# Publications

2018

Larson DT, Fampiou I, Kim G, Kaxiras E. Lithium Intercalation in Graphene-MoS2 Heterostructures. JOURNAL OF PHYSICAL CHEMISTRY C. 2018;122 (43) :24535-24541.Abstract

Two-dimensional (2D) heterostructures are interesting candidates for efficient energy storage devices due to their high carrier capacity by reversible intercalation. We employ here density functional theory calculations to investigate the structural and electronic properties of lithium intercalated graphene/molybdenum disulfide (Gr/MoS2) heterostructures. We explore the extent to which Li intercalates at the interface formed between graphene (Gr) and molybdenum disulfide (MoS2) layers by considering the adsorption and diffusion of Li atoms, the energetic stability, and the changes in the structural morphology of MoS2. We investigate the corresponding electronic structure and charge distribution within the heterostructure at varying concentrations of Li. Our results indicate that the maximum energetically allowed ratio of Li to Mo (Li to C) is 1:1 (1:3) for both the 2H and 1T' phases of MoS2. This is double the Li concentration allowed in graphene bilayers. We find that there is 60% more charge transfer to MoS2 than to Gr in the bilayer heterostructure, which results in a maximum doping of Gr and MoS2 of n(C) = 3.6 x 10(14) cm(-2) and n(MoS2) = 6.0 x 10(14) cm(-2), respectively.

Carr S, Massatt D, Fang S, Cazeaux P, Luskin M, Kaxiras E. Modeling Electronic Properties of Twisted 2D Atomic Heterostructures, in COUPLED MATHEMATICAL MODELS FOR PHYSICAL AND BIOLOGICAL NANOSCALE SYSTEMS AND THEIR APPLICATIONS. Vol 232. Banff Int Res Stat ; 2018 :245-265.Abstract

We present a general method for the electronic characterization of aperiodic 2D materials using ab-initio tight binding models. Specifically studied is the subclass of twisted, stacked heterostructures, but the formalism provided can be implemented for any 2D system without long-range interactions. This new method provides a multi-scale approach for dealing with the ab-initio calculation of electronic transport properties in stacked nanomaterials, allowing for fast and efficient simulation of multi-layered stacks in the presence of twist angles, magnetic field, and defects. We calculate the electronic density of states in twisted bilayer systems of graphene and MX2 transition metal dichalcogenides (TMDCs). We comment on the interesting features of their density of states as a function of twist-angle and local configuration and how these features are experimentally observable. These results support the bilayer twist-angle as a new variable for controlling electronic properties in artificial nanomaterials (''Twistronics'').

Zhang J, Hong H, Zhang J, Fu H, You P, Lischner J, Liu K, Kaxiras E, Meng S. New Pathway for Hot Electron Relaxation in Two-Dimensional Heterostructures. NANO LETTERS. 2018;18 (9) :6057-6063.Abstract

Two-dimensional (2D) heterostructures composed of transition-metal dichalcogenide atomic layers are the new frontier for novel optoelectronic and photovoltaic device applications. Some key properties that make these materials appealing, yet are not well understood, are ultrafast hole/electron dynamics, interlayer energy transfer and the formation of interlayer hot excitons. Here, we study photoexcited electron/hole dynamics in a representative heterostructure, the MoS2/WSe2 interface, which exhibits type II band alignment. Employing time-dependent density functional theory in the time domain, we observe ultrafast charge dynamics with lifetimes of tens to hundreds of femtoseconds. Most importantly, we report the discovery of an interfacial pathway in 2D heterostructures for the relaxation of photoexcited hot electrons through interlayer hopping, which is significantly faster than intralayer relaxation. This finding is of particular importance for understanding many experimentally observed photoinduced processes, including charge and energy transfer at an ultrafast time scale (<1 ps).

Hoyt RA, Montemore MM, Kaxiras E. Nonadiabatic Hydrogen Dissociation on Copper Nanoclusters. JOURNAL OF PHYSICAL CHEMISTRY LETTERS. 2018;9 (18) :5339+.Abstract

Copper surfaces exhibit high catalytic selectivity but have poor hydrogen dissociation kinetics; therefore, we consider icosahedral Cu-13 nanoclusters to understand how nanoscale structure might improve catalytic prospects. We find that the spin state is a surprisingly important design consideration. Cu-13 clusters have large magnetic moments due to finite size and symmetry effects and exhibit magnetization-dependent catalytic behavior. The most favorable transition state for hydrogen dissociation has a lower activation energy than that on single-crystal copper surfaces but requires a magnetization switch from 5 to 3 mu(B). Without this switch, the activation energy is higher than that on single-crystal surfaces. Weak spin-orbit coupling hinders this switch, decreasing the kinetic rate of hydrogen dissociation by a factor of 16. We consider strategies to facilitate magnetization switches through optical excitations, substitution, charge states, and co-catalysts; these considerations demonstrate how control of magnetic properties could improve catalytic performance.

Carr S, Fang S, Jarillo-Herrero P, Kaxiras E. Pressure dependence of the magic twist angle in graphene superlattices. PHYSICAL REVIEW B. 2018;98 (8).Abstract

The recently demonstrated unconventional superconductivity [Cao et al., Nature (London) 556, 43 (2018)] in twisted bilayer graphene (tBLG) opens the possibility for interesting applications of two-dimensional layers that involve correlated electron states. Here we explore the possibility of modifying electronic correlations by the application of uniaxial pressure on the weakly interacting layers, which results in increased interlayer coupling and a modification of the magic angle value and associated density of states. Our findings are based on first-principles calculations that accurately describe the height-dependent interlayer coupling through the combined use of density functional theory and maximally localized Wannier functions. We obtain the relationship between twist angle and external pressure for the magic angle flat bands of tBLG. This may provide a convenient method to tune electron correlations by controlling the length scale of the superlattice.

Shirodkar SN, Mattheakis M, Cazeaux P, Narang P, Soljacic M, Kaxiras E. Quantum plasmons with optical-range frequencies in doped few-layer graphene. PHYSICAL REVIEW B. 2018;97 (19).Abstract

Although plasmon modes exist in doped graphene, the limited range of doping achieved by gating restricts the plasmon frequencies to a range that does not include the visible and infrared. Here we show, through the use of first-principles calculations, that the high levels of doping achieved by lithium intercalation in bilayer and trilayer graphene shift the plasmon frequencies into the visible range. To obtain physically meaningful results, we introduce a correction of the effect of plasmon interaction across the vacuum separating periodic images of the doped graphene layers, consisting of transparent boundary conditions in the direction perpendicular to the layers; this represents a significant improvement over the exact Coulomb cutoff technique employed in earlier works. The resulting plasmon modes are due to local field effects and the nonlocal response of the material to external electromagnetic fields, requiring a fully quantum mechanical treatment. We describe the features of these quantum plasmons, including the dispersion relation, losses, and field localization. Our findings point to a strategy for fine-tuning the plasmon frequencies in graphene and other two-dimensional materials.

Larson DT, Kaxiras E. Raman spectrum of CrI3: An ab initio study. PHYSICAL REVIEW B. 2018;98 (8).Abstract

We study the Raman spectrum of CrI3, a material that exhibits magnetism in a single layer. We employ first-principles calculations within density functional theory to determine the effects of polarization, strain, and incident angle on the phonon spectra of the three-dimensional bulk and the single-layer two-dimensional structure, for both the high- and low-temperature crystal structures. Our results are in good agreement with existing experimental measurements and serve as a guide for additional investigations to elucidate the physics of this interesting material.

Montemore MM, Hoyt R, Kolesov G, Kaxiras E. Reaction-Induced Excitations and Their Effect on Surface Chemistry. ACS CATALYSIS. 2018;8 (11) :10358-10363.Abstract

Despite intensive study of reactions on metals, it is unclear whether electronic excitations play an important role. Here, we show that nonadiabatic effects do indeed play a significant role in N-2 and H-2 dissociation on Ru nanoparticles. We employ nonadiabatic dynamical calculations based on realtime, time-dependent density functional theory to study energy dissipation during these exothermic reaction steps. We find that dissipation of the excess energy into excitation of electrons exceeds thermal dissipation into phonons. For isolated dissociation events, electronic friction can increase reaction barriers; furthermore, the excitations induced by a dissociation event can affect other reacting molecules. Our studies suggest that, for exothermic reactions, metal catalysts in reaction conditions may be constantly experiencing electronic excitations, and these excitations can significantly affect surface chemistry.

Carr S, Massatt D, Torrisi SB, Cazeaux P, Luskin M, Kaxiras E. Relaxation and domain formation in incommensurate two-dimensional heterostructures. PHYSICAL REVIEW B. 2018;98 (22).Abstract

We introduce configuration space as a natural representation for calculating the mechanical relaxation patterns of incommensurate two-dimensional (2D) bilayers. The approach can be applied to a wide variety of 2D materials through the use of a continuum model in combination with a generalized stacking fault energy for interlayer interactions. We present computational results for small-angle twisted bilayer graphene and molybdenum disulfide (MoS2), a representative material of the transition-metal dichalcogenide family of 2D semiconductors. We calculate accurate relaxations for MoS2 even at small twist-angle values, enabled by the fact that our approach does not rely on empirical atomistic potentials for interlayer coupling. The results demonstrate the efficiency of the configuration space method by computing relaxations with minimal computational cost. We also outline a general explanation of domain formation in 2D bilayers with nearly aligned lattices, taking advantage of the relationship between real space and configuration space. The configuration space approach also enables calculation of relaxations in incommensurate multilayer systems.

Cao Y, Fatemi V, Fang S, Watanabe K, Taniguchi T, Kaxiras E, Jarillo-Herrero P. Unconventional superconductivity in magic-angle graphene superlattices. NATURE. 2018;556 (7699) :43+.Abstract

The behaviour of strongly correlated materials, and in particular unconventional superconductors, has been studied extensively for decades, but is still not well understood. This lack of theoretical understanding has motivated the development of experimental techniques for studying such behaviour, such as using ultracold atom lattices to simulate quantum materials. Here we report the realization of intrinsic unconventional superconductivity-which cannot be explained by weak electron-phonon interactions-in a two-dimensional superlattice created by stacking two sheets of graphene that are twisted relative to each other by a small angle. For twist angles of about 1.1 degrees-the first `magic' angle-the electronic band structure of this `twisted bilayer graphene' exhibits flat bands near zero Fermi energy, resulting in correlated insulating states at half-filling. Upon electrostatic doping of the material away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature of up to 1.7 kelvin. The temperature-carrier-density phase diagram of twisted bilayer graphene is similar to that of copper oxides (or cuprates), and includes dome-shaped regions that correspond to superconductivity. Moreover, quantum oscillations in the longitudinal resistance of the material indicate the presence of small Fermi surfaces near the correlated insulating states, in analogy with underdoped cuprates. The relatively high superconducting critical temperature of twisted bilayer graphene, given such a small Fermi surface (which corresponds to a carrier density of about 1011 per square centimetre), puts it among the superconductors with the strongest pairing strength between electrons. Twisted bilayer graphene is a precisely tunable, purely carbon-based, two-dimensional superconductor. It is therefore an ideal material for investigations of strongly correlated phenomena, which could lead to insights into the physics of high-critical-temperature superconductors and quantum spin liquids.

Maier M, Mattheakis M, Kaxiras E, Luskin M, Margetis D. Universal behavior of a dispersive Dirac cone in gradient-index plasmonic metamaterials. PHYSICAL REVIEW B. 2018;97 (3).Abstract

We demonstrate analytically and numerically that the dispersive Dirac cone emulating an epsilon-near-zero (ENZ) behavior is a universal property within a family of plasmonic crystals consisting of two-dimensional (2D) metals. Our starting point is a periodic array of 2D metallic sheets embedded in an inhomogeneous and anisotropic dielectric host that allows for propagation of transverse-magnetic (TM) polarized waves. By invoking a systematic bifurcation argument for arbitrary dielectric profiles in one spatial dimension, we show how TM Bloch waves experience an effective dielectric function that averages out microscopic details of the host medium. The corresponding effective dispersion relation reduces to a Dirac cone when the conductivity of the metallic sheet and the period of the array satisfy a critical condition for ENZ behavior. Our analytical findings are in excellent agreement with numerical simulations.

Xu F, Fampiou I, O'Connor CR, Karakalos S, Hiebel F, Kaxiras E, Madix RJ, Friend CM. Water facilitates oxygen migration on gold surfaces. PHYSICAL CHEMISTRY CHEMICAL PHYSICS. 2018;20 (4) :2196-2204.Abstract

The water-oxygen-gold interface is important in many surface processes and has potential influence on heterogeneous catalysis. Herein, it is shown that water facilitates the migration of atomic oxygen on Au(110), demonstrating the dynamic nature of surface adsorption. We demonstrate this effect for the first time, using in situ scanning tunnelling microscopy (STM), temperature-programmed reaction spectroscopy (TPRS) and first-principles theoretical calculations. The dynamic interaction of water with adsorbed O maintains a high dispersion of O on the surface, potentially creating reactive transient species. At low temperature and pressure, isotopic experiments show that adsorbed oxygen on the Au(110) surface exchanges with oxygen in (H2O)-O-18. The presence of water modulates local electronic properties and facilitates oxygen exchange. Combining experimental results and theory, we propose that hydroxyl is transiently formed via proton transfer from the water to adsorbed oxygen. Hydroxyl groups easily recombine to regenerate water and adsorbed oxygen atoms, the net result of which is migration of the adsorbed oxygen without significant change in its overall distribution on the surface. The presence of water creates a dynamic surface where mobile surface oxygen atoms and hydroxyls are present, which can lead to a better performance of gold catalysis in oxidation reactions.

2017

Vinichenko D, Sensoy MG, Friend CM, Kaxiras E. Accurate formation energies of charged defects in solids: A systematic approach. PHYSICAL REVIEW B. 2017;95 (23).Abstract

Defects on surfaces of semiconductors have a strong effect on their reactivity and catalytic properties. The concentration of different charge states of defects is determined by their formation energies. First-principles calculations are an important tool for computing defect formation energies and for studying the microscopic environment of the defect. The main problem associated with the widely used supercell method in these calculations is the error in the electrostatic energy, which is especially pronounced in calculations that involve surface slabs and two-dimensional materials. We present an internally consistent approach for calculating defect formation energies in inhomogeneous and anisotropic dielectric environments and demonstrate its applicability to the cases of the positively charged Cl vacancy on the NaCl (100) surface and the negatively charged S vacancy in monolayer MoS2.

Madix R, Karakalos S, Xu Y, Tkatchenko A, Kabeer F, Chen W, Kaxiras E, Friend C. Competition for active sites in steady state catalytic reactions; importance of weak van der Waals interactions. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY. 2017;253.

Montemore MM, Montessori A, Succi S, Barroo C, Falcucci G, Bell DC, Kaxiras E. Effect of nanoscale flows on the surface structure of nanoporous catalysts. JOURNAL OF CHEMICAL PHYSICS. 2017;146 (21).Abstract

The surface structure and composition of a multi-component catalyst are critical factors in determining its catalytic performance. The surface composition can depend on the local pressure of the reacting species, leading to the possibility that the flow through a nanoporous catalyst can affect its structure and reactivity. Here, we explore this possibility for oxidation reactions on nanoporous gold, an AgAu bimetallic catalyst. We use microscopy and digital reconstruction to obtain the morphology of a two-dimensional slice of a nanoporous gold sample. Using lattice Boltzmann fluid dynamics simulations along with thermodynamic models based on first-principles total-energy calculations, we show that some sections of this sample have low local O-2 partial pressures when exposed to reaction conditions, which leads to a pure Au surface in these regions, instead of the active bimetallic AgAu phase. We also explore the effect of temperature on the surface structure and find that moderate temperatures (approximate to 300-450 K) should result in the highest intrinsic catalytic performance, in apparent agreement with experimental results. Published by AIP Publishing.

Wen M, Shirodkar SN, Plechac P, Kaxiras E, Elliott RS, Tadmor EB. A force-matching Stillinger-Weber potential for MoS2: Parameterization and Fisher information theory based sensitivity analysis. JOURNAL OF APPLIED PHYSICS. 2017;122 (24).Abstract

Two-dimensional molybdenum disulfide (MoS2) is a promising material for the next generation of switchable transistors and photodetectors. In order to perform large-scale molecular simulations of the mechanical and thermal behavior of MoS2-based devices, an accurate interatomic potential is required. To this end, we have developed a Stillinger-Weber potential for monolayer MoS2. The potential parameters are optimized to reproduce the geometry (bond lengths and bond angles) of MoS2 in its equilibrium state and to match as closely as possible the forces acting on the atoms along a dynamical trajectory obtained from ab initio molecular dynamics. Verification calculations indicate that the new potential accurately predicts important material properties including the strain dependence of the cohesive energy, the elastic constants, and the linear thermal expansion coefficient. The uncertainty in the potential parameters is determined using a Fisher information theory analysis. It is found that the parameters are fully identified, and none are redundant. In addition, the Fisher information matrix provides uncertainty bounds for predictions of the potential for new properties. As an example, bounds on the average vibrational thickness of a MoS2 monolayer at finite temperature are computed and found to be consistent with the results from a molecular dynamics simulation. The new potential is available through the OpenKIM interatomic potential repository at https://openkim.org/cite/MO\_201919462778\_000. Published by AIP Publishing.

Crampton A, Xu F, Kaxiras E, Cai L, Zheng X, Friend C. Heterogeneous photochemical processes on metal oxides. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY. 2017;253.

Granas O, Kolesov G, Kaxiras E. Impact of Vibrations and Electronic Coherence on Electron Transfer in Flat Molecular Wires. MRS ADVANCES. 2017;2 (14) :811-816.Abstract

Electron transfer in molecular wires are of fundamental importance for a range of optoelectronic applications. The impact of electronic coherence and ionic vibrations on transmittance are of great importance to determine the mechanisms, and subsequently the type of wires that are most promising for applications. In this work, we use the real-time formulation of time-dependent density functional theory to study electron transfer through oligo-pphenylenevinylene (OPV) and the recently synthesized carbon bridged counterpart (COPV). A system prototypical of organic photovoltaics is setup by bridging a porphyrin-fullerene dyad, allowing a photo-excited electron to flow between the Zn-porphyrin (ZnP) chromophore and the C60 electron acceptor through the molecular wire. The excited state is described using the fully self-consistent.-SCF method. The state is then propagated in time using the real-time TD-DFT scheme, while describing ionic vibrations with classical nuclei. The charge transferred between porphyrin and C60 is calculated and correlated with the velocity autocorrelation functions of the ions. This provides a microscopic insight to vibrational and tunneling contributions to electron transport in linked porphyrin-fullerene dyads. We elaborate on important details in describing the excited state and trajectory sampling.

Barroo C, Montemore MM, Janvelyan N, Zugic B, Akey AJ, Magyar AP, Ye J, Kaxiras E, Biener J, Bell DC. Macroscopic 3D Nanoporosity Formation by Dry Oxidation of AgAu Alloys. JOURNAL OF PHYSICAL CHEMISTRY C. 2017;121 (9) :5115-5122.Abstract

3D nanoporous metals made by alloy corrosion have attracted much attention due to various promising applications ranging from catalysis and sensing to energy storage and actuation. In this work we report a new process for the fabrication of 3D open nanoporous metal networks that phenomenologically resembles the nano-Kirkendall hollowing process previously reported for Ag/Au nanowires and nano particles, with the difference that the involved length scales are 10-100 times larger. Specifically, we find that dry oxidation of Ag70Au30 bulk alloy samples by ozone exposure at 150 C-omicron stimulates extremely rapid Ag outward diffusion toward the gas/alloy-surface interface, at rates at least 5 orders of magnitude faster than predicted on the basis of reported Ag bulk diffusion values. The micrometer-thick Ag depleted alloy region thus formed transforms into a 3D open nanoporous network morphology upon further exposure to methanol-O-2 at 150 C-omicron. These findings have important implications for practical applications of alloys, for example as catalysts, by demonstrating that large-scale compositional and morphological changes can be triggered by surface chemical reactions at low temperatures, and that dilute alloys such as Au97Ag3 are more resilient against such changes.

Lovchinsky I, Sanchez-Yamagishi JD, Urbach EK, Choi S, Fang S, Andersen TI, Watanabe K, Taniguchi T, Bylinskii A, Kaxiras E, et al. Magnetic resonance spectroscopy of an atomically thin material using a single-spin qubit. SCIENCE. 2017;355 (6324) :503+.Abstract

Two-dimensional (2D) materials offer a promising platform for exploring condensed matter phenomena and developing technological applications. However, the reduction of material dimensions to the atomic scale poses a challenge for traditional measurement and interfacing techniques that typically couple to macroscopic observables. We demonstrate a method for probing the properties of 2D materials via nanometer-scale nuclear quadrupole resonance (NQR) spectroscopy using individual atomlike impurities in diamond. Coherent manipulation of shallow nitrogen-vacancy (NV) color centers enables the probing of nanoscale ensembles down to approximately 30 nuclear spins in atomically thin hexagonal boron nitride (h-BN). The characterization of low-dimensional nanoscale materials could enable the development of new quantum hybrid systems, combining atomlike systems coherently coupled with individual atoms in 2D materials.