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 Eu(2+/3+) are discussed, but no common answer is currently accepted. Here, we present a comparison of the optical properties of a commercially available SrAl(2)O(4):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 Eu(2+) ions are oxidized to Eu(3+) as a consequence of increased exposure during the milling process. These results show evidence only for Eu(2+/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 universally applied to improve optical properties of nanostructures.