%0 Journal Article %J PHYSICAL REVIEW B %D 2016 %T Bounds on nanoscale nematicity in single-layer FeSe/SrTiO3 %A Huang, Dennis %A Webb, Tatiana A. %A Fang, Shiang %A Song, Can-Li %A Chang, Cui-Zu %A Moodera, Jagadeesh S. %A Efthimios Kaxiras %A Hoffman, Jennifer E. %X We use scanning tunneling microscopy (STM) and quasiparticle interference (QPI) imaging to investigate the low-energy orbital texture of single-layer FeSe/SrTiO3. We develop a T-matrix model of multiorbital QPI to disentangle scattering intensities from Fe 3d(xz) and 3d(yz) bands, enabling the use of STM as a nanoscale detection tool of nematicity. By sampling multiple spatial regions of a single-layer FeSe/SrTiO3 film, we quantitatively exclude static xz/yz orbital ordering with domain size larger than delta r(2) = 20 nm x 20 nm, xz/yz Fermi wave vector difference larger than delta k = 0.014 pi, and energy splitting larger than delta E = 3.5 meV. The lack of detectable ordering pinned around defects places qualitative constraints on models of fluctuating nematicity. %B PHYSICAL REVIEW B %V 93 %8 MAR 18 %G eng %N 12 %R 10.1103/PhysRevB.93.125129 %0 Journal Article %J PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES %D 2016 %T Catalyst design for enhanced sustainability through fundamental surface chemistry %A Personick, Michelle L. %A Montemore, Matthew M. %A Efthimios Kaxiras %A Madix, Robert J. %A Biener, Juergen %A Cynthia M. Friend %X Decreasing energy consumption in the production of platform chemicals is necessary to improve the sustainability of the chemical industry, which is the largest consumer of delivered energy. The majority of industrial chemical transformations rely on catalysts, and therefore designing new materials that catalyse the production of important chemicals via more selective and energy-efficient processes is a promising pathway to reducing energy use by the chemical industry. Efficiently designing new catalysts benefits from an integrated approach involving fundamental experimental studies and theoretical modelling in addition to evaluation of materials under working catalytic conditions. In this review, we outline this approach in the context of a particular catalyst-nanoporous gold (npAu)-which is an unsupported, dilute AgAu alloy catalyst that is highly active for the selective oxidative transformation of alcohols. Fundamental surface science studies on Au single crystals and AgAu thin-film alloys in combination with theoretical modelling were used to identify the principles which define the reactivity of npAu and subsequently enabled prediction of new reactive pathways on this material. Specifically, weak van der Waals interactions are key to the selectivity of Au materials, including npAu. We also briefly describe other systems in which this integrated approach was applied. %B PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES %V 374 %8 FEB 28 %G eng %N 2061 %R 10.1098/rsta.2015.0077 %0 Journal Article %J PHYSICAL CHEMISTRY CHEMICAL PHYSICS %D 2016 %T Controlling O coverage and stability by alloying Au and Ag %A Montemore, Matthew M. %A Ekin D. Cubuk %A Klobas, J. Eric %A Schmid, Martin %A Madix, Robert J. %A Cynthia M. Friend %A Efthimios Kaxiras %X One of the most critical factors in oxidation catalysis is controlling the state of oxygen on the surface. Au and Ag are both effective selective oxidation catalysts for various reactions, and their interactions with oxygen are critical for determining their catalytic performance. Here, we show that the state of oxygen on a catalytic surface can be controlled by alloying Au and Ag. Using temperature programmed desorption, density functional theory (DFT), and Monte Carlo simulations, we examine how alloying Au into an Ag(110) surface affects O-2 dissociation, O coverage, and O stability. DFT calculations indicate that Au resides in the second layer, in agreement with previous experimental findings. The minimum ensemble size for O-2 dissociation is 2 Ag atoms in adjacent rows of the second layer. Surprisingly, adsorbed O-2 and the dissociation transition state interact directly with metal atoms in the adjacent trough, such that Au in this position inhibits O-2 dissociation by direct repulsion with oxygen electronic states. Using Monte Carlo simulations based on DFT energetics, we create models of the surface that agree closely with our experimental results. Both show that the O-2 uptake decreases nearly linearly as the Au concentration increases, and no O-2 uptake occurs for Au concentrations above 50%. For Au concentrations greater than 18%, increasing the Au concentration also decreases the stability of the adsorbed O. Based on these results, the O coverage and O stability can be tuned, in some cases independently. We also study how the reactivity of the surface is affected by these factors using CO2 oxidation as a simple test reaction. %B PHYSICAL CHEMISTRY CHEMICAL PHYSICS %V 18 %P 26844-26853 %8 OCT 14 %G eng %N 38 %R 10.1039/c6cp05611c %0 Journal Article %J ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY %D 2016 %T Design principles for sustainable catalysis from fundamental surface chemistry %A Cynthia Friend %A Stavros Karakalos %A Fanny Hiebel %A Yunfei Xu %A Chen, Wei %A Kabeer, Fairoja %A Montemore, Matthew %A Zugic, Branko %A Biener, Juergen %A Alexandre Tkatchenko %A Efthimios Kaxiras %A Robert Madix %B ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY %V 251 %8 MAR 13 %G eng %0 Journal Article %J SURFACE SCIENCE %D 2016 %T Direct visualization of quasi-ordered oxygen chain structures on Au(110)-(1 x 2) %A Hiebel, F. %A Montemore, M. M. %A E. Kaxiras %A C.M. Friend %X The Au(110) surface offers unique advantages for atomically-resolved model studies of catalytic oxidation processes on gold. We investigate the adsorption of oxygen on Au(110) using a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) methods. We identify the typical (empty-states) STM contrast resulting from adsorbed oxygen as atomic-sized dark features of electronic origin. DFT-based image simulations confirm that chemisorbed oxygen is generally detected indirectly, from the binding -induced electronic structure modification of gold. STM images show that adsorption occurs without affecting the general structure of the pristine Au(110) missing-row reconstruction. The tendency to form one-dimensional structures is observed already at low coverage (<0.05 ML), with oxygen adsorbing on alternate sides of the reconstruction ridges. Consistently, calculations yield preferred adsorption on the (111) facets of the reconstruction, on a 3-fold coordination site, with increased stability when adsorbed in chains. Gold atoms with two oxygen neighbors exhibit enhanced electronic hybridization with the O states. Finally, the species observed are reactive to CO oxidation at 200 K and desorption of CO2 leaves a clean and ordered gold surface. (C) 2015 Elsevier B.V. All rights reserved, %B SURFACE SCIENCE %V 650 %P 5-10 %8 AUG %G eng %N SI %R 10.1016/j.susc.2015.09.018 %0 Journal Article %J PHYSICAL REVIEW B %D 2016 %T Dual role of Fe dopants in enhancing stability and charge transfer in (Li0.8Fe0.2)OHFeSe superconductors %A Chen, Wei %A Zeng, Changgan %A Efthimios Kaxiras %A Zhenyu Zhang %X The recently discovered (Li0.8Fe0.2)OHFeSe superconductor provides a new platform for exploiting the microscopic mechanisms of high-T-c superconductivity in FeSe-derived systems. Using density functional theory calculations, we first show that substitution of Li by Fe not only significantly strengthens the attraction between the (Li0.8Fe0.2)OH spacing layers and the FeSe superconducting layers along the c axis but also minimizes the lattice mismatch between the two in the ab plane, both favorable for stabilizing the overall structure. Next, we explore the electron injection into FeSe from the spacing layers and unambiguously identify the Fe-0.2 components to be the origin of the dramatically enhanced interlayer charge transfer. We further reveal that the system strongly favors collinear antiferromagnetic ordering in the FeSe layers, but the spacing layers can be either antiferromagnetic or ferromagnetic depending on the Fe-0.2 spatial distribution. Based on these insights, we predict (Li0.8Co0.2)OHFeSe to be structurally stable with even larger electron injection and potentially higher T-c. %B PHYSICAL REVIEW B %V 93 %8 FEB 25 %G eng %N 6 %R 10.1103/PhysRevB.93.064517 %0 Journal Article %J NANO LETTERS %D 2016 %T Dumbbell Defects in FeSe Films: A Scanning Tunneling Microscopy and First-Principles Investigation %A Huang, Dennis %A Webb, Tatiana A. %A Song, Can-Li %A Chang, Cui-Zu %A Moodera, Jagadeesh S. %A Efthimios Kaxiras %A Hoffinan, Jennifer E. %X The properties of iron-based superconductors (Fe-SCs) can be varied dramatically with the introduction of dopants and atomic defects. As a pressing example, FeSe, parent phase of the highest-T-c Fe-SC, exhibits prevalent defects with atomic-scale ``dumbbell'' signatures as imaged by scanning tunneling microscopy (STM). These defects spoil superconductivity when their concentration exceeds 2.5%. Resolving their chemical identity is a prerequisite to applications such as nanoscale patterning of superconducting/nonsuperconducting regions in FeSe as well as fundamental questions such as the mechanism of superconductivity and the path by which the defects destroy it. We use STM and density functional theory to characterize and identify the dumbbell defects. In contrast to previous speculations about Se adsorbates or substitutions, we find that an Fe-site vacancy is the most energetically favorable defect in Se-rich conditions and reproduces our observed STM signature. Our calculations shed light more generally on the nature of Se capping, the removal of Fe vacancies via annealing, and their ordering into a root 5 x root 5 superstructure in FeSe and related alkali-doped compounds. %B NANO LETTERS %V 16 %P 4224-4229 %8 JUL %G eng %N 7 %R 10.1021/acs.nanolett.6b01163 %0 Journal Article %J PHILOSOPHICAL MAGAZINE %D 2016 %T The effect of Ag, Pb and Bi impurities on grain boundary sliding and intergranular decohesion in Copper %A Georg Schusteritsch %A Kuehne, Thomas D. %A Guo, Zheng Xiao %A Efthimios Kaxiras %X We investigate the changes in grain boundary sliding (GBS) and intergranular decohesion in copper (Cu), due to the inclusion of bismuth (Bi), lead (Pb) and silver (Ag) substitutional impurity atoms at a Sigma 5 (012) symmetric tilt grain boundary (GB), using a first-principles concurrent multiscale approach. We first study the segregation behavior of the impurities by determining the impurity segregation energy in the vicinity of the GB. We find that the energetically preferred sites are on the GB plane. We investigate the intergranular decohesion of Cu by Bi and Pb impurities and compare this to the effect of Ag impurities by considering the work of separation, Ws and the tensile strength, st. Both Ws and st decrease in the presence of Bi and Pb impurities, indicating their great propensity for intergranular embrittlement, whilst the presence of Ag impurities has only a small effect. We consider GBS to assess the mechanical properties in nanocrystalline metals and find that all three impurities strongly inhibit GBS, with Ag having the biggest effect. This suggests that Ag has a strong effect on the mechanical properties of nanocrystalline Cu, even though its effect on the intergranular decohesion properties of coarse-grained Cu is not significant. %B PHILOSOPHICAL MAGAZINE %V 96 %P 2868-2886 %G eng %N 27, A %R 10.1080/14786435.2016.1217360 %0 Journal Article %J JOURNAL OF COMPUTATIONAL SCIENCE %D 2016 %T Effects of Knudsen diffusivity on the effective reactivity Of nanoporous catalyst media %A Montessori, A. %A Prestininzi, P. %A La Rocca, M. %A Falcucci, G. %A S. Succi %A E. Kaxiras %X We investigate the non-equilibrium hydrodynamic effects on the reactivity of a nanoporous catalytic sample. Numerical simulations using the Lattice Boltzmann Method (LBM) show that non-equilibrium effects enhance the reactivity of the porous sample, in agreement with theoretical predictions [1]. In addition, we provide a quantitative assessment of the reactivity in terms of the thickness of the reactive layer inside the nanoporous catalytic sample. Such an assessment constitutes a first step towards integrated simulations encompassing nanoscale reactivity and transport coefficients within a macroscale description of experimental relevance. (C) 2016 Elsevier B.V. All rights reserved. %B JOURNAL OF COMPUTATIONAL SCIENCE %V 17 %P 377-383 %8 NOV %G eng %N 2, SI %R 10.1016/j.jocs.2016.04.006 %0 Journal Article %J PHYSICAL REVIEW B %D 2016 %T Electric field tuning of band offsets in transition metal dichalcogenides %A Huang, Dennis %A Efthimios Kaxiras %X We use first-principles calculations to investigate the band structure evolution of WX2/MoX2 (X = S, Se) heterobilayers under a perpendicular electric field. We characterize the extent to which the type II band alignment in these compounds can be tuned or inverted electrostatically. Our results demonstrate two effects of the stacking configuration. First, different stackings produce different net dipole moments, resulting in band offset variations that are larger than 0.1 eV. Second, based on symmetry constraints that depend on stacking, a perpendicular electric field may hybridizeWX(2) and MoX2 bands that cross at the Brillouin zone corner K. Our results suggest that external electric fields can be used to tune the physics of intralayer and interlayer excitons in heterobilayers of transition metal dichalcogenides. %B PHYSICAL REVIEW B %V 94 %8 DEC 21 %G eng %N 24 %R 10.1103/PhysRevB.94.241303 %0 Journal Article %J PHYSICAL REVIEW B %D 2016 %T Electronic structure theory of weakly interacting bilayers %A Fang, Shiang %A Efthimios Kaxiras %X We derive electronic structure models for weakly interacting bilayers such as graphene-graphene and graphene-hexagonal boron nitride, based on density functional theory calculations followed by Wannier transformation of electronic states. These transferable interlayer coupling models can be applied to investigate the physics of bilayers with arbitrary translations and twists. The functional form, in addition to the dependence on the distance, includes the angular dependence that results from higher angular momentum components in the Wannier p(z) orbitals. We demonstrate the capabilities of the method by applying it to a rotated graphene bilayer, which produces the analytically predicted renormalization of the Fermi velocity, Van Hove singularities in the density of states, and moire pattern of the electronic localization at small twist angles. We further extend the theory to obtain the effective couplings by integrating out neighboring layers. This approach is instrumental for the design of van der Walls heterostructures with desirable electronic features and transport properties and for the derivation of low-energy theories for graphene stacks, including proximity effects from other layers. %B PHYSICAL REVIEW B %V 93 %8 JUN 27 %G eng %N 23 %R 10.1103/PhysRevB.93.235153 %0 Journal Article %J NANO LETTERS %D 2016 %T Enhancing the Hydrogen Activation Reactivity of Nonprecious Metal Substrates via Confined Catalysis Underneath Graphene %A Zhou, Yinong %A Chen, Wei %A Ping Cui %A Zeng, Jiang %A Lin, Zhuonan %A Efthimios Kaxiras %A Zhenyu Zhang %X In the hydrogen evolution reaction (HER), the reactivity as a function of the hydrogen adsorption energy on different metal substrates follows a well-known;volcano curve, peaked a the precious inetal Pt. The goal. of turning nonprecious metals into efficient catalysts for HER and other important chemical reactions is a :fundamental challenge; it is-also of technological significance. Here, we present results toward, achieving Ihis goal by exploiting the synergistic power of marginal catalysis and confined catalysis. Using density functional theory calculations, we first show that the volcano curve stays qualitatively intact when van der Waals attractions between a hydrogen adatom and different metal (111) surfaces are included. We further show that the hydrogen adsorption:energy on the metal: surfaces is weakened by 0.11-0.23 eV when hydrogen is confinedbetween graphene and the metal surfaces, with Ni exhibiting the largest change. Inparticular, we find that the graphene-modified volcano curve peaks around Ni, whose bare surface already possesses moderate (or :marginal) reactivity,i and the corresponding HER rate of grapllene-covered comparable to that of bare Pt. A hydrogen adatom has high mobility within the confined geometry. These findings demonstrate that graphene-coyeted Ni is an appealing effective stable, and economical catalytic platform for HER. %B NANO LETTERS %V 16 %P 6058-6063 %8 OCT %G eng %N 10 %R 10.1021/acs.nanolett.6b02052 %0 Journal Article %J PHYSICAL REVIEW B %D 2016 %T Epsilon-near-zero behavior from plasmonic Dirac point: Theory and realization using two-dimensional materials %A Marios Mattheakis %A Constantinos A. Valagiannopoulos %A Efthimios Kaxiras %X The electromagnetic response of a two-dimensional metal embedded in a periodic array of a dielectric host can give rise to a plasmonic Dirac point that emulates epsilon-near-zero (ENZ) behavior. This theoretical result is extremely sensitive to structural features like periodicity of the dielectric medium and thickness imperfections. We propose that such a device can actually be realized by using graphene as the two-dimensional metal and materials like the layered semiconducting transition-metal dichalcogenides or hexagonal boron nitride as the dielectric host. We propose a systematic approach, in terms of design characteristics, for constructing metamaterials with linear, elliptical, and hyperbolic dispersion relations which produce ENZ behavior, normal or negative diffraction. %B PHYSICAL REVIEW B %V 94 %8 NOV 10 %G eng %N 20 %R 10.1103/PhysRevB.94.201404 %0 Journal Article %J JOURNAL OF CHEMICAL THEORY AND COMPUTATION %D 2016 %T eReaxFF: A Pseudoclassical Treatment of Explicit Electrons within Reactive Force Field Simulations %A Islam, Md Mahbubul %A Grigory Kolesov %A Verstraelen, Toon %A Efthimios Kaxiras %A van Duin, Adri C. T. %X We present a computational tool, eReaxFF, for simulating explicit electrons within the framework of the standard ReaxFF reactive force field method. We treat electrons explicitly in a pseudoclassical manner that enables simulation several orders of magnitude faster than quantum chemistry (QC) methods, while retaining the ReaxFF transferability. We delineate here the fundamental concepts of the eReaxFF method and the integration of the Atom condensed Kohn-Sham DFT approximated to second order (ACKS2) charge calculation scheme into the eReaxFF. We trained our force field to capture electron affinities (EA) of various species. As a proof-of-principle, we performed a set of molecular dynamics (MD) simulations with an explicit electron model for representative hydrocarbon radicals. We establish a good qualitative agreement of EAs of various species with experimental data, and MD simulations with eReaxFF agree well with the corresponding Ehrenfest dynamics simulations. The standard ReaxFF parameters available in the literature are transferrable to the eReaxFF method. The computationally economic eReaxFF method will be a useful tool for studying large-scale chemical and physical systems with explicit electrons as an alternative to computationally demanding QC methods. %B JOURNAL OF CHEMICAL THEORY AND COMPUTATION %V 12 %P 3463-3472 %8 AUG %G eng %N 8 %R 10.1021/acs.jctc.6b00432 %0 Journal Article %J SCIENTIFIC REPORTS %D 2016 %T Establishing the limits of efficiency of perovskite solar cells from first principles modeling %A Granas, Oscar %A Dmitry Vinichenko %A Efthimios Kaxiras %X The recent surge in research on metal-halide-perovskite solar cells has led to a seven-fold increase of efficiency, from similar to 3% in early devices to over 22% in research prototypes. Oft-cited reasons for this increase are: (i) a carrier diffusion length reaching hundreds of microns; (ii) a low exciton binding energy; and (iii) a high optical absorption coefficient. These hybrid organic-inorganic materials span a large chemical space with the perovskite structure. Here, using first-principles calculations and thermodynamic modelling, we establish that, given the range of band-gaps of the metal-halide-perovskites, the theoretical maximum efficiency limit is in the range of similar to 25-27%. Our conclusions are based on the effect of level alignment between the perovskite absorber layer and carrier-transporting materials on the performance of the solar cell as a whole. Our results provide a useful framework for experimental searches toward more efficient devices. %B SCIENTIFIC REPORTS %V 6 %8 NOV 8 %G eng %R 10.1038/srep36108 %0 Journal Article %J 2D MATERIALS %D 2016 %T Excitation intensity dependence of photoluminescence from monolayers of MoS2 and WS2/MoS2 heterostructures %A Kaplan, D. %A Gong, Y. %A Mills, K. %A Swaminathan, V. %A Ajayan, P. M. %A Shirodkar, S. %A E. Kaxiras %X A detailed study of the excitation dependence of the photoluminescence (PL) from monolayers of MoS2 and WS2/MoS2 heterostructures grown by chemical vapor deposition on Si substrates has revealed that the luminescence from band edge excitons from MoS2 monolayers shows a linear dependence on excitation intensity for both above band gap and resonant excitation conditions. In particular, a band separated by similar to 55 meV from the A exciton, referred to as the C band, shows the same linear dependence on excitation intensity as the band edge excitons. A band similar to the C band has been previously ascribed to a trion, a charged, three-particle exciton. However, in our study the C band does not show the 3/2 power dependence on excitation intensity as would be expected for a three-particle exciton. Further, the PL from the MoS2 monolayer in a bilayer WS2/MoS2 heterostructure, under resonant excitation conditions where only the MoS2 absorbs the laser energy, also revealed a linear dependence on excitation intensity for the C band, confirming that its origin is not due to a trion but instead a bound exciton, presumably of an unintentional impurity or a native point defect such as a sulfur vacancy. The PL from the WS2/MoS2 heterostructure, under resonant excitation conditions also showed additional features which are suggested to arise from the interface states at the heteroboundary. Further studies are required to clearly identify the origin of these features. %B 2D MATERIALS %V 3 %8 MAR %G eng %N 1 %R 10.1088/2053-1583/3/1/015005 %0 Journal Article %J PHYSICAL REVIEW LETTERS %D 2016 %T High-Temperature Quantum Anomalous Hall Effect in n-p Codoped Topological Insulators %A Qi, Shifei %A Qiao, Zhenhua %A Deng, Xinzhou %A Ekin D. Cubuk %A Chen, Hua %A Wenguang Zhu %A Efthimios Kaxiras %A Zhang, S. B. %A Xu, Xiaohong %A Zhenyu Zhang %X The quantum anomalous Hall effect (QAHE) is a fundamental quantum transport phenomenon that manifests as a quantized transverse conductance in response to a longitudinally applied electric field in the absence of an external magnetic field, and it promises to have immense application potential in future dissipationless quantum electronics. Here, we present a novel kinetic pathway to realize the QAHE at high temperatures by n-p codoping of three-dimensional topological insulators. We provide a proof-of-principle numerical demonstration of this approach using vanadium-iodine (V-I) codoped Sb2Te3 and demonstrate that, strikingly, even at low concentrations of similar to 2% V and similar to 1% I, the system exhibits a quantized Hall conductance, the telltale hallmark of QAHE, at temperatures of at least similar to 50 K, which is 3 orders of magnitude higher than the typical temperatures at which it has been realized to date. The underlying physical factor enabling this dramatic improvement is tied to the largely preserved intrinsic band gap of the host system upon compensated n-p codoping. The proposed approach is conceptually general and may shed new light in experimental realization of high-temperature QAHE. %B PHYSICAL REVIEW LETTERS %V 117 %8 JUL 27 %G eng %N 5 %R 10.1103/PhysRevLett.117.056804 %0 Journal Article %J JOURNAL OF PHYSICAL CHEMISTRY C %D 2016 %T How Does Nanoporous Gold Dissociate Molecular Oxygen? %A Montemore, Matthew M. %A Madix, Robert J. %A Efthimios Kaxiras %X Nanoporous Au and other dilute AgAu alloys are highly active and selective oxidation catalysts. Their ability to dissociate O-2 is to a large extent unexplained, given that unsupported Au cannot generally dissociate O-2 while large ensembles of Ag atoms (>4) are generally necessary to lower the O-2 dissociation barrier significantly. Here, we identify a site on the surface of dilute AgAu alloys that is stable under reaction conditions and has a low O-2 dissociation barrier, in agreement with experimental measurements. Although Ag generally prefers to disperse throughout Au, the presence of adsorbed O near surface steps creates sites of high local Ag concentration, where the Ag, atoms sit in the rows next to the step Au atoms. O-2 adsorbs on the Au step atoms, but the transition state involves significant Ag-O interaction, resulting in a barrier lower than expected from the adsorption energies of either the initial or final state. %B JOURNAL OF PHYSICAL CHEMISTRY C %V 120 %P 16636-16640 %8 AUG 4 %G eng %N 30 %R 10.1021/acs.jpcc.6b03371 %0 Journal Article %J PHYSICAL REVIEW B %D 2016 %T Li intercalation at graphene/hexagonal boron nitride interfaces %A Shirodkar, Sharmila N. %A Efthimios Kaxiras %X Intercalation of Li in graphite and other layered structures is of interest for highly efficient energy storage devices. In this paper, we determine the extent to which Li intercalates at the different interfaces formed between graphene (G) and hexagonal boron nitride (hBN) heterostructures. We use ab initio calculations to explore in detail the position of the dispersed Li atoms, changes in the structure at the interfaces, energetic stability of the configurations, and the corresponding electronic structure with varying concentrations of the intercalant. We trace the origin of the energetic stability and maximum concentration of Li that intercalates into various layered structures to the ability of the interface to accept electrons. Our calculations indicate that Li intercalates easiest at G/G interfaces, followed by interfaces between G/hBN, whereas Li cannot intercalate in hBN/hBN interfaces. Our results provide a framework for the design of experimental setups with optimal Li intercalation and reveal the implications of intercalation on the dielectric properties of these materials and their possible application in plasmonics. %B PHYSICAL REVIEW B %V 93 %8 JUN 30 %G eng %N 24 %R 10.1103/PhysRevB.93.245438 %0 Journal Article %J JOURNAL OF CHEMICAL THEORY AND COMPUTATION %D 2016 %T Long-Time Dynamics through Parallel Trajectory Splicing %A Perez, Danny %A Ekin D. Cubuk %A Amos Waterland %A Efthimios Kaxiras %A Voter, Arthur F. %X Simulating the atomistic evolution of materials over long time scales is a longstanding challenge, especially for complex systems where the distribution of barrier heights is very heterogeneous. Such systems are difficult to investigate using conventional long-time scale techniques, and the fact that they tend to remain trapped in small regions of configuration space for extended periods of time strongly limits the physical insights gained from short simulations. We introduce a novel simulation technique, Parallel Trajectory Splicing (Par Splice), that aims at addressing this problem through the timewise parallelization of long trajectories. The computational efficiency of Par Splice stems from a speculation strategy whereby predictions of the future evolution of the system are leveraged to increase the amount of work that can be concurrently performed at any one time, hence improving the scalability of the method. ParSplice is also able to accurately account for, and potentially reuse, a substantial fraction of the computational work invested in the simulation. We validate the method on a simple Ag surface system and demonstrate substantial increases in efficiency compared to previous methods. We then demonstrate the power of ParSplice through the study of topology changes in Ag42Cu13 core shell nanoparticles. %B JOURNAL OF CHEMICAL THEORY AND COMPUTATION %V 12 %P 18-28 %8 JAN %G eng %N 1 %R 10.1021/acs.jctc.5b00916 %0 Journal Article %J MICROFLUIDICS AND NANOFLUIDICS %D 2016 %T Mapping reactive flow patterns in monolithic nanoporous catalysts %A Falcucci, Giacomo %A Sauro Succi %A Montessori, Andrea %A Simone Melchionna %A Prestininzi, Pietro %A Barroo, Cedric %A Bell, David C. %A Biener, Monika M. %A Biener, Juergen %A Zugic, Branko %A Efthimios Kaxiras %X The development of high-efficiency porous catalyst membranes critically depends on our understanding of where the majority of the chemical conversions occur within the porous structure. This requires mapping of chemical reactions and mass transport inside the complex nanoscale architecture of porous catalyst membranes which is a multiscale problem in both the temporal and spatial domains. To address this problem, we developed a multiscale mass transport computational framework based on the lattice Boltzmann method that allows us to account for catalytic reactions at the gas-solid interface by introducing a new boundary condition. In good agreement with experiments, the simulations reveal that most catalytic reactions occur near the gas-flow facing side of the catalyst membrane if chemical reactions are fast compared to mass transport within the porous catalyst membrane. %B MICROFLUIDICS AND NANOFLUIDICS %V 20 %8 JUL %G eng %N 7 %R 10.1007/s10404-016-1767-5 %0 Journal Article %J NANO LETTERS %D 2016 %T MoS2 Field-Effect Transistor with Sub-10 nm Channel Length %A Amirhasan Nourbakhsh %A Ahmad Zubair %A Redwan N. Sajjad %A Tavakkoli, Amir K. G. %A Chen, Wei %A Fang, Shiang %A Xi Ling %A Jing Kong %A Dresselhaus, Mildred S. %A Efthimios Kaxiras %A Berggren, Karl K. %A Dimitri Antoniadis %A Tomas Palacios %X Atomically thin molybdenum disulfide (MoS2) is an ideal semiconductor material for field-effect transistors (FETs) with sub-10 nm channel lengths. The high effective mass and large bandgap of MoS2 minimize direct source-drain tunneling, while its atomically thin body maximizes the gate modulation efficiency in ultrashort-channel transistors. However, no experimental study to date has approached the sub-10 nm scale due to the multiple challenges related to nanofabrication at this length scale and the high contact resistance traditionally observed in MoS2 transistors. Here, using the semiconducting-to-metallic phase transition of MoS2, we demonstrate sub-10 nm channel-length transistor fabrication by directed self-assembly patterning of mono- and trilayer MoS2. This is done in a 7.5 nm half-pitch periodic chain of transistors where semiconducting (2H) MoS2 channel regions are seamlessly connected to metallic-phase (1T') MoS2 access and contact regions. The resulting 7.5 nm channel-length MoS2 FET has a low off-current of 10 pA/mu m, an on/off current ratio of >10(7), and a subthreshold swing of 120 mV/dec. The experimental results presented in this work, combined with device transport modeling, reveal the remarkable potential of 2D MoS2 for future sub-10 nm technology nodes. %B NANO LETTERS %V 16 %P 7798-7806 %8 DEC %G eng %N 12 %R 10.1021/acs.nanolett.6b03999 %0 Journal Article %J ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY %D 2016 %T Non-adiabatic effects and electronic excitations during dissociation on catalytic surfaces %A Montemore, Matthew %A Robert Hoyt %A Efthimios Kaxiras %B ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY %V 252 %8 AUG 20 %G eng %0 Journal Article %J ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY %D 2016 %T Non-adiabatic energy dissipation in dissociation on catalytic surface %A Montemore, Matthew %A Robert Hoyt %A Efthimios Kaxiras %B ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY %V 251 %8 MAR 13 %G eng %0 Journal Article %J JOURNAL OF THE AMERICAN CHEMICAL SOCIETY %D 2016 %T Noncovalent Bonding Controls Selectivity in Heterogeneous Catalysis: Coupling Reactions on Gold %A Stavros Karakalos %A Yunfei Xu %A Kabeer, Fairoja Cheenicode %A Chen, Wei %A Rodriguez-Reyes, Juan Carlos F. %A Alexandre Tkatchenko %A Efthimios Kaxiras %A Madix, Robert J. %A Cynthia M. Friend %X Enhancing the selectivity of catalytic processes has potential for substantially increasing the sustainability of chemical production. Herein, we establish relationships between reaction selectivity and molecular structure for a homologous series of key intermediates for oxidative coupling of alcohols on gold using a combination of experiment and theory. We establish a scale of binding for molecules with different alkyl structures and chain lengths and thereby demonstrate the critical nature of noncovalent van der Waals interactions in determining the selectivity by modulating the stability of key reaction intermediates bound to the surface. The binding hierarchy is the same for Au(111) and Au(110), which demonstrates a relative lack of sensitivity to the surface structure. The hierarchy of binding established in this work provides guiding principles for predicting how molecular structure affects the competition for binding sites more broadly. Besides the nature of the primary surface-molecule bonding, three additional factors that affect the stabilities of the reactive intermediates are clearly established: (1) the number of C atoms in the alkyl chain, (2) the presence of C-C bond unsaturation, and (3) the degree of branching of the alkyl group of the adsorbed molecules. We suggest that this is a fundamental principle that is generally applicable to a broad range of reactions on metal catalysts. %B JOURNAL OF THE AMERICAN CHEMICAL SOCIETY %V 138 %P 15243-15250 %8 NOV 23 %G eng %N 46 %R 10.1021/jacs.6b09450 %0 Journal Article %J JOURNAL OF MATERIALS RESEARCH %D 2016 %T Perturbation theory for weakly coupled two-dimensional layers %A Tritsaris, Georgios A. %A Shirodkar, Sharmila N. %A Efthimios Kaxiras %A Cazeaux, Paul %A Luskin, Mitchell %A Plechac, Petr %A Cances, Eric %X A key issue in two-dimensional structures composed of atom-thick sheets of electronic materials is the dependence of the properties of the combined system on the features of its parts. Here, we introduce a simple framework for the study of the electronic structure of layered assemblies based on perturbation theory. Within this framework, we calculate the band structure of commensurate and twisted bilayers of graphene (Gr) and hexagonal boron nitride (h-BN), and of a Gr/h-BN heterostructure, which we compare with reference full-scale density functional theory calculations. This study presents a general methodology for computationally efficient calculations of two-dimensional materials and also demonstrates that for relatively large twist in the graphene bilayer, the perturbation of electronic states near the Fermi level is negligible. %B JOURNAL OF MATERIALS RESEARCH %V 31 %P 959-966 %8 APR 14 %G eng %N 7 %R 10.1557/jmr.2016.99 %0 Journal Article %J JOURNAL OF CHEMICAL THEORY AND COMPUTATION %D 2016 %T Real-Time TD-DFT with Classical Ion Dynamics: Methodology and Applications %A Grigory Kolesov %A Granas, Oscar %A Robert Hoyt %A Dmitry Vinichenko %A Efthimios Kaxiras %X We present a method for real-time propagation of electronic wave functions, within time-dependent density functional theory (RT-TDDFT), coupled to ionic motion through mean-field classical dynamics. The goal of our method is to treat large systems and complex processes, in particular photocatalytic reactions and electron transfer events on surfaces and thin films. Due to the complexity of these processes, computational approaches are needed to provide insight into the underlying physical mechanisms and are therefore crucial for the rational design of new materials. Because of the short time step required for electron propagation (of order similar to 10 attoseconds), these simulations are computationally very demanding. Our methodology is based on numerical atomic-orbital-basis sets for computational efficiency. In the computational package, to which we refer as TDAP-2.0 (Time-evolving Deterministic Atom Propagator), we have implemented a number of important features and analysis tools for more accurate and efficient treatment of large, complex systems and time scales that reach into a fraction of a picosecond. We showcase the capabilities of our method using four different examples: (i) photodissociation into radicals of opposite spin, (ii) hydrogen adsorption on aluminum surfaces, (iii) optical absorption of spin-polarized organic molecule containing a metal ion, and (iv) electron transfer in a prototypical dye sensitized solar cell. %B JOURNAL OF CHEMICAL THEORY AND COMPUTATION %V 12 %P 466-476 %8 FEB %G eng %N 2 %R 10.1021/acs.jctc.5b00969 %0 Journal Article %J NATURE COMMUNICATIONS %D 2016 %T Self-assembly of acetate adsorbates drives atomic rearrangement on the Au(110) surface %A Fanny Hiebel %A Bonggeun Shong %A Chen, Wei %A Madix, Robert J. %A Efthimios Kaxiras %A Cynthia M. Friend %X Weak inter-adsorbate interactions are shown to play a crucial role in determining surface structure, with major implications for its catalytic reactivity. This is exemplified here in the case of acetate bound to Au(110), where the small extra energy of the van der Waals interactions among the surface-bound groups drives massive restructuring of the underlying Au. Acetate is a key intermediate in electro-oxidation of CO2 and a poison in partial oxidation reactions. Metal atom migration originates at surface defects and is likely facilitated by weakened Au-Au interactions due to bonding with the acetate. Even though the acetate is a relatively small molecule, weak intermolecular interaction provides the energy required for molecular self-assembly and reorganization of the metal surface. %B NATURE COMMUNICATIONS %V 7 %8 OCT 12 %G eng %R 10.1038/ncomms13139 %0 Journal Article %J PHYSICAL REVIEW B %D 2016 %T Strain dependence of band gaps and exciton energies in pure and mixed transition-metal dichalcogenides %A Defo, Rodrick Kuate %A Fang, Shiang %A Shirodkar, Sharmila N. %A Tritsaris, Georgios A. %A Dimoulas, Athanasios %A Efthimios Kaxiras %X {{The ability to fabricate 2D device architectures with desired properties, based on stacking of weakly (van der Waals) interacting atomically thin layers, is quickly becoming reality. In order to design ever more complex devices of this type, it is crucial to know the precise strain and composition dependence of the layers' electronic and optical properties. Here, we present a theoretical study of these dependences for monolayers with compositions varying from pure MX2 to the mixed MXY, where M = Mo, W and X %B PHYSICAL REVIEW B %V 94 %8 OCT 27 %G eng %N 15 %R 10.1103/PhysRevB.94.155310 %0 Journal Article %J NATURE PHYSICS %D 2016 %T A structural approach to relaxation in glassy liquids %A S. S. Schoenholz, %A Cubuk, E. D. %A Sussman, D. M. %A E. Kaxiras %A Liu, A. J. %X In contrast with crystallization, there is no noticeable structural change at the glass transition. Characteristic features of glassy dynamics that appear below an onset temperature, T-0 (refs 1-3), are qualitatively captured by mean field theory(4-6), which assumes uniform local structure. Studies of more realistic systems have found only weak correlations between structure and dynamics(7-11). This raises the question: is structure important to glassy dynamics in three dimensions? We answer this question affirmatively, using machine learning to identify a new field, `softness' which characterizes local structure and is strongly correlated with dynamics. We find that the onset of glassy dynamics at T-0 corresponds to the onset of correlations between softness (that is, structure) and dynamics. Moreover, we construct a simple model of relaxation that agrees well with our simulation results, showing that a theory of the evolution of softness in time would constitute a theory of glassy dynamics. %B NATURE PHYSICS %V 12 %P 469+ %8 MAY %G eng %N 5 %R 10.1038/NPHYS3644 %0 Journal Article %J JOURNAL OF PHYSICAL CHEMISTRY B %D 2016 %T Structural Properties of Defects in Glassy Liquids %A Ekin D. Cubuk %A Schoenholz, Samuel S. %A Efthimios Kaxiras %A Liu, Andrea J. %X At zero temperature a disordered solid corresponds to a local minimum in the energy landscape. As the temperature is raised or the system is driven with a mechanical load, the system explores different minima via dynamical events in which particles rearrange their relative positions. We have shown recently that the dynamics of particle rearrangements are strongly correlated with a structural quantity associated with each particle, ``softness'', which we can identify using supervised machine learning. Particles of a given softness have a well-defined energy scale that governs local rearrangements; because of this property, softness greatly simplifies our understanding of glassy dynamics. Here we investigate the correlation of softness with other commonly used structural quantities, such as coordination number and local potential energy. We show that although softness strongly correlates with these properties, its predictive power for rearrangement dynamics is much higher. We introduce a useful metric for quantifying the quality of structural quantities as predictors of dynamics. We hope that, in the future, authors introducing new structural measures of dynamics will compare their proposals quantitatively to softness using this metric. We also show how softness correlations give insight into rearrangements. Finally, we explore the physical meaning of softness using unsupervised dimensionality reduction and reduced curve-fitting, models, and show that softness can be recast in a form that is amenable to analytical treatment. %B JOURNAL OF PHYSICAL CHEMISTRY B %V 120 %P 6139-6146 %8 JUL 7 %G eng %N 26 %R 10.1021/acs.jpcb.6b02144 %0 Journal Article %J ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY %D 2016 %T Structure and reactivity of AgAu Alloys %A Montemore, Matthew %A Efthimios Kaxiras %B ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY %V 251 %8 MAR 13 %G eng %0 Journal Article %J PHYSICAL REVIEW LETTERS %D 2016 %T Superlattice-Induced Insulating States and Valley-Protected Orbits in Twisted Bilayer Graphene %A Cao, Y. %A Luo, J. Y. %A Fatemi, V. %A Fang, S. %A Sanchez-Yamagishi, J. D. %A K. Watanabe %A T. Taniguchi %A E. Kaxiras %A P. Jarillo-Herrero %X Twisted bilayer graphene (TBLG) is one of the simplest van der Waals heterostructures, yet it yields a complex electronic system with intricate interplay between moire physics and interlayer hybridization effects. We report on electronic transport measurements of high mobility small angle TBLG devices showing clear evidence for insulating states at the superlattice band edges, with thermal activation gaps several times larger than theoretically predicted. Moreover, Shubnikov-de Haas oscillations and tight binding calculations reveal that the band structure consists of two intersecting Fermi contours whose crossing points are effectively unhybridized. We attribute this to exponentially suppressed interlayer hopping amplitudes for momentum transfers larger than the moire wave vector. %B PHYSICAL REVIEW LETTERS %V 117 %8 SEP 7 %G eng %N 11 %R 10.1103/PhysRevLett.117.116804 %0 Journal Article %J ACS NANO %D 2016 %T Theory of Graphene Raman Scattering %A Heller, Eric J. %A Yang, Yuan %A Kocia, Lucas %A Chen, Wei %A Fang, Shiang %A Borunda, Mario %A Efthimios Kaxiras %X Raman scattering plays a key role in unraveling the quantum dynamics of graphene, perhaps the most promising material of recent times. It is crucial to correctly interpret the meaning of the spectra. It is therefore very surprising that the widely accepted understanding of Raman scattering, i.e., Kramers Heisenberg Dirac theory, has never been applied to graphene. Doing so here, a remarkable mechanism we term''transition sliding'' is uncovered, explaining the uncommon brightness of overtones in graphene. Graphene's dispersive and fixed Raman bands, missing bands, defect density and laser frequency dependence of band intensities, widths of overtone bands, Stokes, anti -Stokes anomalies, and other known properties emerge simply and directly. %B ACS NANO %V 10 %P 2803-2818 %8 FEB %G eng %N 2 %R 10.1021/acsnano.5b07676