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Böhmler, Miriam (2012): Tip-enhanced near-field optical microscopy on the quasi 1D semiconductors carbon nanotubes and CdSe nanowires. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Optical microscopy is widely used to visualize small structures that can not be seen by the unaided human eye. On the nanoscale, however, the diffraction limit prevents conventional microscopy from studying materials with the required spatial resolution. This work reports on tip-enhanced near-field optical microscopy (TENOM), a technique that allows for nanoscale optical imaging with high detection sensitivity. It exploits the locally enhanced optical fields at a laser illuminated metal tip that acts as an optical antenna. The main aims of this work are to develop a better understanding of the signal enhancement mechanisms in TENOM and to apply the technique to different 1D semiconducting nanostructures, namely single-walled carbon nanotubes (SWCNTs) and cadmium selenide nanowires (CdSe NWs). In the first part, the angular distribution of photoluminescence (PL) emission from SWCNTs with and without the optical antenna is studied by imaging the back focal plane of the microscope objective. Using model calculations, it is shown that the PL of SWCNTs on a dielectric substrate can be described as emission from a single in-plane point dipole despite the quasi 1D structure of the nanotubes. The signal enhancement due to the antenna is connected to a substantial redistribution of the angular emission. A procedure for the individual quantification of the excitation and radiation rate enhancement factors is developed and applied to the experimental data. In the second part, nanoscale optical imaging of CdSe NWs using TENOM is presented for the first time. Spectrally resolved imaging reveals different band gaps for different NWs and variations of the PL energy and intensity along single NWs with energy gradients up to 1 meV/nm. Even bundled NWs can be spatially resolved by their PL and Raman signals. The third part reports on the angular and spectral emission properties of CdSe NWs and the tip-induced changes in a TENOM measurement. In contrast to SWCNTs, two perpendicularly oriented point dipoles are required to describe the angular intensity distribution of PL emission from CdSe NWs sufficiently. Again, tip-induced signal enhancement is accompanied by a spatial redistribution of the emission. The theoretical description is more complex than in the case of SWCNTs, because different radiating dipole orientations in the NW have to be taken into account that can interact with the tip. Finally, investigations of the tip-sample distance dependence of PL and Raman scattering are presented and discussed.