Degradation of and energy transfer in oxide-based microscale and nanoscale phosphors doped with rare-earth elements
Ntwaeaborwa, Odireleng Martin
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This study was aimed at investigating degradation of and energy transfer in oxide based phosphors for application in low voltage field emission displays. Cathodoluminescence degradation of commercial Y203:Eu powder, Y203:Eu thin film and sol-gel SiO2:Ce,Tb nanoparticle powder phosphors was studied by Auger electron spectrosco y (AES) and cathodoluminescence (CL) spectroscopy. Energy transfer from ZnO nanoparticles to Eu3+ and Ce3+ was demonstrated by exciting ZnO-Si02:Eu and ZnO-Si 2:Ce powder phosphors with a 325 nm HeCd laser beam. Energy transfer was also emonstrated between Ce3+ - Eu3+ and Ce3+- Tb3+ ion pairs co-doped in SiO2. The Y203:Eu powder, Y203:Eu thin film and sol-gel SiO2:Ce,Tb nanoparticle powder phosphors were irradiated with an electron beam of energy 2 keV in a vacuum chamber maintained at 1x10-6, 1x10-7 or 1x10-8 Torr O2. The effect of surface coating on the surface and CL intensity degradation was also investigated by coating some of the films with a thin layer of Ta2O5. Degradation of CL intensity of Y20 3:Eu thin film and SiO2:Ce,Tb nanoparticle powder phosphors occurred simultaneously with esorption of oxygen from the surface. In the case of Y203:Eu powder phosphors, there was no desorption of atomic species during electron beam irradiation. Possible mechanisms for the degradation of CL intensity and desorption of atomic species are discused. Energy transfer from ZnO nanopartic!es to Eu3+ and Ce3+ occurred when ZnO-SiO2:Eu and ZnO-SiO2:Ce powders were irradiated with a 325 nm HeCd laser beam at room temperature. Embedding of ZnO nanoparticles in SiO2:Eu and SiO2:Ce resulted in complete suppression of green photoluminescence of the ZnO nanoparticles and a subsequent enhancement of red and blue photoluminescence of Eu3+ and Ce3+ respectively. The blue photoluminescence was enhanced slightly more than the red photoluminescence. Possible mechanisms of energy transfer from ZnO nanoparticles to Ce3+ and Eu3+ ions are discussed. Furthermore, it was demonstrated that for different concentrations of Ce3+ and Eu3+ in SiO2:Ce,Eu, blue photoluminescence of Ce3+ was enhanced by an energy transfer from Eu3+. The enhancement was largest when 0.5 mol% Ce3+ was co-doped with 0.5 mol% Eu3+. In the case of different concentrations of Ce3+ and Tb3+ in SiO2:Ce,Tb, green photoluminescence of Tb3+ was enhanced by an energy transfer fro enhancement was largest when 0.5 mol% Ce3+ was co-doped with 1 mol % Tb3+. Possible mechanisms of energy transfer from Eu3+ to Ce3+, and from Ce3 to Tb3+ are discussed.