# Publications

2017
Cubuk ED, Malone BD, Onat B, Waterland A, Kaxiras E. Representations in neural network based empirical potentials. JOURNAL OF CHEMICAL PHYSICS. 2017;147 (2).Abstract
Many structural and mechanical properties of crystals, glasses, and biological macromolecules can be modeled from the local interactions between atoms. These interactions ultimately derive from the quantum nature of electrons, which can be prohibitively expensive to simulate. Machine learning has the potential to revolutionize materials modeling due to its ability to efficiently approximate complex functions. For example, neural networks can be trained to reproduce results of density functional theory calculations at a much lower cost. However, how neural networks reach their predictions is not well understood, which has led to them being used as a black box'' tool. This lack of understanding is not desirable especially for applications of neural networks in scientific inquiry. We argue that machine learning models trained on physical systems can be used as more than just approximations since they had to learn'' physical concepts in order to reproduce the labels they were trained on. We use dimensionality reduction techniques to study in detail the representation of silicon atoms at different stages in a neural network, which provides insight into how a neural network learns to model atomic interactions. Published by AIP Publishing.
Sensoy MG, Vinichenko D, Chen W, Friend CM, Kaxiras E. Strain effects on the behavior of isolated and paired sulfur vacancy defects in monolayer MoS2. PHYSICAL REVIEW B. 2017;95 (1).Abstract
We investigate the behavior of sulfur vacancy defects, the most abundant type of intrinsic defect in monolayer MoS2, using first-principles calculations based on density functional theory. We consider the dependence of the isolated defect formation energy on the charge state and on uniaxial tensile and compressive strain up to 5%. We also consider the possibility of defect clustering by examining the formation energies of pairs of vacancies at various relative positions, and their dependence on charge state and strain. We find that there is no driving force for vacancy clustering, independent of strain in the material. The barrier for diffusion of S vacancies is larger than 1.9 eV in both charged and neutral states regardless of the presence of other nearby vacancies. We conclude that the formation of extended defects from S vacancies in planar monolayer MoS2 is hindered both thermodynamically and kinetically.
Carr S, Massatt D, Fang S, Cazeaux P, Luskin M, Kaxiras E. Twistronics: Manipulating the electronic properties of two-dimensional layered structures through their twist angle. PHYSICAL REVIEW B. 2017;95 (7).Abstract
The ability in experiments to control the relative twist angle between successive layers in two-dimensional (2D) materials offers an approach to manipulating their electronic properties; we refer to this approach as twistronics.'' A major challenge to theory is that, for arbitrary twist angles, the resulting structure involves incommensurate (aperiodic) 2D lattices. Here, we present a general method for the calculation of the electronic density of states of aperiodic 2D layered materials, using parameter-free Hamiltonians derived from ab initio density-functional theory. We use graphene, a semimetal, and MoS2, a representative of the transition-metal dichalcogenide family of 2D semiconductors, to illustrate the application of our method, which enables fast and efficient simulation of multilayered stacks in the presence of local disorder and external fields. We comment on the interesting features of their density of states as a function of twist angle and local configuration and on how these features can be experimentally observed.
2016
Huang D, Webb TA, Fang S, Song C-L, Chang C-Z, Moodera JS, Kaxiras E, Hoffman JE. Bounds on nanoscale nematicity in single-layer FeSe/SrTiO3. PHYSICAL REVIEW B. 2016;93 (12).Abstract
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.
Personick ML, Montemore MM, Kaxiras E, Madix RJ, Biener J, Friend CM. Catalyst design for enhanced sustainability through fundamental surface chemistry. PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES. 2016;374 (2061).Abstract
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.
Montemore MM, Cubuk ED, Klobas JE, Schmid M, Madix RJ, Friend CM, Kaxiras E. Controlling O coverage and stability by alloying Au and Ag. PHYSICAL CHEMISTRY CHEMICAL PHYSICS. 2016;18 (38) :26844-26853.Abstract
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.
Friend C, Karakalos S, Hiebel F, Xu Y, Chen W, Kabeer F, Montemore M, Zugic B, Biener J, Tkatchenko A, et al. Design principles for sustainable catalysis from fundamental surface chemistry. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY. 2016;251.
Hiebel F, Montemore MM, Kaxiras E, Friend CM. Direct visualization of quasi-ordered oxygen chain structures on Au(110)-(1 x 2). SURFACE SCIENCE. 2016;650 (SI) :5-10.Abstract
Chen W, Zeng C, Kaxiras E, Zhang Z. Dual role of Fe dopants in enhancing stability and charge transfer in (Li0.8Fe0.2)OHFeSe superconductors. PHYSICAL REVIEW B. 2016;93 (6).Abstract
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.
Huang D, Webb TA, Song C-L, Chang C-Z, Moodera JS, Kaxiras E, Hoffinan JE. Dumbbell Defects in FeSe Films: A Scanning Tunneling Microscopy and First-Principles Investigation. NANO LETTERS. 2016;16 (7) :4224-4229.Abstract
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.
Schusteritsch G, Kuehne TD, Guo ZX, Kaxiras E. The effect of Ag, Pb and Bi impurities on grain boundary sliding and intergranular decohesion in Copper. PHILOSOPHICAL MAGAZINE. 2016;96 (27, A) :2868-2886.Abstract
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.
Montessori A, Prestininzi P, La Rocca M, Falcucci G, Succi S, Kaxiras E. Effects of Knudsen diffusivity on the effective reactivity Of nanoporous catalyst media. JOURNAL OF COMPUTATIONAL SCIENCE. 2016;17 (2, SI) :377-383.Abstract
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.
Huang D, Kaxiras E. Electric field tuning of band offsets in transition metal dichalcogenides. PHYSICAL REVIEW B. 2016;94 (24).Abstract
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.
Fang S, Kaxiras E. Electronic structure theory of weakly interacting bilayers. PHYSICAL REVIEW B. 2016;93 (23).Abstract
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.
Zhou Y, Chen W, Cui P, Zeng J, Lin Z, Kaxiras E, Zhang Z. Enhancing the Hydrogen Activation Reactivity of Nonprecious Metal Substrates via Confined Catalysis Underneath Graphene. NANO LETTERS. 2016;16 (10) :6058-6063.Abstract
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.
Mattheakis M, Valagiannopoulos CA, Kaxiras E. Epsilon-near-zero behavior from plasmonic Dirac point: Theory and realization using two-dimensional materials. PHYSICAL REVIEW B. 2016;94 (20).Abstract
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.
Islam MM, Kolesov G, Verstraelen T, Kaxiras E, van Duin ACT. eReaxFF: A Pseudoclassical Treatment of Explicit Electrons within Reactive Force Field Simulations. JOURNAL OF CHEMICAL THEORY AND COMPUTATION. 2016;12 (8) :3463-3472.Abstract
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.
Granas O, Vinichenko D, Kaxiras E. Establishing the limits of efficiency of perovskite solar cells from first principles modeling. SCIENTIFIC REPORTS. 2016;6.Abstract
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.
Kaplan D, Gong Y, Mills K, Swaminathan V, Ajayan PM, Shirodkar S, Kaxiras E. Excitation intensity dependence of photoluminescence from monolayers of MoS2 and WS2/MoS2 heterostructures. 2D MATERIALS. 2016;3 (1).Abstract
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.
Qi S, Qiao Z, Deng X, Cubuk ED, Chen H, Zhu W, Kaxiras E, Zhang SB, Xu X, Zhang Z. High-Temperature Quantum Anomalous Hall Effect in n-p Codoped Topological Insulators. PHYSICAL REVIEW LETTERS. 2016;117 (5).Abstract
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.