Hole Detrapping-Type Persistent Phosphors of RE₂O₂S (RE = La, Gd, Y, Lu) Doped with Eu³⁺-Pr³⁺ and Eu³⁺-Tb³⁺

Abstract

RE₂O₂S:Eu³⁺-Ln³⁺ (RE = La, Gd, Y, Lu; Ln = Pr, Tb) samples were prepared by a solid-state reaction method to develop new red persistent phosphors and to demonstrate the hole detrapping mechanism. All Eu³⁺-singly doped RE₂O₂S show very weak thermoluminescence (TL) glow peaks, while by codoping Pr³⁺ or Tb³⁺ ions, additional strong TL peaks were observed. In the TL spectra and persistent luminescence (PersL) spectra, only Eu³⁺ luminescence lines were observed, but there was no Pr³⁺ and Tb³⁺ luminescence. From the PersL excitation spectra, it is found that PersL is caused after excitation to the charge transfer state of Eu²⁺-S^- in which the hole is in the valence band. These results show that Eu³⁺ acts as a recombination center and Pr³⁺ and Tb³⁺ ions act as hole trap centers. The deeper hole trap depth of Pr³⁺ than that of Tb³⁺ and the RE dependence of hole trap depth are explained using a vacuum referred binding energy diagram considering the nephelauxetic effect. La₂O₂S:Eu³⁺-Pr³⁺ was the best composition among the samples as a persistent phosphor at ambient temperature, showing strong red persistent luminescence in a short time range (>100 mcd/m² for a few seconds).