Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L31: Graphene: Atomic Structure and Lattice Properties |
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Sponsoring Units: DCMP Chair: Dan Finkenstadt, Naval Research Laboratory Room: Morial Convention Center 223 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L31.00001: Nano-Engineering Defect Structures on Graphene Mark Lusk, Lincoln Carr We present a new way of nano-engineering graphene using defect domains. These regions have ring structures that depart from the usual honeycomb lattice, though each carbon atom still has three nearest neighbors. A set of stable domain structures is identified using density functional theory (DFT), including blisters, ridges, ribbons, and meta-crystals. All such structures are made solely out of carbon; the smallest encompasses just 16 atoms. Blisters, ridges and meta-crystals rise up out of the sheet, while ribbons remain flat. In the vicinity of vacancies, the reaction barriers to formation are sufficiently low that such defects could be synthesized through the thermally activated restructuring of coalesced adatoms. These defect domains may offer technological applications associated with the confinement and transport of charge. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L31.00002: Solid State Zwitterions realized on carbon graphenic surface ZhaoHui Huang, Vincent Crespi Zwitterions are single molecular species that combine anionic and cationic groups. Here we consider the prospects for introducing the concept of a zwitterion into the solid state, by combining geometrically incompatible anionic and cationic moieties within a single extended structural element whose covalent rigidity frustrates the close approach of the anionic and cationic regions. Specifically, first principles computations for anionic and cationic groups such as NH3 and CO2 covalently attached to a graphenic surface via linker elements demonstrate long-range charge transfer, the hallmark of a zwitterion, while maintaining overall structural integrity. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L31.00003: Atomic structure of corrugated graphene You Lin, Xiang Gu, Ivan Oleynik, Carter White Following its successful isolation, there has been intense interest in a single sheet of graphite, known as graphene, due to its fundamental physical properties as well as its promising applications in nanoelectronic devices. Recent experimental studies of graphene, freely suspended on nanofabricated scaffolding, found appreciable deviations from a perfect two-dimensional crystalline structure [1]. In this presentation, we discuss results of atomistic modeling of graphene with the occurrence of possible corrugated structures examined under the influence of several factors including sample size and temperature. [1] J.C. Meyer, \textit{et al}, Nature \textbf{446}, 60 (2007) [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L31.00004: \textit{Ab initio} study on atomic and electronic structures of epitaxial graphene Seungchul Kim, Jisoon Ihm, Hyoung Joon Choi, Young-Woo Son Recently, lots of efforts have been devoted to the growth of epitaxial graphene and its geometric and electronic structure measurements. Several models for the interface structure have been suggested but never been successful in explaining the observations from various experiments. Using density functional theory (DFT) calculations, we find an interface atomic structure of graphene on hexagonal silicon carbide which agrees well with the low energy electron diffraction (LEED), scanning tunneling microscopy (STM), as well as angle resolved photo emission spectroscopy (ARPES). Our results clearly resolve the disagreement between $6\times 6$ periodic pattern from STM measurements and the $6\sqrt{3} \times 6\sqrt{3}$ from the LEED measurements. Furthermore, we also investigate the origin of the gap opening at the Dirac point in the present geometry. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L31.00005: Phonon dispersions and vibrational properties of monolayer, bilayer, and trilayer graphene Jia-An Yan, W. Y. Ruan, M. Y. Chou The phonon dispersions of monolayer and few-layer graphene (AB bilayer, ABA and ABC trilayers) are investigated using the density-functional perturbation theory (DFPT). Compared with the monolayer, the optical phonon $E_{2g}$ mode at $\Gamma$ splits into two and three doubly degenerate branches for bilayer and trilayer graphene, respectively, due to the weak interlayer coupling. These modes are of various mode symmetry and exhibit different sensitivity to either Raman or infrared (IR) measurements (or both), and therefore the combination of Raman and IR measurements of the zone-center optical modes should give a clear identification of the layer number as well as the stacking geometry. The splitting is found to be 5 cm$^{-1}$ for bilayer and 2 to 5 cm$^{-1}$ for trilayer graphene. The interlayer coupling is estimated to be about 2 cm$^{-1}$. We found that the highest optical modes at K upshift by about 12 cm$^{-1}$ for bilayer and 18 cm$^{-1}$ for trilayer relative to monolayer graphene. The atomic displacements of these optical eigenmodes are analyzed. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L31.00006: Structural study of the phase evolution of 6H-SiC(0001) by low energy electron microscopy Jiebing Sun, Karsten Pohl, Rudolf M. Tromp, James B. Hannon The surface phase transition of Si-terminated 6H-SiC(0001) upon heat treatment is studied by low energy electron microscopy (LEEM). Bright and dark field imaging demonstrates a direct \textit{in situ} observation of the surface phase evolution, transitions in a sequence from 1$\times $1, 3$\times $3, $\surd $3$\times \surd $3, 6$\surd $3$\times $6$\surd $3 to the graphene phase due to gradually increasing the temperature. Intensity vs. voltage (IV) spectra extracted from single domain diffraction images is used to determine the local surface structure and chemical stoichiometry. Preliminary results from a quantitative dynamical analysis of the LEEM-IV curves show a Si-depleted 1$\times $1 structure and an adatom-trimer-adlayer structure on 3$\times $3 reconstruction. Ongoing work on the structure of the $\surd $3$\times \surd $3 and 6$\surd $3 $\times $ 6$\surd $3 phases is aimed to unraveling the initial growth mechanism of graphene on SiC. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L31.00007: The Initiation of Graphene Growth on SiC(0001)-6H James Hannon, Rudolf Tromp We have studied the evolution of surface morphology on SiC(0001)-6H during annealing at temperatures up to 1250 C using low-energy electron microscopy (LEEM). Surface roughness is dominated by the formation of deep pits or canyons. We show that the canyons form because of the stability of the 6$\surd $3 $\times $ 6$\surd $3 phase, which pins atomic steps during the decomposition of SiC. The density of pits is ultimately determined by how the 6$\surd $3 phase nucleates. Graphene forms preferentially in these pits, where the step density is highest. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L31.00008: Imaging the interface states of epitaxial graphene layers on 6H-SiC G. Sun, Y. Qi, M. Weinert, L. Li Single and bi-layer graphene were epitaxially grown on both the Si- and C-terminations of 6H-SiC. The energy dependence and spatial distribution of their local density of states were investigated using scanning tunneling microscopy and spectroscopy. Of particular interest is the rt3xrt3 reconstructed interface state. Atomically resolved topographs and dI/dV images show clear differences between the single and bi-layer graphene at different length scales. These results will be compared to the electronic and structural properties obtained by first principles calculations. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L31.00009: Interface Studies of Graphene layers on SiC thin films and bulk SiC(0001) Andreas Sandin, J.L. Tedesco, R.J. Nemanich, J.E. (Jack) Rowe A multi-method approach is described based on the well-known high temperature annealing procedure that converts SiC into thin layers of graphite called graphene and has been applied to bulk 6H-SiC(0001) as well as epitaxial thin films of 3C-SiC. Atomic Force Microscopy (AFM) measurements of epitaxial SiC on Si(100) used an UHV Omicron AFM/STM/LEED/Auger multi-probe system. Initial AFM and STM measurements show narrow domains of $\sim $150 nm dimension that have an Auger electron Spectroscopy (AES) signature confirming the formation of graphene on the SiC surface. Our AES measurements show a weak Si KLV transition at $\sim $ 1623 eV which can be used to determine the layer thickness of graphene which is typically $\sim $ 10 {\AA}. Low Energy Electron Diffraction (LEED) also confirms the well-known (6x6) pattern of SiC + graphene reported earlier by several groups.$^{ }$ We find that the conversion temperature appears to be somewhat lower ($\sim $ 1050 -- 1080 \r{ }C) for films of SiC on Si(100) than for bulk SiC (0001) surfaces. This is possibly due to surface dislocations formed during the epitaxial growth process with very large lattice mismatch between SiC and the Si substrate, which provide additional diffusion sites and paths for the surface segregation reaction that forms the graphene layer. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L31.00010: X-ray studies of the rotational fault distributions in multilayer graphene grown on the 4H-SiC$(000\bar{1})$ surface J.E. Mill\'an, J. Hass, F. Varchon, W.A. deHeer, C. Berger, P.N. First, L. Magaud, E.H. Conrad We present x-ray diffraction experiments showing that multilayer graphene grown on 4H-SiC $(000\bar{1})$ (C-face) consists of a high density of rotational stacking faults. The existence of these faults explains why multilayer graphene is electronically similar to an isolated graphene sheet.[1] The faults present themselves as graphite rods rotated through a series of commensurate graphene/SiC angles relative to the SiC rods and as rods shifted due to compressed graphene at the fault boundaries. By analyzing the intensity modulations of these different rods, it is possible to extract information on the distributions of these faults in the film. We will present results from calculations of different models for the fault distribution and compare them against x-ray data for graphene films of different thicknesses. [1] J. Hass, F.Varchon, J. E. Millan-Otoya, M. Sprinkle, W.A. de Heer, C. Berger, P.N. First, L. Magaud, E.H. Conrad (to be published) http://arxiv.org/abs/0706.2134 [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L31.00011: Dispersive Raman Scattering from \textit{n}=1-4 Graphene Layers (\textit{n}GLs) Peter Eklund, Awnish Gupta We present new Raman scattering results from $n$GLs ($n$=1, 2, 3, 4) in the range of 100 - 4500 cm$^{-1}$. Dispersive behavior of Raman peaks was probed at room temperature with 7 laser lines from 1.5-2.7 eV. In addition to the five Raman peaks reported previously, we report on the behavior of five new weaker features that appear in graphene (514.5 nm excitation) at $\sim $1882 cm$^{-1}(\sim $ 125 cm$^{-1}$/eV), $\sim $2035 cm$^{-1 }(\sim $ 177 cm$^{-1}$/eV), $\sim $2218 cm$^{-1 }(\sim $ -43 cm$^{-1}$/eV), $\sim $3174 cm$^{-1 }(\sim $ -40 cm$^{-1}$/eV) and $\sim $4069 cm$^{-1}(\sim $35 cm$^{-1}$/eV). The value in ( ) is the respective band dispersion, or shift with peak position per eV change in the excitation energy. The band dispersion is connected with the ratio of the phonon to Fermi velocity and stems from the double resonance (DR) scattering. New Raman bands that are only observed for $n$GLs ($n>$1) are found at $\sim $1510 cm$^{-1}(\sim $ 16 cm$^{-1}$/eV) and 1737 cm$^{-1}$ ($\sim $ 10 cm$^{-1}$/eV). The identities of the new peaks will be discussed based on DR scattering and the phonon dispersion curve of graphene. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L31.00012: The Thickness Dependence of the Graphene Oxidation Li Liu, Sunmin Ryu, Michelle Tomasik, Elena Stolyarova, Michael Steigerwald, Mark Hybersten, Louis Brus, George Flynn Single-, double-, and triple-layer graphene sheets were heated in an oxygen atmosphere at various temperatures generating nano-sized holes in the sheets. Both AFM topography and Raman spectroscopy indicate that the oxidative reactivity of single-layer sheets is greater than that of thicker sheets. The distribution of hole sizes and STM topography studies suggest that the oxidation reaction is initiated at the pristine carbon surface. Vertical etching of carbon atoms from the graphene surface occurred at a much lower temperature than that from a highly oriented, multi-layer pyrolytic graphite crystal. The mechanism for this thickness dependence of reactivity will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L31.00013: UHV STM/STS and AFM study of oxidized graphene sheets Deepak Pandey, Richard Piner, Ronald Reifenberger Exfoliated oxidized graphene sheets, suspended in an aqueous solution, were deposited on freshly cleaved HOPG and studied by ambient AFM and UHV STM. The AFM images revealed oxidized graphene sheets with a lateral dimension of $\sim $5-10 $\mu $m. The oxidized graphene sheets exhibited different thicknesses and were found to conformally coat the HOPG substrate. Wrinkles and folds induced by the deposition process were clearly observed. Phase imaging and lateral force microscopy showed distinct contrast between the oxidized graphene and the underlying HOPG substrate. The UHV STM studies of oxidized graphene revealed atomic scale periodicity showing a 0.26 nm $\times $ 0.42 nm unit cell over small areas. This periodicity is identified with oxygen atoms bound to the oxidized graphene sheet. I(V) data were taken from oxidized graphene sheets and compared to similar data obtained from bulk HOPG. The dI/dV data from oxidized graphene reveals a reduced density of electron states within $\pm $0.1 V around zero bias. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L31.00014: Scanning Tunneling Spectroscopy (STS) studies of graphene films on an insulating substrate Elena Stolyarova, Li Liu, Mark Hybertsen, Philip Kim, Tony Heinz, George Flynn Scanning Tunneling Spectroscopy has been utilized to study the differences between the electronic structure of a three-dimensional graphite crystal and its two-dimensional building block, graphene. Single and few-layer graphene samples were isolated on a non-conductive silicon dioxide substrate and contacted at the edges with a gold electrode. For single layer flakes current-voltage dependent I(V) curves, recorded at 4.6 K under Ultra-high Vacuum (UHV) conditions, show no additional features (for states far from the Fermi energy) that might be considered characteristic of a weak interaction between graphene and the substrate. No significant spatial inhomogeneity of local sample properties was observed. Evolution of spectroscopic curves as a function of graphene layers will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L31.00015: Generation of atomic scale defects in graphene by high-energy positive-ion bombardment Daniil Stolyarov, Elena Stolyarova, George Flynn, Karl Kusche, Igor Pavlishin, Igor Pogorelsky, Peter Shkolnikov , Vitaly Yakimenko We have demonstrated controllable defect generation in graphene (a single layer of a graphite crystal) and in few atomic layer thick graphitic films. Graphene flakes deposited on a silicon dioxide substrate by mechanical exfoliation were bombarded by a collimated high-energy (1 MeV) particle beam consisting of protons and positively charged ions. This beam was produced by Target Normal Sheath Acceleration (TNSA) upon irradiation of a metal foil with a terawatt CO2 laser pulse. Ions and protons with different mass and energy were separated by a permanent magnet installed between the laser target and the sample. After the exposure the graphene flakes were examined by Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM). Irradiation of graphene films with both protons and positive ions results in the formation of atomic-scale defects without mesoscopic damage to the flake. The density of observed defects depends strongly on the number of atomic layers. This method can be used to modify single chemical bonds in graphene films and to engineer carbon based devices and sensors with tailored properties. [Preview Abstract] |
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