Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q29: Graphene Defects and Functionalization |
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Sponsoring Units: DCMP Chair: Carl Ventrice, College of Nanoscale Science and Engineering, SUNY Albany Room: 603 |
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q29.00001: Direct observation and simulations of atomically resolved low loss images in graphene Myron Kapetanakis, Mark Oxley, Juan-Carlos Idrobo, Wu Zhou, Stephen Pennycook, Sokrates Pantelides Aberration-corrected scanning transmission electron microscopy (STEM) at low voltages provides atomic-resolution imaging of many two-dimensional materials, such as pristine graphene, using core-loss and low-loss spectra. Traditionally, EELS-STEM imaging and density functional theory (DFT) simulations were carried out by two different communities with minimal overlap. One community includes diffraction but ignores solid-state effects in the spectra, while the other includes solid-state effects but leaves out diffraction and interference. Recent work has combined DFT calculations and dynamical scattering to allow the simulation of probe position dependent core-loss spectra. In this talk we describe extension of this work to calculations of STEM images based on low-loss spectroscopy. It is usually assumed that such signals are highly delocalized, since plasmons represent a collective excitation. Considering that not all low-loss excitations are plasmonic in nature, we examine the role of interband transitions in the formation of atomic resolution low-loss images. We compare experimental results that show atomic resolution lattice images of graphene based on low-loss signals with simulations of images based on low-loss scattering potentials. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q29.00002: Simulated non-contact atomic force microscopy for polycyclic aromatic hydrocarbons James Chelikowsky, Minjung Kim Theoretical simulations of non-contact atomic force microscopy (AFM) play an important role in analyzing measured images. Recent non-contact AFM studies on polycyclic aromatic hydrocarbons have achieved atomic resolution that is not observed in other imaging techniques such as scanning tunneling microscopy. In particular, AFM images for these hydrocarbons have resolved bond orders of individual carbon-carbon bonds. Here we present simulated AFM images for several polycyclic aromatic hydrocarbons and explain the role of molecular functionalized AFM tips. Our work is based on solving the electronic structure problem using real space pseudopotentials constructed within density functional theory. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q29.00003: A viable geometry for graphene and single-atom crystals from atoms alone Hamed Pour Imani, Alejandro Pacheco Sanjuan, Zhengfei Wang, Mihajlo Vanevic, Salvador Barraza-Lopez The geometry of a single layer crystals is determined by four invariant parameters from the metric and curvature tensors. We directly study these invariant parameters using a novel framework from atomic positions in terms of angles, areas, vertex and normal vectors from atoms on the lattice for arbitrary elastic regime and atomic conformation, without resorting to differential geometry and continuum elasticity. The results can enable the study the electrical and mechanical properties of atom-thick crystals in a framework complementary to differential geometry and continuum elasticity. [1,2,3] References: 1. ``Graphene's morphology and electronic properties from discrete differential geometry'', Alejandro A. Pacheco Sanjuan, Zhengfei Wang, Hamed Pour Imani, Mihajlo Vanevic and Salvador Barraza-Lopez, Submitted on July 11, 2013. 2. J. V. Sloan et al., Phys. Rev. B 87, 155436 (2013). 3. Barraza-Lopez et al., Solid State Comm 166, 70 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q29.00004: Spatial fluctuations in barrier height at the graphene-silicon carbide Schottky junction Shivani Rajput, Mingxing Chen, Ying Liu, Yaoyi Li, Michael Weinert, Lian Li When graphene is interfaced with a semiconductor, a Schottky contact forms with rectifying properties. Graphene, however, is also susceptible to the formation of ripples upon making contact with another material. In this work, we report intrinsic ripple- and electric field-induced effects at the graphene-semiconductor Schottky junction, by comparing chemical vapor deposited graphene transferred onto semiconductor surfaces of opposite polarization: the hydrogen-terminated Si and C- faces of $\alpha $-SiC. Using scanning tunneling microscopy/spectroscopy and first-principles calculations, we show the formation of a narrow Schottky dipole barrier approximately 10 {\AA} wide, which facilitates the observed effective electric field control of the Schottky barrier height. We further find atomic-scale spatial fluctuations in the Schottky barrier that directly follow the undulation of ripples on both graphene-SiC junctions. These findings reveal fundamental properties of the graphene/semiconductor Schottky junction. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q29.00005: Collective properties of two-dimensional Dirac electron system with a superconducting pairing interaction Daisuke Inotani, Yoji Ohashi, Susumu Okada Recently, the possibility of the superconductivity in graphene are attracting a lot of attention because of its novel properties associated with the pure two-dimensionality, as well as the Dirac fermion nature of the electrons. In this work, we investigate the collective properties of the superconducting graphene. Including the attractive s-wave pairing interaction, as well as the long range Coulomb interaction between the electrons in the tight-binding model for the honeycomb lattice, we calculate the generalized density-density correlation function within the random phase approximation in both normal and superconducting state at T=0. In normal state, we find that a stable collective excitation associated with the superconducting pairing fluctuations appears due to the linear dispersion relation of the electrons. On the other hand, in superconducting state, the phase mode remains stable even at T=0, although the dispersion relation of the phase mode is strongly modified by the Coulomb interaction in the long wave-length region. This result is in contrast to the conventional superconductors in which the phase mode disappears at T=0 by the so-called Anderson-Higgs mechanism. We show that this novel property of the phase mode arises from the pure two-dimensionality of the system. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q29.00006: Interaction phenomena at topological transitions in strongly anisotropic Dirac materials Valeri Kotov It is known that a topological (Lifshitz) transition can take place in graphene, strained uniaxially in the zig-zag direction. At such a transition the spectrum becomes semi-Dirac like, with linear, ultrarelativistic dispersion in one direction, and quadratic momentum dependence in the other. This type of transition also occurs in other materials [1] as well as in artificial graphene lattices [2]. We have found that long-range Coulomb interactions can lead to profound effects at such topological transitions. In particular, an unusually strong log squared renormalization behavior was found in the effective fermion mass, ultimately leading to very strong changes in the shape of the critical fermion spectrum. We also study the stability of such exotic spectrum towards spontaneous gap formation (excitonic transition). Ultimately we find that the interaction effects are much stronger at topological transitions in strongly anisotropic Dirac materials, compared to ``conventional'' isotropic graphene. \\[4pt] [1] G. Montambaux et al, A universal Hamiltonian for the motion and the merging of Dirac cones in a two-dimensional crystal, Eur. Phys. J. B 72, 509 (2009).\\[0pt] [2] M. Bellec et al, Topological transition of Dirac points in a microwave experiment, Phys. Rev. Lett. 110, 033902 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q29.00007: Simulations of noble gases adsorbed on graphene Sidi Maiga, Silvina Gatica We present results of Grand Canonical Monte Carlo simulations of adsorption of Kr, Ar and Xe on a suspended graphene sheet. We compute the adsorbate-adsorbate interaction by a Lennard-Jones potential. We adopt a hybrid model for the graphene-adsorbate force; in the hybrid model, the potential interaction with the nearest carbon atoms (within a distance $r_{\mathrm{nn}})$ is computed with an atomistic pair potential $U_{\mathrm{a}}$; for the atoms at r\textgreater r$_{\mathrm{nn}}$, we compute the interaction energy as a continuous integration over a carbon uniform sheet with the density of graphene. For the atomistic potential $U_{\mathrm{a}}$, we assume the anisotropic LJ potential adapted from the graphite-He interaction proposed by Cole et.al. This interaction includes the anisotropy of the C atoms on graphene, which originates in the anisotropic $\pi $-bonds. The adsorption isotherms, energy and structure of the layer are obtained and compared with experimental results. We also compare with the adsorption on graphite and carbon nanotubes. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q29.00008: Electron supercollimation in graphene using one-dimensional disorder potentials Sangkook Choi, Cheol-Hwan Park, Steven G. Louie Due to its unique electronic structure, electrons in graphene interact with external potential in a counter-intuitive way, manifesting various different interesting characteristics Here we present another surprising, counter-intuitive electron transport phenomenon in graphene. We discovered that electron supercollimation can be induced by 1D disorder potentials. An electron wave packet is guided to propagate undistorted along the fluctuating direction of the external disorder potential, independent of its initial motion. The more disorder, the better is the supercollimation. This robust novel phenomenon is expected to have significant implications in the fundamental understanding of transport in graphene, as well as other materials with Dirac cone physics, and the potential to be exploited in the design of devices based on these materials. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q29.00009: Interaction graphene with metallic and semiconductor surfaces. Ab initio approach to the lattice dynamics Alejandro Molina Sanchez, Ludger Wirtz The interaction of graphene with substrates can alter its electronic and vibrational properties and is relevant for the practical use of graphene. In this work, we describe the graphene-substrate interaction through the theoretical study of the vibrational properties. We focus on three paradigmatic cases where the interaction strength changes gradually: graphene@BN, graphene@Ir(111), and graphene@SiC. We use \textit{ab initio} methods to obtain the phonon band structure, the density of states, and the strength of the electron-phonon coupling. Graphene on boron nitride exhibits a weak interaction but a non-negligible shift of the 2D Raman band. We explain this observation by connecting the increase of the dielectric screening with the softening of the electron-phonon interaction. Graphene on iridium, also displays weak interaction but the substrate is a metal. In this case the electron-electron interaction in graphene is screened by a metal electron gas. In the last case, we study the buffer layer of graphene on silicon carbide. The strong hybridization of graphene with silicon carbide changes substantially the electronic structure of graphene. All the calculations are compared to experimental data of Raman spectroscopy and angle-resolved inelastic electron scattering. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q29.00010: The role of SiO2/graphene interface on morphology of pentacene overlayer Gvido Bratina, Manisha Chhikara We have examined by atomic force microscopy submonolayer of vacuum-evaporated pentacene on exfoliated graphene on SiO$_2$. Two-dimensional (2D) growth is observed on as-transferred graphene. When the samples were heated to 300$^\circ$C for three hours, pentacene formed elongated, three-dimensional (3D) 20-nm-high islands. Strikingly similar pentacene morphology was observed on graphene that was transferred onto SiO$_2$, which was treated by hexamethyldisilazane (HMDS). We have also examined pentacene morphology on many-layer graphene (MLG) that was transferred onto untreated, and HMDS-treated SiO$_2$. In both cases we observed 2D growth. The observed differences in pentacene morphology can be attributed to the changes in graphene surface energy due to different interface layers. Interfacial water layer in the as-transfered graphene samples reduces the surface energy of graphene through the dipole field[1]. This results in 2D pentacene morphology. As the water layer is removed (via HMDS treatment or high-temperature annealing) the graphene surface energy favors 3D growth. As the graphene surface is moved away from the interface on MLG, the effect of the interface is reduced and pentacene grows as 2D layer. [1]J. Sabio, et al., Phys. Rev. B \textbf{77}, 195409 (2008). [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q29.00011: Phase diagram in two-dimensional Hubbard model: variational cluster approximation Armen Kocharian, Kun Fang, Gayanath Fernando, Alexander Balatsky, Kalum Palandage The Variational Cluster Approximation (VCA) is used to rigorously calculate the intrinsic phase diagram in bipartite two-dimensional (2d) Hubbard structures such as square and honeycomb lattice geometries with attraction and repulsion of electrons. The Mott-Hubbard gap, manifested as a smooth metal-insulator transition at finite $U>0$ in both square and honeycomb lattices at half filling ($n=1$), is in agreement with the generic 2d phase diagram. However, a density variation with the chemical potential displays their distinct structural differences away from half filling. Near $n=1$ at equilibrium we found discontinuous transition in square lattices signaling a phase separation instability into an inhomogeneous state with hole rich (metallic) and hole poor ($n=1$-insulating) regions. In contrast, a smooth density transition in honeycomb geometry describes a continuous evolution of homogenous (metallic) state. Incorporation of long-range input in VCA using U$>$0 and U$<$0 models displays antiferromagnetic and superconducting ground states respectively. The implication of VCA results to HTSCs, topological insulators as well as comparison to other studies is discussed. The VCA provides strong support for spontaneous phase separation instability found in our quantum cluster calculations. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q29.00012: Modification of graphene chemistry for metal nanoparticle growth: the effect of substrate selection Anna Zaniewski, Robert Nemanich Graphene and metal nanoparticle composites are a promising class of materials with unique electronic, optical, and chemical properties. In this work, graphene is used as a reducing surface to grow metal nanoparticles out of solution-based metal precursors. The nanoparticle formation is found to strongly depend upon the graphene substrate selection. The studied substrates include silicon oxide, silicon, lithium niobate, and copper. Our results indicate that the chemical properties of graphene depend upon this selection. For example, for the same reaction times and concentration, the reduction of gold chloride to gold nanoparticles on graphene/lithium niobate results in 3{\%} nanoparticle coverage compared to 20{\%} coverage on graphene/silicon and 60{\%} on graphene/copper. This work is supported through the National Science Foundation under Grant {\#} DMR-1206935 . [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q29.00013: Self-Organized Platinum Nanoparticles Elevated on Freestanding Graphene Matthew Ackerman, Peng Xu, Steven Barber, James Schoelz, Dejun Qi, Paul Thibado, Lifeng Dong, Jianhua Yu, Fangfang Xu, Mehdi Neek-Amal, Francois Peeters Freestanding graphene membranes were successfully functionalized with platinum nanoparticles (Pt NPs) using a single-step sputtering deposition process. The membranes were imaged using high-resolution transmission electron microscopy, revealing a homogeneous distribution of uniformly sized, single-crystal Pt NPs that exhibit a preferred orientation and nearest-neighbor distance. The NPs were also found to be partially elevated by the graphene substrate, as deduced from atomic-resolution scanning tunneling microscopy (STM) images. Furthermore, the electrostatic force between the STM tip and sample was utilized to estimate the binding energy of the NPs to the suspended graphene. Local strain accumulation due to elevation during the growth process is thought to be the origin of the NP self-organization. Such detailed insight into the atomic nature of this functionalized system was only possible through the cooperation of dual microscopic techniques combined with molecular dynamics simulations. The findings are expected to shape future approaches to develop high-performance electronics based on nanoparticle-functionalized graphene as well as fuel cells using Pt NP catalysts. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q29.00014: Fracture size effects in defected graphene Alessandro Luigi Sellerio, Stefano Zapperi We investigate fracture in a monolayer graphene with a small concentration of vacancies by molecular dynamics simulations. We simulate monolayers of varying size encompassing more than three decades, and we systematically study the mechanical failure following simulated ``constant engineering strain'' conditions. We compute the fracture strength distribution as a function of system size and defect concentration and compare the results with extreme value statistics. We highlight similarities and differences between the size effects expected in the fracture of macroscopic disordered solids with those observed at the nanoscale. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q29.00015: Effective Properties of Graphene with Large Periodic Anti-dots Bing Zhang, Ping Sheng Antidot graphene is an interesting material system which can exhibit a bandgap. In this work we present the effective properties of graphene with large periodic anti-dots, obtained by solving the Weyl equation numerically, as well as through the k dot p theory calculation. We find the dispersion relation to be hyperbolic in character, leading to an effective mass m* with an altered Fermi velocity v$_{\mathrm{eff}}$ as compared to the pristine case. The gap is exactly given by 2m*c$^{\mathrm{2}}$, where c is the speed of light. The dependence of m* and v$_{\mathrm{eff}}$ on geometric parameters is investigated. A remarkable enhancement of the Coulomb interaction parameter is seen in the antidot graphene, in the region close to the bottom of the band. This can explain the appearance of a Coulomb quasi-gap discovered recently in this material system. [Preview Abstract] |
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