Synthesis and characterization of Ce³⁺ doped silica (SiO₂) ) nanophosphors co-doped with Al³⁺ or Mg²⁺ ions
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Date
2009-11
Authors
Koao, Lehlohonolo Fortune
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University of the Free State (Qwaqwa Campus)
Abstract
In recent studies, amorphous silica (SiO2) has been used as a host matrix for rare-earth ions to prepare luminescent materials that can be used in various light emitting devices. Sol-gel glasses have the potential to hold up to ≥10% dopants without losing their amorphous structure. However, before rare earth (RE) - doped sol-gel glasses can be used as luminescent material, several fluorescence quenching mechanisms must be overcome. There are several quenching mechanisms which are present in all materials that are more serious in sol-gel glasses. The first is cross relaxation which involves energy transfer between RE elements; the others are energy transfer through lattice vibrations and to hydroxyl (OH) groups which are present due to the use of water as the solvent during the preparation process. A few studies have demonstrated that the luminescence intensity of rare-earth doped silica can be improved through incorporation of co-dopants such as Al, TiO2, B and by annealing at high temperatures (e.g. > 500ºC). Following their footsteps and in order to make comparisons, we used aluminum as the codopant in some samples to investigate the effects on luminescence yield for various RE concentrations. We also investigated the effects of magnesium co-dopant and high temperature annealing on the luminescence intensity of rare-earth doped silica. In this work, the highest emission intensity was observed for the sample with a composition of 0.5 mol% Ce3+. Cerium doped silica glasses had broad blue emission corresponding to the D3/2- FJ transition at 445 nm but exhibited apparent concentration quenching after higher concentrations of 0.5 mol% Ce3+. Silica containing Mg2+ or Al3+ ions displayed an increase in luminescence intensity as the Mg2+ or Al3+ to Ce3+ ratio increases for the range investigated but significant luminescence enhancement was observed for Mg2+:Ce ratio greater than 20, while that of Al3+ co-doping had the highest luminescent intensity when the ratio of Al:Ce is 10:1. This enhanced photoluminescence was assigned to an energy transfer from the Mg nanoparticles, to result in enhanced emission from Ce3+. The Al3+ or Mg2+ ions disperses the Ce3+ clusters, enhancing 2F5/2 and 2F7/2 emissions due to increased ion-ion distances and decreased cross-relation.
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Keywords
Nanoparticles, Gelation, Silica gel, Silica, Luminescence, Dissertation (M.Sc. (Physics))--University of the Free State (Qwaqwa Campus), 2009