Phase formation and morphology control of niobium oxide nanopillars

Nanopillar metal oxide thin films offer versatility as ultra high surface area supports and conductors. Metal oxide properties (e.g. stability, conductivity) can be tuned via phase and composition control to achieve desired application-specific functionality. Here we demonstrate phase control of high surface area thin films grown by glancing angle deposition and transformed to desired phases through high temperature annealing in a reducing environment. The post-annealed properties such as stoichiometry, phase, and morphology are shown to be largely dependent on initial film structure and hydrogen forming gas flow rate. Initially amorphous films of approximate stoichiometry Nb2O5 are transformed to NbO2 or NbNxO1−x through annealing. Transformation to oxygen-deficient phases is more easily achieved for films of higher initial porosity. Higher forming gas flow rates result in both increased oxygen removal and significantly less physical degradation of nanostructures. A phase map is included as a guide to phase formation and morphology control in annealed nanopillar niobium oxide films.