Search

The main aim of this project was to investigate the synthesis and characterization of bismuth-doped strontium oxide powder and thin films. Firstly the luminescent properties and stability under electron beam irradiation of the SrO:Bi3+ phosphor powder were investigated and secondly the luminescent properties of SrO:Bi3+ thin films prepared by different techniques were studied. The luminescence from Bi3+ ions can be useful in obtaining blue to red emitting phosphors by using different hosts, when excited by ultraviolet (UV) light due to efficient conversion to longer wavelengths. The energy levels of Bi3+ ions are host dependent. Bi3+ is a low cost activator, which provide strong absorption of UV light and can be efficiently converted to longer wavelengths. These emissions are related to the 3P1 – 1S0 or 1P1 – 1S0 transitions of Bi3+ ions, which are strongly dependent on the host. The alkali-earth oxide phosphors offer a potential low-cost alternative to lanthanide-based blue phosphors. Bi3+ doped strontium oxide (SrO:Bi) phosphor powders were synthesized by the sol-gel combustion method using metal nitrates as precursors and citric acid as fuel. A wide range of temperatures (800 - 1200 °C) and concentrations of Bi3+ (0.05 - 0.7 mol%) were used to determine the optimum sample annealing temperature and Bi3+ concentration. The optimum doping concentration, for a fixed annealing temperature of 1200 °C (2 h), was found to be 0.2 mol% and a further increase in the Bi3+concentration resulted in concentration quenching. Samples of this concentration were annealed at various temperatures and the optimum annealing temperature was found to be 1100 °C (2 h). The X-ray diffraction patterns (XRD) corresponded with the well-known face-centered cubic structure of SrO after high-temperature annealing that ranged between 1100 °C up to 1200 °C. Below 1100 °C strontium hydroxide peaks were also present. Williamson-Hall plots showed that the crystallite size was in the range of ~180 nm., South African Research Chairs Initiative (SARChI), African Laser Center (ALC), University of the Free State

Different concentrations of bismuth doped lanthanum oxide (La2-xO3:Bix) phosphor powder (x = 0.001, 0.002, 0.003, 0.004, 0.006, 0.008 and 0.01) were synthesised by means of the sol-gel combustion method at 250 °C using citric acid as the fuel. The product was annealed at different temperatures and the luminescence properties were investigated. The maximum photoluminescence (PL) emission was obtained for the sample which was doped with x = 0.002 and annealed at 1200 °C in air. The X-ray diffraction (XRD) analysis confirmed that all samples crystallized in the La2O3 hexagonal phase. The scanning electron microscopy (SEM) data showed that the grain size increased with increasing annealing temperature and the shape of the grains changed from rectangular to more round, but faceted, after annealing at 1200 °C in air. Energy dispersive X-ray spectroscopy (EDS) confirmed the chemical composition, while diffuse reflectance spectroscopy was used to study the absorption of the La2-xO3:Bix samples. All the samples were absorbing in the ultraviolet range between 220 to 320 nm. The band gap of the La2O3 pure host sample was obtained from the reflectance data as 5.1 eV. Excitation at a wavelength of 308 nm resulted in a single broad blue luminescence emission band centred at 462 nm. The excitation and emission bands were attributed to transitions between the 1S0 ground state and the 3P1 excited state of Bi3+ ions, with a Stokes shift of 1.35 eV. It was found that the samples no longer exhibited PL after storage of several weeks. Further XRD measurements revealed that the La2O3 had changed to La(OH)3. This is consistent with reports that La2O3 can absorb moisture from the air and transform to La(OH)3, which was observed to occur completely in about a week. Unlike for La2O3:Eu and La2O3:Ho phosphors for which the transformation reduced the luminescence, but did not quench it completely, the luminescence of the degraded La2O3:Bi was negligible so that simply the presence of luminesce, African Laser Centre (ALC)