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Kumar B.,Materials Science and Engineering Program | Kubiak C.P.,Materials Science and Engineering Program
Journal of Physical Chemistry C | Year: 2010

Hydrogen-terminated p-type silicon was used as a photocathode for the selective photoreduction of CO2 to CO in the presence of Re(bipy-But)(CO)3Cl (bipy-But = 4,4′-di-tert-butyl-2,2′-bipyridine) as an electrocatalyst. The reduction of CO2 to CO on p-type silicon was achieved at a potential more than 600 mV lower than that required with a Pt electrode. A Faradaic efficiency of 97 ± 3% and an overall efficiency of 9.3% and 10% for the conversion of monochromatic and polychromatic light, respectively, to electricity were observed for the CO2 photoreduction process. A short-circuit quantum efficiency of 61% for light-to-chemical energy conversion was observed for the conversion of CO2 to CO. © 2010 American Chemical Society.

Sheng W.,Materials Science and Engineering Program | Dore K.,Center for Neural Circuits and Behavior | Alhasan A.H.,University of California at San Diego | Grossman M.,University of California at San Diego | And 2 more authors.
ACS Nano | Year: 2014

Near-infrared (NIR) light-triggered release from polymeric capsules could make a major impact on biological research by enabling remote and spatiotemporal control over the release of encapsulated cargo. The few existing mechanisms for NIR-triggered release have not been widely applied because they require custom synthesis of designer polymers, high-powered lasers to drive inefficient two-photon processes, and/or coencapsulation of bulky inorganic particles. In search of a simpler mechanism, we found that exposure to laser light resonant with the vibrational absorption of water (980 nm) in the NIR region can induce release of payloads encapsulated in particles made from inherently non-photo-responsive polymers. We hypothesize that confined water pockets present in hydrated polymer particles absorb electromagnetic energy and transfer it to the polymer matrix, inducing a thermal phase change. In this study, we show that this simple and highly universal strategy enables instantaneous and controlled release of payloads in aqueous environments as well as in living cells using both pulsed and continuous wavelength lasers without significant heating of the surrounding aqueous solution. © 2014 American Chemical Society.

PubMed | University of California at San Diego and Materials Science and Engineering Program.
Type: | Journal: Acta biomaterialia | Year: 2017

Animal propulsion systems are believed to show high energy and mechanical efficiency in assisting movement compared to artificial designs. As an example, batoid fishes have very light cartilaginous skeletons that facilitate their elegant swimming via enlarged wing-like pectoral fins. The aim of this work is to illustrate the hierarchical structure of the pectoral fin of a representative batoid, the Longnose Skate (Raja rhina), and explain the mechanical implications of its structural design. At the macro level, the pectoral fins are comprised of radially oriented fin rays, formed by staggered mineralized skeletal elements stacked end-to-end. At the micro level, the midsection of each radial element is composed of three mineralized components, which consist of discrete segments (tesserae) that are mineralized cartilage and embedded in unmineralized cartilage. The radial elements are wrapped with aligned, unmineralized collagen fibers. This is the first report of the detailed structure of the ray elements, including the observation of a 3-chain mineralized tesserae. Our analyses demonstrate that this configuration enhances stiffness in multiple directions. A two-dimensional numerical model based on the morphological analysis demonstrated that the tessera structure helps distributing shear, tensile and compressive stress more ideally, which can better support both lift and thrust forces when swimming without losing flexibility. Statement of Significance Batoid fishes have very light cartilaginous skeletons that facilitate their elegant swimming by applying their enlarged wing-like pectoral fins. Previous studies have shown structural features and mechanical properties of the mineralized cartilage skeleton in various batoid fishes. However, the details of the pectoral fin structure at different length scales, as well as the relationship between the mechanical properties and structural design remains unknown. The present work illustrates the hierarchical structure of the pectoral fin of the Longnose Skate (a representative batoid fish) and verifies the materials configuration and structural design increases the stiffness of fin skeleton without a loss in flexibility. These results have implications for the design of strong but flexible materials and bio-inspired autonomous underwater vehicles (AUVs).

Bang W.,Texas A&M University | Kim K.,Texas A&M University | Rathnayaka K.D.D.,Texas A&M University | Teizer W.,Materials Science and Engineering Program | And 2 more authors.
Physica C: Superconductivity and its Applications | Year: 2013

We present studies of the transport properties of a Pb82Bi 18 and Sn superconducting film with an array of parallel nickel or cobalt magnetic nanostripes (500 nm period) deposited on the top of a germanium insulating layer covering the superconducting film surface. The critical current parallel to the stripes is significantly larger than critical current perpendicular to the stripes for Pb82Bi18. © 2013 Elsevier B.V. All rights reserved.

Kumar B.,Materials Science and Engineering Program | Llorente M.,Materials Science and Engineering Program | Froehlich J.,University of California at San Diego | Dang T.,Materials Science and Engineering Program | And 3 more authors.
Annual Review of Physical Chemistry | Year: 2012

The recent literature on photochemical and photoelectrochemical reductions of CO 2 is reviewed. The different methods of achieving light absorption, electron-hole separation, and electrochemical reduction of CO 2 are considered. Energy gap matching for reduction of CO 2 to different products, including CO, formic acid, and methanol, is used to identify the most promising systems. Different approaches to lowering overpotentials and achieving high chemical selectivities by employing catalysts are described and compared. Copyright © 2012 by Annual Reviews. All rights reserved.

Shen J.,Materials Science and Engineering Program | Shen J.,University of California at San Diego | Clemens J.B.,University of California at San Diego | Chagarov E.A.,University of California at San Diego | And 7 more authors.
Surface Science | Year: 2010

The structural and electronic properties of group III rich In 0.53Ga0.47As(001) have been studied using scanning tunneling microscopy/spectroscopy (STM/STS). At room temperature (300 K), STM images show that the In0.53Ga0.47As(001)-(4×2) reconstruction is comprised of undimerized In/Ga atoms in the top layer. Quantitative comparison of the In0.53Ga0.47As(001)- (4×2) and InAs(001)-(4×2) shows the reconstructions are almost identical, but In0.53Ga0.47As(001)-(4×2) has at least a 4× higher surface defect density even on the best samples. At low temperature (77 K), STM images show that the most probable In 0.53Ga0.47As(001) reconstruction is comprised of one In/Ga dimer and two undimerized In/Ga atoms in the top layer in a double (4×2) unit cell. Density functional theory (DFT) simulations at elevated temperature are consistent with the experimentally observed 300 K structure being a thermal superposition of three structures. DFT molecular dynamics (MD) show the row dimer formation and breaking is facilitated by the very large motions of tricoodinated row edge As atoms and z motion of In/Ga row atoms induced changes in As-In/Ga- As bond angles at elevated temperature. STS results show there is a surface dipole or the pinning states near the valence band (VB) for 300 K In0.53Ga0.47As(001)-(4×2) surface consistent with DFT calculations. DFT calculations of the band-decomposed charge density indicate that the strained unbuckled trough dimmers being responsible for the surface pinning. © 2010 Elsevier B.V. All rights reserved.

McKittrick J.,State University of New York at Buffalo | Chen P.-Y.,Materials Science and Engineering Program | Tombolato L.,State University of New York at Buffalo | Novitskaya E.E.,Materials Science and Engineering Program | And 5 more authors.
Materials Science and Engineering C | Year: 2010

Some of the most remarkable materials in terms of energy absorption and impact resistance are not found through human processing but in nature. Solutions to the continuing problems of improved composite technologies may lie in replicating naturally occurring systems. In this review, we examine several mammalian structural materials: bones (bovine femur and elk antler), teeth and tusks from various taxa, horns from the desert big horn sheep, and equine hooves. We establish the relationships between structural and mechanical properties for these materials, with an emphasis on energy absorption mechanisms. We also identify the energy absorbing strategies utilized in these materials. Implementation of these bioinspired design strategies can serve as a basis for the design of new energy absorbent synthetic composite materials. Synthetic constituent materials arranged according to the principles outlined in this work will achieve the same synergistic effects as nature and no longer be confined to the limitations imposed by a mixture law. © 2010 Elsevier B.V.

Shervin S.,Materials Science and Engineering Program | Kim S.-H.,Materials Science and Engineering Program | Kim S.-H.,Hongik University | Asadirad M.,Materials Science and Engineering Program | And 4 more authors.
ACS Photonics | Year: 2016

We show that bending of flexible light-emitting diodes based on polar group III-V nitride structures can function as more than mechanically flexible devices through numerical studies. Controlled external bending can improve internal quantum efficiencies and electron-to-photon conversion efficiencies. Moreover, emission wavelength and color can be changed using active polarization control with external bending strains. We show that applying external strain on InGaN/GaN quantum-well (QW) heterostructures can mitigate or enhance the quantum-confined Stark effect. Significant IQE improvement (≥10%) is estimated by introducing external strain with concave-side up bending of flexible heterostructures. We also show significant wavelength shifts in peak emission. Especially, the In0.35Ga0.65N QW can be tuned in the overall visible spectral range by choosing different curvatures and bending modes. We have suggested the concept of a photoelectromechanical device that can emit different colors depending on the applied external strain varying from red to blue, enabling white light generation with the same active region consisting of QWs with a constant indium mole fraction. © 2016 American Chemical Society.

News Article | January 22, 2016
Site: www.nanotech-now.com

Abstract: Magnetic field-induced helical mode and topological transitions in a topological insulator nanoribbon Luis A. Jauregui1,2, Michael T. Pettes3†, Leonid P. Rokhinson1,2,4, Li Shi3,5 and Yong P. Chen1,2,4* 1 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA. 2 School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA. 3 Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA. 4 Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA. 5 Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, USA. †Present address: Department of Mechanical Engineering, University of Connecticut, Storrs, Connecticut 06269, USA. *e-mail: The spin-helical Dirac fermion topological surface states in a topological insulator nanowire or nanoribbon promise novel topological devices and exotic physics such as Majorana fermions. Here, we report local and non-local transport measurements in Bi2Te3 topological insulator nanoribbons that exhibit quasi-ballistic transport over ?2μm. The conductance versus axial magnetic flux Φ exhibits Aharonov-Bohm oscillations with maxima occurring alternately at half- integer or integer flux quanta (Φ0 = h/e, where h is Planck's constant and e is the electron charge) depending periodically on the gate-tuned Fermi wavevector (kF) with period 2π/C (where C is the nanoribbon circumference). The conductance versus gate voltage also exhibits kF-periodic oscillations, anti-correlated between Φ=0 and Φ0/2. These oscillations enable us to probe the Bi2Te3 band structure, and are consistent with the circumferentially quantized topological surface states forming a series of one-dimensional subbands, which undergo periodic magnetic field-induced topological transitions with the disappearance/appearance of the gapless Dirac point with a one-dimensional spin helical mode. Researchers have created nanoribbons of an emerging class of materials called topological insulators and used a magnetic field to control their semiconductor properties, a step toward harnessing the technology to study exotic physics and building new spintronic devices or quantum computers. Unlike ordinary materials that are either insulators or conductors, topological insulators are paradoxically both at the same time - they are insulators inside but conduct electricity on the surface, said Yong P. Chen, a Purdue University associate professor of physics and astronomy and electrical and computer engineering who worked with doctoral student Luis A. Jauregui and other researchers. The materials might be used for "spintronic" devices and practical quantum computers far more powerful than today's technologies. In the new findings, the researchers used a magnetic field to induce a so-called "helical mode" of electrons, a capability that could make it possible to control the spin state of electrons. The findings are detailed in a research paper that appeared in the advance online publication of the journal Nature Nanotechnology on Jan. 18 and showed that a magnetic field can be used to induce the nanoribbons to undergo a "topological transition," switching between a material possessing a band gap on the surface and one that does not. "Silicon is a semiconductor, meaning it has a band gap, a trait that is needed to switch on and off the conduction, the basis for silicon-based digital transistors to store and process information in binary code," Chen said. "Copper is a metal, meaning it has no band gap and is always a good conductor. In both cases the presence or absence of a band gap is a fixed property. What is weird about the surface of these materials is that you can control whether it has a band gap or not just by applying a magnetic field, so it's kind of tunable, and this transition is periodic in the magnetic field, so you can drive it through many 'gapped' and 'gapless' states." The nanoribbons are made of bismuth telluride, the material behind solid-state cooling technologies such as commercial thermoelectric refrigerators. "Bismuth telluride has been the workhorse material of thermoelectric cooling for decades, but just in the last few years people found this material and related materials have this amazing additional property of being topological insulators," he said. The paper was authored by Jauregui; Michael T. Pettes, a former postdoctoral researcher at the University of Texas at Austin and now an assistant professor in the Department of Mechanical Engineering at the University of Connecticut; Leonid P. Rokhinson, a Purdue professor of physics and astronomy and electrical and computer engineering; Li Shi, BF Goodrich Endowed Professor in Materials Engineering at the University of Texas at Austin; and Chen A key finding was that the researchers documented the use of nanoribbons to measure so-called Aharonov-Bohm oscillations, which is possible by conducting electrons in opposite directions in ring-like paths around the nanoribbons. The structure of the nanoribbon - a nanowire that is topologically the same as a cylinder - is key to the discovery because it allows the study of electrons as they travel in a circular direction around the ribbon. The electrons conduct only on the surface of the nanowires, tracing out a cylindrical circulation. "If you let electrons travel in two paths around a ring, in left and right paths, and they meet at the other end of the ring then they will interfere either constructively or destructively depending on the phase difference created by a magnetic field, resulting in either high or low conductivity, respectively, showing the quantum nature of electrons behaving as waves," Jauregui said. The researchers demonstrated a new variation on this oscillation in topological insulator surfaces by inducing the spin helical mode of the electrons. The result is the ability to flip from constructive to destructive interference and back. "This provides very definitive evidence that we are measuring the spin helical electrons," Jauregui said. "We are measuring these topological surface states. This effect really hasn't been seen very convincingly until recently, so now this experiment really provides clear evidence that we are talking about these spin helical electrons propagating on the cylinder, so this is one aspect of this oscillation." Findings also showed this oscillation as a function of "gate voltage," representing another way to switch conduction from high to low. "The switch occurs whenever the circumference of the nanoribbon contains just an integer number of the quantum mechanical wavelength, or 'fermi wavelength,' which is tuned by the gate voltage of the electrons wrapping around the surface," Chen said. It was the first time researchers have seen this kind of gate-dependent oscillation in nanoribbons and further correlates it to the topological insulator band structure of bismuth telluride. The nanoribbons are said to possess "topological protection," preventing electrons on the surface from back scattering and enabling high conductivity, a quality not found in metals and conventional semiconductors. They were fabricated by researchers at the UT Austin. The measurements were performed while the nanoribbons were chilled to about minus 273 degrees Celsius (nearly minus 460 degrees Fahrenheit). "We have to operate at low temperatures to observe the quantum mechanical nature of the electrons," Chen said. Future research will include work to further investigate the nanowires as a platform to study the exotic physics needed for topological quantum computations. Researchers will aim to connect the nanowires with superconductors, which conduct electricity with no resistance, for hybrid topological insulator-superconducting devices. By further combining topological insulators with a superconductor, researchers may be able to build a practical quantum computer that is less susceptible to the environmental impurities and perturbations that have presented challenges thus far. Such a technology would perform calculations using the laws of quantum mechanics, making for computers much faster than conventional computers at certain tasks such as database searches and code breaking. ### Note to Journalists: A copy of the research paper is available from Emil Venere, Purdue News Service, at 765-494-4709, The research and the team have been supported with funding from the U.S. Defense Advanced Research Projects Agency, Intel Corp., National Science Foundation, Department of Energy and the Purdue Center for Topological Materials. 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 | February 28, 2017
Site: www.materialstoday.com

A team of engineers at the University of Colorado Boulder (CU Boulder) has developed a scalable manufactured metamaterial – an engineered material with extraordinary properties not found in nature – to act as a kind of air conditioning system for structures. It has the ability to cool objects even under direct sunlight with zero energy or water consumption. When applied to a surface, the metamaterial film cools the object underneath by efficiently reflecting incoming solar energy back into space while simultaneously allowing the surface to shed its own heat in the form of infrared thermal radiation. This new material, which is described in a paper in Science, could provide an eco-friendly form of supplementary cooling for thermoelectric power plants, which currently require large amounts of water and electricity to maintain the operating temperatures of their machinery. The material is a glass-polymer hybrid that measures just 50µm thick – slightly thicker than the aluminum foil found in a kitchen – and can be manufactured economically on rolls, making it a potentially viable large-scale technology for both residential and commercial applications. "We feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology," said Xiaobo Yin, co-director of the research and an assistant professor who holds dual appointments in CU Boulder's Department of Mechanical Engineering and the Materials Science and Engineering Program. The material takes advantage of passive radiative cooling, the process by which objects naturally shed heat in the form of infrared radiation without consuming energy. Passive radiation provides some natural night time cooling and is used for residential cooling in some areas, but daytime cooling has historically been more of a challenge. Even a small amount of directly-absorbed solar energy is enough to negate passive radiation. The challenge for the CU Boulder researchers, then, was to create a material that could provide a one-two punch: reflect any incoming solar rays back into the atmosphere while still providing a means of escape for infrared radiation. To solve this, the researchers embedded visibly-scattering but infrared-radiant glass microspheres into a polymer film. They then added a thin silver coating underneath the film to achieve maximum spectral reflectance. "Both the glass-polymer metamaterial formation and the silver coating are manufactured at scale on roll-to-roll processes," said Ronggui Yang, also a professor of mechanical engineering at CU Boulder. During field tests in Boulder and Cave Creek, Arizona, the metamaterial successfully displayed an average radiative cooling power larger than 110W/m2 for a continuous period of 72 hours and larger than 90W/m2 in direct, noon-time sunlight. That cooling power is roughly equivalent to the electricity generated by solar cells over a similar area, but radiative cooling has the advantage that it occurs both day and night. "Just 10–20m2 of this material on the rooftop could nicely cool down a single-family house in summer," said Gang Tan, an associate professor in the University of Wyoming's Department of Civil and Architectural Engineering and a co-author of the paper. In addition to being useful for cooling buildings and power plants, the material could also help to improve the efficiency and lifetime of solar panels. In direct sunlight, panels can heat up to temperatures that hamper their ability to convert solar rays into electricity. "Just by applying this material to the surface of a solar panel, we can cool the panel and recover an additional one to two percent of solar efficiency," said Yin. "That makes a big difference at scale." The engineers have applied for a patent on the technology and are working with CU Boulder's Technology Transfer Office to explore potential commercial applications. They plan to create a 200m2 ‘cooling farm’ prototype in Boulder later this year. "The key advantage of this technology is that it works 24/7 with no electricity or water usage," explained Yang "We're excited about the opportunity to explore potential uses in the power industry, aerospace, agriculture and more." This story is adapted from material from the University of Colorado Boulder, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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