Browsing by Author "Duque, Juan G."

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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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    Diameter dependence of TO phonon frequencies and the Kohn anomaly in armchair single-wall carbon nanotubes
    (American Physical Society, 2014) Telg, Hagen; Hároz, Erik H.; Duque, Juan G.; Tu, Xiaomin; Khripin, Constantine Y.; Fagan, Jeffrey A.; Zheng, Ming; Kono, Junichiro; Doorn, Stephen K.
    We present resonant Raman scattering experiments on nanotube samples enriched in metallic armchair single-wall carbon nanotubes (SWCNTs). We establish the transverse optical (ATO) phonon frequency for the (5,5) through (10,10) armchair species, ranging in diameter from 0.68 to 1.36 nm. The frequencies show a strong diameter dependence similar to that previously observed in semiconducting nanotubes. We show that the ATO frequencies in armchair SWCNTs are dramatically upshifted from those of semiconducting SWCNTs. Furthermore, using electrochemical doping, we demonstrated that the ATO frequencies in armchair SWCNTs are independent of the position of the Fermi level. These results suggest that the upshift is a result of a Kohn anomaly involving a forward-scattering mechanism of electrons close to the Fermi level. This is in contrast to the well-known Kohn anomaly that dominates the downshift of the ALO and E2g phonons in nonarmchair metallic SWCNTs and graphene, respectively.
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    Electrochemistry and self-assembly of complex single-walled carbon nanotube (SWNT) nanostructures
    (2009) Duque, Juan G.; Pasquali, Matteo
    This dissertation explores the optical and electrochemical properties of surfactant stabilized and individually---suspended SWNTs in aqueous media via application of various external stimuli. Resulting effects such as nanostructure formation, photocurrent generation, and inherent nanotube electronic and optical properties are then analyzed. The goal is to engineer SWNT systems which can be tuned by understanding the mechanism of the electrochemical and environmental reactions so that applications in nanophotonics, photovoltaics, and electronics can be effectively exploited. A strategy to obtain a surfactant---polymer protective "shell" that improves the stability and luminescence signal of individual SWNTs is presented. We used literature evidence of emission shifts to understand the interactions between polymers and surfactants and show how morphological changes induced by extrinsic factors distort the SWNT luminescence. We developed an in-situ polymerization which creates an outer shell around the SWNT micelle that resulted in suspensions with stable luminescence at all pH, in saline buffers, and on the surface of living cells. Single molecule imaging and time resolved spectroscopy of individual (6,5) SWNTs demonstrated that SWNT luminescence depends strongly on intrinsic and extrinsic factors such as sample preparation, sample inhomogeneities, defects, and tube synthesis conditions. Moreover, we found compelling spectroscopic evidence of substantial differences in chirality distribution and luminescence properties within HiPco batches. Nanoparticle-nanotube Structures (nanoPaNTs) were fabricated by exploiting the electrochemical properties of SWNTs upon activation with alternating electromagnetic fields. The incident field polarizes the SWNTs at the ends, antenna-like-behavior, which readily drives electrochemical reactions. This process is shown to activate redox reactions preferentially with metallic SWNT and proceed at or near diffusion-limited rates. Electrochemical photocathodes with optical rectifying antennae were developed from an array of vertically grown CNT forests. We demonstrated rectification of AC signals by associating anionic surfactant molecules around the CNT and charge separation in the optical regime that generates measurable, wavelength dependent, electrical current. We show that charge separation drives redox reactions with transition metal salts in SWNT suspensions. The results of this research provide key information on the interaction between SWNTs and electromagnetic fields and insight into the extrinsic and intrinsic factors that affect the optical properties.
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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.