Theoretical and experimental studies of ZnO nanowires grown by vapour phase transport

Abstract

This thesis discusses the growth atmosphere, condensing species and nucleation conditions relevant to vapour phase transport growth of ZnO nanowires. The partial pressure of molecular ZnO in a Zn/O2 mix at normal ZnO growth temperatures is 6 x 10e-7 of the Zn partial pressures. In typical vapour phase transport growth conditions, using carbothermal reduction, the Zn vapour is always undersaturated while the ZnO vapour is always supersaturated. In the case of the ZnO vapour, our analysis suggests that the barrier to nucleation is too large for nucleation of ZnO to take place, which is consistent with experimental evidence that nanostructures will not grow on unseeded areas of substrates. In the presence of suitable accommodation sites, due to ZnO seeds, growth can occur via Zn vapour condensation (followed by oxidation) and via direct condensation of molecular ZnO. The balance between these two condensing species is likely to be a sensitive function of growth parameters. This thesis also examines the relationship between the length and radius of ZnO nanowires grown via VPT and nds that the lengths of the nanowires increase with decreasing radius, supporting the inclusion of a diusion term in a model for the incorporation of molecules into a growing nanowire