ZnO nanostructures were grown on Au-catalyzed Si silicon substrates using vapor phase transport at growth temperatures from 800 to 1150°C. The sample location ensured a low Zn vapor supersaturation during growth. Nanostructures grown at 800 and 850°C showed a faceted rodlike morphology with mainly one-dimensional ͑1D͒ growth along the nanorod axis. Samples grown at intermediate temperatures ͑900, 950, and 1050°C͒ in all cases showed significant three dimensional ͑3D͒ growth at the base of 1D nanostructures. At higher growth temperatures ͑1100 and 1150°C͒ 3D growth tended to dominate resulting in the formation of a porous, nanostructured morphology. In all cases growth was seen only on the Au-coated region. Our results show that the majority of the nanostructures grow via a vapor-solid mechanism at low growth temperatures with no evidence of Au nanoparticles at their tip, in sharp contrast to the morphology expected for the vapor-liquid-solid ͑VLS͒ process often reported as the growth mechanism on Au-catalyzed Si. We see VLS growth only at 900 and 950°C. Transmission electron microscopy data indicate that the nanorods are single crystalline without gross structural defects. Luminescence data reveal strong ultraviolet emission in all samples and weak defect emission in the visible region. We discuss the growth mechanisms with reference to various models in the literature and suggest reasons for VLS growth only in a narrow temperature range. We also discuss the potential effects of the Zn oxidation reaction on the growth morphologies, aspects largely ignored in the general literature on this subject.

This paper discusses the growth atmosphere, condensing species and nucleation conditions relevant to vapour phase transport growth of ZnO nanostructures, including the molecular parameters and thermodynamics of the gas phase ZnO molecule and its importance compared to atomic Zn and molecular O 2 . The partial pressure of molecular ZnO in a Zn/O 2 mix at normal ZnO growth temperatures is ∼ 6 × 10 −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 homogeneous nucleation (or heterogeneous nucleation at unseeded/uncatalysed areas of the substrates) is too large for nucleation of this species 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 (whose flux at the surface, although less than that of Zn vapour, is still sufficient to yield an appreciable nanostructure deposit). The balance between these two

We model the growth of ZnO nanowires via vapour phase transport and examine the relationship predicted between the nanowire length and radius. The model predicts that the lengths of the nanowires increase with decreasing nanowire radii. This prediction is in very good agreement with experimental data from a variety of nanowire samples, including samples showing a broad range of nanowire radii and samples grown using a lithographic technique to constrain the nanowire radius. The close agreement of the model and the experimental data strongly support supporting the inclusion of a surface diffusion term in the model for the incorporation of species into a growing nanowire.

The thermodynamic conditions of the Zn vapour in the growth atmosphere found in carbothermal reduction vapour phase transport growth of ZnO nanostructures are discussed. In typical growth conditions the Zn vapour is undersaturated and will only nucleate at energetically suitable accommodation sites such as Au-coated regions or ZnO buffer layers/seeds. We suggest that growth proceeds by Zn condensation at such sites and subsequent oxidation, consistent with experimental evidence that metal catalysts or pre-deposited ZnO nucleation points are required for ZnO nanostructure growth using carbothermal vapour phase transport.