Investigation of europium concentration dependence on the luminescent properties of borogermanate glasses

Abstract

In order to elucidate the effect of europium content on the optical and luminescent properties of borogermanate glasses, a series of Eu3+ doped 30B2O3-40GeO2-(30-x)Gd2O3 glasses with various doping levels (x=1–9 mol%) have been synthesized and studied with transmission, absorption, photoluminescence and decay time measurements. The transmission spectra proved that the title glasses maintained a high transparency about 80% in the 440 to 900 nm region. Based on the absorption spectra, the optical band gaps obtained from Tauc's plot can be narrowed by increasing content of Eu3+. From the photoluminescence spectra, the strongest red emission has been observed from the 5D0→7F2 level of Eu3+ ions in borogermanate glasses. The strongest emission and excitation intensities of Eu3+ ions are at the doping level of x=7 mol% and then these intensities decrease due to concentration quenching. The red to orange ratio (R/O) of 5D0→7F2 to 5D0→7F1 transitions has been investigated to predict the local environment of Eu3+ ions. Judd-Ofelt (J-O) analyses have been performed from the emission spectra. The values of R/O and Ω2 present an increase with increasing doping level, indicating the lower symmetric environment for Eu3+ ions and higher covalency for Eu-O bond. The emission efficiency calculated from J-O theory is 75% at x=2 mol%. The decay time curves of 6P7/2→8S7/2 transition of Gd3+ ions and 5D0–7F2 transition of Eu3+ ions confirm the energy transfer from Gd3+ to Eu3+ ions.