Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J25: Focus Session: Graphene V: Structure and Raman Spectroscopy |
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Sponsoring Units: DMP Chair: Marcos Pimenta, UFMG, Brasil Room: 327 |
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J25.00001: Quantum and transport scattering times in graphene X. Hong, K. Zou, J. Zhu, A. Posadas, J. Hoffman, C. H. Ahn We study the quantum ($\tau_q$) and transport ($\tau_t$)
scattering times of single layer graphene mechanically
exfoliated on SiO$_2$ and polycrystalline Pb(Zr,Ti)O$_3$ (PZT)
substrates. The PZT substrate exhibits a gating efficiency of
$\sim2\times10^{11}$/cm$^2$/V$_g$(V), corresponding to a
dielectric constant of $\sim$12. We extract $\tau_q$ from the
magnetic field dependence of Shubnikov de Haas oscillations and
$\tau_t$ from the zero field mobility, respectively. For the
PZT-gated graphene, in the density range of $2\times10^{12}
$/cm$^2 |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J25.00002: Electrical and structural properties of chemically modified graphene sheets Dmitriy A. Dikin, Inhwa Jung, Rodney S. Ruoff The chemical exfoliation of graphite through oxidation and then dispersion in a solvent is one of the methods of achieving scalable production of single graphene sheets. We use this method for making chemically modified graphene (CMG) sheets with tunable electronic properties, which can be placed flat on any surface or dispersed in various matrices. CMG sheets share some similarities with pristine graphene and with carbon nanotubes, e.g. tunable electron- and hole-type conductivity is observed in single CMG sheets just above the percolation threshold. CMGs may also be considered as a template for a bottom up development of a new class of materials. We have performed electrical measurements of individual CMG sheets and will discuss their electronic properties and the possible mechanisms of the charge transport in relation to their atomic structure and chemical composition. [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 12:15PM |
J25.00003: Doping, Strain, Orientation and Disorder of Graphene by Raman Spectroscopy Invited Speaker: Raman spectroscopy is a fast and non-destructive method for the characterization of carbons [1]. These show two features: the G and D peaks, around 1580 and 1350cm$^{-1}$ respectively. The G peak corresponds to the doubly degenerate E$_{2g }$phonon at the Brillouin zone centre. The D peak is due to the breathing modes of sp$^{2}$ atoms and requires a defect for its activation [1-5]. It is common for as-prepared graphene not to have enough structural defects for the D peak to be seen [4,6], so that it can only be detected at the edges [6]. The most prominent feature in graphene is the second order 2D peak [6]. This is always seen, since no defects are required for its activation. Its shape distinguishes single and multi-layers [6]. Raman spectroscopy also monitors doping [7-9]. We report the evolution of the Raman spectra of single [7,8] and bi-layer [9] graphene as a function of doping. A Fermi level shift is induced either by applying a bottom gate [7], or by a polymeric top gate [8,9], or naturally happens as a result of charged impurities [10]. This induces a stiffening of the Raman G peak for both hole and electron doping [7]. This is explained including dynamic corrections to the adiabatic Born-Oppenheimer approximation [7]. The phonon renormalization of bilayer graphene has characteristic features compared to single layer. This allows a direct estimation of the interlayer coupling [7-9]. We then consider the effects strain. Uniaxial strain lifts the E$_{2g}$ degeneracy and splits the G peak in two: G$^{+ }$and G$^{-}$. The peaks downshift as a function of strain allows a direct measurement of the Gruneisen parameter [10]. The polarization dependence of the G$^{+}$/G$^{- }$modes is a probe of the crystallographic orientation of the sample [10]. Finally, we consider the effect of disorder [3,4,11] and show how to discriminate between disorder, strain and doping [11]. We will also discuss how the D peak is a signature of $\pi $ electron localisation, and, thus, of gap opening in chemically modified graphene[12]. \\[4pt] 1. A. C. Ferrari, J. Robertson (eds), \textit{Raman spectroscopy in carbons: from nanotubes to diamond}, Theme Issue, Phil. Trans. Roy. Soc. \textbf{362}, 2267 (2004). 2. F. Tuinstra, J.L. Koening, J. Chem. Phys. \textbf{53,} 1126(1970). 3. A. C. Ferrari, J. Robertson Phys Rev B \textbf{61}, 14095 (2000); \textbf{64}, 075414 (2001) 4. A. C. Ferrari Solid State Comm.\textbf{143}, 47 (2007) 5. S. Piscanec et al. Phys. Rev. Lett. \textbf{93}, 185503 (2004) 6. A. C. Ferrari et al. Phys. Rev. Lett. \textbf{97}, 187401 (2006) 7. S. Pisana et al. Nature Mater. \textbf{6}, 198 (2007) 8. A. Das et al, Nature Nano \textbf{3}, 210 (2008). 9. A. Das et al., arXiv:0807.1631v1 (2008) 10. A. C. Ferrari et al. submitted (2008) 11. C. Casiraghi et al. Appl. Phys Lett. \textbf{91}, 233108 (2007) 12. Elias et al. arXiv:0810.4706 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J25.00004: Controlled Structural Strain in Epitaxial Graphene Layers on 6H-SiC and Effects on Surface Morphology Nicola Ferralis, Jason Kawasaki, Roya Maboudian, Carlo Carraro The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investigated by depolarized Raman spectroscopy and electron channeling contrast imaging. The selection of the depolarized component of the scattered light results in a significant increase in the C=C bond signal over the second order SiC Raman signal, which allows us to resolve submonolayer growth, the formation of the buffer layer and a strained graphene layer. The linear strain, measured at room temperature (RT), is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. Whereas film thickness is determined by growth temperature only, the magnitude of the compressive strain and film morphology can be varied by adjusting the growth time at fixed annealing temperature. Annealing times in excess of 8-10 minutes lead to an increase in the mean square roughness of SiC step edges to which graphene films are pinned, resulting in compressively stressed films at RT. Shorter annealing times produce minimal changes in the morphology of the terrace edges and result in nearly stress-free films upon cooling to RT. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J25.00005: Frictional Characteristics of graphene Changgu Lee, Robert Carpick, James Hone The frictional characteristics of graphene were characterized using friction force microscopy (FFM). The frictional force for monolayer graphene is more than twice that of bulk graphite, with 2,3, and 4 layer samples showing a monotonic decrease in friction with increasing sample thickness. Measurements on suspended graphene membranes show identical results, ruling out substrate effects as the cause of the observed variation. Likewise, the adhesion force is identical for all samples. The frictional force is independent of load within experimental uncertainty, consistent with previous measurements on graphite. We consider several possible explanations for the origin of the observed thickness dependence. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J25.00006: Using Defects as Local Electronic Probes of Epitaxial Graphene on SiC Gregory M. Rutter, Kevin D. Kubista, David L. Miller, Ming Ruan, Walter A. de Heer, Phillip N. First, Joseph A. Stroscio Defects play an important role in the transport properties of epitaxial graphene, and understanding this role is essential for realizing potential nanoelectronics based on graphene. In this study, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) performed at 4.2 K are used to measure the local electronic behavior of defects in epitaxial graphene grown on both SiC(0001) and SiC(000-1). Energy-resolved maps of the differential conductance reveal defect-induced standing-wave modulations related to the unique nature of the graphene band structure. In this talk, I will discuss how these defects can be used as a local probe of the graphene electronic properties with the inclusion of an applied magnetic field and the resulting Landau quantization. Supported in part by NSF, NRI-INDEX, and the W. M. Keck Foundation. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J25.00007: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J25.00008: Crystallographic Cuts in Single Layer Graphene Leonardo Campos, Vitor Manfrinato, Javier Sanchez, Jing Kong, Pablo Jarillo-Herrero Graphene consists of a single monolayer of carbon atoms in a honeycomb 2D crystal. It is unique because the electrons are described by the Dirac equation, like ultrarelativistic particles with zero rest mass. According to theoretical predictions, it is possible to create field effect transistor just using narrow (d$<$10nm) nanoribbons. With zigzag edges, graphene nanoribbons can have a large magneto-resistance or could be used to produce a spin valve. With armchair edges it is possible to have an energy gap controllable by electric field. In this work we will show how to use Ni nanoparticles to create crystallographic oriented cuts in exfoliated graphene. Using Raman spectroscopy and electronic measurements of Dirac peaks, we have verified that the graphene, after the high temperature nanocut etching process, are still high quality 2D crystals, indicating that this process can be used to produce graphene nanodevices. Using this method we fabricate oriented nanoribbons and equilateral triangles with varying size. We also present a detailed analysis of the fabrication conditions for controlling the etching characteristics. Last, we present our analysis of the chirality of our nanocuts. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J25.00009: Chiral tunnelling of Dirac electrons in strained graphene A. Garcia-Saravia, G. Cordourier-Maruri, M.E. Cifuentes-Quintal, E. Martinez-Guerra, R. De Coss The behavior of the electrons in graphene is like massless Dirac fermions, which is a consequence of the characteristic energy spectrum of this material (E$\sim $k). Perfect chiral tunnelling is expected when Dirac electrons pass through a step barrier (Klein paradox). However, in a two-dimensional system like graphene, the perfect tunneling is obtained only in a small range of incident angles. In the present work, we have studied the uniaxial deformation as a method of tunning the electronic transmittance in graphene. The effect of the armchair and zigzag strain on graphene was studied by means of first principles calculations, using the Density Functional Theory. For the calculations we used the pseudopotential-LCAO method. We found that the uniaxial deformations, induce an ellipsoidal distortion of the Dirac cones and isotropy breaking of the Fermi velocity. Finally, we used the Dirac--like equation to find the electronic transmittance as a function of the incident angle. We obtain that the strain induces a strong changes in the transmittance when the deformation is perpendicular to the incident axis. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J25.00010: Strain and adhesion of graphene sheets in shallow trenches. Constanze Metzger, Sebastian Remi, Silvia Kusminskiy, Antonio Castro Neto, Anna Swan, Bennett Goldberg Detailed high resolution micro Raman mapping of graphene exfoliated over shallow trenches demonstrates that single layer graphene adheres to the bottom of shallow trenches instead of remaining freely suspended. The analysis shows that the strain is surprisingly uniform. The high resolution Raman mapping of G and 2D Raman shift are consistent with uniaxial strain measurements, and the strain map concurs with our theoretical calculations that predict that a graphene sheet reaches minimal free energy sticking to the trench bottom even if that leads to moderate strain of about 0.6{\%} in the sheet. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J25.00011: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J25.00012: Correlation of optical and topographical measurements with electronic transport properties of epitaxial graphene on Si-face SiC Paul M. Campbell, Joshua Robinson, James C. Culbertson, Joseph L. Tedesco, Glenn G. Jernigan, Rachel L. Myers-Ward, Charles R. Eddy, Jr., D. Kurt Gaskill Epitaxial graphene layers grown on the Si face of (0001) SiC substrates by thermal desorption of Si were studied by Raman spectroscopy. Characteristic D, G, and 2D peaks were observed, and the 2D peak was used to extract layer thickness and film strain. These results, along with measured Hall mobility and topography from AFM, were used to establish those factors that influence the transport properties of graphene devices. The combination of uniform strain and nearly uniform thickness usually results in high-mobility graphene with an average room temperature Hall mobility $>$1000 cm$^{2}$/Vs. In contrast, films with nonuniform strain and thickness typically show lower values of mobility. These results will be useful for optimizing growth conditions to produce uniformly high-mobility graphene on full 2-inch and 3-inch SiC wafers, an agenda that we are now pursuing. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J25.00013: Reconstruction of Vacancy Defects in Graphene and Carbon Nanotube Gun-Do Lee, Euijoon Yoon, Nong-Moon Hwang, Cai-Zhuang Wang, Kai-Ming Ho Recently, various structures of vacancy defects in graphene layers and carbon nanotubes have been reported by high resolution transmission electron microscope (HR-TEM) and those arouse an interest of reconstruction processes of vacancy defects. In this talk, we present reconstruction processes of vacancy defects in a graphene and a carbon nanotube by tight- binding molecular dynamics (TBMD) simulations and by first principles total energy calculations. We~found that a structure of a dislocation defect with two pentagon-heptagon (5-7) pairs in grapheme becomes more stable than other structures when the number of vacancy units is ten and over. The simulation study of scanning tunneling microscopy reveals that the pentagon-heptagon pair defects perturb the wavefunction of electrons near Fermi level to produce the $\surd 3 \times \surd$3 superlattice pattern, which is good agreement with experiment. It is also observed in our tight- binding molecular dynamics simulation that 5-7 pair defects play a very important role in vacancy reconstruction in a graphene layer and carbon nanotubes. [Preview Abstract] |
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