Where is the required lattice match in horizontal growth of nanowires?

The horizontal growth of nanowires (NWs) using the surface-directed vapor-liquid-solid (SVLS) process has been demonstrated for a number of semiconductors and shows the unique ability of eliminating post-growth alignment steps. However, the epitaxial relationship between horizontal NWs and their underlying surface has not been well understood, as it becomes more convoluted in systems with closely matched lattice and crystal symmetry. We have unraveled one of the main mechanisms driving the lateral growth by investigating a highly-mismatched system comprising TiO2 anatase, a 4-fold symmetry crystal, grown on substrates with lower and higher symmetries including sapphire and GaN. Counter-intuitively, our results reveal that the lattice match with substrate 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 non-VLS growth of epitaxial quantum wires. Unlike wurtzite or zinc-blende crystals that have a preferred lattice orientation regardless of the substrate crystal structure, we observe new evidence on strict control of the substrate on shape, faceting and orientation of nanocrystals that could offer a selective route for tailoring TiO2 NW properties and functions at the ensemble level.