Browsing by Author "Haroz, Erik"

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    Curvature Effects on the Optical Transitions of Single-Wall Carbon Nanotubes
    (2013-07-24) Haroz, Erik; Tittel, Frank K.; Xu, Qianfan; Hauge, Robert H.
    Optical transition energies are widely used for providing experimental insight into the electronic band structure of single-wall carbon nanotubes (SWCNTs). While the first and second optical transitions in semiconducting carbon nanotubes have already been heavily studied, due to experimental difficulties in accessing the relevant excitation energy region, little is known about higher lying transitions. Here, I present measurements of the third and fourth optical transitions of small-diameter (0.7-1.2 nm), semiconducting single-wall carbon nanotubes via resonant Raman spectroscopy in the visible deep blue region (415-465 nm) and photoluminescence excitation spectroscopy in the ultraviolet and visible blue optical regions (280-488 nm). Diameter-dependent Raman radial breathing mode features, as well as resonant energy excitation maxima determined by Raman and photoluminescence measurements, are assigned to specific (n,m) nanotube species. The Raman intensity within a given 2n+m branch is found to increase with decreasing chiral angle, consistent with similar measurements for lower order optical states. Additionally, increased excitation line widths and weaker Raman intensities are observed as higher lying transitions are accessed for a given nanotube, in agreement with previous Raman measurements. Chiefly, a scaling law analysis that removes the chiral-angle-dependent contribution to the optical transition energy indicates that the third and fourth transition energies exhibit a significant deviation from the energy trend line observed for the first and second optical transitions, when the transition energies are plotted as a function of nanotube diameter. This deviation can be understood in the context of a change in the competition between exchange and excitonic correction terms. Furthermore, for semiconducting SWCNTs with diameters less than 0.9 nm, an additional deviation is observed that is interpreted as the first observation of crossing-over of the third and fourth transition energy trend lines for a given 2n+m branch and a chirality dependence in the many-body excitonic effects that becomes significant at high nanotube curvatures.
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    Enrichment and Fundamental Optical Processes of Armchair Carbon Nanotubes
    (2013-09-16) Haroz, Erik; Tittel, Frank K.; Xu, Qianfan; Hauge, Robert H.
    The armchair variety of single-wall carbon nanotubes (SWCNTs) is the only nanotube species that behaves as a metal with no electronic band gap and massless carriers, making them ideally suited to probe fundamental questions of many-body physics of one-dimensional conductors as well as to serve in applications such as high-current power transmission cables. However, current methods of nanotube synthesis produce bulk material comprising of a mixture of nanotube lengths, diameters, wrapping angles, and electronic types due to the inability to control the growth process at the nanometer level. As a result, measurements of as-grown SWCNTs produce a superposition of electrical and optical responses from multiple SWCNT species. This thesis demonstrates production of aqueous suspensions composed almost entirely of armchair SWCNTs using a post-synthesis separation method employing density gradient ultracentrifugation (DGU) to separate different SWCNT types based on their mass density and surfactant-specific interactions. Resonant Raman spectroscopy determines the relative abundances of each nanotube species, before and after DGU, by measuring the integrated intensity of the radial breathing mode, the diameter-dependent radial vibration of the SWCNT perpendicular to its main axis, and quantifies the degree of enrichment of bulk nanotube samples to exclusively armchair tubes. Raman spectroscopy of armchair-enriched samples of the G-band mode, which is composed of longitudinal (G-) and circumferential (G+) vibrations oscillating parallel and perpendicular to the tube axis, shows that the G- peak, long-held to be an indicator for the presence of metallic SWCNTs, appears only when electronic resonance with narrow-gap semiconducting SWCNTs occurs and shows only the G+ component in spectra containing only armchair species. Finally, by combining optical absorption measurements with nanotube composition as determined earlier via Raman scattering, peak fitting of absorption spectra indicates that interband transitions of armchair SWCNTs are strongly excitonic as shown by the highly symmetric peak lineshapes, a property normally attributed to semiconductors. Such lineshapes allow classification of armchair SWCNTs as a unique hybrid class of optical nanomaterial. Combining absorption and Raman scattering measurements establishes a distinct optical signature that describes the fundamental optical processes within armchair SWCNTs and lays the foundation for future studies of many-body photophysics and electrical applications.