Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session J30: Optical Properties and Experimental Characterization of Graphene and Related Structures |
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Sponsoring Units: DMP Chair: Marco Fornari, Central Michigan University Room: Morial Convention Center 222 |
Tuesday, March 11, 2008 11:15AM - 11:27AM |
J30.00001: Photo-induced structural dynamics of graphitic carbon studied by ultrafast electron nanocrystallography. Ramani K. Raman, Yoshie Murooka, Ryan A. Murdick, Chong-Yu Ruan We report the studies of photo-induced structural dynamics of graphite and multi-wall carbon nanotubes (MWCNT) using ultrafast electron nanocrystallography. Graphite, upon excitation, contracts along its c-axis causing a reduction of the interlayer distance, which is the first step towards diamondization. MWCNT on the other hand, display an energy-dependent electron-phonon coupling mechanism. Upon excitation at 400nm, the promoted carriers can transfer their excess energy to the lattice rapidly within 5-10 ps whereas at 800nm it takes around 20-30ps for the same. This indicates a more efficient electron-phonon coupling at 400nm where the excited carriers are more strongly coupled to the lattice. Both graphite and MWCNT also exhibit a transient photovoltaic effect where an accumulation of excited charge carriers at the sample interface causes a collective shift of the Bragg peaks. We found that the charge dynamics and atom dynamics are intimately correlated at interfaces. [Preview Abstract] |
Tuesday, March 11, 2008 11:27AM - 11:39AM |
J30.00002: Non-destructive optical characterization of DNA-wrapping of single-walled carbon nanotubes S.E. Snyder, S.V. Rotkin Single-stranded DNA can form a stable hybrid structure with a single-walled carbon nanotube, allowing dispersion of individual nanotubes in aqueous solution and facilitating the development of methods to separate nanotubes by type. Optical and electronic properties of specific DNA-nanotube structures are the focus of our study due to potential optoelectronic device applications. Within a semi-empirical tight-binding approach, we have studied changes in optical absorption of a single-walled carbon nanotube resulting from a helical wrap of ionized single-stranded DNA. The one-electron absorption spectrum for light polarized across the tube is sensitive to bandstructure modulation due to the wrapping. For a non-chiral tube, the helical perturbation generates ``natural'' optical activity in the DNA-nanotube complex, yielding circular dichroism. Symmetry breaking due to the Coulomb potential of the wrap lifts optical selection rules and allows new optical transitions. These optical effects are predicted to serve as qualitative tools to directly identify DNA wrapping. [Preview Abstract] |
Tuesday, March 11, 2008 11:39AM - 11:51AM |
J30.00003: Incommensurately Stacked Graphene Bi-Layer: A Raman Scattering Study Awnish Gupta, Y. Tang, T. Russin, V. Crespi, P. Eklund We report results of Raman scattering studies of two novel graphene systems based on incommensurate stacking of sp$^{2}$ carbon: (1) an incommensurate bi-layer (IBL) formed by folding a graphene sheet onto itself; (2) a graphene scroll formed by rolling up a graphene sheet via a shearing motion between scotch tape and substrate during the micro-mechanical cleaving process . In (1), we have a flat bi-layer system; in (2) it is a gently curved multilayer system -- both should be incommensurate. Interestingly, although no significant D-band is observed in the parent graphene system, the incommensurate contact of the graphene sheet in (1) and (2) leads to strong D-band scattering near 1350 cm$^{-1}$ using 514.5 nm excitation. The dispersion of the D-bands in (1) and (2) is significantly different: scroll ($\sim $38 cm$^{-1}$/eV) and IBL ($\sim $50 cm$^{-1}$/eV). A second Raman band is observed nearby at $\sim $1384 cm$^{-1}$ in both the IBL and the scroll. However, the $\sim $1384 cm$^{-1}$ band is non-dispersive in both cases and is much sharper in the IBL than in the scroll. Our data will be compared to theoretical calculations based on double-resonant (DR) scattering and the electronic states of an IBL. [Preview Abstract] |
Tuesday, March 11, 2008 11:51AM - 12:03PM |
J30.00004: Anharmonic Effects in Raman Scattering Few-Layer Graphene System Timothy Russin, Awnish Gupta, Peter Eklund Result of Raman scattering studies from supported and suspended n-layer graphene films (nGLs) are presented for the temperature range of -190 to 500 $^{\circ}$C. The samples were either supported directly on a Si/SiO$_{2}$ substrate or suspended over lithographically produced trenches; the measurements were performed in a N$_{2}$ atmosphere. For both supported and suspended films, the magnitude of the negative temperature coefficient of the G-band frequency (cm$^{-1}$/K) is found to decrease with number of layers n in the nGL films (i.e., supported 1GL, 2GL and 5GL show -0.037, -0.031, and -0.028 cm$^{-1}$/K, respectively.) The anharmonic coefficients are significantly higher than for highly oriented pyrolytic graphite (HOPG). Surprisingly, the G-band linewidth increases with temperature for the supported films and decreases with temperature for the suspended films. Furthermore, we see evidence for a permanent morphological change at T $\sim $ 200 $^{\circ}$C for supported $n$GLs via new D-band and D'-band scattering. Unsupported films exhibit these changes at higher temperatures. The mechanism and details of the irreversible morphological change(s) is not yet known. [Preview Abstract] |
Tuesday, March 11, 2008 12:03PM - 12:15PM |
J30.00005: Tunneling spectroscopy of single- and double-layer graphene planar tunnel junctions Conor Puls, Neal Staley, Ying Liu It is of fundamental as well as technological concern if there exists an energy gap in single- and double-layer graphene devices. Single-layer graphene is thought to be gapless while double-layer graphene features an energy gap tunable by controlling the charge difference between the two layers. Previously, scanning tunneling microscopy and spectroscopy studies have been employed to examine energy spectra of graphene films. This approach produces local charge inhomogeneity at the probe tip that could significantly alter the local density of states (DOS) in graphene. Planar tunnel junctions provide a probe of the DOS that should not induce such an inhomogeneity in the charge carrier density. We fabricated planar tunnel junctions on single- and double-layer graphene using an ultrathin quartz filament as a shadow mask over mechanically exfoliated graphene as an alternative to lithographic procedures so as to avoid possible contamination in a wet lithography process. We have measured tunneling spectra for both weakly and strongly disordered samples. For single-layer graphene, we observed an unexpected gap. For double-layer graphene, we found a gap and other features in the tunnel spectra by changing the back gate and tunnel junction bias voltages independently - thereby tuning the charge difference between the top and bottom layer - as well as varying magnetic field and temperature. [Preview Abstract] |
Tuesday, March 11, 2008 12:15PM - 12:27PM |
J30.00006: The G* (2450 cm$^{-1})$ Double Resonance Raman Peak in single-, few-layer graphene and DWNTs Alfonso Reina, Hyungbin Son, Federico Villalpando-Paez, Hootan Farhat, Jing Kong, Mildred Dresselhaus The dispersion and skewness of the 2450 cm$^{-1}$ peak in the raman spectra of carbon structures was analyzed. The dispersion of this peak for graphene is smaller in magnitude and of opposite sign than that for the G' ($\sim $2700cm$^{-1})$. This dispersion is independent on number of layers. The peak shows asymmetry (skewness) which increases with E$_{laser}$. The observations can be explained by viewing this double resonance process arising by the scattering with both an iTO and a iLA phonon. The peak becomes more symmetric in DWNTs and it shows a stronger curvature dependence than the G' peak. [Preview Abstract] |
Tuesday, March 11, 2008 12:27PM - 12:39PM |
J30.00007: Probing Non-equilibrium Phonon Dynamics in Graphite by Time-Resolved Raman Spectroscopy Hugen Yan, Daohua Song, Kin Fai Mak, Ioannis Chatzakis, Janina Maultzsch, Tony Heinz Time-resolved Raman spectroscopy has been applied to obtain direct information about phonon lifetimes in graphite. A non-equilibrium population of zone-center optical phonons was produced by the rapid relaxation of charge carriers following photoexcitation of the sample with a femtosecond laser pulse. The subsequent evolution of the phonon population was recorded using the strength of G-mode anti-Stokes Raman scattering from a time-delayed femtosecond probe pulse. A population lifetime for the G-mode phonons of approximately 2 ps was found. Analogous measurements of optical-phonon lifetimes were also conducted in few-layer graphene samples produced by mechanical exfoliation of bulk graphite. Results obtained for graphite and few-layer graphene will be compared with one another, as well as with earlier data on the lifetime of G-mode phonons in single-walled carbon nanotubes [1]. \newline [1] D. Song, F. Wang, G. Dukovic, M. Zheng, E. D. Semke, L. E. Brus, and T. F. Heinz, submitted. [Preview Abstract] |
Tuesday, March 11, 2008 12:39PM - 12:51PM |
J30.00008: Transfer of Graphene to Alternative Substrates Tracy Moore, J.H. Chen, D.R. Hines, E.D. Williams, J. Simspon, A.R.H. Walker Graphene transport properties are limited by charge defects in SiO$_{2 }$, and by large charge density due strong interaction with SiC. We have investigated the transfer of graphene from one substrate to another using high pressures and temperatures to achieve control of the substrate interactions and thus their effects on graphene. The direct transfer from HOPG to alternative substrates PET and PMMA yields mostly multilayer, opaque graphite flakes. Raman signatures of the thinner, translucent flakes on PMMA can be clearly distinguished from the PMMA spectra and show a downshift in the G' peak that occurs around 2700 cm$^{-1}$ and a relative intensity of G to G' peak of approximately one; characteristics of graphene spectra. In addition the transfer from SiO$_{2}$ to alternative substrates occurs readily for PET substrates, and infrequently for PMMA substrates with thicker flakes transferring more readily than thin flakes. Graphene transfer from 1) direct HOPG, 2) flakes on SiO$_{2}$, and 3) the possibility of direct transfer from epitaxial graphene on SiC will be presented, along with the resulting device characteristics. [Preview Abstract] |
Tuesday, March 11, 2008 12:51PM - 1:03PM |
J30.00009: Gap and Impurity induced states on graphene layers Chanyong Hwang, Dohyun Lee, Wondong Kim, Junghwa Yang, Jisang Hong One of the interesting phenomena in graphene is the linear Fermi level crossing at the Dirac point. For the measurement of electronic structure, few layers of graphenes are formed on top of SiC substrate by thermal treatment. As the thickness of graphene layers increases, the formation of the gap near Dirac point is somewhat controversial. Recently this gap has been demonstrated to be tunable by the electric field. We have used angle-resolved photoemission spectroscopy and STM to characterize this gap state and actual morphology of the graphene layers to clarify this controversial issue. In addition, we have shown that the adatom carbon can play an important role in gap state. First principles calculation on this carbon adatom state will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 1:03PM - 1:15PM |
J30.00010: Single Layer Graphene formation on Silicon Oxide surface(001) HeeSung Choi, Young-Kyun Kwon Recently graphene is one of most interesting topics in physics and other research fields. For future nanoelectronics applications, graphene formation becomes an important issue. Here we present our theoretical study of how to make a graphene layer on silicon oxide surfaces. In this work, density functional theory calculations are used to determine atomic structures and energies for graphene formation from various carbon sources, such anthracene, on silicon oxide. We will also preresent optimal graphene formation conditions obtained from our ab inito molecular dynamics simulations. [Preview Abstract] |
Tuesday, March 11, 2008 1:15PM - 1:27PM |
J30.00011: Soldering to a single atomic layer Caglar Girit, Alex Zettl The standard technique to make electrical contact to nanostructures is electron beam lithography. This method has several drawbacks including complexity, cost, and sample contamination. We present a simple technique to cleanly solder submicron sized, Ohmic contacts to nanostructures. To demonstrate, we contact graphene, a single atomic layer of carbon, and investigate low- and high-bias electronic transport. We set lower bounds on the current carrying capacity of graphene. A simple model allows us to obtain device characteristics such as mobility, minimum conductance, and contact resistance. [Preview Abstract] |
Tuesday, March 11, 2008 1:27PM - 1:39PM |
J30.00012: Synthesis of Carbon Nanotubes by Rolling Up Patterned Graphene Nanoribbons Using Selective Atomic Adsorption Decai Yu, Feng Liu We demonstrate a new method for synthesizing Carbon Nanotubes (CNTs), using first principles and classical molecular dynamics simulations. The single-walled nanotubes (SWNTs) are formed by rolling up graphene nanoribbons patterned on graphite films, through adsorption of atoms of varying coverage, which introduces an external stress to drive the folding process. The diameter and chirality of SWNTs can be \textit{a priori} controlled by patterning graphene nanoribbons with predefined width and direction, so that the post-synthesis sorting process is eliminated. Our method allows potentially mass production of identical tubes and easy integration into device structures on a substrate. [Preview Abstract] |
Tuesday, March 11, 2008 1:39PM - 1:51PM |
J30.00013: $^{13}$C NMR studies on the organic zero-gap system, $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ under pressure Kazuya Miyagawa, Motoaki Hirayama, Masafumi Tamura, Kazushi Kanoda We present NMR data for the organic material, $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ under pressure, which is a candidate for zero-gap conductor with cone-like dispersion. The quasi-two-dimensional organic conductor $\alpha$-(BEDT-TTF)$_{2}$I$_{3}$ is known to show peculiar behaviors under high pressure. The resistivity is insensitive to temperature, while the Hall coefficient is strongly dependent on temperature. The band calculation suggests that this system is in the zero gap state with a linear dispersion around the Fermi energy. While the $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ is metallic under ambient pressure, above 5 kbar temperature dependences of resistivity and Hall coefficient are similar to those of $\alpha$-(BEDT-TTF)$_{2}$I$_{3}$. It is remarkable that the graphine like zero-gap state is likely realized in a bulk system. We have performed preliminary investigation into the magnetism of $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ under 8 kbar by $^{13}$C NMR. The external filed is applied to parallel to the conducting layer. In constant to the simple metallic behavior observed under ambient pressure (Korriga's relation holds), the Knight shift vanishes in proportion with temperature and 1/$T_{1}$ shows a steep decrease, which is roughly $T^{3}\sim T^{4}$ down to 4 K . These behaviors are consistent with the zero (or narrow) gap state picture. [Preview Abstract] |
Tuesday, March 11, 2008 1:51PM - 2:03PM |
J30.00014: UHV Growth of Graphene on SiC Paul Campbell, Glenn Jernigan, Keith Perkins, Brenda VanMil, Rachel Myers-Ward, Kurt Gaskill, James Culbertson, Jeremy Robinson, Eric Snow We report graphene growth on Si- and C-face semi-insulating 6H SiC in UHV by thermal Si desorption /reconstruction of the remaining C. The SiC was etched in H$_{2}$ up to 1580 \r{ }C to smooth the surface. XPS shows the H$_{2}$-etched surfaces are covered by an oxide which desorbs at 1000 \r{ }C, resulting in a surface containing excess Si. At 1300 \r{ }C, the surface becomes stoichiometric in Si and C and a $\surd $3 x $\surd $3 R30 LEED pattern is observed. At 1350 \r{ }C, we observe a 6$\surd $3 x 6$\surd $3 R30 LEED pattern develop when graphene has formed, and a 1x1 LEED pattern for graphite films formed at temperatures greater than 1400 \r{ }C. Graphene layers were grown under a variety of temperatures and conditions and characterized using XPS, LEED, AFM, Raman spectroscopy, and Hall effect. Top-gated FETs were fabricated with a wide range of gate lengths (1-25 microns) and gate widths (2-130 microns), and transistor operation was obtained for both single and multiple graphene layers. [Preview Abstract] |
Tuesday, March 11, 2008 2:03PM - 2:15PM |
J30.00015: UHV electron-probing of micro-mechanically cleaved Graphene on SiO$_{2}$ K.R. Knox, S. Wang, P. Kim, R.M. Osgood, T.O. Mentes, M.A.N Orti, A. Locatelli, D. Cvetko, A. Morgante While graphene's distinctive Dirac-cone electronic structure and simple 2D atomic structure have attracted major interest in the physics community, the inherent limitations of isolated graphene samples mounted on an insulating substrates have made it difficult to study such systems with typical UHV probes such as photoemission and low energy electron diffraction (LEED).~ While most single layer graphene transport measurements are done on micro-mechanically extracted samples on SiO$_{2}$, all photoemission and LEED measurements of graphene performed so far have used films grown on SiC substrates.~ In this talk, we will discuss the first results of UHV probes carried out exfoliated graphene bonded to SiO$_{2}$. Using the high spatial resolution of the nanospectroscopy beamline at the ELETTRA synchrotron light source, we have been able to overcome the size limitations, which have prevented previous UHV study of this system. We will discuss the results of our X-ray photoemission (XPS), UV photoemission (UPS) and LEED measurements on single and multilayer graphene samples. [Preview Abstract] |
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