Spray deposition of V4O9 and V2O5 thin films and post-annealing formation of thermochromic VO2

Vanadium oxide (VOx) thin films were deposited at various substrate temperatures (Ts) by spray pyrolysis technique using 0.05 M vanadyl acetylacetonate precursor. V4O9 films are formed at Ts = 300 °C, while mixed V2O5 phases are formed at higher Ts (400 and 500 °C). Annealing in forming gas of V4O9 films shows the formation of higher content of thermochromic VO2 phase than V2O5 films. V4O9 films show little higher electric resistivity (ρ), higher temperature coefficient of resistance (TCR), and higher thermal carrier activation energy (Ea) than V2O5 films. Annealed VOx films show a 2-3 order of magnitude change in ρ, optical transmission switch of 19-39%, and higher Ea than the as deposited films. Annealed films deposited at Ts = 500 °C presents a high TCR of −4.6%K−1. Optical absorption, electronic transitions, and energy gaps of the formed VOx phases have been discussed in relation to its electronic band structure.