Insights into microstructural features governing Ce3+ luminescence efficiency in sol-gel silica glasses

The radioluminescence (RL) properties of Ce-doped silica glasses prepared by sol-gel method were investigated as a function of the dopant amount in a wide range of concentrations (Ce:Si molar ratios from 1 x 10-5 to 5 x 10-2). The effects of xerogel densification temperature and of a postdensification thermal treatment were also considered. In order to understand the microstructural features governing Ce3+ luminescence efficiency, optical absorption measurements in the ultraviolet, visible, and infrared regions and time-resolved photoluminescence experiments were performed. The complex dependence of RL intensity upon rare-earth (RE) concentration and thermal treatment was attributed to the role of OH vibrations as nonradiative recombination channels, as well as to the formation of RE aggregates in the silica matrix. Specifically, the segregation of CeO2 nanoparticles, in which Ce4+ does not supply any radiative emission, has been revealed by infrared absorption measurements, in agreement with previous Raman, transmission electron microscopy, and X-ray diffraction data. Moreover, an optical absorption band, centered at 2.4 eV and whose intensity increases with the square of cerium concentration, was observed and tentatively assigned to an intervalence electron-transfer transition involving Ce3+-Ce4+ dimers. Postdensification thermal treatments markedly reduce the intensity of this band and increase RL intensity. The relationship between the 2.4 eV band and the RL properties will be outlined. In particular, the increase in RL intensity will be discussed, mainly as a consequence of microscopic modifications leading to an improvement of the charge-transfer efficiency toward emitting centers.