Nanostructured MoOx films deposited on c-plane sapphire

Molybdenum oxide films were deposited on alpha-Al2O3 (0001) at 580 degrees C using MoO3 from a conventional molecular beam epitaxy Knudsen cell. A relatively smooth film (RMS roughness 1.1 nm) was deposited in 1 min at 580 degrees C using a Knudsen cell temperature of 620 degrees C; however, after 15 min deposition under these conditions, isolated islands (30-50 nm wide x 10-20 nm tall) develop that are stable to annealing at 600 degrees C for 60 min. XPS evidenced that the films are oxygen deficient with an average formula of MoO2.7. The authors infer that this oxygen deficiency is responsible for their thermal stability and may have significant effects on their catalytic and electronic properties. In contrast, stoichiometric MoO3 films deposited at 400 degrees C sublime completely during annealing at 600 degrees C.