Santos EJG, Kaxiras E.
Electric-Field Dependence of the Effective Dielectric Constant in Graphene. Nano Letters. 2013;13 :898-902.
AbstractThe 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.
nanolett_13_898-902_2013.pdf 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.
AbstractSilicon 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
nanolett_13_2011-2015_2013.pdf 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.
AbstractSnS 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).
japplphys_113_233507_2013.pdf Malone BD, Kaxiras E.
Quasiparticle band structures and interface physics of SnS and GeS. Physical Review B. 2013;87 :245312.
AbstractWe 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.
physrevb.87.245312_2013.pdf 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.
AbstractUsing 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.
nanolett_13_509-514_2013.pdf 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.
AbstractWe 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.
ijhpca-keyes-4-83_2013.pdf