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Abstract

This paper discusses the growth atmosphere, condensing species, and nucleation conditions relevant to vapor 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 O2. The partial pressure of molecular ZnO in a Zn/O2 mix at normal ZnO growth temperatures is ∼6 × 10–7 of the Zn partial pressures. In typical vapor phase transport growth conditions, using carbothermal reduction, the Zn vapor is always undersaturated while the ZnO vapor is always supersaturated. In the case of the ZnO vapor, 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 vapor condensation (followed by oxidation) and via direct condensation of molecular ZnO (whose flux at the surface, although less than that of Zn vapor, is still sufficient to yield an appreciable nanostructure deposit). The balance between these two condensing species is likely to be a sensitive function of growth parameters and could explain both the diversity of reported nanostructure morphologies and the challenges to be faced in developing reproducible and scalable growth systems for specific applicable morphologies.

Theoretical Analysis of Nucleation and Growth of ZnO Nanostructures in Vapor Phase Transport Growth

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Abstract

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