Thermochromic VO2 Thin Films: Solution-Based Processing, Improved Optical Properties, and Lowered Phase Transformation Temperature

This paper describes a solution-phase synthesis of high-quality vanadium dioxide thermochromic thin films. The films obtained showed excellent visible transparency and a large change in transmittance at near-infrared (NIR) wavelengths before and after the metal-insulator phase transition (MIPT). For a 59 not thick single-layer VO2 thin film, the integral values of visible transmittance (T-int) for metallic (M) and semiconductive (S) states were 54.1% and 49.1%, respectively, while the NIR switching efficiencies (Delta T) were as high as 50% at 2000 nm. Thinner films can provide much higher transmittance of visible light, but they suffer from an attenuation of the switching efficiency in the near-infrared region. By varying the film thickness, ultrahigh T-int values of 75.2% and 75.7% for the M and S states, respectively, were obtained, while the Delta T at 2000 nm remained high. These results represent the best data for VO2 to date. Thicker films in an optimized ranee can give enhanced NI R switching efficiencies and excellent NI R blocking abilities; in a particularly impressive experiment, one film provided near-zero NIR transmittance in the switched state. The thickness-dependent performance suggests that VO2 will be of great use in the objective-specific applications. The reflectance and emissivity at the wavelength range of 2.5-25 mu m before and after the Nil PT were dependent on the film thickness; large contrasts were observed for relatively thick films. This work also showed that the Nil PT temperature can be reduced simply by selecting the annealing temperature that induces local nonstoichiometry; a MIPT temperature as low as 42.7 degrees C was obtained by annealing the film at 440 degrees C. These properties (the high visible transmittance, the large change in infrared transmittance, and the near room-temperature MIPT) suggest that the current method is a landmark in the development of this interesting material toward applications in energy-saving smart windows.