The optical and electrical properties of titanium dioxide
This thesis describes the investigation of shallow and deep trap states in single crystal rutile end commercial pigments. The processes involved when rutile acts as a photocatalyst are examined and their relevance to the pigment industry is discussed. Single crystals have been grown by the plasma flame fusion method. Oxidised (colourless) and reduced (blue) crystals were grown by altering the position of the growth pedestal relative to the plasma. Crystals doped with aluminum, niobium, manganese and iron introduced via the powder feed material were successfully grown as single crystals. The techniques of photoconductivity, photoluminascene, photon stimulated current, thermally stimulated conductivity (TSC) and thexmoluminescene (TL) have been used to probe deep and shallow levels in the single crystal rutile. The kinetic responses of PL and PC have been explained consistently in terms of a charge transfer model by considering the hole traffic through the luminescence centre as well as the electron concentration in the conduction band, and realising that the Cr(^3+) luminescence centre is only one of the many recombination centres present in these crystals, A study of TSC and TL in the nominally 'undoped' single crystals reveals the presence of six discrete electron trapping levels. The TSC and TL curves are analysed and consistent values of the trapping parameters are obtained. Limitations of the simple insulator model and its application to single crystal rutile are discussed. Crystals from different sources all contain the same three trapping levels with ionisation energies of 0.1), 0.24 and 0o37 eV. The density of these traps increase as the crystals are chemically reduced. An investigation of manganese, chromium, iron, cobalt and nickel diffusion doped single crystal rutile reveals the presence of the 0,19 eV trap in chromium doped crystals and the 0.41 eV trap in nickel, chromium, iron and cobalt doped crystals.