Crystallographic Orientation(s) of Vanadium Dioxide Nano-Grains on Various Single-Crystal Sapphire Substrates

Vanadium dioxide (VO$_{2}$) is a material of particular interest due to its reversible structural semiconductor to metal phase transition near room temperature (\~ 68 $^{\circ}$C) and its accompanied hysteresis. Electron Back-Scattered Diffraction (EBSD) was used to study the orientation of the crystalline VO$_{2}$ grains deposited on three cuts of sapphire (a-, c-, and r-cuts) by pulsed laser deposition. EBSD showed a predominant family of crystallographic relationships present in all cuts of sapphire wherein the rutile VO$_{2}$ \{001\} planes tend to lie parallel to the substrate's \{10-10\} and the rutile VO$_{2}$ \{100\} planes lie parallel to the substrate's \{1-210\} and \{0001\}. This family of relationships accounts for the majority of the VO$_{2}$ grains observed on all the studied sapphire substrates. However, due to the symmetry of the substrate, there were variations of these same relationships that prevent a single epitaxy from taking place in these cuts as the VO$_{2}$ grains did orient themselves with equivalent out-of plane directions in the substrate.