Controlling the assembly of nanocrystalline ZnO films by a transient amorphous phase in solution

The chronological sequence of the precipitation and crystallization of Zn species in methanol solutions containing poly(vinyl pyrrolidone) (PVP) was studied with dynamic light scattering, electron microscopy, and chemical analysis and correlated with the film formation on substrates. It was found that the polymer inhibited uncontrolled precipitation of ZnO and promoted the formation of smooth ZnO films on substrates. The reaction solution was quenched from 60 degrees C to room temperature after different periods of time, It could be shown that amorphous needle-like particles formed in the first place. This intermediate and metastable phase dissolved, and aggregated ZnO nanocrystals formed concurrently, whereby the transformation rate was determined by the PVP concentration. The formation of high-quality ZnO films, with respect to uniformity and low surface roughness, on substrate surfaces modified by self-assembled monolayers was strongly related to the presence of the intermediate amorphous phase in solution. Uncontrolled crystallization at low PVP concentrations resulted in defective and rough deposits. Very high PVP concentrations resulted in rapid aggregation of the zinc oxide nanocrystals in solution and a strongly decreased deposition rate. Control of the reaction conditions through intermediate amorphous phases commonly occurs in biomineralization processes. The employment of this mechanism could facilitate the development of novel chemical bath deposition processes of artificial nanostructured materials.