Abstract
SrAl2O4 that is optically activated by Eu2+, often additionally co-doped with Dy3+, is a non-radioactive persistent phosphor which is known for its excellent afterglow properties. It has found various applications, e.g. in the watch industry, for security signs, in medical diagnostics, and in photovoltaics. The monoclinic SrAl2O4 was synthesized in polycrystalline form and structurally characterized. Its luminescence and afterglow properties were studied. Wavelength-dependent thermoluminescence experiments were performed on SrAl2O4:Eu and SrAl2O4:Eu,Dy polycrystalline samples. Substitution of Sr2+ by Eu2+ on two different Sr sites in the crystal is associated with blue and green Eu2+ emission. Excitation at 445 nm allows to selectively excite one of the two different Eu2+ ions, whereas excitation at 375 nm excites both Eu2+ ions. Incorporation of dysprosium increases significantly (by a factor of about 4 to 8) the total number of traps involved in the afterglow of this persistent phosphor. Increasing the temperature at which the samples are irradiated (loaded) from 173 K to 248 K reveals that many new traps can only be occupied or activated at higher temperatures, leading to a strong increase of the integrated thermoluminescence intensity, in particular for the Dy-codoped samples. The results of this study reveal that the diversity of traps leading to the long afterglow is much larger than previously reported in the literature. We propose that the presence of dysprosium induces an excitation-induced charge-transfer reaction Eu2+ + Dy3+ -> Eu3+ + Dy2+. However, the principal traps responsible for the efficient afterglow are temperature-activated and appear to be associated with the green-emitting Eu2+ ion on the Sr2 site coupled to a nearby dysprosium ion.