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Gokus, Tobias Dominik (2011): Time-Resolved Photoluminescence and Elastic White Light Scattering Studies of Individual Carbon Nanotubes and Optical Characterization of Oxygen Plasma Treated Graphene. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

In the course of this work the excited state dynamics of individual single-walled carbon nanotubes (SWCNTs) were studied by a combination of confocal PL spectroscopy and time correlated single photon counting (TCSPC) measurements. Nonradiative decay channels dominate the excited state dynamics of SWCNTs leading to low photoluminescence (PL) quantum yields and PL decay times on the picosecond timescale. Knowledge about the microscopic nature of these decay channels is crucial to improve the material properties. The measurements on the single nanotube level revealed large tube-to-tube variations of PL decay times, which could be attributed to different defect densities for different tubes. For the present SWCNT material the PL decay times only depend weakly on the nanotube length. SWCNT material synthesized by using a cobalt-molybdenum catalyst (CoMoCAT) systematically display short monoexponential PL decays, while the PL decay dynamics of SWCNTs produced high pressure decomposition of carbon monoxide process (HiPco) is either mono or biexponential depending on the respective local environment of the nanotube. The transition from a bi- to monoexponential PL decay can be explained by synthesis-dependent differences in defect densities. This defect related nonradiative decay channels reduce the amplitude of one decay component below the experimental detection limit. It is further shown, that photo-induced defects and gold atoms adsorbed to the sidewalls of SWCNTs are shown to alter the PL properties of individual SWCNTs. Additional low-energy PL satellite bands arise in the spectra. Their origin can be attributed to emission from nominally dark excitons which are ”brightened” due to defect facilitated mixing of intrinsic states with different parity/spin. The role of defects in the brightening process was investigated by time-resolved PL measurements and complementary Raman spectroscopy. Based on its energy separation and the unusually slow PL decay dynamics the lowest energy satellite band can be assigned to the radiative recombination of a triplet exciton. In a second project a common-path interference scattering approach (iSCAT) utilizing a conventional inverted laser scanning confocal microscope combined with a photonic crystal fibre as a supercontinuum white light source is successfully tested for its capabilities for elastic scattering imaging and spectroscopy of individual SWCNTs. Finally, it is shown that single layer graphene can selectively be turned luminescent upon exposure to a mild oxygen plasma. The treatment leads to a strong and spatially uniform PL which is characterized by a single, broad PL band extending from the visible to the near infrared spectral region. The analysis of the defect related Raman I(D)/I(G) intensity ratio indicates the formation of nanometer sized islands for which the sp2 conjugated lattice of graphene is still preserved. Emission of quantum confined states within these islands is discussed as a possible origin of the PL.