Waugh, M.R.; (2011) The synthesis, characterisation and application of transparent conducting thin films. Doctoral thesis , UCL (University College London).

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Abstract

Transparent conducting thin films of metal oxides, doped metal oxides, and carbon nanotubes (CNTs), have been produced using various deposition techniques, including: Aerosol Assisted Chemical Vapour Deposition (AACVD), Atmospheric Pressure Chemical Vapour Deposition (APCVD), and Spray Coating. The resultant thin films were tested for their performance in a number of applications, including: Low emissivity (‘Low-E’) glazing, photovoltaic electrode materials, gas sensing and photocatalysis. AACVD was shown as a viable, and attractive, deposition technique for the synthesis of tin oxide, and doped tin oxide thin films, which allows for controllable doping levels, crystallinity, and surface structure. The tailoring of these physical attributes allows for enhanced performance of the functional properties of the films, whereby, a lower growth temperature produced highly transparent, highly conductive coatings with a low haze value for ‘Low-E’ applications, whereas, higher growth temperatures produced the high electrical conductivity, transparency, and light scattering properties required for high performance electrodes in thin film photovoltaics. In addition, a dual coating methodology was developed using both AACVD, and APCVD, to grow tin oxide thin films in a rapid timescale, but with modified surface structures showing changes to the short range waviness, kurtosis, and the surface area. Growth of carbon nanotubes, using CVD, was investigated over a range of metal catalysts, with varying Pauling electronegativity values, and over a range of temperature, methane, and hydrogen conditions. A growth mechanism has been postulated, whereby, the electronegativity of the metal catalyst, and the solubility and diffusion of the carbon through that catalyst, affects the type and properties of the carbon structure produced. To the authors knowledge, this is the first reported growth of MWCNTs using a chromium solo-metal catalyst, and the first reported growth of the unique ‘carbon nanofibres’ which were produced using gold and silver metal catalysts. Functionalisation of SWCNTs using a microwave reflux process was shown to yield sulphonate and sulphone modified nanotubes, which are highly soluble in water and able to undergo spray coating to produce carbon nanotube, nanonet transparent conducting thin films. The functionalisation process was shown to be reversible upon heating of the modified nanotubes. AACVD has been deemed unable to produced doped zinc oxide transparent conducting films. However, undoped zinc oxide films were produced. They displayed a high photocatalytic action in the degredation of stearic acid, and a UV light induced superhydrophilicity. The modification and deposition techniques, established throughout this work, were utilised to form transparent, hybrid, metal oxide-CNT coatings, for gas sensing. The hybrid materials displayed enhanced response times to combustible target gases, which has been attributed to the catalytic effects of the exposed carbon nanotube surfaces; and to the spillover of adsorbed oxygen from the active nanotubes to the metal oxide surface.

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