Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J28: Focus Session: Carbon Nanotube Optics III |
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Sponsoring Units: DMP Chair: Anna Swan, Boston University Room: Colorado Convention Center 302 |
Tuesday, March 6, 2007 11:15AM - 11:51AM |
J28.00001: Resonance Raman of Single-Wall Carbon Nanotubes Invited Speaker: The use of resonance Raman spectroscopy to study and characterize single-wall carbon nanotubes (SWNTs) will be discussed. The achievements and limitations of the technique for metrology purposes will be presented, addressing the importance of the excitonic nature of the optical transitions. We use the technique do understand the effect of carbon nanotube doping. The efforts to extend the Kataura plot to larger tube diameters and higher optical transitions not only extend our characterization capability, but also sheds light into the nature of the optically active levels. Experimental results that have not been predicted by solid state approaches are understood on the basis of quantum chemical calculations. It is also interesting to discuss some results on nano-ribbons and their relations to carbon nanotubes. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J28.00002: Raman Studies of Exciton-Phonon Coupling in Carbon Nanotubes: Quantitation of Bundled vs. Isolated Behavior Stephen Doorn, Andrew Shreve, Sergei Tretiak, Zhengtang Luo, Fotios Papadimitrakopoulos Exciton-phonon and electron-phonon coupling are important for a number of carbon nanotube optical and transport behaviors and have recently drawn attention for their role in chirality- dependent intensities observed in radial breathing mode (RBM) Raman spectra. Given the importance of these effects, there is a need to quantitate the magnitude of the exciton-phonon coupling. We present a Raman transform analysis of RBM fundamental and overtone intensities that yield the magnitude of coupling for five specific nanotube chiralities. These results agree with values predicted through quantum chemical calculations and indicate that non-Condon effects may be important in describing nanotube transitions. We extend the analysis of the coupling to bundled nanotube samples and find it decreases significantly in these sample types. We also discuss the coupling behavior of a new class of intermediate frequency modes (IFMs) that display step-wise dispersive behavior. These IFMs are associated with coupling between the E11 and E22 transitions. Bundling is found to increase the coupling observed for these modes. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J28.00003: Exciton-phonon interaction and Raman intensity of carbon nanotubes Riichiro Saito, Jie Jiang, Ado Jorio, Kentaro Sato, Gene Dresselhaus, Millie Dresselhaus Using extended tight binding framework, the exciton states and exciton-phonon interaction are calculated for understanding optical properties of single wall carbon nanotubes. Resonance Raman intensity for first and second order Raman processes are calculated as a function of $(n,m)$ with use of exciton wavefunctions. Chirality, type and diameter dependence of Raman intensity is now fully given. In particular, the dark exciton plays an important role for second-order, intervalley, resonance Raman processes. Although the exciton-phonon interaction is not so different from the electron-phonon interaction, the optical absorption (emission) is enhanced significantly by the localized exciton wavefunctions.\\ \ \\ References: J. Jiang et al, Phys. Rev. B, in press. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J28.00004: Tunable Electron-Phonon Coupling in Isolated Metallic Carbon Nanotubes Observed by Raman Scattering Yang Wu, Janina Maultzsch, Ernst Knoesel, Bhupesh Chandra, Mingyuan Huang, Matt Sfeir, Louis Brus, James Hone, Tony Heinz Metallic single-walled carbon nanotubes can exhibit significant broadening of the high-energy (G) mode Raman features. In contrast to narrow Raman widths for semiconducting nanotubes, full widths in excess of 50/cm are commonly observed in metallic nanotubes. Different possible physical origins have been proposed in previous literatures. In this paper, we demonstrate the ability to modify the Raman linewidth by electrostatic gating. Using measurements of individual suspended nanotubes, we find that either a positive or negative shift in the Fermi energy by an applied electrostatic field can reduce the linewidth by more than a factor of two. The results can be understood in terms of blocking vertical electronic transitions (electron-hole pair generation) possible for the zone-center phonons in an unperturbed nanotube, but not in a nanotube with a sufficiently shifted Fermi level. A simple model is presented to explain the experimental results. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J28.00005: Ultrafast Spectroscopy of Phonons in Single-Walled Carbon Nanotubes Erik Haroz, David Hilton, Junichiro Kono, Robert Hauge, Ki-Ju Yee, Yong-Sik Lim, Stephen Doorn Recently, we observed coherent phonons (CPs) of the radial breathing mode (RBMs) in semiconducting single-walled carbon nanotubes (SWNTs) suspended as individuals in aqueous surfactant (1). We demonstrated CP spectroscopy as a powerful method for determining phonon and exciton energies in an ensemble of SWNTs with different chiralities. Here, we extend these ultrafast optical studies on various types of nanotube samples including films and solutions. In order to provide new insight into CP decay mechanisms, we systematically investigated the temperature dependence of CP amplitude, frequency, and lifetime from 4 -300 K while changing the pump/probe photon energy. We also investigated how bundling affects CP line widths. Furthermore, we compared the intensity dependence of CPs resonant with the $E_{11}$ and $E_{22}$ transitions by studying the excitation profile for specific RBMs, focusing particularly on the excitation line width and shape. 1) Y. S. Lim \textit{et al}., Nano Letters, published electronically November 2, 2006. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J28.00006: Theory of coherent phonons in carbon nanotubes Gary Sanders, Chris Stanton We develop a general theory for the generation of coherent phonons in single wall carbon nanotubes or arbitrary chirality. Coherent phonons are generated in the nanotube via the deformation potential electron-phonon interaction with photogenerated carriers. In our theory the electronic states are treated in a third nearest neighbor tight binding formalism which gives a good description of the states over the entire nanotube Brillouin zone while the nanotube phonon states are treated in a valence force field model that includes bond-stretching, in-plane and out-of-plane bond-bending, and bond-twisting interactions. In the tight-binding electron-phonon interaction, all two center integrals out to fourth nearest neighbors are retained. The equations of motion for the coherent phonon amplitudes are obtained in a density matrix formalism and we find that the coherent phonon amplitudes satisfy driven oscillator equations for each value of the phonon wavevector. We will discuss excitation strengths for different coherent phonon modes and compare to recent experiments. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J28.00007: Intrinsic BWF-lineshape Observed by Raman Scattering in Isolated Metallic Carbon Nanotubes Janina Maultzsch, Yang Wu, Ernst Knoesel, Bhupesh Chandra, Mingyuan Huang, Matt Sfeir, Louis Brus, James Hone, Tony Heinz Broadened and asymmetric lineshapes for Raman scattering in the high-energy (or G) modes of metallic carbon nanotubes have been reported for many years. There remains, however, controversy about whether this behavior is an intrinsic feature of metallic nanotubes or is induced by perturbations. To address this issue, we have examined isolated metallic nanotubes suspended in air, with chiral indices determined independently by Rayleigh scattering and Raman measurements of the radial breathing mode. Our data show that strong broadening (to FWHM $>$ 50/cm) and weak asymmetry are typical of the high-energy Raman modes, with lineshapes describable by a Breit-Wigner-Fano (BWF) form. Significant variation in peak width and Raman shift is, however, observed as a function of the nanotube chiral index. Indeed, some metallic nanotubes have lineshapes and widths that are very similar to those of semiconducting nanotubes. We will discuss the observed variation and the origin of the BWF lineshape. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J28.00008: Bundling and Electronic Effects on the BWF Feature for Doped and Undoped Carbon Single-wall Nanotubes Jeff Blackburn, Timothy McDonald, Chaiwat Engtrakul, Anne Dillon, Michael Heben In this contribution we examine the role of bundling and electronic effects on the Breit-Wigner-Fano (BWF) Raman component for dispersions of undoped and boron-doped (p-type) SWNTs in various surfactants. Interestingly, we find that the intensity of the BWF component is sensitive to the degree of SWNT debundling, solution pH, doping level, charge transfer with redox active molecules, and differences in the SWNT-surfactant interactions, all of which lead to varying degrees of charge localization at the nanotube surface. In several cases, we observe a strong BWF component in the metallic Raman spectrum even for dispersions of highly isolated SWNTs. In general, our results, coupled with results from the literature, suggest that the presence and intensity of the BWF feature is sensitive to any changes in the magnitude of dielectric screening, whether from tube-tube interactions in bundles, from charge injection or depletion, or from charge polarization from tube-molecule interactions. These results suggest that, contrary to practice in some recent studies, the existence or lack of a BWF feature should not be used alone as a measure of SWNT aggregation. They also provide information regarding the nature of surfactant-nanotube interactions, SWNT redox chemistry, and nanotube separations. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J28.00009: First-principles study of resonant Raman spectroscopy in graphite and carbon nanotubes David Prendergast, Jack Deslippe, Steven Louie Resonant Raman spectroscopy is an increasingly used experimental tool for the characterization of carbon nanotubes (CNTs). It explores the coupling of optical, electronic, and vibrational modes in these quasi-one-dimensional systems. Using first-principles methods we calculate the electron-photon and -phonon matrix elements necessary to estimate the first-order Raman cross-section. For graphite, the non-interacting quasiparticle spectrum is sufficient, however, for CNTs, the excitonic spectrum and wave functions require an accurate description of electron-hole correlation. We calculate excitonic effects by solving the Bethe-Salpeter equation, using as input the quasiparticle spectrum obtained within the GW approximation to the electron self-energy. We analyze the exciton-phonon coupling in CNTs and its impact on the resonant Raman cross-section. [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J28.00010: Signature of the electron-phonon interaction in the electron spectral function of graphene Cheol Hwan Park, Feliciano Giustino, Marvin L. Cohen, Steven G. Louie The spectral function of graphene has been measured with high energy and momentum resolution by angle-resolved photoelectron spectroscopy. It has been proposed that the measured spectral function exhibits combined signatures from electron-phonon, electron-electron, and electron-plasmon interactions. We here present a first-principle investigation of the contribution to the electron self-energy of graphene arising from the electron-phonon interaction. We compute the electron self-energy treating the graphene bandstructure within density functional theory, the lattice dynamics within density functional perturbation theory, and the electron-phonon interaction within the Migdal approximation. Due to its peculiar cone-shaped bandstructure, the electron-phonon contribution to the electron self-energy of graphene shows qualitative differences as compared to the case of ordinary bulk metals. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J28.00011: 1st and 2nd order Raman scattering from n-Graphene Layer (nGL) Films on Silicon Substrates. Awnish Gupta, Gugang Chen, Peter Eklund Results of room temperature Raman scattering experiments on graphene and n-graphene layer films (nGLs) will be presented [1]. We find that the G band at $\sim $ 1582 cm-1 exhibits an interesting upshift in frequency with 1/n which we tentatively assign to a surface strain phenomenon connected with surface roughness of the substrate and compensated by the increase in stiffness of the nGL with increasing n. Interesting n-specific bands are observed in the $\sim $1350 cm-1 (or D-band) region which may correlate with deviations from planarity of the nGL. The second order scattering is very interesting and for small n (n$<$4) the (2D' or G') band intensity at $\sim $ 2700 cm-1 is actually higher than the first-order G-band scattering. The shape of this band is sensitive to n and thus can be used to identify n without an AFM measurement. Whereas, the 2D' band is sensitive to n, the 2nd order 2G band $\sim $ 3248 cm-1 is independent of n. These observations will be discussed in terms of the phonon and electronic dispersion of nGLs. 1 A. Gupta, G. Chen, P. Joshi, S. Tadigadapa and P.C. Eklund, `` Raman Scattering from High-Frequency Phonons in Supported n-Graphene Layer Films'' NanoLett (in Press). [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J28.00012: Raman Scattering Study of the Graphene-Substrate Interaction. Peter Eklund, Awnish Gupta, Gugang Chen We report on Raman scattering studies of graphene and few graphene layer films (i.e., $n$GLs, where $n $is the number of graphene layers in the film). $n$GL films (n=1-3, 25) were prepared by mechanical transfer from HOPG to various substrates (SiO2:Si, Au, Ag, cleaved Mica, and free-standing films). For metallic substrates we observed a clear G-band frequency downshift relative to that observed when the $n$GL is on SiO2:Si. This downshift is interpreted in terms of a chemical charge transfer of electrons from the metallic substrate to the nGL. Interestingly, the position and shape of the 2D' (or G') band at $\sim $ 2700 cm$^{-1}$ is found insensitive to the substrate interaction. [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J28.00013: Raman Spectroscopy of single and double layer graphene Yan Yin, Sebastian Remi, Svetlana Anissimova, Anna Swan, Bennett Goldberg Dirac fermions and novel quantum Hall effects. Graphene also holds the promise of one day replacing silicon in microchips. Early Raman scattering has identified basic features of the G-band and D-band, where the former shows intensity dependence associated with addition of single layers, and the later displays significant intensity only for the single and double layer systems. We have performed room temperature Raman scattering with a spatial resolution of 0.5 microns consistent with this work. More recently, Pinczuk and Kim and co-workers have shown low-temperature Raman scattering that displays evidence of electron density dependent screening. We investigate the interlayer hopping with temperature-dependent Raman scattering and in our low-temperature Raman, we specifically investigate the novel coupling and edge states predicted by Castro Neto and co-workers. [Preview Abstract] |
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