|Title:||Luminescence of phosphors based on novel oxides for field emission display|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Applied Physics|
|Pages:||x, 90, xii leaves : ill. (some col.) ; 31 cm.|
|Abstract:||This thesis presents studies on the synthesis, structural and optical characterization of rare-earth doped tungstates and binary oxide phosphors. Photoluminescence (PL) and low-voltage cathodoluminescence (CL) are used to investigate these promising phosphors for use in field emission display. The imperfection performance of sulfide-base phosphors has motivated us to investigate the more chemically and physically stable oxide-based phosphors. In our work, a variety of properties of the phosphors were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectra, low-voltage CL, PL spectra and lifetime. In our work, the luminescent properties of BaWO₄-based and ZnWO₄-based phosphors were investigated. Rare-earth ions (Eu³⁺ , Tb³⁺ , Tm³⁺) doped BaWO₄phosphors were prepared by a polyol-mediated method. Spherical morphology of the scheelite-structured particles was observed. While excited by ultra-violet (UV) light or low-voltage electron beam, red, green and blue emissions were observed in Eu³⁺, Tb³⁺ and Tm³⁺ doped phosphors, respectively. These emissions are attributed to the characteristic intraconfigurational 4f transitions in Eu³⁺, Tb³⁺ and Tm³⁺. Among them, the luminance of BaWO₄: Tb³⁺ was as high as 4866 cd/m² under excitation of electron beam (4.5 kV). The investigation on the chromaticity of BaWO₄: Tm³⁺ indicated that it had extremely high color purity (97%) and showed its potential as a promising blue-emitting phosphor. For the Eu³⁺ doped BaWO₄, very high intensity ratio of ⁵D₀ → ⁷F₂and ⁵Do → ⁷F₁transitions (R-value) indicates that Eu³⁺ ions may locate at a crystallographic site without inversion symmetry. Wolframite nanostructural ZnWO₄and ZnWO₄: Eu³⁺ were synthesized by refluxing in a water solution and followed by annealing at different temperatures. Different colors of emission were observed from the ZnWO₄- based phosphors. Blue-green CL emission of the annealed ZnWO₄is attributed to the highly emissive WO⁶⁻₆ ion complex. As-synthesized and annealed ZnWO₄: Eu³⁺ phosphors exhibit red and white emissions, respectively. The dynamic of emissions between WO⁶⁻₆ ion complex and Eu³⁺ ions with increasing annealing temperature were discussed. Luminescence of binary oxides was also studied in this work. A urea precipitation method was used to synthesize nanostructural ZrO₂: RE³⁺ (RE=Eu, Tb, Tm) phosphors. The characteristic red-green-blue (RGB) emissions of the rare earth ions were observed in the PL spectrum. Comparing to BaWO₄: Eu³⁺, ZrO₂: Eu³⁺ phosphor has a relatively small R-value, which indicates that Eu³⁺ ions may be distorted slightly and occupy a site without inversion symmetry. Similar result was observed in the CL spectrum. Regarding the Tm³⁺ doped ZrO₂, both emissions from defects of ZrO₂and Tm³⁺ ions was observed; however, such emission was observed in CL. Intrinsic emission in ZnO was also studied via PL and CL. Different morphologies of ZnO particles were synthesized by a hydrothermal method. Defect-related emission of the ZnO particles as a function of crystal size and morphology was discussed.|
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