Structural and optical properties of tungsten-doped vanadium dioxide films

Thin films of tungsten (W)-doped thermochromic vanadium dioxide (VO₂) were deposited onto the soda-lime glass and fused silica by radio frequency magnetron sputtering. The doped VO₂ films were characterized by X-ray diffraction, optical transmittance measurement, and near field optical microscopy with Raman. X-ray diffraction patterns show that the (011) peak of W-doped thermochromic VO₂ film shifts to a lower diffraction angle with the increase of W concentration. The optical measurements indicated that the transmittance change (ΔT) at wavelength of 2500 nm drops from 65% (ΔT at 35 ℃ and 80 ℃ for undoped VO₂ film) to 38% (ΔT at 30 ℃ and 42 ℃ for the doped VO₂ film). At the same time, phase transition temperature drops from 65 ℃ to room temperature or lower with the increase of W concentration. Near field optical microscopy image shows that the surface of W-doped VO₂ film is smooth. Raman results show that the main Raman modes of W-doped VO₂ are centered at 614 cm^-1, the same as that of undoped VO₂, suggesting no Raman mode changes for lightly W-doped VO₂ at room temperature, due to no phase transition appearing under this condition.