Effect of annealing treatment on optical properties and microstructural variation of WO3/Ag/WO3 multilayer nano-films

Structural and optical properties of WO3/Ag/WO3 nano-multilayer compositeswere investigated for heat mirror applications. WO3/Ag/WO3 thin films were fabricated through a physical vapour deposition method by using electron-beam evaporation at the vacuum chamber at 10(-5) Torr. WO3 nano-layer was fabricated at 40 nm. Annealing treatment was carried out at 100, 200, 300 and 400 degrees C for 1 h after the deposition of first layer of WO3 on the glass. On WO3 film, Ag nano-layers with 10, 12 or 14 nm thickness were deposited. Individual layers morphology was investigated using atomic force microscopy (AFM) and deduced that a smoother layer can be achieved after the annealing at 300 degrees C. Ellipsometry analysis was executed to determine both layers, Ag film thickness and inter-diffusion between the WO3-Ag-WO3 layers. It was inferred that there was almost no interfering among the WO3-WO3 layers in the samples with 12 and 14 nm Ag thickness; while silver was deposited on the annealed WO3 layer at 300 degrees C. UV-visible spectrophotometer showed that the annealing treatment of the first WO3 layer enhanced the transparency of films in the visible region. The innovations of the present study have been based on the annealing of the films and finding an optimum thickness for the Ag film at 12-14 nm. Heat mirrors efficiency was assessed according to the principle of their optical behaviour and optimum performance obtained for 14 nm of Ag film, deposited on annealed tungsten oxide at 300 degrees C.