Where is the required lattice match in horizontal growth of nanowires?
The surface-directed growth of nanowires (NWs) has been demonstrated for a number of semiconductors and is anticipated to be applicable to a wider group of materials. However, the epitaxial relationship between NWs and their underlying surface has not been well understood, as it becomes more convoluted in systems with closely matched lattice and crystal symmetry. To unravel this relationship, here we investigate a highly-mismatched system comprising TiO2 anatase, a cubic crystal with a 4-fold symmetry, grown on substrates with lower and higher symmetries including sapphire and GaN. Our results reveal that the key lattice match, counter-intuitively, exists along the width of the NWs. We demonstrate the first set of examples that rule out the requirement for having a lattice match along the NW growth axis, which is observed in the growth of epitaxial quantum wires using non-VLS based processes. Unlike wurtizte crystals such as ZnO that have a preferred growth direction regardless of the substrate crystal orientation, in the case of horizontal TiO2 NWs, the lattice symmetry of the underlying GaN and a-sapphire substrates directly impacts NW facet formation resulting in hexagonal and square cross-sections, respectively. TiO2 heteroepitaxy using the SVLS growth shows great potential for realization of nanotextured surfaces with tailored crystallographic phases and facets.