Vanadium Dioxide: Bulk and Thin Films for Device Applications
In the present era, energy consumption is more than the energy that is generated for which renewable as well as non-renewable sources are used. In order overcome this lag, researchers are developing smart materials (thermochromic smart window and metamaterial) where energy generation is enough to deal with the energy demand in order to have a better efficiency. Vanadium dioxide (VO2) is a transition metal oxide which is extensively studied for smart window and metamaterial applications. It undergoes a first-order phase (semiconductor to metal) transition at 68 degC (TSMT). During the transition, a change in resistance as well reflectance (IR region) is observed. Due to the complexity in the synthesis, achieving phase pure VO2 is difficult. Earlier in the group, VO2 powders has been synthesised using Solution Combustion Synthesis (SCS) and thin films using Ultrasonic Nebulised Spray Pyrolysis of Aqueous Combustion Mixture (UNPSACM) and CVD. W and Mo was doped in VO2 where the TSMT was reduced to 25 degC with 2 at. % of W dopant. But upon doping, only 40 % of visible transparency was observed, thereby making VO2 unsuitable for smart window application. Further, the thin films showed the roughness value of 0.2 micron-m and 18 nm with UNSPACM and CVD respectively, making VO2 unsuitable for optical applications. In this study, we report the effect of scandium doping on bulk (by SCS) and thin films (by UNSPACM) on SMT and optical characteristics of VO2 to use it for smart window applications. We observed that Sc doping induced the blue-shift in the optical spectrum without showing any change in the transition temperature (TSMT). Different synthesis techniques like DC Reactive Sputtering and Pulsed Laser Deposition (PLD) were explored for getting optimized condition to achieve phase pure VO2 in single-step and reducing the roughness of vanadium dioxide thin films for smart window and metamaterial application. DC Reactive Sputtering was found to be a two-step synthesis process to achieve phase pure VO2. The roughness was found to be 9.8 nm which was less than that observed earlier in the group. PLD was further explored, where it was found to be one-step synthesis process to get phase pure VO2 and roughness value was reduced to 3.8 nm. Based on the best conditions observed, photodetection measurements were done on thin films which showed promising results with that reported in the literature.