Investigation of the Emission Mechanism in Milled SrAl2O4:Eu, Dy Using Optical and Synchrotron X-ray Spectroscopy

There currently exists much debate as to the active state related to the "long afterglow" effect in europium doped oxide materials. Redox couples that consist of Eu+/2+ and Eu2+/3+ are discussed, but no common answer is currently accepted. Here, we present a comparison of the optical properties of a commercially available SrAl2O4:Eu, Dy phosphor, as a function of nanoparticle size reduction via dry mechanical milling. X-ray and optical spectroscopic data indicate a significant decrease in phosphorescence efficiency and an increase in laser stimulated emission efficiency as near surface Eu2+ ions are oxidized to Eu3+ as a consequence of increased exposure during the milling process. These results show evidence only for Eu2+/3+ oxidation states, suggesting the mechanism related to long afterglow effect does not arise from Eu+ species. We also suggest that size reduction, as a rule, cannot be improve optical properties of nanostructures.