Effect of excitation wavelength(blue vs near UV)and dopant concentrations on afterglow and fast decay of persistent phosphor SrAl2O4∶Eu2+,Dy3+

The persistent phosphor SrAl2O4∶Eu2+,Dy3+is the subject of numerous investigations.One often neglected aspect is that in this phosphor,as well as in Sr4Al14O25∶Eu2t,Dy3+,there are two different Sr2+sites which can be occupied by the dopant Eu2+ions.We first introduce a general scheme of possible energy transfers in these persistent phosphor materials including explicitly both europium ions.This scheme is used as a generic starting point to study experimentally specific pathways.We illustrate this application with the study of the effect of excitation wavelength(444 and 382 nm)on the afterglow of differently doped SrAl2O4∶Eu2+,Dy3+samples,as well as on the emission decay curves.With the same excitation intensity under 444 nm excitation,the resulting afterglow intensity is stronger than under near UV excitation.At 382 nm,Eu2+ions on both Sr2+sites in SrAl2O4 are excited,but at room tem-perature the blue emission is quenched,leading to a loss of photons.The observed effects can further be associated with the ratio of Eu2+ions and trap states which are modulated by the concentrations of Eu2+and Dy3+in SrAl2O4,as well as by temperature.Increasing the nominal Dy3+content from 0.1 mol％to 0.5 mol％with respect to Sr results in the doubling of the integrated afterglow intensity and confirms thus that Dy3+ions are indeed involved in the trapping process.The concentration of trap states is much lower than the concentration of Eu2+ions,as even with low excitation densities,a plateau of integrated afterglow intensity(corresponding to the total number of accessible traps)is reached.We postulate that an important fraction of excited Eu2+ions can potentially transfer their energy to trap states.Once that all traps are filled or in a dynamical filling-depletion process under illumination(with thermal and/or optical depletion processes),for the remaining Eu2+a"normal"steady-state emission is observed.The luminescence decay curves at 520 nm measured at 77 K show a mono-exponential decay with a common lifetime of about 1140 ns for all 5 samples under 437 nm excitation,while under 375 nm excitation,a feed process originating from the energy transfer between Eu2+ions is demonstrated.Under 375 nm excitation,the non-exponential decay observed at 440 nm can be quantitatively associated to a F?rster energy transfer process with R0=1.58(8)nm.For the overall understanding of the afterglow processes,it appears that one has to consider the individual contributions of all active ions on different lattice sites.