Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B22: Focus Session: Graphene Structure: Local Probes |
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Sponsoring Units: DMP Chair: Vikas Berry, Kansas State University Room: Portland Ballroom 252 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B22.00001: Electron Diffraction and Imaging Measurements of the Morphology of Suspended and Supported Graphene Kevin Knox, Andrea Locatelli, Dean Cvetko, Tevfik Mentes, Miguel Nino, Shancai Wang, Mehmet Yilmaz, Philip Kim, Richard Osgood, Alberto Morgante Crystal deformations in graphene are known to adversely affect conductivity and increase local chemical reactivity. Thus, an understanding of the surface morphology of graphene is essential to making high quality devices. The morphology of supported graphene has been extensively studied with STM and AFM. However, since these techniques are accompanied by the application of elastic forces they cannot be extended to suspended graphene. In this talk, we will describe a non-invasive approach to examine the corrugation in suspended and supported exfoliated graphene using a combination of low energy electron microscopy (LEEM) and micro-spot low energy electron diffraction (LEED). LEEM measurements allow real space imaging of surface features above 10 nm. A careful analysis of LEED measurements reveals details of surface morphology down to interatomic distances. Thus, by combining these two probes we are able to access both the microscopic and mesoscopic regimes to obtain a full characterization of the corrugation in graphene. [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B22.00002: Modified Electronic Structure of Distorted Graphene on Ru(0001) : STM and STS Study Won-Jun Jang, Jong Keon Yoon, Howon Kim, Se-Jong Kahng Graphene is an attractive material that has special electronic features such as massless carriers, Dirac cones and integer quantum Hall effect. Graphene can be epitaxially grown on metal surfaces. Electronic structure of Graphene on Ru(0001) was studied using scanning tunneling microscopy and scanning tunneling spectroscopy. A monolayer graphene on Ru(0001) has periodically rippled structures due to the difference in the lattice parameters of substrate and graphene, and shows inhomogeneities of charge distribution. The monolayer graphene can have extra inhomogeneities that are possibly made by the geometrical or electronic variation of substrate, which is explained by our spectroscopy data. [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B22.00003: Substrate effects on a graphene membrane Silvia Viola Kusminskiy, David Campbell, Antonio Castro Neto, Francisco Guinea We study the effect of the substrate on the configuration of a graphene membrane within elastic theory. We analyze different forms of spatial disorder of the substrate and its coupling to graphene's phonon modes. In-plane and flexural modes of the graphene membrane are taken into account. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:27PM |
B22.00004: Exotic properties of graphene - based membranes Invited Speaker: Freestanding graphene, i.e. a graphene membrane, is an only one atom thick membrane with exceptional crystalline quality. Here, we report about optical and structural properties of freestanding graphene and chemically modified graphene membranes. Even though graphene is only one atom thick, we found that it absorbs a considerable amount of light (2.3{\%}), which is defined by the fine structure constant, the parameter that describes the coupling between light and relativistic electrons. We also used graphene membranes as a scaffold for synthesizing new materials. Whereas three-dimensional graphite is an inert material, graphene is said to be highly reactive to different atomic species. By exposing graphene to atomic hydrogen, we were able to convert sp2 hybridized graphene to sp3 hybridized graphane. The atomic structures of graphene membranes, as well as chemically modified graphene membranes, were investigated using atomic resolution transmission electron microscopy and electron diffraction. These studies clarified that after chemical modification the lattice constant and the corrugation of freestanding graphene is changed considerably. I will also give an overview of our latest results on structural and electronic properties of newly developed graphene derivatives. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B22.00005: Topological defects in graphene: dislocations and grain boundaries Oleg Yazyev, Steven Louie Topological defects in graphene, dislocations and grain boundaries, are still not well understood despites the considerable number of experimental observations. We introduce a general approach for constructing dislocations in graphene characterized by arbitrary Burgers vectors as well as grain boundaries, covering the whole range of possible misorientation angles. By using ab initio calculations we investigate thermodynamic, electronic and transport properties of grain boundaries, finding energetically favorable large-angle symmetric configurations, strong tendency towards out-of-plane deformation in the small-angle regimes, pronounced effects on the electronic structure, and two distinct behaviors in the electronic transport - either perfect reflection or high transparency for low-energy charge carriers depending on the grain boundary structure. Our results show that dislocations and grain boundaries are important intrinsic defects in graphene which may be used for engineering graphene-based functional devices. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B22.00006: Imaging surface potential variations in graphene on Si-face SiC Alexandra Curtin, Michael S. Fuhrer, J. L. Tedesco, R. L. Myers-Ward, P. M. Campbell, C. R. Eddy, D. K. Gaskill Kelvin Probe Microscopy (KPM) is used to map the surface potential of graphene monolayers grown on the Si-face of SiC wafers. Variations in surface potential are observed which are not correlated to any topographical features on the graphene surface. The observed variation in surface potential has an RMS value of $\sim $17 meV, and correspond to variations in carrier concentration on the order of 10$^{10}$-10$^{11}$ cm$^{-2}$. We will discuss the implications of these measurements on interpretation of transport data on ungated graphene on Si-face SiC. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B22.00007: Local Probe Investigation of Metal Adsorbates on a Gated Graphene Device Regis Decker, Victor W. Brar, Hans-Michael Solowan, Yuanbo Zhang, Alex Zettl, Michael F. Crommie Understanding the scattering properties of electrons from adsorbates and defects in graphene is important for controlling the behavior of different graphene nanostructure-based devices. Here we report a scanning tunneling microscopy and spectroscopy study of metal adsorbates on a single monolayer of graphene. In our experiments the graphene is placed on a layer of insulating SiO$_{2}$ that sits above a doped silicon back-gate electrode. We will discuss our observations of the electronic local density in the vicinity of metal atoms, as well as how these properties respond to electrical gating of the graphene monolayer with respect to the silicon back-gate electrode. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B22.00008: Defects in epitaxial graphene on SiC(0001) created by Ar plasma Ying Liu, Lian Li Defects are created upon the exposure of epitaxial graphene on SiC(0001) to Ar plasma at room temperature. The atomic and electronic structures of these defects are studied by scanning tunneling microscopy (STM), which reveals two predominant types of structures for these defects. Their induced modulations on the local density of states are further studied by Fourier transform STM. The results suggest that vacancies are created, with Ar atoms nearby trapped between the graphene sheets and SiC substrate, forming Ar-vacancy complexes. Changes in the defect electronic structures during STM imaging can be attributed to the dissociation and recombination of these complexes. [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B22.00009: Magnetism of nitrogen-vacancy complexes in epitaxial graphene on SiC(0001) S. H. Rhim, Y. Qi, G. F. Sun, Y. Liu, M. Weinert, L. Li We present a study of the magnetism of nitrogen impurity--vacancy complexes in epitaxial graphene on SiC(0001) using a combination of density functional calculations and scanning tunneling microscopy. Nitrogen-vacancy complexes are created by means of a nitrogen plasma. From the calculated energetics and by comparison with STM, the most probable configuration is found to have the vacancy and substitutional nitrogen at the nearest neighbor site. This configuration is non-magnetic, whereas other higher energy N-vacancy configurations (and the isolated vacancy) are magnetic. Adatoms such as H, C, and N, in combination with the nitrogen-vacancy complex, show a variety of magnetic behaviors, depending on both the adatom and its position. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B22.00010: Piezo force microscopy studies of ferroelectric copolymer P (VDF-TrFE) on epitaxial graphene/SiC(0001) Miraj Uddin, Lian Li Graphene, a sheet of sp$^{2}$-bonded carbon atoms densely packed in a honeycomb lattice, has a linear band dispersion near the Dirac points that allows the direct control of carrier density and type via electric field. The integration of graphene with a ferroelectric material that exhibits non-volatile field effects and where the polarization domains can be reconfigured can further enhance its applications in multifunctional devices. In this work, we studied morphology and piezo response of thin films of a ferroelectric polymer polyvinylidene fluoride with trifluoroethylene, P(VDF-TrFE), deposited on epitaxial graphene on SiC(0001) by spin coating, using atomic force microscopy (AFM) and piezo force microscopy. We have demonstrated that polarization domains of the ferroelectric polymer films can be switched by the electric field produced by a biased conducting AFM tip. ~We also found that the polarization domains of the P(VDF-TrFE) films show good stability, facilitating its potential use as a nonvolatile gate to control the conductivity of graphene. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B22.00011: Atomic-scale imaging and manipulation of ridges on epitaxial graphene on 6H-SiC(0001) G.F. Sun, J.F. Jia, Q.K. Xue, L. Li The graphitization of hexagonal SiC surfaces provides a viable alternative for the synthesis of graphene on a wafer. Ridges are often observed on epitaxial graphene, particularly during the later stages of growth. These ridges (or ``wrinkles'') introduce ripples in the otherwise atomically flat graphene sheet, which likely causes scattering and reduces its carrier mobility. The origin of these features, however, is largely under debate. In this work, we show, by atomically resolved STM imaging, that ridges are in fact bulged regions of the graphene layer, as a result of bending and buckling to relieve the compressive strain between the graphene and SiC substrate [1]. We further demonstrate that their length, direction, and distribution can be manipulated and even new ones created by the pressure exerted by the STM tip. We have further determined a lower limit of terrace size for ridge formation to be $\sim $80 nm, and demonstrated the growth of nearly ridge-free graphene films on vicinal substrates. \\[4pt] [1] Sun et al., Nanotechnology \textbf{20, }355701 (2009). [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B22.00012: Nanomechanics of Graphene, Silicene and Boron Nitride ribbons: From honeycomb structure to atomic chains Mehmet Topsakal, Salim Ciraci This first-principles study of elastic and plastic deformation of graphene, silicene and boron nitride (BN) honeycomb nanoribbons under uniaxial tension reveals interesting features. In the course of stretching, the electronic and magnetic properties can be strongly modified. Under plastic deformation, the honeycomb structure changes irreversibly and offers a number of new structures and functionalities. Interesting cage like structures, even suspended atomic chains, a truly one-dimensional system offering unique mechanical, electronic and transport properties, can be derived between two honeycomb flakes. Present work elaborates on the recent experiments by Jin \textit{et al.,} Phys. Rev. Lett. \textbf{102}, 205501 (2009) deriving carbon chains from graphene and furthermore predicts the similar formation of BN and Si atomic chains. [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B22.00013: Local Density of States in Disordered Graphene: a tight binding calculation Donald Priour, Euyheon Hwang, Sankar Das Sarma We calculate the local density of states for graphene where disorder has the form of hydrogenation defects. We use the results to theoretically simulate the images of STM studies of this system for various defect densities. Operating within a tight binding framework, we calculate the density of electronic states in the Coherent Phase Approximation (CPA). Ultimately, we make no approximation and obtain the Green's function directly (we exploit sparseness of the appropriate system of equations to examine large systems sizes) for specific realizations of disorder in the graphene honeycomb lattice. We discuss the accuracy of the CPA results in the context of the full Green's function calculation. We present simulated STM images obtained in our calculations. This work has been partially supported by US-ONR and NSF-NRI-SWAN. [Preview Abstract] |
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