Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W20: Graphene Spectroscopy |
Hide Abstracts |
Sponsoring Units: DMP DCMP Chair: Tony Heinz, Columbia University Room: C120-122 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W20.00001: Effect of doping on the Raman lineshape and intensity of graphene Cinzia Casiraghi, Denis M. Basko, Andrea C. Ferrari Graphene can be doped by applying a gate voltage [1-2]. Doping strongly affects the G and 2D Raman peaks: i) the G peak upshifts for increasing doping, while its width decreases [1]; ii) the 2D upshifts for p-doping, while it downshifts for n-doping [2]. iii) the ratio between the 2D and G peaks intensity decreases for increasing doping [2]. The 2D intensity is strongly affected by the electron-electron scattering rate, which increases with doping [3]. Similar Raman peaks variations were observed for non-gated samples, as an effect of charged impurities [4]. Here we use the 2D peak intensity variation with doping to extract the electron-phonon scattering rate [3,5]. We note that in non-gated samples, where the Fermi level shift is induced by charged impurities, we can probe the 2D peak dependence much closer to the Dirac point than in gated ones [5]. We find an electron-phonon coupling scattering rate of 60 ps$^{-1}$ at 2.41 eV excitation energy [3,5]. \\[4pt] [1] S. Pisana et al, Nature Mat. 6, 198 (2007)\\[0pt] [2] A. Das et al., Nature Nanotech. 3, 210 (2008)\\[0pt] [3] D. M. Basko et al. PRB 80, 165413 (2009)\\[0pt] [4] C. Casiraghi et al., APL 91, 233108 (2007)\\[0pt] [5] C. Casiraghi, arXiv:0908.4480 [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W20.00002: Identifying the orientation of edge of graphene using G band Raman spectra K. Sasaki, S. Murakami, R. Saito, M.S. Dresselhaus, K. Takai, T. Mori, T. Enoki, K. Wakabayashi The electron-phonon (el-ph) matrix element may depend on position in graphene since an electronic wave function is position dependent. In particular, the el-ph matrix element near the edges of graphene can differ from those in the bulk. We are pursuing our studies on this point in relation to Raman spectroscopy. Our theoretical studies on the Raman G band near the edges of graphene show that the Raman intensity is enhanced when the polarization of Raman laser is parallel (perpendicular) to the armchair (zigzag) edge. This asymmetry between the armchair and zigzag edges is useful in identifying the orientation of the edge of graphene. Kohn anomalies near the edges of graphene are also reported on. The analysis of the pseudospin reveals that the LO phonon mode undergoes a strong Kohn anomaly both for the armchair and zigzag edges, and that only the LO (TO) mode is Raman active mode near the armchair (zigzag) edge. (ref.) K. Sasaki et al., PRB 80, 155450 (2009); K. Sasaki et al., arXiv:0911.1593. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W20.00003: Study of defectivity in suspended graphene by analysis of G-phonon Raman line-widths Gayathri Rao, Ji Ung Lee, Robert Geer Exfoliated graphene supported on substrate have degraded performance due to scattering from charge impurities and impact device performance. However to understand the impact of defectivity on graphene it is essential to eliminate any substrate induced effects. This makes the study of defectivity on suspended graphene using Raman spectroscopy very attractive. The defect-induced Raman D peak gives great insight into study of defects. There is also increase in G line-widths due to increased EPC(electron phonon coupling) on defect introduction. In this work we have carried out electron beam irradiation of suspended graphene at controlled doses. Post Raman analysis on the irradiated samples indicated the evolution of D peaks. This increase in the D peak in suspended graphene is almost linear when compared to the monotonic increase in ID/IG ratio of supported graphene. The line widths of the G peak show significant variation. The results are contradictory to those on the substrate indicating large influence of substrate. Results of line-width variation of G-peak of suspended graphene along with comparison to substrate supported graphene are presented. [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W20.00004: Power dependence of Raman spectrum of graphene Hyerim Moon, Duhee Yoon, Seoungwoo Woo, Young-Woo Son, Hyeonsik Cheong Raman spectroscopy has been a useful tool to indentify the number of graphene layers. The thermal conductivity of graphene, the influence of the substrate, and the doping effect have been investigated by using Raman spectroscopy. Although Raman spectroscopy is widely used as a standard characterization tool, the excitation laser power varies for each experiment. In this work, we investigated the variation of the Raman spectrum of single-layer graphene as a function of the excitation laser power. Graphene samples were isolated from natural graphite flakes using mechanical cleavage. The 514.5 nm (2.41 eV) line of an Ar ion laser was used as the excitation source. The power was varied from 0.2 mW to 20 mW. Micro-Raman spectroscopy was performed with a 0.6 N.A. objective lens. The frequency and the full-width-at-half-maximum (FWHM) of Raman G and 2D bands were found to vary significantly as a function of the laser power. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W20.00005: Temperature Dependent Ultraviolet Raman Spectroscopy of Monolayer Graphene Films Irene Calizo, Brent Sperling, Joshua Giltinan, James Maslar, Angela R. Hight Walker Strong interest in Raman spectroscopy lies in that it delivers a wealth of information about the structure of graphene. Raman spectroscopy allows for the identification of graphene layers and disorder. We have produced monolayer graphene films with lateral dimensions on the order of centimeters on copper foils by chemical vapor deposition (CVD). Temperature dependent Raman spectra were obtained at multiple wavelengths including ultraviolet and visible and at temperatures from 10 K to 300 K. Here we present the effect of excitation wavelength and temperature on the Raman spectrum of CVD grown graphene and compare it to theoretical predictions. The changes in Raman linewidth and positions provide powerful information about anharmonicity and electron-phonon interactions. The obtained results are important for Raman nanometrology of graphene. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W20.00006: Raman study of turbostratic graphene grown via chemical vapor deposition on metals Daniel R. Lenski, Michael S. Fuhrer We have grown graphene of varying thickness on metal foils (copper and nickel) and thin films (nickel) via chemical vapor deposition (CVD). We use micro-Raman spectroscopy to characterize these graphene films, both as-grown on metal and transferred to oxide substrates. Raman spectra on multi-layer CVD graphene films show the characteristic single Lorentzian 2D peak of monolayer graphene, providing strong evidence of interlayer rotational disorder in multi-layer graphene prepared by CVD on metals. We discuss the evolution of the Raman features in CVD graphene with film thickness. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W20.00007: Dispersions of graphene sheets characterized by Raman spectroscopy Angela Hight Walker, Irene Calizo, Guangjun Cheng, Xiaomin Tu, Jeff Simpson, Ming Zheng Using expertise garnered on carbon nanotube separation, work is underway to demonstrate quality graphene dispersions and low defect sheets in the liquid phase. Several combinations of surfactants, solutions, starting graphite material, and separation protocols are being explored to determine the optimal chemical and physical environments. Raman spectroscopy is used as the quality indicator both for the numbers of layers and the defect density. Additional characterization with AFM and UV-Vis confirms our findings. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W20.00008: ABSTRACT WITHDRAWN |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W20.00009: First Direct Observation of a Nearly Ideal Graphene Band Structure Mike Sprinkle, Y. Hu, J. Hicks, A. Tejeda, A. Taleb-Ibrahimi, P. Le F\`{e}vre, F. Bertran, C. Berger, W.A. de Heer, E.H. Conrad Angle-resolved photoemission and x-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC$(000\bar{1})$ surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films causes adjacent graphene layers to electronically decouple, leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K-point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low-density faults. Recent results are discussed. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W20.00010: ARPES Study of Quasifreestanding Graphene David Siegel, Choonkyu Hwang, Xiaozhu Yu, Alexei Fedorov, Walt de Heer, Claire Berger, Alessandra Lanzara Graphene films grown on the carbon face of SiC have been shown to be decoupld from adjacent layers by rotational faults, making this an ideal system to study the electronic properties of freestanding graphene. By using high resolution angle-resolved photoemission spectroscopy, we provide the first full study of the electronic structure, stacking, and many-body interactions of these freestanding samples. We also discuss the evolution of the electronic structure and many body interactions as a function of doping. These results provide critical insights into the intrinsic properties of freestanding graphene sheets. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W20.00011: New approach to electronic structure of graphene studied by ARPES W. S. Jung, Chul Kim, C. S. Leem, Seung Ryong Park, Y. K. Kim, Y. Y. Koh, Changyoung Kim, E. Rotenberg ARPES has traditionally considered as a tool of electronic structural studies in the momentum space. One may speculate that the spectroscopic data obtained in the real and momentum spaces can be converted from one to the other as wave functions in real and momentum spaces are converted from one to the other through the fourier transform. In scanning tunneling spectroscopy (STS), the spectroscopic data obtain in the real space is fourier transformed to produced electronic structure in real space. However there is no exact analogy between the STS and ARPES cases. We try to transform the graphene ARPES spectra to real space by using atomic orbital of tight binding calculation. From this analysis, we can extract the information of phase which is lost in ARPES measurements. Additionally, we can plot the distribution of electrons in real space. We will discuss about the meaning of phase which is related to pseudospin in graphene. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W20.00012: Measuring the quantum properties of graphene using angle-resolved photoemission spectroscopy Choongyu Hwang, Hui Zhai, David A. Siegel, Shuyun Zhou, Alexei V. Fedorov, Dung-Hai Lee, Alessandra Lanzara Graphene, single carbon layer, exhibits novel properties based on its spinor eigenstates stemming from two sublattices carbon atoms. By using angle-resolved photoemission spectroscopy, we study the origin of the intensity distribution in the constant energy maps as a function of graphene thickness and polarization, and discuss the results in terms of quantum interference effect. Our findings provide one example of quantum properties of graphene and deeper understanding of the photoemission process. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W20.00013: Quasiparticle Interference and Anisotropy in Photoemission Spectroscopy on Graphene Michelle Yong, David Siegel, Shuyun Zhou, Chris Jozwiak, Alessandra Lanzara A novel way of analyzing angle-resolved photoemission data from epitaxial graphene is presented resulting in new insights into relationships between quasiparticle interference, allowed scattering states, and anisotropy in the photoemission data. Dependences on polarization, film thickness and binding energy are also considered. We also present theoretical simulations to compare freestanding graphene with epitaxial graphene, in order to better understand the role of the substrate interaction in quasiparticle interference. In addition, a comparison with the known results of scanning tunneling spectroscopy is made to confirm and contrast features detectable by both real and momentum space probes. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W20.00014: Anomalous temperature-dependent shift in Fermi energy of epitaxial graphene on silicon carbide studied by photoluminescence spectroscopy and angle resolved photoemission spectroscopy Sebastien Lounis, David Siegel, Robert Broesler, Eugene Haller, Alessandra Lanzara Photoluminescence spectroscopy (PL) and angle resolved photoemission spectroscopy (ARPES) have been used to study the interaction between epitaxially grown graphene and the silicon carbide substrate. We report evidence of an anomalous temperature dependent shift of the Fermi energy with a maximum at 65K. At this temperature, a similarly anomalous onset of the photoluminescence spectra is observed. These results are explained by the formation of a Schottky barrier at the graphene/silicon carbide interface, which is also responsible for the large electron doping of epitaxially grown graphene films. Finally, we discuss how the interaction between incident photons and the Schottky barrier could potentially be harnessed for future optical applications based on our results. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W20.00015: Measurement of the electronic density of states versus oxygen coverage in oxidized graphene Tanesh Bansal, Aditya Mohite, Charudatta Galande, Anchal Srivastava, Hemant Shah, Pulickel Ajayan, Bruce Alphenaar The electronic distribution of states for graphene oxide (GO) is thought to vary with oxygen concentration, eventually resulting in the formation of a band-gap. Theory suggests that there are a number of stable oxidative states for GO that produce characteristic electronic state distributions. Here, we describe a direct experimental probe of the GO density of states as a function of oxygen coverage using capacitive photocurrent spectroscopy. The Hummer's method was used to oxidize the graphene samples and standard reduction techniques were followed to vary the oxidation coverage on the samples. Three stable peaks in the density of states were observed across a large range of oxygen concentrations. Evidence for these features was also observed in CVD grown graphene, presumably due to unintentional oxidation. The energy and intensity of the three peaks along with the overall intensity of the photocurrent varies with the oxygen coverage. This information can be used to identify the stable oxidative states, and determine the oxygen coverage of the GO. [Preview Abstract] |
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