Substrate Chemistry and Lattice Effects in Vapor Transport Growth of Vanadium Dioxide Microcrystals

Vapor-phase transport is a rapid, inexpensive method of growing nano- and microscale single crystals of vanadium dioxide, a correlated-electron material with a metal–insulator transition at ∼70 °C. Many growth parametersincluding time, temperature, precursor, ambient conditions, and substratehave been explored, and a variety of crystal morphologies has been produced, with most emphasis given to oriented nanowires. However, a comprehensive strategy for predicting/controlling the crystal morphology is still evolving. Here, we investigate the role of the substrate in platelet growth, highlighting three important types of interactions: chemical reactions at the surface, lattice matching effects, and surface energy. We present results on four different cuts of sapphire (Al₂O₃) and three of yttria-stabilized zirconia (YSZ) to differentiate the roles of these mechanisms. Each has significant effects: chemical reactions leading to Al-doped VO₂ on Al₂O₃ and the formation of YVO₄ on YSZ, lattice match producing preferred orientations on both, and high surface energy promoting growth of larger microcrystals. We suggest a framework for relating crystal morphology, orientation, and doping to substrate properties, in order to use intentional choice of the substrate to engineer the size, shape, orientation, and strain state of VO₂ single crystals, a crucial step toward realizing VO₂ crystal-based devices.