Towards high quality V2O5 and VO2 thin films for gas sensing, smart windows and optoelectronic applications
The global demand for improved living standards coupled with rising pollutant or hazardous gases levels, safety and security demands have led to increased demand for miniaturized gas sensors and photodetectors. Production of conventional commercial photodetectors like InGaAs and Si usually involves expensive high vacuum techniques like MBE requiring special substrates like sapphire for their fabrication. Thus, there is urgent need to develop low-cost highly sensitive gas sensors and photodetectors to meet various applications. The choice of a material for those applications is crucial. Besides stability, cost-effectiveness and portability, a good sensor should exhibit high sensitivity and selectivity with fast response and recovery times at low limit of analyte detection. Vanadium oxides, particularly V2O5 and VO2 are potential candidates in that regard due to their unique properties. The layered structural nature of V2O5, its large optical band gap, thermal and chemical stability, excellent electrochromic and thermochromic properties makes it favorable for gas sensing. VO2 on the other hand has great potential for use in optoelectronic, switching and energy storage devices due to its ability to undergo reversible first order phase transition at a transition temperature (Tc) of 68 oC, accompanied by drastic change in its infrared reflectance and a change in resistance of 3-5 orders of magnitude. In this study, the synthesis of V2O5 and VO2 thin films by low-cost scalable techniques; Ultrasonic Nebulised Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM), DC Reactive Sputtering and Chemical Vapor Deposition is reported. Phase identification and confirmation was done by XRD and Raman techniques. In-house built gas sensing system was fabricated to study the humidity and gas sensing properties of V2O5 films. Films deposited at 350 oC showed high sensitivity (90.8 % at 76 % RH) towards humidity at room temperature. The films exhibited high sensitivity and selectivity towards ethanol when subjected to various volatile organic compounds (VOCs) at 280 oC. Strategies of reducing Tc of VO2 while maintaining its transition strength for smart windows applications were explored through Ce-W co-doping. Ce helped to hold the transition strength as W reduced the Tc. The effect of dopants, substrates and deposition techniques on SMT of VO2 films are discussed. The potential of VO2 films as low-cost IR photodetector was demonstrated. The VO2 thin films exhibited promising IR photoresponse properties when subjected to 1064/1550 nm laser.