Browsing by Author "Naumov, Anton Viatcheslavovich"

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    Advanced Characterization and Optical Properties of Single-Walled Carbon Nanotubes and Graphene Oxide
    (2011) Naumov, Anton Viatcheslavovich; Weisman, R. Bruce
    Photophysical, electronic, and compositional properties of single-walled carbon nanotubes (SWCNTs) and bulk nanotube samples were investigated together with graphene oxide photoluminescence. First, we studied the effect of external electric fields on SWCNT photoluminescence. Fields of up to 10 7 V/m caused dramatic, reversible decreases in emission intensity. Quenching efficiency was proportional to the projection of the field on the SWCNT axis, and showed inverse correlation with optical band gap. The magnitude of the effect was experimentally related to exciton binding energy, as consistent with a proposed field-induced exciton dissociation model. Further, the electronic composition of various SWCNT samples was studied. A new method was developed to measure the fraction of semiconducting nanotubes in as- grown or processed samples. SWCNT number densities were compared in images from near-IR photoluminescence (semiconducting species) and AFM (all species) to compute the semiconducting fraction. The results provide important information about SWCNT sample compositions that can guide controlled growth methods and help calibrate bulk characterization techniques. The nature of absorption backgrounds in SWCNT samples was also studied. A number of extrinsic perturbations such as extensive ultrasonication, sidewall functionalization, amorphous carbon impurities, and SWCNT aggregation were applied and their background contributions quantified. Spectral congestion backgrounds from overlapping absorption bands were assessed with spectral modeling. Unlike semiconducting nanotubes, metallic SWCNTs gave broad intrinsic absorption backgrounds. The shape of the metallic background component and its absorptivity coefficient were determined. These results can be used to minimize and evaluate SWCNT absorption backgrounds. Length dependence of SWCNT optical properties was investigated. Samples were dispersed by ultrasonication or shear processing, and then length-fractionated by gel electrophoresis or controlled ultrasonication shortening. Fractions from both methods showed no significant absorbance variations with SWCNT length. The photoluminescence intensity increased linearly with length, and the relative quantum yield gradually increased, approaching a limiting value. Finally, a strong pH dependence of graphene oxide photoluminescence was observed. Sharp and structured excitation/emission features resembling the spectra of molecular fluorophores were obtained in basic conditions. Based on the observed pH-dependence and quantum calculations, these spectral features were assigned to quasi-molecular fluorophores formed by the electronic coupling of oxygen-containing addends with nearby graphene carbon atoms.
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    Electric field quenching of single-walled carbon nanotube photoluminescence
    (2008) Naumov, Anton Viatcheslavovich; Weisman, R Bruce
    The effect of an electric field on the photoluminescence of single-walled carbon nanotubes was investigated. Individual SWNTs embedded in polymeric film were deposited on microscope slides with electrodes. The fluorescence intensity and spectra of single semiconducting SWNTs were acquired. When SWNTs in polymer were subjected to electric fields of up to 10 7 V/m, a drastic decrease in their fluorescence intensity was observed. The effect was reversible and reproducible. SWNT fluorescence intensity was well approximated by inverse hyperbolic cosine of the electric field, with a single quenching parameter k. It was shown that the effect was induced by the electric field parallel to SWNT axis while the perpendicular component did not produce detectable quenching. The quenching process was found to be enhanced for long nanotubes. Bulk sample fluorescence studies indicated that the electric field quenching also becomes stronger with the decrease in bandgap energy. Potential theoretical models will be discussed.