Luminescence investigation of trivalent rare earth ions in sol-gel derived SiO₂ and ZnO co-doped SiO₂:Pr³⁺
Mhlongo, Gugu Hlengiwe
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Tb3+-Pr3+ , Ce3+-Pr3+ , and Eu3+-Pr3+ ion pairs co-doped in SiO2 were successfully synthesized using a sol gel method to produce rare earth activated oxide nanophosphors. Green emitting ZnO nanoparticles were also successfully embedded into single doped Pr3+ in SiO2 matrix resulting in a red emitting ZnO.SiO2:Pr3+ nanocomposite. The phosphor powders were produced by drying the precursor gels at room temperature followed by annealing at 600 oC in ambient air. Based on the X-ray diffraction results, it was found that the SiO2 was amorphous regardless of the incorporation of Pr3+, Ce3+, Tb3+, Eu3+ ions and nanocrystalline ZnO or annealing at 600 oC. The particle morphology of powder phosphors was observed from field emission scanning electron microscopy and high resolution transmission electron microscope images. The field emission scanning electron microscopy revealed that the particles of the synthesized phosphors were mostly spherical and agglomerated. In addition, the morphology and distribution of SiO2 nanoparticles were not influenced by the presence of different rare affected by the presence of rare-earth ions in the matrix. The high resolution transmission electron microscope on the other hand confirmed the homogenous dispersion of the rare-earth ions incorporated in the amorphous SiO2 matrix. The presence of these ions in SiO2 host was confirmed by the energy dispersive X-ray spectroscopy. The energy transfer from ZnO to Pr3+ which was evidenced by quenching of green emission from ZnO resulting in an enhanced red emission from Pr3+ under both low electron beam and vacuum ultra violet excitation was demonstrated. For Pr3+-Ce3+ ion pair, the red emission form Pr3+ was slowly quenched while that from Ce3+ was slightly enhanced with increasing Ce3+ concentration. Such results indicate the energy transfer from Pr3+ to Ce3+. In the case of SiO2:Tb3+/Eu3+ co-doped with Pr3+, the cathodoluminescence and photoluminescence intensities of Pr3+-Tb3+ and Pr3+-Tb3+ were strongly quenched with Pr3+ co-doping. We also investigated the effect of beam voltage and current on the cathodoluminescence intensity from the powder phosphors as well as their cathodoluminescence intensity degradation under prolonged electron bombardment in the cathodoluminescence spectroscopy.