Ashraf, S.; (2006) Aerosol assisted chemical vapour deposition of tungsten and molybdenum oxide thin films. Doctoral thesis , University of London.

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Abstract

Tungsten oxide and molybdenum oxide thin films have been deposited by aerosol assisted chemical vapor deposition (AACVD) using polyoxometalates, large ionic clusters which would be unsuitable for use in conventional atmospheric pressure chemical vapor deposition due to their low volatility. AACVD reactions of polyoxotungstates resulted in the formation of either fully oxidized yellow or partially reduced blue tungsten oxide films. Raman spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis indicated that there is a correlation between the stoichiometry of the films and the randomisation of the crystallites blue films have a partially reduced W(XX stoichiometry and show preferred orientation along the direction, whereas yellow films are fully oxidized WO3 and composed of randomly orientated crystallites. Depositions using polyoxotungstates containing heteroatoms, such as niobium, tantalum and titanium afforded doped tungsten oxide films in which the W:heteroatom ratio of the precursor was retained. This was also observed for films deposited using mixtures comprising of a doped polyoxometalate in conjunction with an undoped polyoxometalate demonstrating that polyoxometalates provide a route to the deposition of films with a highly controlled level of doping. Polyoxometalates have also been used as precursors to molybdenum oxide films. The resulting films were comprised of either M0O3, M0O2 or a mixture of the two oxides, with the M0O2 phase predominating with increasing temperature and distance from the precursor inlet. Results are also presented on the AACVD reactions of tungsten hexacarbonyl, W(CO)6j in a variety of organic solvents. The tungsten oxide films function as gas sensitive resistors for detecting traces of ethanol and nitrogen dioxide in air, showing responses exceeding those of commercially available screen printed sensors, with a faster speed of response and lower optimum temperature for nitrogen dioxide detection.

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