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Qian, Huihong (2008): Tip-enhanced Near-Field Optical Spectroscopy on Single-Walled Carbon Nanotubes. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

High resolution optical methods overcome the diraction limit, a step essential for understanding the physical and chemical properties of nanostructures. In this work, I applied tip-enhanced near-eld optical microscopy (TENOM) to study the optical properties of single-wall carbon nanotubes (SWNTs) with nanoscale spatial resolution. Simultaneously obtained near-eld Raman scattering and photoluminescence (PL) data is shown to provide information with unprecedented detail on the nanotube structure and the resulting phonon and exciton properties. Near-eld PL is found to be more localized along single nanotubes than Raman scattering in most cases due to defects and environmental perturbations. By detecting near-eld PL spectra, my work has shown exciton energy variations along the same nanotubes induced by the environment. The local PL energy response to DNA-wrapping reveals large DNA-induced redshifts of the exciton energy that are two times higher than indicated by spatially averaging confocal microscopy. Exciton energy transfer between two semiconducting nanotubes is observed for the rst time limited to small distances because of competing fast non-radiative relaxation. The transfer mechanism is explained by F�orster-type electromagnetic near-eld coupling. In addition, towards the end of a nanotube, PL decay is observed on a length scale of 15-40 nm which is attributed to exciton propagation followed by additional non-radiative relaxation at the nanotube end. The dierent enhancement mechanisms of Raman scattering and PL lead to dierent enhancement factors of the two signals. The PL enhancement can be stronger than the Raman enhancement because of the very low initial quantum yield of nanotubes. The signal enhancement of Raman scattering and PL is also found to exhibit dierent tip-sample distance dependencies because of the PL quenching eects from the gold tip. The results achieved in my thesis highlight the enormous capabilities of TENOM for the investigation of nanoscale surfaces.