Generation of ZnO nanowires with varied densities and lengths by tilting a substrate

Recently, ZnO nanowires have received wide attention. Existing fabrication approaches mainly focused on generating such nanowires with uniform density and length over a substrate. On the other hand, ZnO nanowires with gradients of density and length over a single substrate could make the corresponding devices have varied properties. Therefore, in this work, we developed a simple approach to generate ZnO nanowires of varied morphology on a common substrate. This was achieved by creating a boundary layer of non-uniform thickness which affected the amount of reactants reaching the surface during the growth. This was done by tilting the substrate relative to the incoming gas flow. Here, we first presented theoretical background of the critical idea, and then simulated the thicknesses of boundary layers for different substrate tilt angles. Finally, based on these theoretical and numerical investigations, we fabricated two types of surfaces, which had one- and two-tier wire structures, respectively.