Growth and Characterisation of ZnO Nanostructures: Excitonic Properties and Morphology

Abstract

The growth mechanism of aligned ZnO nanostructures, grown by the vapour phase transport (VPT) growth method, specifically with nanorod and nanowall morphologies has been studied. The thesis begins with an introductory chapter on ZnO nanostructures and related topics, and the second chapter introduces the various experimental techniques used. The main thesis work involves five distinct studies. Firstly, the conditions to grow nanorod and nanorod/nanowall structures on sapphire using a ZnO/graphite powder mixture as a growth source are studied and the optimum conditions for each morphology identified. Secondly, the effects on ZnO nanostructure growth on sapphire of using activated carbon and carbon black powders, rather than graphite powder are studied. Nanostructures can be grown at significantly lower temperatures with carbon black and activated carbon, though with different morphologies, compared to graphite. Thirdly, low temperature cathodoluminescence spectroscopy measurements of ZnO nanostructures grown on Si substrate are presented. These data show significant inhomogeneity in the spatial distribution of emission throughout the sample for the Al-related donor bound exciton emission at 3.3605 eV and the Al-related emissions are compared to the other spectral features seen for these samples. The possible origin of this inhomogeneity is discussed. Fourthly, the microscopic origin of a unique photoluminescence peak at ~3.367 eV, which is known as surface exciton peak, has been studied in detail and its behaviour is studied after samples have been subjected to various post-growth treatments such as plasma treatment, UV exposure in vacuum and exposure to high voltages. Finally, post-growth passivation of nanostructures has been done using PVP and HF on ZnO nanostructure samples. The effects of these chemicals on the optical emission from these samples are studied and the potential for these to act as effective passivation agents is discussed. The thesis concludes with a summary of the work done, some general conclusions and comments on possible future directions