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.
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.
Yoon M.,Pohang University of Science and Technology |
Srirambalaji R.,Pohang University of Science and Technology |
Kim K.,Pohang University of Science and Technology
Chemical Reviews | Year: 2012
An overview of on metal-organic frameworks (MOF)-based asymmetric heterogeneous catalysis is discussed by presenting the state of the art in this field and discussing its potential and limitations. Tanaka et al. reported the synthesis of a copper-BINOL-based chiral framework and its catalytic activity in the asymmetric ring-opening reaction of epoxides with amines. Lin group successfully synthesized chiral porous coordination polymers for asymmetric catalysis using both MPD and MPP strategies, they were amorphous, which made the detailed structural characterization of the active sites impossible. Later the group succeeded in synthesizing a crystalline homochiral MOF using a pyridyl functionalized BINOL linker. Jeong and co-workers also utilized a similar dual functional enantiopure linker in which the two phenyl rings are orthogonal to each other to avoid the steric repulsion between the hydroxy and methyl groups, to synthesize a homochiral MOF containing chiral dihydroxy groups on the linkers of the framework.
Kim W.Y.,Pohang University of Science and Technology |
Kim K.S.,Pohang University of Science and Technology
Accounts of Chemical Research | Year: 2010
(Figure Presented) With the advance of nanotechnology, a variety of molecules, from single atoms to large-scale structures such as graphene or carbon nanotubes, have been investigated for possible use as molecular devices. Molecular orbitals (MOs) are a key ingredient in determining the transport properties of molecules, because they contain all the quantum mechanical information of molecular electronic structures and offer spatial conduction channels for electron transport. Therefore, the delicate modulation of the MOs enables us to tune the performance of electron transport through the molecule. Electric and magnetic fields are powerful and readily accessible means for that purpose. In this Account, we describe the effects of external fields on molecular electronic and spintronic devices. Quantum transport through a molecule that connects source and drain electrodes depends strongly on the alignment of molecular energy levels with respect to the chemical potentials at both electrodes. This dependence results from the energy levels being exploited in resonant tunneling processes when the molecule is weakly coupled to the electrodes in the molecular junction. Molecular energy levels can be shifted by the Stark effect of an external electric field. For a molecule with no permanent dipóle moment, the polarizability is the primary factor determining the energy shift of each MO, according to the second-order Stark effect; more polarizable MOs undergo a larger energy shift Interestingly, even a small shift may lead to a completely nontrivial result. For example, we show a magnetic on-off switching phenomenon of a molecule controlled by an electric field. If a molecule has a nonmagnetic ground state but a highly polarizable magnetic excited state with an energy slightly above the ground state, the magnetic exdted state can have lower energy than the ground state under a sufficiently strong electric field. A magnetic field is normally used to control spin orientation in a ferromagnetic system. Here we show that the magnetic field can also be used to control MOs. A graphene nanoribbon with zig-zag-shaped edges (ZGNR) has a ferromagnetic spin ordering along the edges, and the spin states have unique orbital symmetries. Both spin polarizations and orbital symmetries can simultaneously be controlled by means of an external magnetic field. The ZGNR spin-valve devices incorporating this effect are predicted to show an extreme enhancement (compared with conventional devices) of magnetoresistance due to the double spin-filtering process. In such a system, spins are filtered not only by spin matching-mismatching between both electrodes as in normal spin-valve devices, but also by the orbital symmetry matching-mismatching. Thus, a new type of magnetoresistance, and with extremely large values, so-called super-magnetoresistance (distinct from the conventional tunneling or giant magnetoresistance), is available with this method. MOs are at the heart of understanding and tuning transport properties in molecular systems. Therefore, investigating the effects of external fields on MOs is important not only for understanding fundamental quantum phenomena in molecular devices but also for practical applications in the development of interactive devices. © 2010 American Chemical Society.
Bhadeshia H.K.D.H.,University of Cambridge |
Bhadeshia H.K.D.H.,Pohang University of Science and Technology
Progress in Materials Science | Year: 2012
A casual metallurgist might be forgiven in believing that there are but a few basic types of steels used in the manufacture of some of the most technologically important engineering components, the rolling bearings. First the famous 1C-1.5Cr steel from which the majority of bearings are made. Its structure is apparently well-understood and the focus is on purity in order to avoid inclusions which initiate fatigue during rolling contact. Then there is the M50 steel and its variants, from which bearings which serve at slightly higher temperatures in aeroengines are manufactured, based on secondary-hardened martensite. The casual metallurgist would be wrong; there is a richness in the subject which inspires deep study. There are phenomena which are little understood, apparently incommensurate observations, some significant developments and other areas where convincing conclusions are difficult to reach. The subject seemed ready for a critical assessment; hence, this review. The structure and properties of bearing steels prior to the point of service are first assessed and described in the context of steelmaking, manufacturing and engineering requirements. This is followed by a thorough critique of the damage mechanisms that operate during service and in accelerated tests. © 2011 Elsevier Ltd. All rights reserved.
Marti-Rujas J.,Italian Institute of Technology |
Kawano M.,Pohang University of Science and Technology
Accounts of Chemical Research | Year: 2013
Porous coordination networks are materials that maintain their crystal structure as molecular "guests" enter and exit their pores. They are of great research interest with applications in areas such as catalysis, gas adsorption, proton conductivity, and drug release. As with zeolite preparation, the kinetic states in coordination network preparation play a crucial role in determining the final products. Controlling the kinetic state during self-assembly of coordination networks is a fundamental aspect of developing further functionalization of this class of materials. However, unlike for zeolites, there are few structural studies reporting the kinetic products made during self-assembly of coordination networks. Synthetic routes that produce the necessary selectivity are complex.The structural knowledge obtained from X-ray crystallography has been crucial for developing rational strategies for design of organic-inorganic hybrid networks. However, despite the explosive progress in the solid-state study of coordination networks during the last 15 years, researchers still do not understand many chemical reaction processes because of the difficulties in growing single crystals suitable for X-ray diffraction: Fast precipitation can lead to kinetic (metastable) products, but in microcrystalline form, unsuitable for single crystal X-ray analysis. X-ray powder diffraction (XRPD) routinely is used to check phase purity, crystallinity, and to monitor the stability of frameworks upon guest removal/inclusion under various conditions, but rarely is used for structure elucidation. Recent advances in structure determination of microcrystalline solids from ab initio XRPD have allowed three-dimensional structure determination when single crystals are not available. Thus, ab initio XRPD structure determination is becoming a powerful method for structure determination of microcrystalline solids, including porous coordination networks. Because of the great interest across scientific disciplines in coordination networks, especially porous coordination networks, the ability to determine crystal structures when the crystals are not suitable for single crystal X-ray analysis is of paramount importance.In this Account, we report the potential of kinetic control to synthesize new coordination networks and we describe ab initio XRPD structure determination to characterize these networks' crystal structures. We describe our recent work on selective instant synthesis to yield kinetically controlled porous coordination networks. We demonstrate that instant synthesis can selectively produce metastable networks that are not possible to synthesize by conventional solution chemistry. Using kinetic products, we provide mechanistic insights into thermally induced (573-723 K) (i.e., annealing method) structural transformations in porous coordination networks as well as examples of guest exchange/inclusion reactions. Finally, we describe a memory effect that allows the transfer of structural information from kinetic precursor structures to thermally stable structures through amorphous intermediate phases.We believe that ab initio XRPD structure determination will soon be used to investigate chemical processes that lead intrinsically to microcrystalline solids, which up to now have not been fully understood due to the unavailability of single crystals. For example, only recently have researchers used single-crystal X-ray diffraction to elucidate crystal-to-crystal chemical reactions taking place in the crystalline scaffold of coordination networks. The potential of ab initio X-ray powder diffraction analysis goes beyond single-crystal-to-single-crystal processes, potentially allowing members of this field to study intriguing in situ reactions, such as reactions within pores. © 2012 American Chemical Society.
Ree M.,Pohang University of Science and Technology
Macromolecular Rapid Communications | Year: 2014
For advanced functional polymers such as biopolymers, biomimic polymers, brush polymers, star polymers, dendritic polymers, and block copolymers, information about their surface structures, morphologies, and atomic structures is essential for understanding their properties and investigating their potential applications. Grazing incidence X-ray scattering (GIXS) is established for the last 15 years as the most powerful, versatile, and nondestructive tool for determining these structural details when performed with the aid of an advanced third-generation synchrotron radiation source with high flux, high energy resolution, energy tunability, and small beam size. One particular merit of this technique is that GIXS data can be obtained facilely for material specimens of any size, type, or shape. However, GIXS data analysis requires an understanding of GIXS theory and of refraction and reflection effects, and for any given material specimen, the best methods for extracting the form factor and the structure factor from the data need to be established. GIXS theory is reviewed here from the perspective of practical GIXS measurements and quantitative data analysis. In addition, schemes are discussed for the detailed analysis of GIXS data for the various self-assembled nanostructures of functional homopolymers, brush, star, and dendritic polymers, and block copolymers. Moreover, enhancements to the GIXS technique are discussed that can significantly improve its structure analysis by using the new synchrotron radiation sources such as third-generation X-ray sources with picosecond pulses and partial coherence and fourth-generation X-ray laser sources with femtosecond pulses and full coherence. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Posco, Pohang University of Science and Technology | Date: 2014-03-17
The present patent application describes a cantilever for atomic force microscopy (AFM), which includes a cantilever body having a fixed end and a free end, the free end having a surface region being chemically modified by a dendron in which a plurality of termini of the branched region of the dendron are bound to the surface, and a terminus of the linear region of the dendron is functionalized.
Pohang University of Science and Technology | Date: 2015-01-27
The display driving circuit including a type detector for receiving a data packet including a 2-bit embedded signal, in which a clock signal embedded in a data signal, and outputting a first reference clock or a second reference clock different from the first reference clock according to a type of the data packet, a window generator for receiving multi-phase clocks and providing to the type detector a first window reference and a second window reference different from the first window reference to be used in determining the type of the data packet, a buffer for delaying the first reference clock by a first interval and delaying the second reference clock by a second interval different from the first interval, and a multiplexer for multiplexing the delayed first and second reference clocks and outputting a multiplexed reference clock may be provided.
Kim S.Y.,Pohang University of Science and Technology
Nature communications | Year: 2010
Proton exchange fuel cells (PEFCs) have the potential to provide power for a variety of applications ranging from electronic devices to transportation vehicles. A major challenge towards economically viable PEFCs is finding an electrolyte that is both durable and easily passes protons. In this article, we study novel anhydrous proton-conducting membranes, formed by incorporating ionic liquids into synthetic block co-polymer electrolytes, poly(styrenesulphonate-b-methylbutylene) (S(n)MB(m)), as high-temperature PEFCs. The resulting membranes are transparent, flexible and thermally stable up to 180 °C. The increases in the sulphonation level of S(n)MB(m) co-polymers (proton supplier) and the concentration of the ionic liquid (proton mediator) produce an overall increase in conductivity. Morphology effects were studied by X-ray scattering and electron microscopy. Compared with membranes having discrete ionic domains (including Nafion 117), the nanostructured membranes revealed over an order of magnitude increase in conductivity with the highest conductivity of 0.045 S cm(-1) obtained at 165 °C.
Massachusetts General Hospital, Pohang University of Science and Technology | Date: 2014-09-09
One aspect of the present disclosure relates to a remotely controllable lens device. The lens device can include a lens material and a circuit physically coupled to the lens material. The circuit can be configured to be powered based on an energy signal from a power source to perform a function (e.g., release of drug, generation of electromagnetic radiation, detection of electromagnetic radiation, and/or control of an optical refractive property). For example, the power source can be an external power source and/or an auto-powered source. The external power source can be, for example, a power source that utilizes synchronized magnetic flux phase coupling (e.g., WiTricity). The auto-powered source can be provided on-site (e.g., on-chip) using a harvesting system (e.g., solar-cell, photo-cell, piezoelectric, etc.)