Pulsed laser ablated WO3 nanocrystalline and iso-epitaxial thin films for electrochromic and gas sensing applications

Tungsten trioxide thin films were deposited by Pulsed Laser Deposition Technique under different substrate temperatures and oxygen partial pressures. The substrates were maintained in temperature range 303-923 K and the oxygen partial pressure was maintained in the range 10-300 mTorr. The properties and performance of devices based on WO3 thin films are generally to the composition, structure and morphology of the films which in turn dependent on the deposition conditions and process parameters used during the growth of the films. Hence WO3 films were deposited on ITO coated glass, polished single crystal (100) SrTiO3 substrates under various deposition conditions for their use in electrochromic and gas sensing applications. A systematic characterization has been carried out to study their compositional, structural, morphological, electrical and optical properties. The WO3 thin found to be nanocrystalline at a substrate temperature of 673 K in an oxygen partial pressure of 100 mTorr with an average grain of 58 nm. In the AFM pictures of these films a very porous and granular morphology was observed with an average root mean square roughness of 10 nm. Also the individual grains are clearly visible and were in good contact with each other showing no tint or no color variation with viewing angle. These films exhibited a high degree transmission of >80% above the fundamental absorption edge with a band gap of 3.31 eV. The electrochromic properties of these films were studied by dry lithiation and electrochromic cell. Dry lithiation studies of these films an effective optical modulation in the near infrared region than in the visible region of the spectrum upon maximum lithiation of 20 nm. The electrochemical measurements revealed a optical modulation with charge injection of 12.5 mC/cm2. The coloration efficiency of these films using these two methods are nearly equal. The WO3 thin films deposited on STO at a substrate 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 electrical conduction was a continuous variation with temperature. The activation energy of these films was 0.78 eV. These films were used for NO2 and NH3 gas sensing applications. These films showed 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. (author)