Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B24: Focus Session: Nanotube Characterization |
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Sponsoring Units: DMP Chair: Cary Yang, Santa Clara University Room: 326 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B24.00001: Raman spectroscopy of individual freestanding single-walled carbon nanotubes of defined chiral structure Invited Speaker: We review the main information that we have obtained from Raman spectroscopy experiments combined with electron diffraction experiments on individual freestanding single-walled carbon nanotubes. This information concerns: the radial breathing mode vs diameter relationship; the dependence of the frequency and lineshape of the G-modes in semiconducting and metallic tubes; the evaluation of the optical transition energies for individual freestanding SWNTs. These experimental Raman results obtained on well-identified individual SWNTs are compared with other experimental data and theoretical predictions. From these data, we can define Raman criteria that allow identifying carbon nanotubes from their Raman features only. We show the efficiency of this approach: (i) to assign the (n,m) indices of individual freestanding single-walled carbon nanotubes, and (ii) to identify the (n,m) tubes organized in a small bundle. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B24.00002: Raman Studies of Exciton and Exciton-Phonon Coupling Behavior in Metallic Single-Walled Carbon Nanotubes Invited Speaker: A scaling law analysis of carbon nanotube transition energies has been found to be valuable in revealing new electronic behaviors for the third and fourth transitions in semiconducting nanotubes. In the work presented here, we discuss resonance Raman data obtained for the E$_{11}^{M}$ and E$_{22}^{M}$ transitions of a broad diameter range (0.7 - 4 nm) of metallic carbon nanotubes. We show that application of the scaling law analysis to transition energies for metallic nanotubes suggests that the transitions are excitonic in nature and that relative scaling of electron self-energies and exciton binding energies in metallic nanotubes closely matches that found in semiconductors. This similarity in behavior can be understood in terms of similar regions of the Brillouin zone being sampled by E$_{11}^{M}$ and E$_{11}^{S}$ and E$_{22}^{S}$ (and by E$_{22}^{M}$ and E$_{33}^{S}$ and E$_{44}^{S})$. Additionally, for large diameter nanotubes ($>$ 1.3 nm) we now observe the previously elusive upper branch signatures for several chiralities for both E$_{11}^{M}$ and E$_{22}^{M}$ excitation. These results are discussed as a consequence of the nodal behavior of exciton-phonon coupling. Also, while theoretical calculations for the ($n,m)$-dependent matrix elements predict the RBM intensity should decrease with increasing diameter; the opposite behavior is observed experimentally. We show this to be a consequence of an increase in the resonance Raman broadening factor $\Gamma $ as diameter decreases. Finally, we present Raman excitation data from surfactant suspensions highly enriched in metallic nanotubes via density gradient ultracentrifugation. Specifically, we will focus on the evolution of G-band behavior over a wide range of chiralities enabled by these new sample types. The variable behavior of the Breit-Wigner-Fano line in these enriched ensemble samples will be discussed. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B24.00003: Resonance Raman Spectroscopy of Armchair Single-Walled Carbon Nanotubes Erik Haroz, William Rice, Benjamin Lu, Robert Hauge, Donny Magana, Stephen Doorn, Pasha Nikolaev, Sivaram Arepalli, Junichiro Kono We performed resonance Raman spectroscopy studies of metallic single-walled carbon nanotubes (SWNTs), including armchair SWNTs from (6,6) through (10,10). The measurements were carried out with excitation of 440-850 nm on aqueous ensemble samples of SWNTs enriched in metallic species. From this, we generated Raman excitation profiles (REPs) of the radial breathing mode and compare the REPs of armchairs and other metallic species. Additionally, we measured REPs of the G-band mode and observed how the Breit-Wigner-Fano line shape of the G$^{-}$ peak evolves in peak position, width and intensity relative to the G$^{+}$ peak as different metallic nanotubes are excited. By combining these studies with absorption and photoluminescence excitation spectroscopy studies, we present a comprehensive examination of the optical signatures of metallic SWNTs. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B24.00004: Direct Measurement of the D-Mode and G-Mode Optical Phonon Lifetimes in Single Wall Carbon Nanotubes Hugen Yan, Daohua Song, Tony Heinz Time-resolved Raman spectroscopy has been applied to determine the population lifetime of both zone-center and zone-edge optical phonons. Non-equilibrium populations of these phonons were produced by the rapid relaxation of charge carriers following photoexcitation of the nanotube sample with a femtosecond laser pulse. The temporal evolution of these phonon populations was recorded using the strength of antiStokes Raman scattering in G-mode (for the zone-center phonons) and D-mode (for the zone-edge phonons) as a function of the time delay of the fs probe pulse. A longer lifetime was found for the D-mode than for the G-mode phonons, a result consistent with recent ab-initio calculations of the anharmonic decay of these phonons [1]. We also report on the transient mode populations for the zone-center and zone-edge phonons that result from carrier cooling. [1] N. Bonini, M. Lazzeri, N. Marzari, and F. Mauri, Phys. Rev. Lett. \textbf{99}, 176802 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B24.00005: Investigation of Nanotube Growth Mechanisms via \textit{In-Situ} Spectroscopy Rahul Rao, David Liptak, Roberto Acosta, Benji Maruyama Analysis of single-walled carbon nanotubes (SWNTs) during growth via Raman spectroscopy offers a unique approach to understand their growth mechanism, which remains unclear due to large variability of parameters in synthesis methods. In our technique the SWNTs are synthesized via chemical vapor deposition inside an environmental cell coupled to an automated stage. Growth occurs from catalyst nanoparticles on thermally isolated islands within substrates. \textit{In-situ }micro-Raman spectroscopy is performed on the radial breathing mode and D/G bands of the growing SWNTs where the excitation laser also serves as a localized heat source for SWNT growth. Computer control over substrate temperature and position, feed gas composition, and chamber pressure enable rapid real-time exploration of SWNT growth parameter space. Comparison of nanotube nucleation and growth kinetics from various metallic catalyst particles will be presented and implications for nanotube catalyst design will be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B24.00006: Characterisation of Carbon Nano-Materials with the Confocal Raman AFM Klaus Weishaupt, Thomas Dieing, Matthias Kress, Ute Schmidt Graphene and carbon nanotubes represent perfect model systems for fundamental research. Carbon nanotubes have proven to be unique systems for the study of Raman spectra in one-dimensional systems. Although the diameter of single walled carbon nanotubes (SWCNT) is far below the optical resolution limit, its unique optical and spectroscopic properties due to the one-dimensional confinement of electronic and phonon states leads to resonant enhancement of the corresponding photophysical process. Characteristic for SWCNT only are the radial breathing modes (RBM) providing information about the diameter of the carbon nanotube. The position and width of the G-band is used to distinguish between metallic and semiconducting SWCNT and to probe the charge transfer arising from doping a SWCNT. The G' band, characteristic for interlayer coupling in graphite, arises from phonon resonance in SWCNT. Graphene shows similar unique properties and is a perfect model system for Raman spectroscopy in a two-dimensional system. The combination of two different analytical techniques such as confocal Raman microscopy and atomic force microscopy (AFM) in one instrument, allow the topographical and optical characterization of carbon nano-materials. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B24.00007: Resonance Raman Scattering for Quantification of the Bundling of SWNTs Tao Liu, Zhiwei Xiao The strong attractive van der Waals interaction induces individual SWNTs to form bundles or ropes. It has been demonstrated both experimentally and theoretically that, the various physical properties of SWNTs, e.g., photoluminescence, electrical and electronic, and mechanical, strongly depends upon their bundling states. Upon comparative studies of SWNT dispersions with the preparative ultracentrifuge method, which is a newly developed characterization technique by us for quantifying the structures of SWNTs in a dispersion, and resonance Raman scattering, we demonstrate that the bundling states for a given SWNT dispersion can be quantified with the latter technique. In this presentation, the preparative ultracentrifuge method for studying the processing-structure-property relationships of SWNT dispersion will be introduced. The mechanisms of using resonance Raman scattering to quantify the bundling states of SWNTs will be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B24.00008: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 1:39PM - 1:51PM |
B24.00009: Electrical Characterization of Carbon Nanotube Bundles Synthesized from Chemical Vapor Deposition of Ferrocene C. Wolfe, R. Shah, X. Zhang, X. An, S. Kar, S. Talapatra We employed a chemical vapor deposition technique, which used ferrocene both as the catalyst as well as the carbon source, to grow films of carbon nanotubes (CNT). The CNT films obtained using this procedure were characterized using Raman Spectroscopy and Transmission Electron Microscopy which indicated the presence of thin diameter carbon nanotubes as well as single walled CNT ropes. Electrical transport measurements performed on long ropes of CNTs extracted from these bulk films will be presented and will be discussed in the framework of transport theories of quasi-one dimensional systems. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B24.00010: Coherent Phonon Dynamics in Single-Walled Carbon Nanotubes L.G. Booshehri, E.H. Haroz, J. Kono, Y.S. Lim, J.H. Kim, K.J. Yee, G.D. Sanders, C.J. Stanton Understanding how electrons and phonons relax in energy and momentum is one of the current goals in carbon nanotube spectroscopy as well as an important step towards developing novel electronic and optoelectronic devices based on carbon nanotubes. Recent ultrafast pump-probe spectroscopy studies of single-walled carbon nanotubes (SWNTs) have successfully detected coherent phonon dynamics, but the dominant dephasing mechanism that occurs with decoherence of phonon mode oscillations has yet to be understood. Our previous work demonstrating ultrafast coherent phonon spectroscopy of the radial breathing mode (RBM) of semiconducting SWNTs provided a powerful method for determining phonon energies in an ensemble of SWNTs. We now extend our previous studies to provide new insight into the dephasing mechanisms of coherent phonons in SWNTs. Here, we systematically investigated the temperature, polarization, and wavelength dependence of coherent phonon dephasing times, amplitude, and frequency for various types of nanotube film and solution samples. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B24.00011: Microscopic theory of coherent phonon spectroscopy of carbon nanotubes G.D. Sanders, C.J. Stanton, J.K. Kim, K.J. Yee, Y.S. Lim, E.H. Haroz, L.G. Booshehri, J. Kono, R. Saito Using pump-probe spectroscopy with pulse shaping techniques, we study coherent phonons in chirality-specific semiconducting single-walled carbon nanotubes. The signals are resonantly enhanced when the pump photon energy coincides with an exciton resonance, and provides information on the chirality-dependence of light absorption, phonon generation, and phonon-induced band structure modulation. We develop a microscopic theory for generation and detection of coherent phonons in carbon nanotubes. We find that coherent phonon amplitudes satisfy a driven oscillator equation with a driving term depending on photoexcited carrier density. We compare theory with experiment and find that our model predicts correct overall trends in the relative strength of the coherent phonon signal both within and between different mod(n-m,3)=2 families. We predict that phonon intensities are considerably weaker in mod(n-m,3)=1 tubes in comparison with mod(n-m,3)=2 tubes, also in agreement with experiment. [Preview Abstract] |
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