templates. The wrinkled templates, which have a surface topography that can be tuned via changes in the mechanical strain, are used as both a template to align the nanowires and as a stamp to transfer the aligned nanowires to target substrates. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Hwang I.,Pohang University of Science and Technology | Sheen J.,Massachusetts General Hospital | Sheen J.,Harvard University | Muller B.,University of Zürich
Annual Review of Plant Biology | Year: 2012

Despite long-standing observations on diverse cytokinin actions, the discovery path to cytokinin signaling mechanisms was tortuous. Unyielding to conventional genetic screens, experimental innovations were paramount in unraveling the core cytokinin signaling circuitry, which employs a large repertoire of genes with overlapping and specific functions. The canonical two-component transcription circuitry involves His kinases that perceive cytokinin and initiate signaling, as well as His-to-Asp phosphorelay proteins that transfer phosphoryl groups to response regulators, transcriptional activators, or repressors. Recent advances have revealed the complex physiological functions of cytokinins, including interactions with auxin and other signal transduction pathways. This review begins by outlining the historical path to cytokinin discovery and then elucidates the diverse cytokinin functions and key signaling components. Highlights focus on the integration of cytokinin signaling components into regulatory networks in specific contexts, ranging from molecular, cellular, and developmental regulations in the embryo, root apical meristem, shoot apical meristem, stem and root vasculature, and nodule organogenesis to organismal responses underlying immunity, stress tolerance, and senescence. © 2012 by Annual Reviews. All rights reserved.

Cho J.,Pohang University of Science and Technology | Lee J.,Kyung Hee University
Expert Systems with Applications | Year: 2013

As the number of new products developed by new technologies has increased, the importance of the commercialization of new technology products has become crucial to manufactures in the successful delivery of valuable new products and services. This study classified success factors for commercialization of new products and analyzed which factors should be primarily considered. Based on the literature review and Delphi method, we identified four decision areas and further prioritized the sixteen factors under a hierarchy model structured by fuzzy AHP (analytic hierarchy process) approach. The FAHP is conducted by 111 R&D and business experts working at the world's major players in machinery industry; using the priorities of success factors derived by FAHP, we devise an example of commercialization assessment model. The paper drives the assessment initiatives of the new product development in manufactures and provides them with practical implications about the commercialization of new technology product. © 2013 Elsevier Ltd. All rights reserved.

Jung S.-M.,Pohang University of Science and Technology
ISIJ International | Year: 2014

The carbothermic reduction of titanomagnetite (TTM) was investigated from a kinetic viewpoint in the temperature range of 1 000 to 1 150°C employing thermogravimetric analysis (TGA) and quadruple mass spectrometry (QMS). The method of evaluating the fractional reduction in the carbothermic reduction of TTM by TGA was validated. The carbon in char consumed for the reduction of TTM was fully gasified into CO and CO2, which indicates that the TGA can be coupled to QMS from the viewpoint of mass balance of carbon. The carbon gasification reaction was activated when the Fe3O4 in TTM, wustite and Fe coexist at the fractional reduction of 0.21, indicating that Fe-catalyzed nature of Fe was confirmed for the carbon gasification. The activation energy for the reduction in TTM to wustite was evaluated to be 241 kJ/mol and the initial reduction stage is believed to be limited by carbon gasification. In the current study, it was considered that the changeover in reaction mechanism might be carried out from carbon gasification to the reduction in wustite to Fe by CO during the isothermal reduction of TTM with char.

Park J.E.,Pohang University of Science and Technology | Son M.,Pohang University of Science and Technology | Hong M.,Pohang University of Science and Technology | Lee G.,Pohang University of Science and Technology | Choi H.C.,Pohang University of Science and Technology
Angewandte Chemie - International Edition | Year: 2012

Down to the wire: Pentacene exhibits crystal-plane-dependent photoluminescence (PL) activity, as demonstrated in highly crystalline 1Da wires and 2Da disks, which were selectively synthesized using the vaporization-condensation-recrystallization (VCR) process. Although pentacene 1Da wires and 2Da disks have identical triclinic crystal structures, PLa activity is observed only from pentacene 1Da wires owing to the presence of "PL-active" (010) planes. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Jeong B.,Daum Communications Corporation | Lee J.,Pohang University of Science and Technology | Cho H.,Pohang University of Science and Technology
Information Sciences | Year: 2010

Memory-based collaborative filtering (CF) makes recommendations based on a collection of user preferences for items. The idea underlying this approach is that the interests of an active user will more likely coincide with those of users who share similar preferences to the active user. Hence, the choice and computation of a similarity measure between users is critical to rating items. This work proposes a similarity update method that uses an iterative message passing procedure. Additionally, this work deals with a drawback of using the popular mean absolute error (MAE) for performance evaluation, namely that ignores ratings distribution. A novel modulation method and an accuracy metric are presented in order to minimize the predictive accuracy error and to evenly distribute predicted ratings over true rating scales. Preliminary results show that the proposed similarity update and prediction modulation techniques significantly improve the predicted rankings. © 2009 Elsevier Inc. All rights reserved.

Lee J.H.,Pohang University of Science and Technology | Gupta S.,Pohang University of Science and Technology | Jeong W.,Pohang University of Science and Technology | Rhee Y.H.,Pohang University of Science and Technology | Park J.,Pohang University of Science and Technology
Angewandte Chemie - International Edition | Year: 2012

Shine a light: A fluorescent light-induced synthetic method for the title compounds has been developed and the chemoselective nature of the reaction is highlighted by the observation of the cis/trans isomers of various N-unsubstituted imines. The synthetic utility of this method is demonstrated by the one-pot imine formation/asymmetric allylation sequence of benzyl azide catalyzed by 1. (Ipc=isopinocampheyl). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kim H.H.,Pohang University of Science and Technology | Chung Y.,Pohang University of Science and Technology | Lee E.,Pohang University of Science and Technology | Lee S.K.,Pohang University of Science and Technology | Cho K.,Pohang University of Science and Technology

Transferring graphene without water enables water-sensitive substrates to be used in graphene electronics. A polymeric bilayer (PMMA/PBU) is coated on graphene as a supporting layer for the water-free transfer process and as an excellent passivation layer that enhances device operation. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kim Y.-J.,Pohang University of Science and Technology | Song M.-G.,Pohang University of Science and Technology | Cho Y.-S.,Pohang University of Science and Technology | Im G.-H.,Pohang University of Science and Technology
IEEE Transactions on Wireless Communications | Year: 2013

In spectrum-sharing-based cognitive radio networks, multiple secondary systems can access a licensed spectrum to better utilize scarce radio resources. When the multiple secondary transmitters are co-located, the weighted sum-rate of the secondary users (SUs) is mainly limited by the inter-cell interference (ICI). With limited cooperation among co-located secondary transmitters, we propose an algorithm for decentralized beamforming with power allocation via dual decomposition. To maximize the weighted sum-rate of the SUs, the proposed decentralized algorithm efficiently mitigates the ICI by the undesired leakage power limitation at each secondary transmitter. Because the channel information is not perfectly known at the transmitter in practical applications, we also develop a decentralized robust beamformer. To efficiently design the robust beamformer, a convex problem is formulated by semi-definite relaxation. Simulation results show that the proposed algorithm with perfect channel state information (P-CSI) efficiently maximizes the weighted sum-rate performance by the undesired leakage power limitation. For an imperfect CSI with a small error bound, the proposed robust beamformer approaches the performance of a P-CSI case, without causing harmful interference to the primary user. © 2002-2012 IEEE.

Aal M.I.A.E.,Zagazig University | Aal M.I.A.E.,Pohang University of Science and Technology | Kim H.S.,Pohang University of Science and Technology
Materials and Design | Year: 2014

In this paper, Al-7. wt% Si alloy was processed via high pressure torsion (HPT) at an applied pressure 8. GPa for 10 revolutions at room temperature. The microstructure and hardness of the HPT samples were investigated and compared with those of the as-cast samples. The wear properties of as-cast and the HPT samples under dry sliding conditions using different sliding distances and loads were investigated by reciprocated sliding wear tests.The HPT process successfully resulted in nanostructure Al-7. wt% Si samples with a higher microhardness due to the finer Al matrix grains and Si particles sizes with more homogeneous distribution of the Si particles than those in the as-cast samples.The wear mass loss and coefficient of friction values were decreased after the HPT process. The wear mechanism was observed to be adhesive, delamination, plastic deformation bands and oxidization in the case of the as-cast alloy. Then, the wear mechanism was transformed into a combination of abrasive and adhesive wear after the HPT process. The oxidization cannot be considered as a mechanism that contributes to wear in the case of HPT samples, because O2 was not detected in all conditions. © 2013 Elsevier Ltd.

Oh J.-E.,Pohang University of Science and Technology | Cho Y.-W.,Pohang University of Science and Technology | Scarcelli G.,Harvard University | Kim Y.-H.,Pohang University of Science and Technology
Optics Letters | Year: 2013

We demonstrate sub-Rayleigh limit imaging of an object via speckle illumination. Imaging beyond the conventional Rayleigh limit is achieved by illuminating the object with pseudothermal light that exhibits a random speckle pattern. An object image is reconstructed from the second-order correlation measurement and the resolution of the image, which exceeds the Rayleigh limit, is shown to be related to the size of the speckle pattern that is tied to the lateral coherence length of the pseudothermal light. © 2013 Optical Society of America.

Park J.,Pohang University of Science and Technology | Jung S.W.,Pohang University of Science and Technology | Jung M.-C.,Pohang University of Science and Technology | Yamane H.,Japan Institute for Molecular Science | And 2 more authors.
Physical Review Letters | Year: 2013

We investigated Pt-induced nanowires on the Si(110) surface using scanning tunneling microscopy (STM) and angle-resolved photoemission. High resolution STM images show a well-ordered nanowire array of 1.6 nm width and 2.7 nm separation. Angle-resolved photoemission reveals fully occupied one-dimensional (1D) bands with a Rashba-type split dispersion. Local dI/dV spectra further indicate well-confined 1D electron channels on the nanowires, whose density of states characteristics are consistent with the Rashba-type band splitting. The observed energy and momentum splitting of the bands are among the largest ever reported for Rashba systems, suggesting the Pt-Si nanowire as a unique 1D giant Rashba system. This self-assembled nanowire can be exploited for silicon-based spintronics devices as well as the quest for Majorana fermions. © 2013 American Physical Society.

Lee J.,Pohang University of Science and Technology | Hwang S.-W.,Pohang University of Science and Technology
Information Systems | Year: 2014

Skyline queries have recently received considerable attention as an alternative decision-making operator in the database community. The conventional skyline algorithms have primarily focused on optimizing the dominance of points in order to remove non-skyline points as efficiently as possible, but have neglected to take into account the incomparability of points in order to bypass unnecessary comparisons. To design a scalable skyline algorithm, we first analyze a cost model that copes with both dominance and incomparability, and develop a novel technique to select a cost-optimal point, called a pivot point, that minimizes the number of comparisons in point-based space partitioning. We then implement the proposed pivot point selection technique in the existing sorting- and partitioning-based algorithms. For point insertions/deletions, we also discuss how to maintain the current skyline using a skytree, derived from recursive point-based space partitioning. Furthermore, we design an efficient greedy algorithm for the k representative skyline using the skytree. Experimental results demonstrate that the proposed algorithms are significantly faster than the state-of-the-art algorithms. © 2013 Elsevier Ltd.

Ham J.,Pohang University of Science and Technology | Lee J.-L.,Pohang University of Science and Technology
Advanced Energy Materials | Year: 2014

Ormoclear/Ag/WO3 (OAW) films with ultrahigh transparency are designed for application in organic solar cells (OSCs). When the thicknesses of Ag and WO3 are fixed at 8 nm and 30 nm, respectively, excellent transparency that is independent of Ormoclear thickness is successfully achieved by employing soft materials instead of inorganic dielectrics. Oxygen plasma treatment prior to the deposition of Ag introduces the polar functional groups on the Ormoclear surface, which results in increasing the surface hydrophilicity, thereby enhancing the wettability of Ag. From the surface modification, OAW exhibits low sheet resistance (4.8 ohm sq-1), high transmittance of up to 96.3% at 535 nm, and enhanced efficiency of 7.63% of OSCs. Moreover, nanoimprint lithography is used to prepare a well-ordered nanopatterned OAW with dimple diameter of 90 nm, leading to further increase in photocurrent density by 17%, compared to that with a planar indium tin oxide (ITO) electrode. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Boyman O.,University of Zürich | Krieg C.,University of Zürich | Homann D.,University of Colorado at Denver | Sprent J.,Garvan Institute of Medical Research | Sprent J.,Pohang University of Science and Technology
Cellular and Molecular Life Sciences | Year: 2012

Homeostasis in the immune system encompasses the mechanisms governing maintenance of a functional and diverse pool of lymphocytes, thus guaranteeing immunity to pathogens while remaining selftolerant. Antigen-naïve T cells rely on survival signals through contact with self-peptide-loaded major histocompatibility complex (MHC) molecules plus interleukin (IL)-7. Conversely, antigen-experienced (memory) T cells are typically MHC-independent and they survive and undergo periodic homeostatic proliferation through contact with both IL-7 and IL-15. Also, non-conventional γδ T cells rely on a mix of IL-7 and IL-15 for their homeostasis, whereas natural killer cells are mainly dependent on contact with IL-15. Homeostasis of CD4 + T regulatory cells is different in being chiefly regulated by contact with IL-2. Notably, increased levels of these cytokines cause expansion of responsive lymphocytes, such as found in lymphopenic hosts or following cytokine injection, whereas reduced cytokine levels cause a decline in cell numbers. © 2011 Springer Basel AG.

Kim D.E.,Pohang University of Science and Technology | Kim M.-H.,Pohang University of Science and Technology
International Journal of Heat and Fluid Flow | Year: 2011

An experimental investigation of turbulent heat transfer in vertical upward and downward supercritical CO2 flow was conducted in a circular tube with an inner diameter of 4.5mm. The experiments were performed for bulk fluid temperatures from 29 to 115°C, pressures from 74.6 to 102.6bar, local wall heat fluxes from 38 to 234kW/m2, and mass fluxes from 208 to 874kg/m2s. At a moderate wall heat flux and low mass flux, the wall temperature had a noticeable peak value for vertical upward flow, but increased monotonically along the flow direction without a peak value for downward flow. The ratios of the experimental Nusselt number to the value obtained from a reference correlation were compared with Bo* and q+ distributions to observe the buoyancy and flow-acceleration effects on heat transfer. In the experimental range of this study, the flow acceleration predominantly affected the heat-transfer phenomena. Based on an analysis of the shear-stress distribution in the turbulent boundary layer and the significant variation of the specific heat across the turbulent boundary layer, a new heat-transfer correlation for vertical upward and downward flow of supercritical pressurized fluid was developed; this correlation agreed with various experimental datasets within ±30%. © 2010 Elsevier Inc.

Kim C.S.,Hanyang University | Choi S.H.,Pohang University of Science and Technology | Bang J.H.,Hanyang University
ACS Applied Materials and Interfaces | Year: 2014

Despite recent significant strides in understanding various processes in quantum dot-sensitized solar cells (QDSSCs), little is known about the intrinsic electrocatalytic properties of copper sulfides that are the most commonly employed electrocatalysts for the counter electrode of QDSSCs. Given that the physical properties of copper sulfides are governed by their stoichiometry, the electrocatalytic activity of copper sulfides toward polysulfide reduction may also be dictated by their compositions. Using a new, simple approach to prepare robust copper sulfide films based on chemical bath deposition (CBD), we were able to delicately control the compositions of copper sulfides, which allowed us to perform a systematic investigation to gain new insight into copper sulfide-based electrocatalysts. The electrocatalytic activity is indeed dependent on the compositions of copper sulfides: Cu-deficient films (CuS and Cu1.12S) are superior to Cu-rich films (Cu1.75S and Cu1.8S) in their electrocatalytic activity. In addition, the stability of the Cu-deficient electrocatalysts is substantially better than that of the Cu-rich counterparts. © 2014 American Chemical Society.

Kim T.-H.,Pohang University of Science and Technology | Yeom H.W.,Pohang University of Science and Technology
Physical Review Letters | Year: 2012

We investigated phase defects in a quasi-one-dimensional commensurate charge-density wave (CDW) system, an In atomic wire array on Si(111), using low temperature scanning tunneling microscopy. The unique fourfold degeneracy of the CDW state leads to various phase defects, among which intrinsic solitons are clearly distinguished. The solitons exhibit a characteristic variation of the CDW amplitude with a coherence length of about 4 nm, as expected from the electronic structure, and a localized electronic state within the CDW gap. While most of the observed solitons are trapped by extrinsic defects, moving solitons are also identified and their novel interaction with extrinsic defects is disclosed. © 2012 American Physical Society.

Yi S.,Pohang University of Science and Technology | Chung W.J.,Kongju National University | Heo J.,Pohang University of Science and Technology
Journal of the American Ceramic Society | Year: 2014

Commercial Ce3+:YAG phosphors were embedded in glass frits. Thermal condition for the viscous sintering of the composite materials was optimized. The phosphor-glass composites had maximum external efficiency of 30% and maximum light extraction efficiency of 39%. Color temperatures of the composites composed of fluorescent glass frits containing Eu3+ and Mn2+ combined with blue LEDs shifted from ∼7000 to ∼4000 K. © 2013 The American Ceramic Society.

Min S.K.,Pohang University of Science and Technology | Kim W.Y.,Pohang University of Science and Technology | Kim W.Y.,KAIST | Cho Y.,Pohang University of Science and Technology | Kim K.S.,Pohang University of Science and Technology
Nature Nanotechnology | Year: 2011

Devices in which a single strand of DNA is threaded through a nanopore could be used to efficiently sequence DNA. However, various issues will have to be resolved to make this approach practical, including controlling the DNA translocation rate, suppressing stochastic nucleobase motions, and resolving the signal overlap between different nucleobases. Here, we demonstrate theoretically the feasibility of DNA sequencing using a fluidic nanochannel functionalized with a graphene nanoribbon. This approach involves deciphering the changes that occur in the conductance of the nanoribbon as a result of its interactions with the nucleobases via π-π stacking. We show that as a DNA strand passes through the nanochannel, the distinct conductance characteristics of the nanoribbon (calculated using a method based on density functional theory coupled to non-equilibrium Green function theory 18-20) allow the different nucleobases to be distinguished using a data-mining technique and a two-dimensional transient autocorrelation analysis. This fast and reliable DNA sequencing device should be experimentally feasible in the near future. © 2011 Macmillan Publishers Limited. All rights reserved.

Choi J.,Pohang University of Science and Technology | Song S.,Pohang University of Science and Technology | Kang G.,Pohang University of Science and Technology | Park T.,Pohang University of Science and Technology
ACS Applied Materials and Interfaces | Year: 2014

We systematically investigated the charge transport properties of doubly or singly open-ended TiO2nanotube arrays (DNT and SNT, respectively) for their utility as electrodes in dye-sensitized solar cells (DSCs). The SNT or DNT arrays were transferred in a bottom-up (B-up) or top-up (T-up) configuration onto a fluorine-doped tin oxide (FTO) substrate onto which had been deposited a 2 μm thick TiO2nanoparticle (NP) interlayer. This process yielded four types of DSCs prepared with SNTs (B-up or T-up) or DNT (B-up or T-up). The photovoltaic performances of these DSCs were analyzed by measuring the dependence of the charge transport on the DSC geometry. High resolution scanning electron microscopy techniques were used to characterize the electrode cross sections, and electrochemical impedance spectroscopy was used to characterize the electrical connection at the interface between the NT array and the TiO2NP interlayer. We examined the effects of decorating the DNT or SNT arrays with small NPs (sNP@DNT and sNP@SNT, respectively) in an effort to increase the extent of dye loading. The DNT arrays decorated with small NPs performed better than the decorated SNT arrays, most likely because the Ti(OH)4precursor solution flowed freely into the array through the open ends of the NTs in the DNT case but not in the SNT case. The sNP@DNT-based DSC exhibited a better PCE (10%) compared to the sNP@SNT-based DSCs (6.8%) because the electrolyte solution flow was not restricted, direct electron transport though the NT arrays was possible, the electrical connection at the interface between the NT array and the TiO2NP interlayer was good, and the array provided efficient light harvesting. © 2014 American Chemical Society.

Kim J.,Pohang University of Science and Technology | Kim K.-S.,Pohang University of Science and Technology | Jhi S.-H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2012

Material with a nontrivial topology in its electronic structure enforces the existence of helical Dirac fermionic surface states. We discover emergent topological phases in the stacked structures of topological insulator and band insulator layers where the surface Dirac fermions interact with each other with a particular helicity ordering. Using first-principles calculations and a model Lagrangian, we explicitly demonstrate that such helicity ordering occurs in real materials of ternary chalcogen compounds and determines their topological-insulating phase. Our results reveal the rich collective nature of interacting surface Dirac fermions and pave the way for utilizing topological phases for technological devices such as nonvolatile memories. © 2012 American Physical Society.

Lee J.K.,Hoseo University | Kim M.-J.,Pohang University of Science and Technology
Journal of Molecular Catalysis B: Enzymatic | Year: 2011

The room temperature solid-phase ionic liquid (RTSPIL) co-lyophilized enzyme exhibited markedly enhanced activity in organic solvent. The enzyme co-lyophilized with a dodecyl-imidazolium salt was 660-fold more active compared to its RTSPIL-free counterpart. The activity enhancement by RTSPILs was mainly attributable to the reduced particle sizes and improved dispersion of enzymes suspended in organic solvent. Also, the RTSPIL co-lyophilized enzyme displayed significantly enhanced enantioselectivity. Its enantioselectivity was 2.5-fold higher than that of its RTSPIL-free counterpart. © 2010 Elsevier B.V. All rights reserved.

Lee J.S.,Pohang University of Science and Technology | Weon B.M.,Pohang University of Science and Technology | Je J.H.,Pohang University of Science and Technology | Fezzaa K.,Argonne National Laboratory
Physical Review Letters | Year: 2012

When a liquid drop impacts a solid surface, air is generally entrapped underneath. Using ultrafast x-ray phase-contrast imaging, we directly visualized the profile of an entrapped air film and its evolution into a bubble during drop impact. We identified a complicated evolution process that consists of three stages: inertial retraction of the air film, contraction of the top air surface into a bubble, and pinch-off of a daughter droplet inside the bubble. Energy transfer during retraction drives the contraction and pinch-off of a daughter droplet. The wettability of the solid surface affects the detachment of the bubble, suggesting a method for bubble elimination in many drop-impact applications. © 2012 American Physical Society.

Lakshminarasimhan N.,Pohang University of Science and Technology | Lakshminarasimhan N.,CSIR - Central Electrochemical Research Institute | Bokare A.D.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology
Journal of Physical Chemistry C | Year: 2012

Two mesoporous TiO 2 samples (M1-TiO 2 and M2-TiO 2) with different morphologies were synthesized, and the photocatalytic and photoelectrochemical properties of both TiO 2 and their photoplatinized counterparts (0.05, 0.1, and 1.0 wt % of Pt) were systematically investigated. Electron microscopic analysis showed that M1-TiO 2 consists of densely packed nanoparticles forming spherical secondary particles (0.5 to 1.0 μm), whereas M2-TiO 2 is made up of loosely agglomerated nanoparticles. Subsequently, this morphological difference led to the formation of different Pt clusters (photodeposited on them): large Pt nanoparticles on M1-TiO 2 versus well-dispersed smaller Pt nanoparticles on M2-TiO 2. The photocatalytic activities of platinized M1-TiO 2 and M2-TiO 2 were investigated for H2 production and 4-chlorophenol degradation. Whereas M1-TiO 2 exhibited the highest photoactivity with 0.1 wt % Pt loading, the activity of M2-TiO 2 increased with increasing Pt loading (up to 1.0 wt %). The critical role of surface Pt morphology on the photocatalytic behavior of M1-TiO 2 and M2-TiO 2 was investigated using electrochemical impedance spectroscopy and photocurrent measurements. In the case of M1-TiO 2, an increase in Pt cluster size enhanced the charge-transfer resistance and reduced the interfacial electron transfer efficiency, whereas the same loading of Pt on M2-TiO 2 effectively enhanced the interfacial charge transfer. This dissimilar interfacial charge-transfer kinetics for M1-TiO 2 and M2-TiO 2 indicates that the TiO 2 microstructure controls the photodeposited Pt morphology, which subsequently affects the photocatalytic activity. This study reveals that the agglomerated state of TiO 2 nanoparticles can be an important parameter in determining the photocatalytic activity in both the suspension and film states. © 2012 American Chemical Society.

Choi C.W.,Pohang University of Science and Technology | Yu D.I.,Pohang University of Science and Technology | Kim M.H.,Pohang University of Science and Technology
International Journal of Heat and Mass Transfer | Year: 2011

An aspect ratio is an important parameter for two-phase flow in a rectangular microchannel. To study the aspect ratio effect on the flow pattern, pressure drop and void fraction, experiments of adiabatic liquid water and nitrogen gas two-phase flow in rectangular microchannels were conducted. The widths and heights of rectangular microchannels are 510 μm × 470 μm, 608 μm × 410 μm, 501 μm × 237 μm and 503 μm × 85 μm. Therefore, the aspect ratios of the rectangular microchannels are 0.92, 0.67, 0.47 and 0.16; and the hydraulic diameters of the rectangular microchannels were 490, 490, 322 and 143 μm, respectively. Experimental ranges were liquid superficial velocities of 0.06-1.0 m/s and gas superficial velocities of 0.06-71 m/s. Visible rectangular microchannels were fabricated using a photosensitive glass. And pressure drop in microchannels was directly measured through embedded ports. The visualization of the flow pattern was carried out with a high-speed camera and a long distance microscope. Typical flow patterns in the rectangular microchannels observed in this study were bubble flow, transitional flow (multiple flow) and liquid ring flow. As the aspect ratio decreased, the bubble flow regime became dominant due to the confinement effect and the thickness of liquid film in corner was decreased. A void fraction in the rectangular microchannels has a linear relation with the volumetric quality. And the two-phase flow becomes homogeneous with decreasing aspect ratio owing to the reduction of the liquid film thickness. Like Zhang et al.'s [19] correlation, as the confinement number increased, the C-value in Lockhart and Martinelli correlation decreased. And a frictional pressure drop in the rectangular microchannels was highly related with the flow pattern. © 2010 Elsevier Ltd. All rights reserved.

Park J.W.,Pohang University of Science and Technology | Kang M.H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2012

We demonstrate by using density functional calculations that the In/Si(111)-√7×√3 surface consists of an In double layer, contrary to the prevailing idea that the In overlayer on this surface is just one atom thick and thus can be used to represent the ultimate 2D limit of metal overlayer properties. The double-layer In structure is sound energetically and microscopically and, above all, well reproduces the measured photoemission band structure that could not be fairly compared with any single-layer In model. The present double-layer model urges a reconsideration on the recent experimental claims that the In overlayer properties were pushed to a single-layer limit in this surface. © 2012 American Physical Society.

Kim H.,Pohang University of Science and Technology | Hwang I.,Pohang University of Science and Technology | Yong K.,Pohang University of Science and Technology
ACS Applied Materials and Interfaces | Year: 2014

For stable quantum dot-sensitized solar cells, an oligomer-contained gel electrolyte was employed with a carbon-based counter electrode and a hierarchically shelled ZnO photoelectrode. Poly(ethylene glycol) dimethyl-ether (PEGDME) was added to the polysulfide electrolyte to enhance the stability of the methanol-based electrolyte. In addition, the nanocomposite gel electrolyte with fumed silica was used, which provided a solid three-dimensional network. A quantum-dot-modified ZnO nanowire photoanode enhanced the visible light harvesting, and a Pt/CNT-RGO counter electrode increased the catalytic activity. The oligomer gel electrolyte prevented the liquid electrolyte from leaking, and the carbon-based counter electrode retarded chemical poisoning at the counter electrode. The optimized cell exhibited 5.45% photoelectric conversion efficiency with long-term stability demonstrated over 5000 s operation time. © 2014 American Chemical Society.

Kim J.,Pohang University of Science and Technology | Choi W.,Pohang University of Science and Technology
Applied Catalysis B: Environmental | Year: 2011

A new hybrid modification method of TiO2 photocatalyst was developed and investigated. TiO2 modified with both phosphates and platinum nanoparticles on its surface (P-TiO2/Pt) was prepared by a simple two-step method using phosphoric acid (as phosphate source) and chloroplatinic acid (as platinum source). The coexistence of phosphate and platinum on the surface of TiO2 was confirmed by X-ray photoelectron spectroscopy and transmission electron micrography. P-TiO2/Pt showed a significantly higher photocatalytic activity than any of bare TiO2, P-TiO2, and Pt/TiO2 for the degradation of phenolic compounds (4-chlorophenol; bisphenol A; 2,4-dichlorophenoxyacetic acid (2,4-D)). In particular, P-TiO2/Pt minimized the production of toxic intermediate (2,4-dichlorophenol) during the photocatalytic degradation of 2,4-D. In accordance with the higher photocatalytic activities of P-TiO2/Pt, both the production of OH radicals and the photocurrent collection in the suspension were markedly enhanced upon the simultaneous platinization and phosphation of TiO2. The two surface species acted synergically to enhance the photocatalytic activity. The surface phosphation that should replace the surface hydroxyl groups on TiO2 favors the formation of unbound OH radicals instead of surface-bound OH radicals while the surface platinization accelerates the electron transfer with retarding the charge recombination. The phosphation of TiO2 was stable over a wide range of pH due to the strong chemical bonding of phosphate on TiO2 whereas the surface fluorination of TiO2, which can be similarly compared with the phosphation in its photocatalytic effect, is active only at acidic pH. P-TiO2/Pt showed a higher photocatalytic activity than Pt/TiO2 for the degradation of 2,4-D even at pH 11, under which condition the effect of surface fluorination of Pt/TiO2 completely disappeared. © 2011 Elsevier B.V.

Choi C.,Pohang University of Science and Technology | Kim M.,Pohang University of Science and Technology
International Journal of Heat and Fluid Flow | Year: 2011

Numerous pressure drop correlations for microchannels have been proposed; most of them can be classified as either a homogeneous flow model (HFM) or a separated flow model (SFM). However, the predictions of these correlations have not been compared directly because they were developed in experiments conducted under a range of conditions, including channel shape, the number of channels, channel material and the working fluid. In this study, single rectangular microchannels with different aspect ratios and hydraulic diameters were fabricated in a photosensitive glass. Adiabatic water-liquid and Nitrogen-gas two-phase flow experiments were conducted using liquid superficial velocities of 0.06-1.0. m/s, gas superficial velocities of 0.06-72. m/s and hydraulic diameters of 141, 143, 304, 322 and 490 μm. A pressure drop in microchannels was directly measured through embedded ports. The flow pattern was visualized using a high-speed camera and a long-distance microscope. A two-phase pressure drop in the microchannel was highly related to the flow pattern. Data were used to assess seven different HFM viscosity models and ten SFM correlations, and new correlations based on flow patterns were proposed for both HFMs and SFMs. © 2011 Elsevier Inc.

Ahn S.,Pohang University of Science and Technology | Jung S.Y.,Pohang University of Science and Technology | Seo E.,Pohang University of Science and Technology | Lee S.J.,Pohang University of Science and Technology
Biomaterials | Year: 2011

Time-resolved dynamic imaging of bio-fluids can provide valuable information for clinical diagnosis and treatment of circulatory disorders. Quantitative information on non-transparent blood flows can be directly obtained by particle-tracing dynamic X-ray imaging, which needs better spatial resolution and enhanced image contrast compared to static imaging. For that use, tracer particles tagging along the flow streams are critically required. In this study, taking the advantage of high X-ray absorption, gold nanoparticles (AuNPs) are incorporated into human red blood cells (RBC) to produce contrast-enhanced tracers designed for dynamic X-ray imaging of blood flows. RBCs are advantageous tracers for blood flow measurements since they are natural and primary components of blood. The loading efficiency of AuNPs into RBCs is investigated in terms of the surface properties of the AuNPs. The AuNP-incorporated RBC provides a potential in the dynamic X-ray imaging of blood flows which can be used for clinical applications. © 2011 Elsevier Ltd.

Ramasamy E.,Pohang University of Science and Technology | Jo C.,Pohang University of Science and Technology | Anthonysamy A.,Pohang University of Science and Technology | Jeong I.,Pohang University of Science and Technology | And 2 more authors.
Chemistry of Materials | Year: 2012

Ordered mesoporous titanium nitride-carbon (denoted as OM TiN-C) nanocomposite with high surface area (389 m 2 g -1) and uniform hexagonal mesopores (ca. 5.5 nm) was facilely synthesized via the soft-template method. As a structure-directing agent, Pluronic F127 triblock copolymer formed an ordered structure with inorganic precursors, resol polymer, and prehydrolyzed TiCl 4, followed by a successive heating at 700 °C under nitrogen and ammonia flow. In this study, the amorphous carbon within the parent OM TiO 2-C acted as a rigid support, preventing structural collapse during the conversion process of TiO 2 nanocrystals to TiN nanocrystals. The OM TiN-C was then successfully applied as counter electrode material in dye-sensitized solar cells (DSCs). The organic electrolyte disulfide/thiolate (T 2/T -) was introduced to study the electrocatalytic property of the OM TiN-C nanocomposite. Because of the existence of TiN nanocrystals and the defect sites of the amorphous carbon, the DSCs using OM TiN-C as a counter electrode showed 6.71% energy conversion efficiency (platinum counter electrode DSCs: 3.32%) in the organic electrolyte system (T 2/T -). Furthermore, the OM TiN-C counter electrode based DSCs showed an energy conversion efficiency of 8.41%, whereas the DSCs using platinum as a counter electrode showed a conversion efficiency of only 8.0% in an iodide electrolyte system. The superior performance of OM TiN-C counter electrode resulted from the low charge transfer resistance, enhanced electrical conductivity, and abundance of active sites of the OM TiN-C nanocomposite. Moreover, OM TiN-C counter electrode showed better chemical stability in organic electrolyte compared with the platinum counter electrode. © 2012 American Chemical Society.

Lee I.J.,Pohang University of Science and Technology | Kim B.H.,Pohang University of Science and Technology
Chemical Communications | Year: 2012

Pairs of pyrene-modified deoxyadenosine ( PyA) units induce a stable interstrand i-motif structure, which can be characterized by a change in the fluorescence λ max, with an exciplex emission that is not observable in its single-strand structure. © 2012 The Royal Society of Chemistry.

Kwon W.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Chemical Communications | Year: 2012

Highly luminescent graphitic carbon quantum dots (GQDs) are synthesized employing reverse micelles as nanoreactors. This method offers size tunability and narrow size distribution without any unpractical size separation process. Also, high quantum yields of maximum 35% at the 360 nm excitation wavelength are achieved. © 2012 The Royal Society of Chemistry.

Jeong Y.K.,Pohang University of Science and Technology | Lee J.-H.,Pohang University of Science and Technology | Ahn S.-J.,Pohang University of Science and Technology | Jang H.M.,Pohang University of Science and Technology
Chemistry of Materials | Year: 2012

An epitaxially constrained LuFeO 3 (LFO) thin film was fabricated by adopting a suitable hexagonal template. Optimized unit-cell crystal structure of the hexagonally constrained h-LFO layer was obtained by carrying out first-principles density-functional theory (DFT) calculations using the Vienna ab initio simulation package (VASP). XRD data indicated that the h-LFO layer grown on the YSZ is also epitaxial with a 6-fold hexagonal symmetry. DFT calculations indicate that Fe 3+ spins in the ground-state form a so-called 120° triangular spin structure on the a-b plane. This configuration is largely consistent with a typical spin structure for triangular AFM spins in frustrated magnetic systems such as hexagonal manganites with triangular sublattice. An epitaxially constrained LuFeO 3 thin film fabricated by adopting a hexagonal template exhibits room temperature ferroelectricity with the remanent polarization of 6.5 μC/cm2 at 300 K.

Ahn H.S.,Pohang University of Science and Technology | Jo H.J.,Pohang University of Science and Technology | Kang S.H.,Pohang University of Science and Technology | Kim M.H.,Pohang University of Science and Technology
Applied Physics Letters | Year: 2011

It is well known that nanoparticles deposited on a heating surface during nanofluid boiling can change the characteristics of the heating surface and increase the critical heat flux (CHF) dramatically. We considered a new approach to investigate the nanoparticle surface effect on CHF enhancement using surfaces modified with artificial micro/nanostructures similar to deposited nanoparticle structures. We examined the effect of the surface wettability and liquid spreading ability on the CHF. The results demonstrated that the CHF enhancement on the modified surfaces was a consequence of both the improved surface wettability and the liquid spreading ability of the artificial micro/nanostructures. © 2011 American Institute of Physics.

Reunchan P.,Asia Pacific Center for Theoretical Physics | Jhi S.-H.,Pohang University of Science and Technology
Applied Physics Letters | Year: 2011

First-principles calculations are carried out to study the role of various metal atoms on porous graphene for molecular hydrogen (H2) adsorption. The binding sites of each metal atom on porous graphene are investigated and the binding energies are determined. It is shown that H 2 exhibits different adsorption characteristics onto alkaline, alkaline-earth, or transition metals in porous graphene. In particular, Ca-decorated porous graphene is investigated and found to be feasible for high-capacity hydrogen storage. Our results provide a general picture on the interactions of H2 with porous graphene decorated with various metals. © 2011 American Institute of Physics.

Lee J.,Carnegie Mellon University | You D.,Carnegie Mellon University | You D.,Pohang University of Science and Technology
Journal of Computational Physics | Year: 2013

A fully-implicit ghost-cell immersed boundary method for simulations of flow over complex moving bodies on a Cartesian grid is presented. The present immersed boundary method is highly capable of controlling the generation of spurious force oscillations on the surface of a moving body, thereby producing an accurate and stable solution. Spurious force oscillations on the surface of an immersed moving body are reduced by alleviating spatial and temporal discontinuities in the pressure and velocity fields across non-grid conforming immersed boundaries. A sharp-interface ghost-cell immersed-boundary method is coupled with a mass source and sink algorithm to improve the conservation of mass across non-grid conforming immersed boundaries. To facilitate the control for the temporal discontinuity in the flow field due to a motion of an immersed body, a fully-implicit time-integration scheme is employed. A novel backward time-integration scheme is developed to effectively treat multiple layers of fresh cells generated by a motion of an immersed body at a high CFL number condition. The present backward time-integration scheme allows to impose more accurate and stable velocity vectors on fresh cells than those interpolated. The effectiveness of the present fully-implicit ghost-cell immersed boundary method coupled with a mass source and sink algorithm for reducing spurious force oscillations during simulations of moving body problems is demonstrated in a number of test cases.©2012 Elsevier Inc.

Kittlaus E.A.,Yale University | Shin H.,Yale University | Shin H.,Pohang University of Science and Technology | Rakich P.T.,Yale University
Nature Photonics | Year: 2016

Both Kerr and Raman nonlinearities are radically enhanced by tight optical-mode confinement in nanoscale silicon waveguides. Counterintuitively, Brillouin nonlinearities - originating from coupling between photons and acoustic phonons - are exceedingly weak in these same nonlinear waveguides. Strong Brillouin interactions have only recently been realized in a new class of optomechanical structures that control the interaction between guided photons and phonons. Despite these major advances, appreciable Brillouin-based optical amplification has yet to be observed in silicon. Using a membrane-suspended waveguide, we report large Brillouin amplification in silicon for the first time, reaching levels greater than 5dB for modest pump powers, and demonstrate a record low (5mW) threshold for net amplification. This work represents an important step towards the realization of high-performance Brillouin lasers and amplifiers in silicon. © 2016 Macmillan Publishers Limited. All rights reserved.

Choi K.-Y.,Asia Pacific Center for Theoretical Physics | Choi K.-Y.,Pohang University of Science and Technology | Huang Q.-G.,CAS Institute of Theoretical Physics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We study the standard model Higgs field as a source for the primordial curvature perturbation, particularly in the curvaton and modulated reheating scenario. We conclude that the Higgs cannot play as a curvaton due to the small energy density when it decays, however the modulated reheating by Higgs can be a viable scenario. In the latter case, the non-Gaussianity is inevitably generated and strongly constrains the type of potential of inflaton field and the Higgs-dependent interaction term. For the quadratic potential of the inflaton field with decay rate which nonlinearly depends on the Higgs vacuum expectation value, the contribution of the Higgs field to the primordial curvature perturbation must be less than 8%. © 2013 American Physical Society.

Weon B.M.,Pohang University of Science and Technology | Je J.H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2012

During bubble or droplet coalescence, there is a puzzling tendency for the coalesced bubble or droplet to be preferentially placed closer to the larger of its two parents. We confirm that this preference is a function of parent size ratio by directly visualizing coalescing air bubbles on an oil-water interface and coalescing water droplets immersed in oil. We find that the final position of the coalesced sphere is controlled by surface energy release and is related to the parent size ratio by a power-law relationship. © 2012 American Physical Society.

Kim K.-W.,Pohang University of Science and Technology | Moon J.-H.,Korea University | Lee K.-J.,Korea University | Lee H.-W.,Pohang University of Science and Technology
Physical Review Letters | Year: 2012

Magnetization dynamics in a ferromagnet can induce a spin-dependent electric field through a spin motive force. Spin current generated by the spin-dependent electric field can in turn modify the magnetization dynamics through spin-transfer torque. While this feedback effect is usually weak and thus ignored, we predict that in Rashba spin-orbit coupling systems with a large Rashba parameter α R, the coupling generates the spin-dependent electric field [±(α Rm e/e)(z×-m/-t)], which can be large enough to modify the magnetization dynamics significantly. This effect should be relevant for device applications based on ultrathin magnetic layers with strong Rashba spin-orbit coupling. © 2012 American Physical Society.

Xu X.-Q.,Sungkyunkwan University | Han J.H.,Sungkyunkwan University | Han J.H.,Pohang University of Science and Technology
Physical Review Letters | Year: 2012

Hydrodynamic theory of the spinor BEC condensate with Rashba spin-orbit coupling is presented. A close mathematical analogy of the Rashba-Bose-Einstein condensate model to the recently developed theory of chiral magnetism is found. Hydrodynamic equations for mass density, superfluid velocity, and the local magnetization are derived. The mass current is shown to contain an extra term proportional to the magnetization direction, as a result of the Rashba coupling. Elementary excitations around the two known ground states of the Rashba-Bose-Einstein condensate Hamiltonian, the plane-wave, and the stripe states, are worked out in the hydrodynamic framework, highlighting the cross coupling of spin and superflow velocity excitations due to the Rashba term. © 2012 American Physical Society.

Han J.-H.,Pohang University of Science and Technology | Lee I.-B.,Pohang University of Science and Technology
Applied Energy | Year: 2011

In a power-generation system, power plants as major CO2 sources may be widely separated, so they must be connected into a comprehensive network to manage both electricity and CO2 simultaneously and efficiently. In this study, a scalable infrastructure model is developed for planning electricity generation and CO2 mitigation (EGCM) strategies under the mandated reduction of GHG emission. The EGCM infrastructure model is applied to case studies of Korean energy and CO2 scenarios in 2020; these cases consider combinations of prices of carbon credit and total electricity demand fulfilled by combustion power plants. The results highlight the importance of systematic planning for a scalable infrastructure by examining the sensitivity of the EGCM infrastructure. The results will be useful both to help decision makers establish a power-generation plan, and to identify appropriate strategies to respond to climate change. © 2011 Elsevier Ltd.

Kim Y.-D.,Pohang University of Science and Technology | Choi S.,Pohang University of Science and Technology
Journal of Machine Learning Research | Year: 2014

Bayesian matrix factorization (BMF) is a popular method for collaborative prediction, because of its robustness to overfitting as well as of being free from cross-validation for fine tuning of regularization parameters. In practice, however, due to its cubic time complexity with respect to the rank of factor matrices, existing variational inference algorithms for BMF are not well suited to web-scale datasets where billions of ratings provided by millions of users are available. The time complexity even increases when the side information, such as user binary implicit feedback or item content information, is incorporated into variational Bayesian matrix factorization (VBMF). For instance, a state of the arts in VBMF with side information, is to place Gaussian priors on user and item factor matrices, where mean of each prior is regressed on the corresponding side information. Since this approach introduces additional cubic time complexity with respect to the size of feature vectors, the use of rich side information in a form of high-dimensional feature vector is prohibited. In this paper, we present a scalable inference for VBMF with side information, the complexity of which is linear in the rank K of factor matrices. Moreover, the algorithm can be easily parallelized on multi-core systems. Experiments on large-scale datasets demonstrate the useful behavior of our algorithm such as scalability, fast learning, and prediction accuracy.

Kim W.,Pohang University of Science and Technology | Tachikawa T.,Osaka University | Monllor-Satoca D.,Pohang University of Science and Technology | Kim H.-I.,Pohang University of Science and Technology | And 2 more authors.
Energy and Environmental Science | Year: 2013

Tungsten trioxide (WO3) is being investigated as one of the most promising materials for water oxidation using solar light. Its inherent surface-related drawbacks (e.g., fast charge recombination caused by surface defect sites, the formation of surface peroxo-species, etc.) are nowadays being progressively overcome by different methods, such as surface passivation and the deposition of co-catalysts. Among them, the role of surface passivation is still poorly understood. Herein, transparent WO3 (electrodeposited) and Al2O3/WO3 (prepared by atomic layer deposition, ALD) thin film electrodes were employed to investigate the role of an alumina overlayer by using both photoelectrochemical and laser flash photolysis measurements. Films with a 5 nm-alumina overlayer (30 ALD cycles) showed an optimum photoelectrochemical performance, portraying a 3-fold photocurrent and Faradaic efficiency enhancement under voltage biases. Moreover, IPCE measurements revealed that alumina effect was only significant with an applied potential ca. 1 V (vs. Ag/AgCl), matching the thermodynamic potential for water oxidation at pH 1 (0.97 V vs. Ag/AgCl). According to the investigation of electron accumulation through optical absorption measurements, the alumina overlayer dominantly decreased the number of electron trapping sites on the WO3 surface, eventually facilitating photoelectron transfer to the external circuit in the presence of a positive bias. In addition, the laser flash photolysis measurements of WO3 and Al2O 3/WO3 thin films clearly showed that the electron trapping decreased in the presence of the alumina overlayer whereas the hole trapping relatively increased with alumina, facilitating water photooxidation and rendering a more sluggish recombination process. These results provide a physical insight into the passivation process that could be used as a guideline for further development of efficient photoanodes in artificial photosynthesis. © 2013 The Royal Society of Chemistry.

Kwon W.,Pohang University of Science and Technology | Kim J.-M.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Journal of Materials Chemistry A | Year: 2013

Carbonaceous materials have received much attention as alternative catalysts for platinum in dye-sensitized solar cells. Recently, various forms of carbon materials have been intensely investigated due to their low cost, excellent electrochemical stability, reasonable catalytic activity, and process adaptability. In this review, we introduce the current research issues in carbon counter electrodes including the equivalent circuit analysis, the electrochemical properties, the photovoltaic performances, and the research outlook. In this regard, the electrochemical properties of selected carbon materials are compared with each other by means of the impedance spectroscopy and equivalent circuit analysis. Also, fabrication methods and related photovoltaic performance are discussed. This knowledge will offer valuable insight to inspire investigation of carbon materials and encourage a multitude of applications ranging from all-carbon electrodes to flexible devices. © 2013 The Royal Society of Chemistry.

Mayavan S.,Korea Advanced Institute of Science and Technology | Jang H.-S.,Korea Advanced Institute of Science and Technology | Lee M.-J.,Korea Advanced Institute of Science and Technology | Choi S.H.,Pohang University of Science and Technology | Choi S.-M.,Korea Advanced Institute of Science and Technology
Journal of Materials Chemistry A | Year: 2013

Pt NPs were in situ synthesised on poly(sodium styrene sulfonate) functionalized graphene supports (PSS-G) in aqueous solution. We investigate the reduction of graphene oxide, PSS adsorption on reduced graphene, and Pt NP functionalization by X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure studies (XAFS), Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy. The as-prepared Pt on PSS-G sample (Pt-PSS-G) was used directly as a catalyst ink without further treatment. The use of PSS as a stabilizer prevents stacking of reduced graphene sheets, binds Pt NPs, and promotes mass transport of reaction species. The as-prepared Pt-PSS-G exhibits higher activity and stability for methanol oxidation reaction than Pt NPs supported on pristine graphene sheets (Pt-G). The higher activity is due to the presence of Pt NPs on the surface of the PSS-G support, which provides an integrated electron and mass transport pathway for every Pt NP. This work realizes both scalable and greener production of highly efficient catalysts, and would be valuable for practical applications of graphene based fuel cell catalysts. © 2013 The Royal Society of Chemistry.

Gunjakar J.L.,Ewha Womans University | Kim I.Y.,Ewha Womans University | Lee J.M.,Ewha Womans University | Lee N.-S.,Pohang University of Science and Technology | Hwang S.-J.,Ewha Womans University
Energy and Environmental Science | Year: 2013

Highly efficient photocatalysts for visible light-induced O2 generation are synthesized via an electrostatically derived self-assembly of Zn-Cr-LDH 2D nanoplates with graphene 2D nanosheets. In the obtained nanohybrids, the positively charged Zn-Cr-LDH nanoplates are immobilized on the surface of negatively charged graphene nanosheets with the formation of a highly porous stacked structure. A strong electronic coupling of the subnanometer-thick Zn-Cr-LDH nanoplates with reduced graphene oxide (RGO)/graphene oxide (GO) nanosheets gives rise not only to the prominent increase of visible light absorption but also to a remarkable depression of the photoluminescence signal. The self-assembled Zn-Cr-LDH-RGO nanohybrids display an unusually high photocatalytic activity for visible light-induced O 2 generation with a rate of ∼1.20 mmol h-1 g -1, which is far superior to that of the pristine Zn-Cr-LDH material (∼0.67 mmol h-1 g-1). The fact that pristine Zn-Cr-LDH is one of the most effective visible light photocatalysts for O2 production with unusually high quantum efficiency of 61% at λ = 410 nm highlights the excellent functionality of the Zn-Cr-LDH-RGO nanohybrids as visible light active photocatalysts. The Zn-Cr-LDH-RGO nanohybrid shows a higher photocatalytic activity than the Zn-Cr-LDH-GO nanohybrid, providing strong evidence for the superior advantage of the hybridization with RGO. The present findings clearly demonstrate that graphene nanosheets can be used as an effective platform for improving the photocatalytic activity of 2D nanostructured inorganic solids. © 2013 The Royal Society of Chemistry.

Jung G.H.,Pohang University of Science and Technology | Lee J.-L.,Pohang University of Science and Technology
Journal of Materials Chemistry A | Year: 2013

The mechanism of the formation of gap states in bathocuproine (BCP) used as an electron transport layer (ETL) in poly(3-hexylthiophene) and the fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) based solar cell is investigated. The photo-generated electrons in the active layer are extracted to a cathode electrode through the gap states of BCP located near the lowest unoccupied molecular orbital (LUMO) level of the electron acceptors. In the most extensively used BCP, the origin of the formation of gap states is not well understood. We show that gap states in BCP are induced by out-diffusion of P3HT molecules into the BCP layer. A Secondary Ion Mass Spectrometry (SIMS) depth profile reveals that the out-diffusion of P3HT occurs during the deposition of BCP on the P3HT:PCBM active layer. The mixing of P3HT molecules with the BCP layer induces a chemical reaction between BCP and P3HT to produce the N-S bonds (between the nitrogen from BCP and the sulfur from P3HT), which acts as gap states in BCP. The power conversion efficiency increased from 0.03% to 3.0% as the deposition rate is reduced from 5.0 Å s-1 to 0.1 Å s-1. This increase originates from the change in the diffusion length of P3HT with the deposition rate of BCP, leading to an increase in conductivity as well as the alignment of the Fermi level through the gap states. © 2013 The Royal Society of Chemistry.

Dong W.J.,Pohang University of Science and Technology | Jung G.H.,Pohang University of Science and Technology | Lee J.-L.,Pohang University of Science and Technology
Solar Energy Materials and Solar Cells | Year: 2013

Effects of oxygen plasma (O2 plasma) treatment of ITO on the characteristics of solution-processed molybdenum oxide (MoO3) hole extraction layer in bulk hetero-junction organic photovoltaics (OPVs) are studied. The chemical composition of O2 plasma-treated ITO was determined using monochromatic X-ray photoelectron spectroscopy (XPS). The valence band energies were investigated by ultraviolet photoemission spectroscopy (UPS) measurements. XPS and UPS measurements reveal that O 2 plasma treatment of bare ITO film was found to incorporate the polar surface species such as (O2)2-, resulting in an increase of both workfunction from 4.62 to 5.05 eV and polar surface energy from 27 to 38 mN/m. The high work function results in efficient hole transport at the ITO/MoO3 interfaces. The highly polar surface is readily available for uniform coating of MoO3 on ITO. Electrical conductivity of oxidized ITO changes four orders of magnitude from 2.4×10-2 to 4.08×102 S/cm, depending on O2 plasma pressure conditions. Thus, the ITO/MoO3 interface dominates the series resistance of OPVs fabricated with poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The presence of (O2)2- states in the ITO/MoO3 interface in OPVs is suggested to play a significant role in controlling the device lifetime as well as the efficiency of OPV. © 2013 Elsevier B.V.

Seo H.S.,Pohang University of Science and Technology | Yun D.W.,Pohang University of Science and Technology | Kim K.Y.,Pohang University of Science and Technology
International Journal of Hydrogen Energy | Year: 2013

The effect of Nb on the oxidation kinetics, electrical conductivity and Cr evaporation behavior of FSS has been discussed depending on the Nb content and oxygen active element such as Ti and Si. Nb in ferritic stainless steel is saturated during heat treatment as NbO2 at the outermost oxide scale and as both Nb2O5 and Laves phase near the oxide scale/alloy interface. Excess Nb (>4.7 wt%) suppresses precipitation of Nb2O5, because of rapid Laves phase growth. Nb enhances selective Ti oxidation, whereas Ti retards Nb2O5 precipitation near the scale/alloy interface. On the other hand, Si suppresses Nb enrichment near the scale/alloy interface and it reduces the precipitation of both Nb2O5 and Laves phase. Nb also suppresses Si enrichment and the formation of continuous Si oxide at the scale/alloy interface. Co-addition of Nb and Ti is effective to decrease the electrical resistance and Cr evaporation rate of oxide scale. Copyright © 2012, Hydrogen Energy Publications, LLC.

Yun D.-J.,Samsung | Ra H.,Pohang University of Science and Technology | Rhee S.-W.,Pohang University of Science and Technology
Renewable Energy | Year: 2013

Conductive and catalytic multiwalled carbon nanotube (MWCNT)/poly(3,4-ethylenedioxythiophene) polymerized with poly(4-styrenesulfonate) (PEDOT:PSS) composite films were prepared by dispersing MWCNT in the PEDOT:PSS aqueous solution using ultrasonication without any oxidation process. The effect of the MWCNT loading in the PEDOT:PSS solution was studied in terms of the conductivity and catalytic activity. The MWCNT/PEDOT:PSS composite films showed much lower sheet resistance in the range of 730-2150 Ω and higher surface roughness in the range of 25-110 nm than the pure PEDOT:PSS film (980 kΩ and 0.82 nm) depending on the MWCNT content in the composite. The high conductivity and roughness of MWCNT/PEDOT:PSS films enhanced its performance as a catalytic counter electrode in the dye-sensitized solar cell (DSSC). The interface resistance between the MWCNT/PEDOT:PSS film and the electrolyte was evaluated with electrochemical impedance spectroscopy (EIS) and it was confirmed that as the content of MWCNT in PEDOT:PSS solution was increased, the resistance was decreased. The cell efficiency of DSSC with MWCNT/PEDOT:PSS counter electrode was highly improved as the MWCNT concentration in MWCNT/PEDOT:PSS film increases and especially, the DSSC with FTO-free spray coated MWCNT/PEDOT:PSS counter electrode showed excellent sell performance (Jsc: 10.9 mA/cm 2, F.F.: 60.6% and n: 5.4%), which is comparable with that with platinum on FTO counter electrode system (Jsc: 13.9 mA/cm 2, F.F.: 62.8% and n: 7.1%). © 2012 Elsevier Ltd.

Sahni V.,Inter-University Center for Astronomy and Astrophysics | Shafieloo A.,Asia Pacific Center for Theoretical Physics | Shafieloo A.,Pohang University of Science and Technology | Starobinsky A.A.,Kazan Federal University
Astrophysical Journal Letters | Year: 2014

Baryon acoustic oscillations (BAOs) allow us to determine the expansion history of the universe, thereby shedding light on the nature of dark energy. Recent observations of BAOs in the Sloan Digital Sky Survey (SDSS) DR9 and DR11 have provided us with statistically independent measurements of H(z) at redshifts of 0.57 and 2.34, respectively. We show that these measurements can be used to test the cosmological constant hypothesis in a model-independent manner by means of an improved version of the Om diagnostic. Our results indicate that the SDSS DR11 measurement of H(z) = 222 ± 7 km s-1Mpc-1at z = 2.34, when taken in tandem with measurements of H(z) at lower redshifts, imply considerable tension with the standard ΛCDM model. Our estimation of the new diagnostic Omh 2from SDSS DR9 and DR11 data, namely, Omh 20.122 ± 0.01, which is equivalent to Ω0m h 2for the spatially flat ΛCDM model, is in tension with the value Ω0m h 2= 0.1426 ± 0.0025 determined for ΛCDM from Planck+WP. This tension is alleviated in models in which the cosmological constant was dynamically screened (compensated) in the past. Such evolving dark energy models display a pole in the effective equation of state of dark energy at high redshifts, which emerges as a smoking gun test for these theories. © 2014. The American Astronomical Society. All rights reserved..

Kim C.,Pohang University of Science and Technology
2016 International Conference on Computing, Networking and Communications, ICNC 2016 | Year: 2016

As full duplex radio communication is introduced, it is expected to double the throughput theoretically over full duplex radio for one-to-one communication. However, in the practical environment, it is hard to find such an application that demands the same amount of traffic in the both directions at the same time. To enhance the throughput as much as possible, we need to arrange medium accesses in a different way, not like the contention-based MAC. In this paper, we propose an efficient MAC protocol to maximize throughput for OFDM-based full duplex radio. Our proposed MAC protocol finds stations with uplink traffic in the hidden relation with the station for downlink traffic and provides them transmission opportunities to balance the amount of traffic in the both direction. Our protocol requires one OFDM subcarrier only. Our protocol can achieve higher throughput as compared with that of legacy half duplex MAC protocol and that of basic full duplex MAC protocol. © 2016 IEEE.

Yoo J.,Pohang University of Science and Technology | Pyo J.,Pohang University of Science and Technology | Je J.H.,Pohang University of Science and Technology
Nanoscale | Year: 2014

We report for the first time single nanowires (NWs) with ambipolar (positive/negative) photoresponse that changes sign depending on the illumination wavelength. The single NWs were grown by the meniscus-guided method using inorganic (ZnO nanoparticles)-organic (PEDOT:PSS) hybrid materials. The ambipolar photoresponse of the single NWs enabled us to develop an unprecedented spectrum-discriminating NW photodetector array. This journal is © the Partner Organisations 2014.

Choi Y.,Pohang University of Science and Technology | Beak M.,Pohang University of Science and Technology | Yong K.,Pohang University of Science and Technology
Nanoscale | Year: 2014

Photoanodes prepared using CuInS2/CdS/ZnO nanowires were fabricated by a solution-based process for constructing a photo-driven hydrogen generation system. For efficient light harvesting and photoexcited charge collection, ZnO nanowire (NW) photoanode arrays were co-sensitized with CdS and CuInS2 (CIS). A CdS layer was deposited on the ZnO NW via successive ion layer adsorption and reaction (SILAR), and the CIS layer was prepared by depositing a molecular precursor solution onto the CdS/ZnO NW. The generated anodic photocurrent was increased with the subsequent deposition of the CIS and CdS layers. Ultraviolet photoelectron spectroscopy analysis revealed cascade type-II band alignments for the CIS/CdS/ZnO NW photoanodes, which enabled efficient electron collection. Our heterostructure photoelectrode has generated a greatly improved photocurrent density of 13.8 mA cm-2 at 0.3 V vs. SCE under 1 sun illumination. This journal is © the Partner Organisations 2014.

Kim G.,Pohang University of Science and Technology | Jo C.,Pohang University of Science and Technology | Kim W.,Pohang University of Science and Technology | Chun J.,Pohang University of Science and Technology | And 3 more authors.
Energy and Environmental Science | Year: 2013

A rapid and relatively large-scale production of ultrathin TiO2 nanodisks was achieved under mild conditions by developing a novel and simple sol-gel process occurring at the interface of an organic solvent and ice. Owing to the ultrathin structure and unusually high surface area (>400 m 2 g-1), the TiO2 nanodisks exhibited high reversible capacity (191.4 mA h g-1 at 0.2 C) and excellent rate performance (58% capacity retention at 20 C) as an anode in lithium ion batteries. © The Royal Society of Chemistry 2013.

Choi M.,Pohang University of Science and Technology | Yong K.,Pohang University of Science and Technology
Nanoscale | Year: 2014

Vertically aligned high-quality single crystalline brookite TiO2 nanoarrays were synthesized for the first time using an environmentally benign one-step hydrothermal reaction. They have a unique bullet-shaped structure which has a length of 700-1000 nm and a width of 150-250 nm with a sharpened tip structure. By adjusting the concentration of NaOH in hydrothermal reaction, we could also synthesize other types of TiO2 nanostructures including anatase TiO2 nanotubes/nanowires. The morphologies and crystal structures of the products were confirmed by scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. Their vertically aligned structures facilitate their application as photoanodes in photoelectrochemical cells, and the photoelectrochemical properties such as photocurrent density and open circuit voltage were measured in a three-electrode electrochemical cell with TiO2 nanoarrays, Ag/AgCl and a Pt flag as the working, reference and counter electrodes, respectively, incorporating a 0.1 M NaOH electrolyte solution. The fabricated brookite TiO2 nanoarrays exhibited a highly enhanced photocurrent density and a longer electron lifetime compared with anatase TiO2 nanoarrays with similar lengths. This journal is © The Royal Society of Chemistry.

Kim T.-W.,Pohang University of Science and Technology | Kim T.-W.,Korea Advanced Institute of Science and Technology | Najjar R.G.,Pennsylvania State University | Lee K.,Pohang University of Science and Technology
Global Biogeochemical Cycles | Year: 2014

Precipitation over the ocean surface in the vicinity of industrialized and populated coastlines can increase the ocean nitrate concentration and consequently enhance ocean primary productivity. Using satellite data and a meteorological reanalysis product, we evaluated the impact of precipitation events on the chlorophyll a concentration in coastal and offshore waters located downwind of the eastern United States. We found that in low-nutrient areas (defined as having nitrate concentrations < 1 μM) precipitation events were associated with increased levels of chlorophyll a (up to approximately 15%), but in high-nutrient areas (nitrate concentrations > 1 μM) they were associated with decreased levels. These contrasting responses of chlorophyll a concentration to precipitation were attributed to the correlation of precipitation with wind speed and to other factors (nutrients and light) limiting phytoplankton growth. Increases in wind speed accompanied by precipitation events typically deepen the mixed layer, which can entrain additional nutrients into the mixed layer but simultaneously reduce light availability. We suggest that in nutrient-depleted areas (south of 36°N) the added nutrients were a dominant factor increasing the chlorophyll a concentration, whereas in the nutrient-replete areas (north of 36°N), where phytoplankton growth was light limited, reduced light availability was the dominant factor determining reduced chlorophyll a concentration. Our results indicate that an increase in wind speed accompanied by precipitation events was a major contributor to the observed changes in chlorophyll a concentration during wet days, whereas the wet deposition of pollutant nitrogen slightly increased the chlorophyll a concentration (< 5%) only in nutrient-depleted areas. Key Points Precipitation considerably enhanced phytoplankton biomass in low-nutrient area Increase in wind speed during wet day was a major factor increasing productivity The atmospheric N deposition was less important in increasing productivity ©2013. American Geophysical Union. All Rights Reserved.

Kim J.,Pohang University of Science and Technology | Lee S.-J.,Pohang University of Science and Technology | De Cooman B.C.,Pohang University of Science and Technology
Scripta Materialia | Year: 2011

The effect of Al on the stacking fault energy (SFE) of Fe-18Mn-0.6C twinning-induced plasticity steel was investigated by means of weak-beam dark-field transmission electron microscopy. The SFE of Fe-18Mn-0.6C steel was measured to be 13 ± 3 mJ m-2 and the actual increase in SFE due to adding 1 wt.% Al was approximately +11.3 mJ m-2. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Lee M.-Y.,Pohang University of Science and Technology | Park S.-J.,Pohang University of Science and Technology | Park K.,Pohang University of Science and Technology | Kim K.S.,Pohang University of Science and Technology | And 2 more authors.
ACS Nano | Year: 2011

Target-specific intracellular delivery of small interfering RNA (siRNA) is regarded as one of the most important technologies for the development of siRNA therapeutics. In this work, a cysteamine modified gold nanoparticles (AuCM)/siRNA/polyethyleneimine (PEI)/hyaluronic acid (HA) complex was successfully developed using a layer-by-layer method for target-specific intracellular delivery of siRNA by HA receptor mediated endocytosis. Atomic force microscopic and zeta potential analyses confirmed the formation of a AuCM/siRNA/PEI/HA complex having a particle size of ca. 37.3 nm and a negative surface charge of ca. -12 mV. With a negligible cytotoxicity, AuCM/siRNA/PEI/HA complex showed an excellent target-specific gene silencing efficiency of ca. 70% in the presence of 50 vol % serum, which was statistically much higher than that of siRNA/Lipofectamine 2000 complex. In the competitive binding tests with free HA, dark-field bioimaging and inductively coupled plasma-atomic emission spectroscopy confirmed the target-specific intracellular delivery of AuCM/siRNA/PEI/HA complex to B16F1 cells with HA receptors. Moreover, the systemic delivery of AuCM/siRNA/PEI/HA complex using apolipoprotein B (ApoB) siRNA as a model drug resulted in a significantly reduced ApoB mRNA level in the liver tissue. Taken together, AuCM/siRNA/PEI/HA complex was thought to be developed as target-specific siRNA therapeutics for the systemic treatment of various liver diseases. © 2011 American Chemical Society.

Bae J.H.,Pohang University of Science and Technology | Prasada Rao A.K.,Sreenidhi Institute of Science and Technology | Kim K.H.,POSCO | Kim N.J.,Pohang University of Science and Technology
Scripta Materialia | Year: 2011

Park S.M.,Pohang University of Science and Technology | Kim D.S.,Pohang University of Science and Technology

A free-standing nanofiber membrane can be simultaneously fabricated, patterned, and integrated with electrolyte-assisted electrospinning (ELES). The fluidic nature of the electrolyte collector enables flexible patterning and facile integration of the free-standing nanofiber membrane on complex substrates from a 2D flat surface to a 3D curved geometry via ELES. The structural integrity and performance of the free-standing nanofiber membrane are verified, and this plays a crucial role for future applications, including organ-on-a-chip, tissue scaffolds, and biosensors. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kwon W.,Pohang University of Science and Technology | Kim Y.-H.,Pohang University of Science and Technology | Lee C.-L.,Gwangju Institute of Science and Technology | Lee M.,Pohang University of Science and Technology | And 5 more authors.
Nano Letters | Year: 2014

Size-controlled graphene quantum dots (GQDs) are prepared via amidative cutting of tattered graphite. The power of this method is that the size of the GQDs could be varied from 2 to over 10 nm by simply regulating the amine concentration. The energy gaps in such GQDs are narrowed down with increasing their size, showing colorful photoluminescence from blue to brown. We also reveal the roles of defect sites in photoluminescence, developing long-wavelength emission and reducing exciton lifetime. To assess the viability of the present method, organic light-emitting diodes employing our GQDs as a dopant are first demonstrated with the thorough studies in their energy levels. This is to our best knowledge the first meaningful report on the electroluminescence of GQDs, successfully rendering white light with the external quantum efficiency of ca. 0.1%. © 2014 American Chemical Society.

Park J.W.,Pohang University of Science and Technology | Seo Y.J.,Chonbuk National University | Kim B.H.,Pohang University of Science and Technology
Chemical Communications | Year: 2013

Systematic modification of the 4A loop region of the Rb gene with PyA fluorophore units allows discrimination of the fluorescence signals corresponding to structural dynamics from single-stranded to i-motif structures. © 2014 The Royal Society of Chemistry.

Han J.M.,Seoul National University | Jeong S.J.,Seoul National University | Park M.C.,Seoul National University | Kim G.,Seoul National University | And 6 more authors.
Cell | Year: 2012

Amino acids are required for activation of the mammalian target of rapamycin (mTOR) kinase, which regulates protein translation, cell size, and autophagy. However, the amino acid sensor that directly couples intracellular amino acid-mediated signaling to mTORC1 is unknown. Here we show that leucyl-tRNA synthetase (LRS) plays a critical role in amino acid-induced mTORC1 activation by sensing intracellular leucine concentration and initiating molecular events leading to mTORC1 activation. Mutation of LRS amino acid residues important for leucine binding renders the mTORC1 pathway insensitive to intracellular levels of amino acids. We show that LRS directly binds to Rag GTPase, the mediator of amino acid signaling to mTORC1, in an amino acid-dependent manner and functions as a GTPase-activating protein (GAP) for Rag GTPase to activate mTORC1. This work demonstrates that LRS is a key mediator for amino acid signaling to mTORC1. © 2012 Elsevier Inc.

Lee S.J.,Pohang University of Science and Technology | Hwang B.G.,Pohang University of Science and Technology | Kim H.K.,Pohang University of Science and Technology
Planta | Year: 2013

Plants have efficient water-transporting vascular networks with a self-recovery function from embolism, which causes fatal discontinuity in sap flow. However, the embolism-refilling process in xylem vessel is still unclear. The water-refilling processes in the individual xylem vessels of excised Arabidopsis roots were visualized in this study using synchrotron X-ray micro-imaging technique with high spatial resolution up to 1 μm per pixel and temporal resolution up to 24 fps. In normal continuous water-refilling process, we could observe various flow patterns affected by the morphological structures of the xylem vessels, especially when water passed through perforation plates. A simple criterion based on the variation in dynamic pressure was suggested to evaluate the contribution of individual perforation plates to the water-refilling process. Meanwhile, the water-refilling embolized sections of xylem vessels through radial pathways were also observed. Separated water columns were formed from this discontinuous water-refilling process and the water influx rates through the radial pathways were estimated to be 478 and 928 μm3 s-1. The dynamic behavior of the separated water columns were quantitatively analyzed from the stoppage of volume growth to the translational phase. These water-refilling processes in excised roots of Arabidopsis may shed light on understanding the water refilling in the embolism vessels of intact plants and the interconnectivity of xylem vessel networks in vascular plants. © 2013 Springer-Verlag Berlin Heidelberg.

Huh S.,Seoul National University | Park J.,Pohang University of Science and Technology | Kim Y.S.,Sungkyunkwan University | Kim K.S.,Pohang University of Science and Technology | And 2 more authors.
ACS Nano | Year: 2011

We fabricated a highly oxidized large-scale graphene platform using chemical vapor deposition (CVD) and UV/ozone-based oxidation methods. This platform offers a large-scale surface-enhanced Raman scattering (SERS) substrate with large chemical enhancement in SERS and reproducible SERS signals over a centimeter-scale graphene surface. After UV-induced ozone generation, ozone molecules were reacted with graphene to produce oxygen-containing groups on graphene and induced the p-type doping of the graphene. These modifications introduced the structural disorder and defects on the graphene surface and resulted in a large chemical mechanism-based signal enhancement from Raman dye molecules [rhodamine B (RhB), rhodamine 6G (R6G), and crystal violet (CV) in this case] on graphene. Importantly, the enhancement factors were increased from ∼10 3 before ozone treatment to ∼10 4, which is the largest chemical enhancement factor ever on graphene, after 5 min ozone treatment due to both high oxidation and p-doping effects on graphene surface. Over a centimeter-scale area of this UV/ozone-oxidized graphene substrate, strong SERS signals were repeatedly and reproducibly detected. In a UV/ozone-based micropattern, UV/ozone-treated areas were highly Raman-active while nontreated areas displayed very weak Raman signals. © 2011 American Chemical Society.

Soare S.C.,Technical University of Cluj Napoca | Barlat F.,Pohang University of Science and Technology
European Journal of Mechanics, A/Solids | Year: 2011

As shown recently in (Soare and Barlat, 2010. Convex polynomial yield functions. J. Mech., Phys. Solids, 58, 1804-1818), the principal values based yield function Yld2004, proposed in (Barlat et al., 2005. Linear transformation based anisotropic yield function. Int. J. Plast., 21, 1009-1039), is polynomial for integer exponents. Based on this observation, a new algorithm is proposed for implementing symmetric yield functions formulated in terms of principal values. The algorithm is tested here by simulating with a commercial FE code the cylindrical deep drawing of two aluminum sheets. It is found that the classical description of the in-plane directional properties of the sheet (uniaxial r-values and yield stresses), even if modeled correctly by the yield function, is not sufficient for a unique characterization of the predicted earing profile. For certain combinations of the directional properties the r-value in biaxial stressing has to be considered for a correct calibration of the material model. This in turn requires a finer detail in yield surface modeling and, to achieve it, an ad-hoc extension of Yld2004 is constructed. In combination with the proposed implementation algorithm, the extension is shown to be a useful research tool, having some interesting modeling capabilities and satisfactory FE runtime. © 2011 Elsevier Masson SAS. All rights reserved.

Yun H.-J.,Gyeongsang National University | Choi H.H.,Pohang University of Science and Technology | Kwon S.-K.,Gyeongsang National University | Kim Y.-H.,Gyeongsang National University | Cho K.,Pohang University of Science and Technology
Chemistry of Materials | Year: 2014

A new donor-acceptor organic semiconducting co-polymer (PDPP-TAT) containing acetylene linkages based on dithienyl-diketopyrrolopyrrole (tDPP) has been synthesized and compared with a tDPP-based co-polymer (PDPP-TVT) containing vinylene linkages. The sp-hybridized carbons in the acetylene linkages result in favorable overlap of the electron wave functions of the tDPP units along the main chain. Further, the π-conjugation of PDPP-TAT was found to be highly insensitive to the chain conformation, in contrast to that of PDPP-TVT. As a result, PDPP-TAT provides favorable charge transport for electrons as well as holes, and enables facile charge transport in amorphous and tie-molecular regions connecting its crystalline domains. PDPP-TAT exhibits ambipolar characteristics with a high electron/hole mobility ratio (μe/μh) of ∼0.3 in field-effect transistors, whereas PDPP-TVT exhibits unipolar characteristics with a μe/ μh value that is a factor of 30 lower. Our results demonstrate that the conformation sensitivity of charge transport is a vital factor in the electrical performances of actual organic transistor devices. © 2014 American Chemical Society.

Jo C.,Pohang University of Science and Techno