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News Article | May 4, 2017
Site: www.eurekalert.org

IMAGE:  Schematic of the pathway describing the evolution of adsorbed ethene (top left) to graphene (bottom left). The sequence of intermediates identified in the study and their respective appearance temperatures are... view more An international team of scientists has developed a new way to produce single-layer graphene from a simple precursor: ethene - also known as ethylene - the smallest alkene molecule, which contains just two atoms of carbon. By heating the ethene in stages to a temperature of slightly more than 700 degrees Celsius -- hotter than had been attempted before - the researchers produced pure layers of graphene on a rhodium catalyst substrate. The stepwise heating and higher temperature overcame challenges seen in earlier efforts to produce graphene directly from hydrocarbon precursors. Because of its lower cost and simplicity, the technique could open new potential applications for graphene, which has attractive physical and electronic properties. The work also provides a novel mechanism for the self-evolution of carbon cluster precursors whose diffusional coalescence results in the formation of the graphene layers. The research, reported as the cover article in the May 4 issue of the Journal of Physical Chemistry C, was conducted by scientists at the Georgia Institute of Technology, Technische Universität München in Germany, and the University of St. Andrews in Scotland. In the United States, the research was supported by the U.S. Air Force Office of Scientific Research and the U.S. Department of Energy's Office of Basic Energy Sciences. "Since graphene is made from carbon, we decided to start with the simplest type of carbon molecules and see if we could assemble them into graphene," explained Uzi Landman, a Regents' Professor and F.E. Callaway endowed chair in the Georgia Tech School of Physics who headed the theoretical component of the research. "From small molecules containing carbon, you end up with macroscopic pieces of graphene." Graphene is now produced using a variety of methods including chemical vapor deposition, evaporation of silicon from silicon carbide - and simple exfoliation of graphene sheets from graphite. A number of earlier efforts to produce graphene from simple hydrocarbon precursors had proven largely unsuccessful, creating disordered soot rather than structured graphene. Guided by a theoretical approach, the researchers reasoned that the path from ethene to graphene would involve formation of a series of structures as hydrogen atoms leave the ethene molecules and carbon atoms self-assemble into the honeycomb pattern that characterizes graphene. To explore the nature of the thermally-induced rhodium surface-catalyzed transformations from ethene to graphene, experimental groups in Germany and Scotland raised the temperature of the material in steps under ultra-high vacuum. They used scanning-tunneling microscopy (STM), thermal programed desorption (TPD) and high-resolution electron energy loss (vibrational) spectroscopy (HREELS) to observe and characterize the structures that form at each step of the process. Upon heating, ethene adsorbed onto the rhodium catalyst evolves via coupling reactions to form segmented one-dimensional polyaromatic hydrocarbons (1D-PAH). Further heating leads to dimensionality crossover - one dimensional to two dimensional structures - and dynamical restructuring processes at the PAH chain ends with a subsequent activated detachment of size-selective carbon clusters, following a mechanism revealed through first-principles quantum mechanical simulations. Finally, rate-limiting diffusional coalescence of these dynamically self-evolved cluster-precursors leads to condensation into graphene with high purity. At the final stage before the formation of graphene, the researchers observed nearly round disk-like clusters containing 24 carbon atoms, which spread out to form the graphene lattice. "The temperature must be raised within windows of temperature ranges to allow the requisite structures to form before the next stage of heating," Landman explained. "If you stop at certain temperatures, you are likely to end up with coking." An important component is the dehydrogenation process which frees the carbon atoms to form intermediate shapes, but some of the hydrogen resides temporarily on, or near, the metal catalyst surface and it assists in subsequent bond-breaking process that lead to detachment of the 24-carbon cluster-precursors. "All along the way, there is a loss of hydrogen from the clusters," said Landman. "Bringing up the temperature essentially 'boils' the hydrogen out of the evolving metal-supported carbon structure, culminating in graphene." The resulting graphene structure is adsorbed onto the catalyst. It may be useful attached to the metal, but for other applications, a way to remove it will have to be developed. Added Landman: "This is a new route to graphene, and the possible technological application is yet to be explored." Beyond the theoretical research, carried out by Bokwon Yoon and Landman at the Georgia Tech Center for Computational Materials Science, the experimental work was done in the laboratory of Professor Renald Schaub at the University of St. Andrews and in the laboratory of Professor Ueli Heiz and Friedrich Esch at the Technische Universität München. Other co-authors included Bo Wang, Michael König, Catherine J. Bromley, Michael-John Treanor, José A. Garrido Torres, Marco Caffio, Federico Grillo, Herbert Frücht, and Neville V. Richardson. The work at the Georgia Institute of Technology was supported by the Air Force Office of Scientific Research through Grant FA9550-14-1-0005 and by the Office of Basic Energy Sciences of the U.S. Department of Energy through Grant FG05-86ER45234. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsoring organizations. CITATION: Bo Wang, et al., "Ethene to Graphene: Surface Catalyzed Chemical Pathways, Intermediates, and Assembly," (Journal of Physical Chemistry C). http://dx.


News Article | May 5, 2017
Site: www.cemag.us

An international team of scientists has developed a new way to produce single-layer graphene from a simple precursor: ethene — also known as ethylene — the smallest alkene molecule, which contains just two atoms of carbon. By heating the ethene in stages to a temperature of slightly more than 700 degrees Celsius — hotter than had been attempted before – the researchers produced pure layers of graphene on a rhodium catalyst substrate. The stepwise heating and higher temperature overcame challenges seen in earlier efforts to produce graphene directly from hydrocarbon precursors. Because of its lower cost and simplicity, the technique could open new potential applications for graphene, which has attractive physical and electronic properties. The work also provides a novel mechanism for the self-evolution of carbon cluster precursors whose diffusional coalescence results in the formation of the graphene layers. The research, reported as the cover article in the May 4 issue of the Journal of Physical Chemistry C, was conducted by scientists at the Georgia Institute of Technology, Technische Universität München in Germany, and the University of St. Andrews in Scotland. In the United States, the research was supported by the U.S. Air Force Office of Scientific Research and the U.S. Department of Energy’s Office of Basic Energy Sciences. “Since graphene is made from carbon, we decided to start with the simplest type of carbon molecules and see if we could assemble them into graphene,” explains Uzi Landman, a Regents’ Professor and F.E. Callaway endowed chair in the Georgia Tech School of Physics who headed the theoretical component of the research. “From small molecules containing carbon, you end up with macroscopic pieces of graphene.” Graphene is now produced using a variety of methods including chemical vapor deposition, evaporation of silicon from silicon carbide — and simple exfoliation of graphene sheets from graphite. A number of earlier efforts to produce graphene from simple hydrocarbon precursors had proven largely unsuccessful, creating disordered soot rather than structured graphene. Guided by a theoretical approach, the researchers reasoned that the path from ethene to graphene would involve formation of a series of structures as hydrogen atoms leave the ethene molecules and carbon atoms self-assemble into the honeycomb pattern that characterizes graphene. To explore the nature of the thermally-induced rhodium surface-catalyzed transformations from ethene to graphene, experimental groups in Germany and Scotland raised the temperature of the material in steps under ultra-high vacuum. They used scanning-tunneling microscopy (STM), thermal programed desorption (TPD) and high-resolution electron energy loss (vibrational) spectroscopy (HREELS) to observe and characterize the structures that form at each step of the process. Upon heating, ethene adsorbed onto the rhodium catalyst evolves via coupling reactions to form segmented one-dimensional polyaromatic hydrocarbons (1D-PAH). Further heating leads to dimensionality crossover — one dimensional to two dimensional structures — and dynamical restructuring processes at the PAH chain ends with a subsequent activated detachment of size-selective carbon clusters, following a mechanism revealed through first-principles quantum mechanical simulations. Finally, rate-limiting diffusional coalescence of these dynamically self-evolved cluster-precursors leads to condensation into graphene with high purity. At the final stage before the formation of graphene, the researchers observed nearly round disk-like clusters containing 24 carbon atoms, which spread out to form the graphene lattice. “The temperature must be raised within windows of temperature ranges to allow the requisite structures to form before the next stage of heating,” Landman explains. “If you stop at certain temperatures, you are likely to end up with coking.” An important component is the dehydrogenation process which frees the carbon atoms to form intermediate shapes, but some of the hydrogen resides temporarily on, or near, the metal catalyst surface and it assists in subsequent bond-breaking process that lead to detachment of the 24-carbon cluster-precursors.  “All along the way, there is a loss of hydrogen from the clusters,” says Landman. “Bringing up the temperature essentially ‘boils’ the hydrogen out of the evolving metal-supported carbon structure, culminating in graphene.” The resulting graphene structure is adsorbed onto the catalyst. It may be useful attached to the metal, but for other applications, a way to remove it will have to be developed. Adds Landman: “This is a new route to graphene, and the possible technological application is yet to be explored.” Beyond the theoretical research, carried out by Bokwon Yoon and Landman at the Georgia Tech Center for Computational Materials Science, the experimental work was done in the laboratory of Professor Renald Schaub at the University of St. Andrews and in the laboratory of Professor Ueli Heiz and Friedrich Esch at the Technische Universität München. Other co-authors included Bo Wang, Michael König, Catherine J. Bromley, Michael-John Treanor, José A. Garrido Torres, Marco Caffio, Federico Grillo, Herbert Frücht, and Neville V. Richardson. The work at the Georgia Institute of Technology was supported by the Air Force Office of Scientific Research through Grant FA9550-14-1-0005 and by the Office of Basic Energy Sciences of the U.S. Department of Energy through Grant FG05-86ER45234. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsoring organizations.


Young S.M.,Center for Computational Materials Science | Kane C.L.,University of Pennsylvania
Physical Review Letters | Year: 2015

Graphene is famous for being a host of 2D Dirac fermions. However, spin-orbit coupling introduces a small gap, so that graphene is formally a quantum spin Hall insulator. Here we present symmetry-protected 2D Dirac semimetals, which feature Dirac cones at high-symmetry points that are not gapped by spin-orbit interactions and exhibit behavior distinct from both graphene and 3D Dirac semimetals. Using a two-site tight-binding model, we construct representatives of three possible distinct Dirac semimetal phases and show that single symmetry-protected Dirac points are impossible in two dimensions. An essential role is played by the presence of nonsymmorphic space group symmetries. We argue that these symmetries tune the system to the boundary between a 2D topological and trivial insulator. By breaking the symmetries we are able to access trivial and topological insulators as well as Weyl semimetal phases. © 2015 American Physical Society.


Kim Y.C.,Center for Computational Materials Science | Mittal J.,Lehigh University
Physical Review Letters | Year: 2013

A statistical mechanical theory is presented to predict the effects of macromolecular crowding on protein association equilibria, accounting for both excluded volume and attractive interactions between proteins and crowding molecules. Predicted binding free energies are in excellent agreement with simulation data over a wide range of crowder sizes and packing fractions. It is shown that attractive interactions between proteins and crowding agents counteract the stabilizing effects of excluded volume interactions. A critical attraction strength, for which there is no net effect of crowding, is approximately independent of the crowder packing fraction. © 2013 American Physical Society.


News Article | April 11, 2016
Site: www.nrl.navy.mil

U.S. Naval Research Laboratory (NRL) research physicist, Dr. Alexander Efros, is bestowed the honor of Fellow by the Materials Research Society (MRS), recognizing a record of success in research in the field of optical science materials and technology. Most notably, Efros is recognized for his pioneering and fundamental contributions to the theory of low dimensional semiconductor structures, which have established the basic theoretical concepts that today are used by everyone for describing the electronic and optical properties of nanocrystal quantum dots, nanowires and nanoplatelets. These concepts are also commonly used in the development of novel nanocrystal-based optical devices, which are smaller, cheaper, more efficient, and consume less energy than the traditional devices. "Dr. Efros' rooted commitment to materials science theory and his profound knowledge of solid-state physics are notably recognized and respected by the scientific community worldwide," said Dr. Michael Mehl, head, NRL Center for Computational Materials Science. "His work has given us the ability to understand and control the behavior of semiconductor nanostructures, leading to new and exciting applications in lasers, light emitting diodes, and photovoltaics." In the early 1980s Efros and his colleagues, A. I. Ekimov and A. A. Onuschenko, discovered semiconductor nanocrystals while studying doped glasses. They also explained the origin of the size-dependent optical properties of nanocrystals, and established that nanocrystals act as 'artificial atoms' that opened the door to a new class of optical materials. Like atoms, nanocrystals have discrete optical energy spectra that are tunable over a wide spectral range by varying their size. At the same time these atoms can be manipulated with nanometer precision to form nanocrystal molecules and solids. Nanocrystals also can serve as dopants in nanocrystalline solids. As a result, researchers now have an unlimited number of new atomic elements available to create new materials. Based on size controlled tunable emission spectra and great structural and chemical flexibility, the nanocrystals have potential in the creation of a new class of tunable quantum dot lasers and light emitting diodes that function from the far infrared to deep ultraviolet wavelength ranges. An additional application is the labeling of biological molecules by nanocrystals, which are being developed as ultra-sensitive detectors and sensors for use in neuron and drug transport imaging, and the detection of dangerous chemical agents. The high optical stability of the nanocrystals allow for higher efficiency in quantum dot displays. In very recent developments of this area, researchers from Samsung Electronics demonstrated in 2012 a full-color quantum-dot display monitor that is brighter than liquid-crystal displays and consumes less than a fifth of the power. Receiving a Master of Science degree in physical engineering in 1973 and a Ph.D. in physics in 1978, both from the Technical University in Leningrad, USSR, Efros became senior researcher at the Ioffe Institute in Leningrad. From 1990 to 1992 he was senior researcher at the Technical University of Munich and a visiting scientist at Massachusetts Institute of Technology (MIT). In 1993 Efros came to NRL as a consultant and in 1999 become senior researcher in NRL's Materials Science and Technology Division. Efros has authored and co-authored more than 190 articles in refereed journals and holds two patents. He has given more than 80 invited talks at international meetings and more than 190 at various universities and laboratories. He is co-editor of two books on nanoscale semiconductors and co-organizer of many conferences on this topic and is a Fellow of the American Physical Society. Efros has received five Alan Berman publication awards, an NRL Patent Award (2003), the NRL Sigma Xi Award for Pure Science (2006), the E. F. Gross Medal of the D. S. Rozhdestvensky Optical Society of Russia (2013 with A. I. Ekimov and A. A. Onuschenko) the R.W. Wood Prize of the Optical Society of America (2006, with L. E. Brus and A. I. Ekimov), the Humboldt Research Award for Senior U.S. Scientists (2008), and the Dolores M. Etter Top Navy Scientists and Engineers of the Year Award (2009). About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.


Kim Y.C.,Center for Computational Materials Science | Hummer G.,U.S. National Institute of Diabetes and Digestive and Kidney Diseases
Biochimica et Biophysica Acta - Bioenergetics | Year: 2012

Cytochrome c oxidase is an efficient energy transducer that reduces oxygen to water and converts the released chemical energy into an electrochemical membrane potential. As a true proton pump, cytochrome c oxidase translocates protons across the membrane against this potential. Based on a wealth of experiments and calculations, an increasingly detailed picture of the reaction intermediates in the redox cycle has emerged. However, the fundamental mechanism of proton pumping coupled to redox chemistry remains largely unresolved. Here we examine and extend a kinetic master-equation approach to gain insight into redox-coupled proton pumping in cytochrome c oxidase. Basic principles of the cytochrome c oxidase proton pump emerge from an analysis of the simplest kinetic models that retain essential elements of the experimentally determined structure, energetics, and kinetics, and that satisfy fundamental physical principles. The master-equation models allow us to address the question of how pumping can be achieved in a system in which all reaction steps are reversible. Whereas proton pumping does not require the direct modulation of microscopic reaction barriers, such kinetic gating greatly increases the pumping efficiency. Further efficiency gains can be achieved by partially decoupling the proton uptake pathway from the active-site region. Such a mechanism is consistent with the proposed Glu valve, in which the side chain of a key glutamic acid shuttles between the D channel and the active-site region. We also show that the models predict only small proton leaks even in the absence of turnover. The design principles identified here for cytochrome c oxidase provide a blueprint for novel biology-inspired fuel cells, and the master-equation formulation should prove useful also for other molecular machines. This article is part of a Special Issue entitled: Respiratory Oxidases. © 2011 Elsevier B.V. All rights reserved.


Roycki B.,U.S. National Institute of Diabetes and Digestive and Kidney Diseases | Kim Y.C.,Center for Computational Materials Science | Hummer G.,U.S. National Institute of Diabetes and Digestive and Kidney Diseases
Structure | Year: 2011

We developed and implemented an ensemble-refinement method to study dynamic biomolecular assemblies with intrinsically disordered segments. Data from small angle X-ray scattering (SAXS) experiments and from coarse-grained molecular simulations were combined by using a maximum-entropy approach. The method was applied to CHMP3 of ESCRT-III, a protein with multiple helical domains separated by flexible linkers. Based on recent SAXS data by Lata et al. (J. Mol. Biol. 378, 818, 2008), we constructed ensembles of CHMP3 at low- and high-salt concentration to characterize its closed autoinhibited state and open active state. At low salt, helix α5 is bound to the tip of helices α1 and α2, in excellent agreement with a recent crystal structure. Helix α6 remains free in solution and does not appear to be part of the autoinhibitory complex. The simulation-based ensemble refinement is general and effectively increases the resolution of SAXS beyond shape information to atomically detailed structures. © 2011 Elsevier Ltd All rights reserved.


Hoang K.,North Dakota State University | Johannes M.D.,Center for Computational Materials Science
Journal of Materials Chemistry A | Year: 2014

We report a comprehensive first-principles study of the thermodynamics and transport of intrinsic point defects in layered oxide cathode materials LiMO2 (M = Co, Ni), using density-functional theory and the Heyd-Scuseria-Ernzerhof screened hybrid functional. We find that LiCoO 2 has a complex defect chemistry; different electronic and ionic defects can exist under different synthesis conditions, and LiCoO2 samples free of cobalt antisite defects can be made under Li-excess (Co-deficient) environments. A defect model for lithium over-stoichiometric LiCoO2 is also proposed, which involves negatively charged lithium antisites and positively charged small (hole) polarons. In LiNiO2, a certain amount of Ni3+ ions undergo charge disproportionation and the concentration of nickel ions in the lithium layers is high. Tuning the synthesis conditions may reduce the nickel antisites but would not remove the charge disproportionation. In addition, we find that LiMO2 cannot be doped n- or p-type; the electronic conduction occurs via hopping of small polarons and the ionic conduction occurs via migration of lithium vacancies, either through a monovacancy or divacancy mechanism, depending on the vacancy concentration. © 2014 the Partner Organisations.


Erwin S.C.,Center for Computational Materials Science
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We recently proposed that impurity doping in colloidally grown semiconductor nanocrystals is often controlled primarily by kinetics rather than by thermodynamics. In this "trapped-dopant" model the diffusion of an impurity through a nanocrystal is negligible at colloidal growth temperatures. Consequently, an impurity can only be incorporated as a dopant into a growing nanocrystal if it first adsorbs on the surface and is then overgrown. This surface adsorption can be complicated by a competing process: the binding of the impurity by surfactant molecules and other agents added to the growth solution to passivate the nanocrystal and control its growth. Here we use density-functional theory to study the interplay and outcome of these processes for the doping of PbSe nanocrystals by a number of candidate dopants (Mn, Co, Cl, In, Cd, Tl, etc.) in the presence of two widely used growth additives (oleic acid and hexadecylamine). The results suggest that successful doping requires making a trade-off between surface adsorption (which favors small dopants) and interior trapping (which favors large dopants). Moreover, the widely used growth agent oleic acid binds strongly to almost all dopants, suggesting that the standard growth procedure may require modification for successful doping to be realized. © 2010 The American Physical Society.


Bernstein N.,Center for Computational Materials Science
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2012

Frette and Hemmer recently showed that for a simple model for the boarding of an airplane, the mean time to board scales as a power law with the number of passengers N and the exponent is less than 1. They note that this scaling leads to the prediction that the "back-to-front" strategy, where passengers are divided into groups from contiguous ranges of rows and each group is allowed to board in turn from back to front once the previous group has found their seats, has a longer boarding time than would a single group. Here I extend their results to a larger number of passengers using a sampling approach and explore a scenario where the queue is presorted into groups from back to front, but allowed to enter the plane as soon as they can. I show that the power law dependence on passenger numbers is different for large N and that there is a boarding time reduction for presorted groups, with a power law dependence on the number of presorted groups. © 2011 Published by the American Physical Society.

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