Structural, morphological, and optical properties of W‐doped VO 2 thin films prepared by sol‐gel spin coating method

Thermochromic VO 2 thin films were deposited on soda‐lime glass via sol‐gel method. Doping was done through adding tungstic acid solution to the vanadium solution precursor. Grazing incidence x‐ray diffractometer ( GIXRD ) results showed that VO 2 and V 6 O 13 phases were formed together in the heat‐treated sample. According to the GIXRD result of the W‐doped sample, only VO 2 remained. Field‐emission scanning electron microscopy ( FESEM ) micrographs showed that the VO 2 grain size decreased from about 70 to about 25 nm for undoped film and 2 wt% W‐doped films, respectively. Atomic force microscopy ( AFM ) results showed that the root mean square roughness for the film with 180 nm thickness was about 18 nm, and 2 wt% W‐doped film had a smoother surface. Diffuse reflectance spectroscopy ( DRS ) results showed that the band gap energy for undoped, 1 wt% W‐ doped, and 2 wt% W‐doped VO 2 thin films was 1.7, 1.3, and 0 eV, respectively. Four‐point probe resistivity measurements showed a significant decrement, from approximately 1 MΩ at 15°C to <100 Ω at 80°C. Regarding Vis‐ NIR spectroscopy results, maximum optical transmission for undoped and W‐doped films was approximately 75% and 35%, respectively.