Influence of activator concentration on green-emitting Tb3+-doped materials derived by sol-gel method

In this study, visible-emitting Tb3+-doped silica xerogels and β-PbF2-based glass-ceramic materials with different concentration of optically active dopant have been investigated. After annealing process at 350 °C, the formation of β-PbF2 crystal phase dispersed within silica host of studied samples was examined based on X-ray diffraction measurements. Luminescence characteristics were carried out by monitoring an excitation and emission spectra as well as analyzing luminescence decay kinetics from the 5D4 excited state of Tb3+ ions. Photoluminescence measurements have been performed using three different excitation wavelengths from the UV range: λexc=351 nm, λexc=369 nm and λexc=378 nm, according to the 7F6→5L9, 7F6→5L10 and 7F6→5D3 optical transitions of Tb3+ions, respectively. In result, the well-resolved 5D4→7FJ (J =3-6) bands have been registered. For all prepared samples, a luminescence intensity of the 5D4→7F5 green band located at λem=543 nm is significantly more prominent than that of the 5D4→7F6 blue peak with maximum at λem=488 nm and the green-to-blue (G/B) ratio was calculated and analyzed. It was observed that the G/B-factor values were slightly increased with Tb3+ dopant content; moreover, the G/B values were strongly correlated with the symmetry of local environment around Tb3+ ions and significantly increased after heat-treatment. Additionally, the prolongation of Tb3+ luminescence lifetimes from the 5D4 excited state were observed for β-PbF2 glass-ceramic samples compared to precursor xerogel materials. Generally, obtained results clearly indicated the beneficial influence of heat-treatment process as well as increasing concentration of Tb3+ ions (up to 8.74 mass %) on luminescence properties of studied silica sol-gel phosphors.