Pulsed laser deposited nanocrystalline and iso-epitaxial tungsten trioxide thin films for electrochromic and gas sensing applications

Tungsten trioxide (WO3), in thin film configuration, has received much attention owing to its notifying structural, optical and electrical properties and exceptional industrial applications in electrochromic and gas sensing devices. In the present study, WO3 thin films are deposited by Pulsed Laser Deposition Technique on ITO coated glass and polished single crystal (100) SrTiO3 substrates under different deposition conditions. The compositional, structural, morphological, electrical and optical properties are studied systematically. The WO3 thin films deposited at a substrate temperature of 673 K in an oxygen partial pressure of 100 mTorr were found to be nanocrystalline with an average grain size of 58 nm. These films exhibited a high degree of optical transmission (>80%) above the fundamental absorption edge with an optical band gap of 3.31 eV. The cyclic voltammogram of these nanocrystalline thin films exhibited a clear one step electrochromism, transparent to blue on oxidation and blue to transparent on reduction with good cycling stability. The coloration efficiency of these films was found to be 125 cm2/C. The WO3 thin films deposited on STO substrate at a temperature of 873 K and in an oxygen partial pressure of 150 mTorr were found to have (001) plane epitaxy between WO3 films and SrTiO3 films suggesting that the films are crystallographically aligned with the substrate planes. The AFM data demonstrated the iso-epitaxial columnar growth of the films. The films exhibited a maximum sensitivity of 15 towards 10 ppm NO2 at an operating temperature of 673 K and a maximum sensitivity of 2.0 towards 100 ppm NH3 gas at an operating temperature of 523 K respectively. These films responded reversibly to NO2 and NH3 pollutant gases. (author)