Luminescence characteristics of Er3+ ions in ZnO-Ta2O5/Nb2O5/ZrO2-B2O3 glass system-A case study of energy transfer from ZnO to Er3+ ions

Zinc borate glasses doped with 2.0 mol% of Er2O3 and mixed with thermally resistant transition metal oxides viz., Ta2O5, Nb2O5 and ZrO2 were synthesized. Optical absorption and photoluminescence spectra and decay profiles of these glasses were recorded at ambient temperature. The spectra were characterized using Judd-Ofelt theory and various radiative parameters viz., transition probability, A, branching ratio, beta, and the radiative lifetime, tau, of principal emission transitions of Er3+ ions in these glasses were evaluated. The results indicated a significant enhancement in the intensity of green emission (related to S-4(3/2) -> I-4(15/2) transition) of Er3+ ions due to co-doping of the glass system with Ta2O5, Nb2O5 and ZrO2. Quantitative analysis of the results pointed out that ZrO2 is the most favorable oxide among the three co-dopants for achieving the highest quantum efficiency of green emission. Results were further analyzed based on the structural modifications taking place in the vicinity of Er3+ ions due to co-doping with Ta2O5, Nb2O5 and ZrO2 and the reasons for such improvement were identified by means of IR and Raman spectroscopy studies. Additionally, the S-4(3/2) energy level of Er3+ ions (from which the green emission is originated) seemed to be additionally populated by the energy transfer from the excited state of ZnO and contributed to the amplification of green emission.