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Georgi, Carsten (2011): Exciton Mobility and Localized Defects in Single Carbon Nanotubes Studied with Tip-Enhanced Near-Field Optical Microscopy. Dissertation, LMU München: Fakultät für Chemie und Pharmazie



In this work, single-walled carbon nanotubes (SWNTs) have been studied using tip-enhanced near-field optical microscopy (TENOM). This technique provides a sub-diffraction spatial resolution of 15 nm on the basis of strong local signal enhancement, which allows for nanoscale imaging of the photoluminescence (PL) intensity and energy along single semiconducting SWNTs. Thereby, the mobility of excitons and their interaction with defects and spatial exciton energy variations can be directly visualized. Similarly, the local Raman scattering properties of metallic SWNTs have been investigated, revealing the microscopic relation of localized defects and the resulting Raman D-band intensity. The first part of the thesis presents a newly developed numerical description of exciton mobility and local quenching at defect sites, accounting also for the TENOM imaging process. This highly flexible model is used to quantitatively evaluate experimental observations such as photo-induced PL blinking and strong spatial PL intensity variations of single semiconducting SWNTs. The main finding is that exciton propagation can be described as ne-dimensional diffusion with a diffusion length of 100 nm for the studied nanotubes, determined independently from both the PL blinking characteristics and the direct visualization using high-resolution TENOM. The temporal and spatial PL variations result from efficient exciton quenching at localized defects and the nanotube ends. The second part reports on the first observation of exciton localization in SWNTs at room temperature, leading to strongly confined and bright PL emission. Localization results from narrow exciton energy minima with depths of more than 15 meV, evidenced by energy-resolved near-field PL imaging. Complementary simulations using a modified numerical model accounting for energy gradients are in good agreement, predicting a significant directed diffusion towards energy minima yielding locally enhanced exciton densities. The energy variations are attributed to inhomogeneous DNA-wrapping of the nanotubes, used for their separation during sample preparation. In the last part, the microscopic relation between the defect-induced Raman D-band and the defect density has been investigated for metallic SWNTs. The length scale of the D-band scattering process in the vicinity of defects was imaged with TENOM for the first time and found to be about 2 nm. Furthermore, localized defects have been photo-generated intentionally by the strong fields at the tip while recording the evolution of the local Raman spectrum. Based on this data, a quantitative relation could be determined, that is highly relevant for the characterization of carbon nanotubes via Raman spectroscopy.