| In this thesis, the photoluminescence (PL) from various kinds of single ZnO nanostructures is studied by near-field scanning optical microscopy (NSOM). PL from individual single ZnO nanostructures provides unambiguous information for our further analysis on the optical mechanisms of the UV near band edge (NBE) emission and the visible defect-related (DE) luminescence.;With several fabrication methods, we obtained ZnO nanostructures of various optical qualities: only NBE (Sample 1: nanowires (NW) by carbon thermal method; Sample 2: NW by Zn evaporation method), NBE+DE (Tetropods in Sample 2; Sample 3: NW by thermal method; Sample 4: NW by hydrothermal method), and only DE luminescence (Sample 5: Y-shape by ZnO thermal method). The importance of PL measurement on single nanostructure rather than the ensembles is shown.;Post-treatments were applied on the samples to investigate the types and distributions of the defects in ZnO. For the optically perfect ZnO NW (Sample 1), aging in ambiences may damage the morphology, but would neither quench the NBE nor generate DE luminescence. As to the hydrothermal Sample 4, boiling in water at 350ºC increases the UV intensity by about ten times and depresses the DE/NBE ratio from 4 to less than 0.1.;Spatially-resolved PL investigations were carried out on single ZnO nanowires, tetrapods and nanocrystals at room temperature (RT) and low temperature (LT). At RT, multiple DE emission peaks were observed at the regions near the core of tetrapods, with no detectable DE emission from nanowires grown simultaneously, suggesting that the green luminescence may not exclusively come from surface states, but might also from bulk defects. At LT, the fractional intensity for bound exciton transitions (BX) was shown to be correlated to the size in all the observed ZnO nanostructures. This size dependency is attributed to the inhomogeneous density distribution of the defects as binding sites for BX in the ZnO nanostructures, in good agreement with a simple model calculation. |
