New route for modification of thermochromic properties of vanadium dioxide films via high-energy X-ray irradiation

To study the effect of high-energy X-ray irradiation on the thermochromic properties of VO2 films, we prepared VO2 films on fused silica substrates by direct-current reactive magnetron sputtering and exposed them to X-ray irradiation at various doses in the range of 0-9000 Gy. X-ray diffraction analysis showed that an appropriate irradiation dose (1500 Gy) can enhance the crystallization of VO2 films, whereas a higher dose (9000 Gy) would reduce the crystallinity slightly. XPS measurement indicated that the VO2 film irradiated at 1500 Gy had the maximum V4+ content (71.64%) among the film samples. Moreover, atomic force microscopy and scanning electron microscopy measurements suggested that the VO2 film samples treated at 1500 Gy had higher surface roughness than other films, possibly owing to the localized annealing effect at lower doses. The visible and infrared transmittance of the irradiated and pristine samples were characterized and analyzed. The results demonstrated that high-energy X-ray irradiation strongly influenced the room-temperature infrared transmittance of the VO2 films, and the visible transmittance of the films decreased monotonously as the irradiation dose increased owing to the formation of color centers. In addition, the phase transition temperature of the VO2 film samples increased from 48 degrees C for the pristine sample to 53 degrees C for the sample irradiated at 1500 Gy, and then decreased continuously to 43 degrees C for the sample irradiated at 9000 Gy. These findings provided valuable insight into modification of the thermochromic properties of vanadium dioxide films by X-ray irradiation.