Browsing by Author "Walker, Angela R. Hight"

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    Asymmetric excitation profiles in the resonance Raman response of armchair carbon nanotubes
    (American Physical Society, 2015) Hároz, Erik H.; Duque, Juan G.; Barros, Eduardo B.; Telg, Hagen; Simpson, Jeffrey R.; Walker, Angela R. Hight; Khripin, Constantine Y.; Fagan, Jeffrey A.; Tu, Xiaomin; Zheng, Ming; Kono, Junichiro; Doorn, Stephen K.
    We performed tunable resonance Raman spectroscopy on samples highly enriched in the (5,5), (6,6), (7,7), and (8,8) armchair structures of metallic single-wall carbon nanotubes. We present Raman excitation profiles (REPs) for both the radial breathing mode and G-band phonons of these species. G-band excitation profiles are shown to resolve the expected incoming and outgoing resonances of the scattering process. Notably, the profiles are highly asymmetric, with the higher-energy outgoing resonance weaker than the incoming resonance. These results are comparable to the asymmetric excitation profiles observed previously in semiconducting nanotubes, introduce a different electronic type, and broaden the structural range over which the asymmetry is found to exist. Modeling of the behavior with a third-order quantum model that accounts for the k dependence in energies and matrix elements, without including excitonic effects, is found to be insufficient for reproducing the observed asymmetry. We introduce an alternative fifth-order model in which the REP asymmetry arises from quantum interference introduced by phonon-mediated state mixing between the EM11 and K-momentum excitons. Such state mixing effectively introduces a nuclear coordinate dependence in the transition dipole moment and thus may be viewed as a non-Condon effect from a molecular perspective. This result unifies a molecularlike picture of nanotube transitions (introduced by their excitonic nature) with a condensed matter approach for describing their behavior.
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    Fundamental optical processes in armchair carbon nanotubes
    (The Royal Society of Chemistry, 2013) Haroz, Erik H.; Duque, Juan G.; Tu, Xiaomin; Zheng, Ming; Walker, Angela R. Hight; Hauge, Robert H.; Doorn, Stephen K.; Kono, Junichiro; Richard E. Smalley Institute for Nanoscale Science and Technology
    Single-wall carbon nanotubes provide ideal model one-dimensional (1-D) condensed matter systems in which to address fundamental questions in many-body physics, while, at the same time, they are leading candidates for building blocks in nanoscale optoelectronic circuits. Much attention has been recently paid to their optical properties, arising from 1-D excitons and phonons, which have been revealed via photoluminescence, Raman scattering, and ultrafast optical spectroscopy of semiconducting carbon nanotubes. On the other hand, dynamical properties of metallic nanotubes have been poorly explored, although they are expected to provide a novel setting for the study of electronﾖhole pairs in the presence of degenerate 1-D electrons. In particular, (n,n)-chirality, or armchair, metallic nanotubes are truly gapless with massless carriers, ideally suited for dynamical studies of TomonagaﾖLuttinger liquids. Unfortunately, progress towards such studies has been slowed by the inherent problem of nanotube synthesis whereby both semiconducting and metallic nanotubes are produced. Here, we use post-synthesis separation methods based on density gradient ultracentrifugation and DNA-based ion-exchange chromatography to produce aqueous suspensions strongly enriched in armchair nanotubes. Through resonant Raman spectroscopy of the radial breathing mode phonons, we provide macroscopic and unambiguous evidence that density gradient ultracentrifugation can enrich ensemble samples in armchair nanotubes. Furthermore, using conventional, optical absorption spectroscopy in the nearinfrared and visible range, we show that interband absorption in armchair nanotubes is strongly excitonic. Lastly, by examining the G-band mode in Raman spectra, we determine that observation of the broad, lower frequency (G!) feature is a result of resonance with non-armchair “metallic” nanotubes. These !ndings regarding the fundamental optical absorption and scattering processes in metallic carbon nanotubes lay the foundation for further spectroscopic studies to probe many-body physical phenomena in one dimension.