Experimental Validation of the Geometrical Selection Model for Hydrothermally Grown Zinc Oxide Nanowire Arrays

Zinc oxide nanowire arrays were hydrothermally grown on a Si(100) substrate coated with randomly oriented seed crystallites to characterize the process of geometrical selection. The theory suggests that randomly oriented rod-like crystallites can be grown into a film or array of nanowires with the maximal growth rate direction approximately normal to the substrate; in the case of ZnO, this is the c-axis direction. To examine this growth phenomenon experimentally, ZnO nanowire arrays with a random initial orientation were grown, and the number of wires that survived the geometrical selection up to a certain distance, h, from the substrate was measured. The resulting number density of the survived wires decreased as h –0.8, while the geometrical selection model predicted the decrease to be ∼h –1. As developed originally, the model can also apply universally to other three-dimensional (3D) crystal ensembles besides ZnO. Understanding geometrical selection will allow assessment of if and when this theory can be used to obtain films with certain characteristics, such as the orientation and scattering of the nanowire array, that is relevant for specific applications.