# Publications

2016
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.
Montemore MM, Madix RJ, Kaxiras E. How Does Nanoporous Gold Dissociate Molecular Oxygen?. JOURNAL OF PHYSICAL CHEMISTRY C. 2016;120 (30) :16636-16640.Abstract
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.
Shirodkar SN, Kaxiras E. Li intercalation at graphene/hexagonal boron nitride interfaces. PHYSICAL REVIEW B. 2016;93 (24).Abstract
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.
Perez D, Cubuk ED, Waterland A, Kaxiras E, Voter AF. Long-Time Dynamics through Parallel Trajectory Splicing. JOURNAL OF CHEMICAL THEORY AND COMPUTATION. 2016;12 (1) :18-28.Abstract
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.
Falcucci G, Succi S, Montessori A, Melchionna S, Prestininzi P, Barroo C, Bell DC, Biener MM, Biener J, Zugic B, et al. Mapping reactive flow patterns in monolithic nanoporous catalysts. MICROFLUIDICS AND NANOFLUIDICS. 2016;20 (7).Abstract
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.
Nourbakhsh A, Zubair A, Sajjad RN, Tavakkoli AKG, Chen W, Fang S, Ling X, Kong J, Dresselhaus MS, Kaxiras E, et al. MoS2 Field-Effect Transistor with Sub-10 nm Channel Length. NANO LETTERS. 2016;16 (12) :7798-7806.Abstract
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.
Montemore M, Hoyt R, Kaxiras E. Non-adiabatic effects and electronic excitations during dissociation on catalytic surfaces. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY. 2016;252.
Montemore M, Hoyt R, Kaxiras E. Non-adiabatic energy dissipation in dissociation on catalytic surface. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY. 2016;251.
Karakalos S, Xu Y, Kabeer FC, Chen W, Rodriguez-Reyes JCF, Tkatchenko A, Kaxiras E, Madix RJ, Friend CM. Noncovalent Bonding Controls Selectivity in Heterogeneous Catalysis: Coupling Reactions on Gold. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. 2016;138 (46) :15243-15250.Abstract
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.
Tritsaris GA, Shirodkar SN, Kaxiras E, Cazeaux P, Luskin M, Plechac P, Cances E. Perturbation theory for weakly coupled two-dimensional layers. JOURNAL OF MATERIALS RESEARCH. 2016;31 (7) :959-966.Abstract
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.
Kolesov G, Granas O, Hoyt R, Vinichenko D, Kaxiras E. Real-Time TD-DFT with Classical Ion Dynamics: Methodology and Applications. JOURNAL OF CHEMICAL THEORY AND COMPUTATION. 2016;12 (2) :466-476.Abstract
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.
Hiebel F, Shong B, Chen W, Madix RJ, Kaxiras E, Friend CM. Self-assembly of acetate adsorbates drives atomic rearrangement on the Au(110) surface. NATURE COMMUNICATIONS. 2016;7.Abstract
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.
Defo RK, Fang S, Shirodkar SN, Tritsaris GA, Dimoulas A, Kaxiras E. Strain dependence of band gaps and exciton energies in pure and mixed transition-metal dichalcogenides. PHYSICAL REVIEW B. 2016;94 (15).Abstract
{{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
S. S. Schoenholz, Cubuk ED, Sussman DM, Kaxiras E, Liu AJ. A structural approach to relaxation in glassy liquids. NATURE PHYSICS. 2016;12 (5) :469+.Abstract
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.
Cubuk ED, Schoenholz SS, Kaxiras E, Liu AJ. Structural Properties of Defects in Glassy Liquids. JOURNAL OF PHYSICAL CHEMISTRY B. 2016;120 (26) :6139-6146.Abstract
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.