Publications by Year: 2013

Santos EJG, Kaxiras E. Electrically Driven Tuning of the Dielectric Constant in MoS2 Layers. ACS Nano. 2013;12 :10741-10746.Abstract

The properties of two-dimensional materials, such as molybdenum disulfide, will play an important role in the design of the next generation of electronic devices. Many of those properties are determined by the dielectric constant which is one of the fundamental quantities used to characterize conductivity, refractive index, charge screening, and capacitance. We predict that the effective dielectric constant (ε) of few-layer MoS2 is tunable by an external electric field (Eext). Through first-principles electronic structure calculations, including van der Waals interactions, we show that at low fields (Eext < 0.01 V/Å) ε assumes a nearly constant value ∼4 but increases at higher fields to values that depend on the layer thickness. The thicker the structure, the stronger the modulation of ε with the electric field. Increasing of the external field perpendicular to the dichalcogenide layers beyond a critical value can drive the system to an unstable state where the layers are weakly coupled and can be easily separated. The observed dependence of ε on the external field is due to charge polarization driven by the bias. Implications on the optical properties as well as on the electronic excitations are also considered. Our results point to a promising way of understanding and controlling the screening properties of MoS2 through external electric fields.

Tritsaris GA, Kaxiras E, Meng S, Wang E. Adsorption and Diffusion of Lithium on Layered Silicon for Li-Ion Storage. Nano Letters. 2013;13 :2258-2263.Abstract

The energy density of Li-ion batteries depends critically on the specific charge capacity of the constituent electrodes. Silicene, the silicon analogue to graphene, being of atomic thickness could serve as high-capacity host of Li in Li-ion secondary batteries. In this work, we employ first-principles calculations to investigate the interaction of Li with Si in model electrodes of free-standing single-layer and double-layer silicene. More specifically, we identify strong binding sites for Li, calculate the energy barriers accompanying Li diffusion, and present our findings in the context of previous theoretical work related to Liion storage in other structural forms of silicon: the bulk and nanowires. The binding energy of Li is ∼2.2 eV per Li atom and shows small variation with respect to Li content and silicene thickness (one or two layers) while the barriers for Li diffusion are relatively low, typically less than 0.6 eV. We use our theoretical findings to assess the suitability of two-dimensional silicon in the form of silicene layers for Li-ion storage.

Wang WL, Kaxiras E. Efficient calculation of the effective single-particle potential and its application in electron microscopy. Physical Review B. 2013;87 :085103.Abstract

We present an efficient method for obtaining the effective single-particle potential for electrons within density functional theory (DFT). In contrast to the independent atom model (IAM) often used to interpret microscopy experiments, our method includes the contributions from charge redistribution and exchange-correlation interactions in a realistic system. The method allows calculation of the effective potential through the widely used pseudopotential formalism instead of the costly all-electron calculations. A transferable and spin-independent core potential for each element is calculated once and for all. The nonlinear exchange-correlation interaction is treated explicitly. This scheme can be readily implemented in pseudopotential DFT codes. We demonstrate the usefulness of our method by calculating the scattering potential and simulating images of nitrogen point defects in graphene for transmission electron microscopy (TEM). The results are in good agreement with experiments.

Santos EJG, Kaxiras E. Electric-Field Dependence of the Effective Dielectric Constant in Graphene. Nano Letters. 2013;13 :898-902.Abstract

The dielectric constant of a material is one of the fundamental features used to characterize its electrostatic properties such as capacitance, charge screening, and energy storage capability. Graphene is a material with unique behavior due to its gapless electronic structure and linear dispersion near the Fermi level, which can lead to a tunable band gap in bilayer and trilayer graphene, a superconducting-insulating transition in hybrid systems driven by electric fields, and gatecontrolled surface plasmons. All of these results suggest a strong interplay between graphene properties and external electric fields. Here we address the issue of the effective dielectric constant (ε) in N-layer graphene subjected to out-ofplane (Eext ) and in-plane (Eext ) external electric fields. The value of ε has attracted interest due to contradictory reports from theoretical and experimental studies. Through extensive first-principles electronic structure calculations, including van der Waals interactions, we show that both the out-of-plane (ε) and the in-plane (ε) dielectric constants depend on the value of applied field. For example, ε⊥ and ε∥ are nearly constant (∼3 and ∼1.8, respectively) at low fields (Eext < 0.01 V/Å) but increase at higher fields to values that are dependent on the system size. The increase of the external field perpendicular to the graphene layers beyond a critical value can drive the system to a unstable state where the graphene layers are decoupled and can be easily separated. The observed dependence of ε and ε on the external field is due to charge polarization driven by the bias. Our results point to a promising way of understanding and controlling the screening properties of few-layer graphene through external electric fields.

Cubuk ED, Wang WL, Zhao K, Vlassak JJ, Suo Z, Kaxiras E. Morphological Evolution of Si Nanowires upon Lithiation: A First- Principles Multiscale Model. Nano Letters. 2013;13 :2011-2015.Abstract

Silicon is a promising anode material for high-capacity Li-ion batteries. Recent experiments show that lithiation of crystalline silicon nanowires leads to highly anisotropic morphologies. This has been interpreted as due to anisotropy in equilibrium interface energies, but this interpretation does not capture the dynamic, nonequilibrium nature of the lithiation process. Here, we provide a comprehensive explanation of experimentally observed morphological changes, based on first-principles multiscale simulations. We identify reaction paths and associated structural transformations for Li insertion into the Si {110} and {111} surfaces and calculate the relevant energy barriers from density functional theory methods. We then perform kinetic Monte Carlo simulations for nanowires with surfaces of different orientations, which reproduce to a remarkable degree the experimentally observed profiles and the relative reaction front rates

Tritsaris GA, Malone BD, Kaxiras E. Optoelectronic properties of single-layer, double-layer, and bulk tin sulfide: A theoretical study. Journal of Applied Physics. 2013;113 :233507.Abstract

SnS is a metal monochalcogenide suitable for use as absorber material in thin film photovoltaic cells. Its structure is an orthorhombic crystal of weakly coupled layers, each layer consisting of strongly bonded Sn-S units. We use first-principles calculations to study model single-layer, double-layer, and bulk structures of SnS in order to elucidate its electronic structure. We find that the optoelectronic properties of the material can vary significantly with respect to the number of layers and the separation between them: the calculated band gap is wider for fewer layers (2.72 eV, 1.57 eV, and 1.07 eV for single-layer, double-layer, and bulk SnS, respectively) and increases with tensile strain along the layer stacking direction (by ∼55 meV/1% strain).

Malone BD, Kaxiras E. Quasiparticle band structures and interface physics of SnS and GeS. Physical Review B. 2013;87 :245312.Abstract

We perform first-principles, density-functional-theory calculations of the electronic structure for the layered bulk materials SnS and GeS which are of interest for photovoltaic applications. Band gap corrections are computed within the GW approximation to the electron self-energy. The resulting quasiparticle gaps in both SnS and GeS are in excellent agreement with the majority of existing experimental measurements. In order to better understand the possible use of GeS layers as a carrier-confining barrier within a SnS-based photovoltaic device, we compute the band offsets for different orientations of a SnS/GeS heterojunction. We find the valence band offsets to be almost independent of interfacial direction while the conduction band offsets show a strong anisotropy as a result of the variation in the band gap caused by epitaxial strain along the different directions.

Chen W, Santos EJG, Zhu W, Kaxiras E, Zhang Z. Tuning the Electronic and Chemical Properties of Monolayer MoS2 Adsorbed on Transition Metal Substrates. Nano Letters. 2013;13 :509-514.Abstract

Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS2 adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS2 overlayer. We find that, for each of the metal substrates, the contact nature is of Schottky-barrier type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS2 layer. The enhanced binding of hydrogen, by as much as ∼0.4 eV, is attributed in part to a stronger H−S coupling enabled by the transferred charge from the substrate to the MoS2 overlayer, and in part to a stronger MoS2-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS2-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.

Keyes DE, McInnes LC, Woodward C, Gropp W, Myra E, et al. Multiphysics Simulations: Challenges and Opportunities. International Journal of High Performance Computing Applications. 2013;27 (1) :4-83.Abstract

We consider multiphysics applications from algorithmic and architectural perspectives, where “algorithmic” includes both mathematical analysis and computational complexity, and “architectural” includes both software and hardware environments. Many diverse multiphysics applications can be reduced, en route to their computational simulation, to a common algebraic coupling paradigm. Mathematical analysis of multiphysics coupling in this form is not always practical for realistic applications, but model problems representative of applications discussed herein can provide insight. A variety of software frameworks for multiphysics applications have been constructed and refined within disciplinary communities and executed on leading-edge computer systems. We examine several of these, expose some commonalities among them, and attempt to extrapolate best practices to future systems. From our study, we summarize challenges and forecast opportunities.