Shape Engineering Driven by Selective Growth of SnO on Doped GaO Nanowires.

Tailoring the shape of complex nanostructures requires control of the growth process. In this work, we report on the selective growth of nanostructured tin oxide on gallium oxide nanowires leading to the formation of SnO/GaO complex nanostructures. GaO nanowires decorated with either crossing SnO nanowires or SnO particles have been obtained in a single step treatment by thermal evaporation. The reason for this dual behavior is related to the growth direction of trunk GaO nanowires. GaO nanowires grown along the [001] direction favor the formation of crossing SnO nanowires. Alternatively, SnO forms rhombohedral particles on [110] GaO nanowires leading to skewer-like structures. These complex oxide structures were grown by a catalyst-free vapor-solid process. When pure Ga and tin oxide were used as source materials and compacted powders of GaO acted as substrates, [110] GaO nanowires grow preferentially. High-resolution transmission electron microscopy analysis reveals epitaxial relationship lattice matching between the GaO axis and SnO particles, forming skewer-like structures. The addition of chromium oxide to the source materials modifies the growth direction of the trunk GaO nanowires, growing along the [001], with crossing SnO wires. The SnO/GaO junctions does not meet the lattice matching condition, forming a grain boundary. The electronic and optical properties have been studied by XPS and CL with high spatial resolution, enabling us to get both local chemical and electronic information on the surface in both type of structures. The results will allow tuning optical and electronic properties of oxide complex nanostructures locally as a function of the orientation. In particular, we report a dependence of the visible CL emission of SnO on its particular shape. Orange emission dominates in SnO/GaO crossing wires while green-blue emission is observed in SnO particles attached to GaO trunks. The results show that the GaO-SnO system appears to be a benchmark for shape engineering to get architectures involving nanowires via the control of the growth direction of the nanowires.