Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H15: 2D and Graphene - Electronic and Atomic StructureFocus
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Sponsoring Units: DMP Room: 314 |
Tuesday, March 15, 2016 2:30PM - 2:42PM |
H15.00001: One-dimensional metallic wires at phase-engineered boundaries in two-dimensional materials Marco Gibertini, Nicola Marzari At the interfaces between systems with different electric polarization, free carriers appear in order to screen the resulting polarization charges associated with the resulting polar discontinuity. This mechanism is believed to be at the origin of the two-dimensional electron gas emerging at oxide interfaces and provides the basis for manifold exciting novel phenomena. Recently, it has been shown that similar processes take place also in two-dimensional materials, where one-dimensional wires of free carriers are induced at planar interfaces between materials with different in-plane polarization or at the edges of polar nanoribbons. Here we show by first-principles simulations that some two-dimensional polar materials can display a metastable non-polar phase, so that boundaries between the stable and metastable phases support a polar discontinuity and the resulting one-dimensional metallic wires. We provide several approaches to engineer such phase boundaries by locally inducing metastable phases in a single parent crystal. We finally show how this novel strategy to engineer polar discontinuities in two dimensions offers unprecedented opportunities to efficiently manipulate and reconfigure the emerging one-dimensional metallic wires or switch their conducting state. [Preview Abstract] |
Tuesday, March 15, 2016 2:42PM - 2:54PM |
H15.00002: First-principles study of structural properties of SiO$_2$ bilayers Andrei Malashevich, Sohrab Ismail-Beigi, Eric I. Altman Two dimensional (2D) materials draw a tremendous amount of interest because they exhibit unique physical properties due to reduced dimensionality. Recently, SiO$_2$ 2D bilayer systems were discovered. The structure of these bilayers is formed by two mirror-image planes of corner-sharing SiO$_4$ tetrahedra and does not have a direct relation to bulk SiO$_2$ systems. SiO$_2$ bilayers may be obtained in crystalline or amorphous forms. In the crystalline form, the bilayers are constructed from six-membered rings of corner-sharing SiO$_4$ tetrahedra. The amorphous form has rings of various sizes typically in the range from four to nine Si atoms in the ring. These structures may be of practical interest as atomically thin membranes and molecular sieves. In our work, we study the effect of strain and doping on the crystalline structure of SiO$_2$ bilayers using density functional theory. We analyze the stability of structures depending on the ring size and establish strain and doping conditions that may render the structures with large ring sizes stable. This work is supported by the National Science Foundation through grants MRSEC NSF DMR-1119826 and NSF DMR-1506800. [Preview Abstract] |
Tuesday, March 15, 2016 2:54PM - 3:06PM |
H15.00003: The Pyrite Structure of PdS$_{2}$ and PdSe$_{2}$ Monolayers Arunima K. Singh, Richard G. Hennig There has been a rising interest in two-dimensional (2D) materials due to a range of extraordinary electronic, optical and mechanical properties which are different from their bulk counterparts. The structure, stability and electronic properties of 2D PdS$_{2}$ and PdSe$_{2}$ have been investigated in the past in the well-known hexagonal 1T and 2H structures. However, bulk PdS$_{2}$ and PdSe$_{2}$ are layered compounds with individual rhombohedral pyrite-type monolayers vertically stacked with van-der Waals forces. Using density functional theory simulations, and five different functionals, we compare the energetic stability of 2D PdS$_{2}$ and PdSe$_{2}$ pyrite structure with the 1T and 2H structures. We find that the PdS$_{2}$ is most stable in the pyrite structure, whereas the PdSe$_{2}$ is most stable in the 1T structure with the pyrite structure closely competing in energy. The fundamental band gap of these compounds as a function of the structure, number of layers, the stacking arrangement and in-layer strain has been investigated. The pyrite structures of PdS$_{2}$ and PdSe$_{2}$ are found to be semi-conducting with indirect band gaps, and effective masses comparable to that of monolayer MoS$_{2}$; thus are potential candidates for nano-electronic applications. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H15.00004: Electronic and Mechanical Properties of Hydrogenated Irradiated and Amorphous Graphene Asanka Weerasinghe, Ashwin Ramasubramaniam, Dimitrios Maroudas Defect engineering and chemical functionalization of graphene are promising routes for fabrication of carbon nanostructures and 2D metamaterials with unique properties and function. Here, we use hydrogenation of irradiated, including irradiation-induced amorphous, graphene as a means of studying chemical functionalization effects on its electronic structure and mechanical response. We use molecular-dynamics simulations based on a reliable bond-order potential to prepare the hydrogenated configurations and carry out dynamic deformation tests at constant strain rate and temperature. Our mechanical tests show that hydrogenation does not affect the ultimate tensile strength (UTS) of the irradiated graphene sheet if the hydrogenated C atoms remain sp$^{\mathrm{2}}$-hybridized; however, upon inducing sp$^{\mathrm{3}}$ hybridization of these C atoms, UTS decreases by about 10 GPa. Furthermore, the fracture strain of the irradiated structure decreases by up to 30{\%} upon hydrogenation independent of the hybridization type. We also report results for the electronic structure of hydrogenated configurations based on a density-functional tight-binding approach and assess the potential for tuning the electronic properties of these defective, functionalized graphenes. [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H15.00005: Effect of radial stretch on vibration characteristics of single-layered circular graphene sheets Gunjan Pahlani, Deepti Verma, Shakti Gupta Vibrations of single-layered circular graphene sheets are studied using molecular mechanics (MM) simulations. Interactions between bonded and non-bonded atoms are prescribed using MM3 potential. Frequencies of different modes of vibration are computed from the eigenvalues and eigen vectors of mass weighted Hessian of the system. This study is performed on graphene sheets of various diameters. A linear continuum membrane model for predicting vibrational frequencies is studied using finite element (FE) method. Frequencies for several modes computed from continuum and molecular model matched well for moderate values of radial stretch, however, with increased stretch those deviated from each other significantly. In particular for higher values of stretch the MM simulations predict considerably lower values of frequencies compared to that found from FE simulations. Also, at higher values of stretch the frequency vs. stretch curve obtained from MM simulations showed a hardening behavior which could not be captured by the linear continuum model. We have also found a similar behavior in two-layered graphene sheets using MM simulations. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H15.00006: A Tight Binding Approach to Strain in Monolayer Transition-Metal Dichalcogenides Alexander Pearce, Guido Burkard We present a model of the electronic properties of the monolayer transition-metal dichalcogenides based on a tight binding approach which includes the effects of strain and curvature of the crystal lattice. Mechanical deformations of the lattice change bond lengths leading directly to corrections in the electronic Hamiltonian, while curvature of the crystal lattice mixes the orbital structure of the electronic Bloch bands. We first present an effective low energy Hamiltonian describing the electronic properties near the K point in the Brillouin zone, then present the corrections to this Hamiltonian due to arbitrary mechanical deformations and curvature in a way which treats both effects on an equal footing. This analysis finds that local area variations of the lattice allow for tuning of the band gap and effective masses, where the application of uniaxial strain decreases the magnitude of the direct band gap at the K point. Additionally, strain induced bond length modifications create a fictitious gauge field but with a coupling that is smaller than seen in related materials like graphene. Whereas curvature of the lattice leads to appearance of both an effective in-plane magnetic field which couples to spin degrees of freedom and a Rashba-like spin-orbit coupling. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H15.00007: Effective tight-binding model for transition metal dichalcogenides Yen-Hung Ho, Miguel Cazalilla, Hector Ochoa For transition metal dichalcogenides, various band models have been developed to describe the novel subband features. In this work, we propose a new effective minimum-band model by preforming a canonical transformation on the full-band Hamiltonian. We found that, depending on the form of transformation, both the $\Gamma$- and $K$-valley electrons can be well captured, including the frequency and band effective mass. And, for the full-band parameters used, starting from Wannier function basis set leads to a better result than from Slater-Koster basis set. A close inspection of the transformation projection also enables us to extract the modification on the site energy, as well as the orbital hopping between several nearest neighboring atoms. Instead of pure empirical fitting, our effective models preserve rich orbital physics inside, which is shown to be versatile in studying a variety of fundamental physical properties. [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H15.00008: Structural and electronic properties of a single layered $\alpha $-tetragonal B$_{\mathrm{50}}$ sheet. Cherno Kah, Ming Yu, Chakram S Jayanthi, Shiyu Wu Ultrathin single-crystalline boron nanosheets with $\alpha $-tetragonal B$_{\mathrm{50}}$ symmetry ($\alpha $-t-B$_{\mathrm{50}})$ have recently been synthesized [1]. In this presentation, the relaxed structure of this new type of boron sheet is determined using a robust self-consistent and environment-dependent semi-empirical Hamiltonian developed within the LCAO framework that includes MD and power quenching schemes. Upon relaxation, the sheet symmetry is broken and the icosahedral B$_{\mathrm{12}}$ units in the sheet are found to be distorted. This stability of the sheet was investigated through a calculation of the vibrational frequencies. The sheet electronic density of states exhibits no energy gap at the Fermi level, suggesting a metallic character similar to that of the bulk $\alpha $-t-B$_{\mathrm{50}}$. Finally, the cohesive energy of the $\alpha $-t-B$_{\mathrm{50}}$ sheet is found to be higher than that of the recently reported icosahedral B$_{\mathrm{12}}$-$\delta_{\mathrm{6\thinspace }}$sheet [2]. [1] Adv. Sci. 2, 1500023 (2015) [2] Nanotechnology 26, 405701 (2015) [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H15.00009: Ab Initio Based 2D Continuum Mechanics -- Sensitivity Prediction for Contact Resonance Atomic Force Microscopy Based Structure Fingerprints Qing Tu, Bj\"orn Lange, J. Marcelo J. Lopes, Stefan Zauscher, Volker Blum Contact resonance AFM is demonstrated as a powerful tool for mapping differences in the mechanical properties of 2D materials and heterostructures, permitting to resolve surface and subsurface structural differences of different domains. Measured contact resonance frequencies are related to the contact stiffness of the combined tip-sample system. Based on first principles predicted elastic properties and a continuum approach to model the mechanical impedance, we find contact stiffness ratios between different domains of few-layer graphene on 3C-SiC(111) in excellent agreement with experiment. We next demonstrate that the approach is able to quantitatively resolve differences between other 2D materials domains, e.g., for h-BN, MoS$_2$ and MoO$_3$ on graphene on SiC. We show that the combined effect of several materials parameters, especially the in-plane elastic properties and the layer thickness, determines the contact stiffness, therefore boosting the sensitivity even if the out-of-plane elastic properties are similar. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H15.00010: Substrate induced phase transformation of monolayer transition metal dichalcogenides Shudun Liu, Xiaojun Fu, Wenguang Zhu Using density functional theory calculations, we investigate the effects of a metal substrate on the structural and electronic properties of a monolayer of transition metal dichalcogenide (TMD). We find that a suitable choice of substrate can induce a transformation of the phase of the monolayer from 2H to 1T. We will discuss the impact of the results on some earlier studies of TMD/metal contacts as well as potential applications of our system in catalysis. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H15.00011: DFT calculation of Landau levels in 2D crystals: from black phosphorus to dichalcogenides Jose Lado, Joaquin Fernandez Rossier We present a method to calculate the Landau levels and the corresponding edge states of two dimensional (2D) crystals using as a starting point their electronic structure as obtained from standard density functional theory (DFT). The DFT Hamiltonian is represented in the basis of maximally localized Wannier functions [1]. This defines a tight-binding Hamiltonian for the bulk that can be readily used to describe other structures, such as ribbons, provided that atomic scale details of the edges are ignored. The effect of the orbital magnetic field is described using the Peierls substitution. By implementing this approach in a ribbon geometry we recover known results for graphene, MoS$_2$ [2] and black phosphorous [3]. We apply this method to predict the Landau level spectrum of MoSSe. Our procedure can readily be used in any other 2D crystal, and provides an alternative to effective mass descriptions. \ [1] A. A. Mostofi, J. R. Yates, Y.-S. Lee, I. Souza, D. Vanderbilt and N. Marzari Comput. Phys. Commun. 178, 685 (2008) [2] Habib Rostami and Reza Asgari, Phys. Rev. B 91, 075433 (2015) [3] J. M. Pereira, Jr. and M. I. Katsnelson, Phys. Rev. B 92, 075437 (2015) [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H15.00012: A tight-binding model for MoS$_2$ monolayers Emilia Ridolfi, Duy Le, Talat Rahman, Eduardo Mucciolo, Caio Lewenkopf We propose an accurate tight-binding parametrization for the band structure of MoS$_2$ monolayers near the main energy gap. We introduce a generic and straightforward derivation for the band energies equations that could be employed for other monolayer dichalcogenides. A parametrization that includes spin–orbit coupling is also provided. The proposed set of model parameters reproduce both the correct orbital compositions and location of valence and conductance band in comparison with ab initio calculations. The model gives a suitable starting point for realistic large-scale atomistic electronic transport calculations. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H15.00013: Probing the uniaxial strains in MoS$_{\mathrm{2}}$ using polarized Raman spectroscopy: A first-principles study Danna Doratotaj, Jeffrey R. Simpson, Jia-An Yan Characterization of strain in two-dimensional crystals is important for understanding their properties and performance. Using first-principles calculations, we study the effects of uniaxial strain on the Raman-active modes in monolayer MoS$_{\mathrm{2}}$. We show that the in-plane $E'$ mode at 384 cm$^{\mathrm{-1}}$ and the out-of-plane $A_{1}'$ mode at 403 cm$^{\mathrm{-1\thinspace }}$can serve as fingerprints for the uniaxial strain in this material. Specifically, under a uniaxial strain, the doubly degenerate $E'$ mode splits into two non-degenerate modes: the $E'_{\mathrm{\vert \vert }}$ and $E'_{\bot }_{\mathrm{\thinspace }}$modes. The frequency of the $E'_{\mathrm{\vert \vert }}$ mode blue-shifts for a compressive strain, but red-shifts for a tensile strain. In addition, due to the strain-induced anisotropy in the MoS$_{\mathrm{2}}$ lattice, the polarized Raman spectra of the $E'_{\mathrm{\vert \vert }}$ and $E'_{\bot }_{\mathrm{\thinspace }}$modes exhibit distinct angular dependence for specific laser polarization setups, allowing for a precise determination of the direction of the strain with respect to the crystallographic orientation. Furthermore, we find that the polarized Raman intensity of the $A_{1}'$ mode also shows evident dependence on the applied strain, providing additional effective clues for determining the direction of the strain even without knowledge of the crystallographic orientation. Thus, polarized Raman spectroscopy offers an efficient non-destructive way to characterize the uniaxial strains in monolayer MoS$_{\mathrm{2}}$. [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H15.00014: Computational study of electronic and thermal properties of single-layer molybdenum disulphide folded nanostructure Jie Peng, Peter Chung Single-layer Molybdenum disulphide ($SLMoS_{2}$), a two-dimensional transition-metal dichalcogenide with a large band gap and high mobility, is considered to be a next generation material for transistors and optoelectronic devices. We present recent results on the electronic and thermal behavior of $SLMoS_{2}$ folded nanostructures. Through an approach that uses both molecular dynamics (MD) and density functional theory (DFT), we estimate the stable equilibrium structure of folded sheets as well as the related phonon and electronic band structures. The MD simulations are based on a Stillinger-Weber potential and the DFT simulations employ projector augmented wave (PAW) pseudopotentials using generalized gradient approximation (GGA) and local density approximation (LDA). The structure is examined as a function of folding orientation, layer number and system size. Mechanisms of the phonon transport and electronic band gap properties in such a mechanically distorted atomic-layer nanostructure will be discussed. [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H15.00015: Tight binding approach to study electronic properties of MoS$_{\mathrm{2}}$/WS$_{\mathrm{2}}$ heterostructure Namita Narendra, Ki Wook Kim The heterostructure interface of MoS$_{\mathrm{2}}$/WS$_{\mathrm{2}}$ is being increasingly studied in recent years for its electronic and optical properties. The ability to tune properties of few-layer transition metal dichalcogenides (TMDs) by strain engineering provides a significant incentive to further explore these material interfaces. It has been shown that misorientation in bulk MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ can also alter the electronic properties. Tight binding allows us to calculate the transport properties of MoS$_{\mathrm{2}}$/WS$_{\mathrm{2}}$ interface for all the angles of misorientation, unlike the computationally limited first principles approach. In this work, the tight binding parameters for the bulk are extracted from first principles and the heterostructure model is verified. A detailed study of variation of electronic properties of MoS$_{\mathrm{2}}$/WS$_{\mathrm{2}}$ interface with respect to addition of strain and number of layers of MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ is carried out. The extension of tight binding model to misoriented MoS$_{\mathrm{2}}$/WS$_{\mathrm{2}}$ interface is demonstrated. [Preview Abstract] |
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