Metal oxide thin films for optoelectronic applications

This thesis details the use of aerosol assisted chemical vapour deposition to deposit transparent conducting oxide thin films. Transparent conducting oxides are a special class of materials that exhibit high optical transparency as well as good electrical conductivity, two properties usually in contradiction with each other. The combination of these properties in one material has established an essential role for transparent conducting oxides in a range of applications such as flat screen displays, photovoltaic cells, gas sensors, low-emissive coatings and light emitting diodes. Aerosol assisted chemical vapour deposition is increasingly becoming recognised as a simple, low-cost and reliable technique for depositing thin films. It involves generating an aerosol mist from a solution containing the precursors that is transported with the aid of an inert or reactive carrier gas into the reaction chamber where deposition takes place on a heated substrate. Two of the attractive features of this method are its versatility in allowing the use of precursors that are not suitable for conventional chemical vapour deposition methods as the method depends on solubility rather than volatility and the facility to use multiple precursors simultaneously within a single vessel. The focus of this work is on doping and co-doping of metal oxide thin films, namely ZnO and SnO2, to enhance their optoelectronic properties. The ZnO films were doped with group III elements aluminium or gallium, and the SnO2 films were doped with multivalent elements antimony or tungsten. All four systems were co-doped by introducing fluorine to replace the oxygen ion in the lattice. Fluorine was used as the co-dopant because of its established use in fluorine doped tin(IV) oxide transparent conducting oxides, a commercially available product. Co-doping has received less attention compared with single cation doping largely because of the limitations of other deposition methods. The rationale for co-doping is that it would allow greater tuning of the optoelectronic properties of the transparent conducting oxides to suit specific applications. All films synthesised in this investigation were characterised using a wide range of techniques including X-ray diffraction, energy and/or wavelength dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-visible-near infrared spectroscopy and Hall effect measurements

Enhanced Bactericidal Activity of Silver Thin Films Deposited via Aerosol-Assisted Chemical Vapor Deposition

Silver thin films were deposited on SiO2-barrier-coated float glass, fluorine-doped tin oxide (FTO) glass, Activ glass, and TiO2-coated float glass via AACVD using silver nitrate at 350 °C. The films were annealed at 600 °C and analyzed by X-ray powder diffraction, X-ray photoelectron spectroscopy, UV/vis/near-IR spectroscopy, and scanning electron microscopy. All the films were crystalline, and the silver was present in its elemental form and of nanometer dimension. The antibacterial activity of these samples was tested against Escherichia coli and Staphylococcus aureus in the dark and under UV light (365 nm). All Ag-deposited films reduced the numbers of E. coli by 99.9% within 6 h and the numbers of S. aureus by 99.9% within only 2 h. FTO/Ag reduced bacterial numbers of E. coli to below the detection limit after 60 min and caused a 99.9% reduction of S. aureus within only 15 min of UV irradiation. Activ/Ag reduced the numbers of S. aureus by 66.6% after 60 min and TiO2/Ag killed 99.9% of S. aureus within 60 min of UV exposure. More remarkably, we observed a 99.9% reduction in the numbers of E. coli within 6 h and the numbers of S. aureus within 4 h in the dark using our novel TiO2/Ag system

Highly Conductive Tungsten-Doped Tin(IV) Oxide Transparent Electrodes Delivered by Lattice-Strain Control

Alternatives to tin-doped indium oxide transparent electrodes are needed to meet the growing demand for modern electronic devices. Here, we present a chemical vapor deposition route to tungsten-doped SnO2 thin films with resistivities as low as 5.9 × 10–4 Ω cm and electron mobilities as high as 30 cm2 V–1 s–1. Le Bail fitting of the XRD data showed that the substitutional dopant, tungsten(V) causes minimal distortion to the SnO2 unit cell due to its radius closely matching that of tin(IV). Furthermore, crystallographic preferential orientation in the [200] direction that is thought to facilitate a high mobility was also seen. X-ray photoelectron spectroscopy analysis suggests that W is present in the +5 state, as opposed to +6, therefore minimizing ionized impurity scattering, hence also helping achieve the observed high electron mobilities. The tungsten-doped films had optical band gaps of 3.7 eV, thus enabling transparency to visible light