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Large Excitonic Effects In Monolayers Of Molybdenum And Tungsten Dichalcogenides, Ashwin Ramasubramaniam
Large Excitonic Effects In Monolayers Of Molybdenum And Tungsten Dichalcogenides, Ashwin Ramasubramaniam
Ashwin Ramasubramaniam
Quasiparticle band structures and optical properties of MoS2, MoSe2, MoTe2, WS2, and WSe2 monolayers are studied using the GW approximation in conjunction with the Bethe-Salpeter equation (BSE). The inclusion of two-particle excitations in the BSE approach reveals the presence of two strongly bound excitons (A and B) below the quasiparticle absorption onset arising from vertical transitions between a spin-orbit-split valence band and the conduction band at the K point of the Brillouin zone. The transition energies for monolayer MoS2, in particular, are shown to be in excellent agreement with available absorption and photoluminescence measurements. Excitation energies for the remaining monolayers …
Tunable Band Gaps In Bilayer Graphene-Bn Heterostructures, Ashwin Ramasubramaniam, Doron Naveh, Elias Towe
Tunable Band Gaps In Bilayer Graphene-Bn Heterostructures, Ashwin Ramasubramaniam, Doron Naveh, Elias Towe
Ashwin Ramasubramaniam
We investigate band-gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV, and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. The calculations suggest that the graphene-boron nitride heterostructures could provide a viable route to graphene-based electronic devices.
Interatomic Potentials For Hydrogen In A-Iron Based On Density Functional Theory, Ashwin Ramasubramaniam, Emily A. Carter, Mitsuhiro Itakura
Interatomic Potentials For Hydrogen In A-Iron Based On Density Functional Theory, Ashwin Ramasubramaniam, Emily A. Carter, Mitsuhiro Itakura
Ashwin Ramasubramaniam
We present two interatomic potentials for hydrogen in α–iron based on the embedded atom method potentials for iron developed by Mendelev et al. Philos. Mag. 83 3977 (2003) and Ackland et al. J. Phys.: Condens. Matter 16 S2629 (2004). Since these latter potentials are unique among existing iron potentials in their ability to produce the same core structure for screw dislocations as density functional theory (DFT) calculations, our interatomic potentials for hydrogen in iron also inherit this important feature. We use an extensive database of energies and atomic configurations from DFT calculations to fit the cross interaction of hydrogen with …
Substrate-Induced Magnetism In Epitaxial Graphene, Ashwin Ramasubramaniam, V. B. Shenoy, N. V. Medhekar
Substrate-Induced Magnetism In Epitaxial Graphene, Ashwin Ramasubramaniam, V. B. Shenoy, N. V. Medhekar
Ashwin Ramasubramaniam
Magnetism in graphene is of fundamental as well as technological interest, with potential applications in molecular magnets and spintronic devices. While defects and/or adsorbates in freestanding graphene nanoribbons and graphene sheets have been shown to cause itinerant magnetism, controlling the density and distribution of defects and adsorbates is in general difficult. We show from first principles calculations that graphene buffer layers on SiC(0001) can also show intrinsic magnetism. The formation of graphene-substrate chemical bonds disrupts the graphene pi-bonds and causes localization of graphene states near the Fermi level. Exchange interactions between these states lead to itinerant magnetism in the graphene …
Edge-Stress Induced Warping Of Graphene Sheets And Nanoribbons, Ashwin Ramasubramaniam, Y. W. Zhang, C. D. Reddy, V. B. Shenoy
Edge-Stress Induced Warping Of Graphene Sheets And Nanoribbons, Ashwin Ramasubramaniam, Y. W. Zhang, C. D. Reddy, V. B. Shenoy
Ashwin Ramasubramaniam
We show that edge stresses introduce intrinsic ripples in freestanding graphene sheets even in the absence of any thermal effects. Compressive edge stresses along zigzag and armchair edges of the sheet cause out-of-plane warping to attain several degenerate mode shapes. Based on elastic plate theory, we identify scaling laws for the amplitude and penetration depth of edge ripples as a function of wavelength. We also demonstrate that edge stresses can lead to twisting and scrolling of nanoribbons as seen in experiments. Our results underscore the importance of accounting for edge stresses in thermal theories and electronic structure calculations for freestanding …
Three-Dimensional Simulations Of Self-Assembly Of Hut Shaped Si-Ge Quantum Dots, Ashwin Ramasubramaniam, V. B. Shenoy
Three-Dimensional Simulations Of Self-Assembly Of Hut Shaped Si-Ge Quantum Dots, Ashwin Ramasubramaniam, V. B. Shenoy
Ashwin Ramasubramaniam
This article presents the results of three-dimensional modeling of heteroepitaxial thin film growth with the objective of understanding recent experiments on the early stages of quantum dot formation in SiGe/Si systems. We use a continuum model, based on the underlying physics of crystallographic surface steps, to study the growth of quantum dots, their spatial ordering and coarsening behavior. Using appropriate parameters, obtained from atomistic calculations, the (100) orientation is found to be unstable under compressive strains. The surface energy now develops a minimum at an orientation that may be interpreted as the (105) facet observed in SiGe/Si systems. This form …