Annealing effect on the formation of nanocrystals in thermally evaporated tungsten oxide thin films

The effect of thermal annealing on tungsten oxide (WO3) thin films deposited by vacuum evaporation was investigated. The properties of films were studied in terms of annealing temperature and annealing time. It was found that the crystallinity and physical properties of WO3 films were changed by annealing temperature as low as 100 °C. The X-ray diffraction (XRD) and Raman spectroscopy indicated that the as-deposited films were composed of nanometer size grains. While the crystal structure remained monoclinic, the size of crystals changed from nanometers to hundreds of nanometers by annealing in 100–600 °C range. It was also found that the films annealed at 500 °C for 5 h had low surface roughness, good adhesion and high optical transmittance whereas films annealed at 600 °C were delaminated. Surface topography was also investigated with atomic force microscopy (AFM). The optical gaps calculated from the transmission spectra were in good agreement with those reported for crystalline WO3 films.