Influence of Monomer Flux and Temperature on Morphology of Indium Oxide Nanocrystals during a Continuous Growth Synthesis

The flux of molecular or atomic species to the crystal surface has a significant influence on its growth. For nanocrystals, little is known about the influence of the growth processes on morphology despite the fact that their properties depend critically on atomic or nanoscale variations in composition and structure. A continuous growth method was used to carefully control the addition of a precursor to examine the influence of flux on the growth of indium oxide nanocrystals. Detailed analysis of the morphologies by high-resolution transmission electron microscopy showed that growth occurs by monomer addition to nanocrystal surfaces and not through particle coalescence events. High flux causes the growth of single-crystal, branched morphologies, whereas low flux results in uniform, faceted morphologies. As the reaction temperature increases, the growth process becomes less sensitive to the flux; at higher temperatures, branched growth does not occur until significantly higher fluxes are reached. A model is proposed that explains how surface diffusion of reactive species plays a key role in determining the growth and morphology of nanocrystals. At high fluxes, these species are more prone to nucleate new islands on the surface of the growing nanocrystals, leading to single-crystal, yet branched, morphologies. At lower fluxes, surface diffusion is efficient, resulting in growth at step edges that produce faceted nanocrystals. The studies suggest that flux is an important consideration in commonly employed nanocrystal synthesis methods. The proposed model can be used to establish conditions for controlling crystalline quality and morphology of nanocrystals in future syntheses.