Mechanism for the long afterglow in Eu2O3 and Nd2O3 co-doped SrAl2O4 single crystals

Abstract

Persistent luminescence is reported for a series of SrAl₂O₄ single crystals co-doped with 1.0 mol% Eu₂O₃ and (0.5–3.0 mol%) Nd₂O₃. Crystals were successfully grown under an Ar atmosphere in an optical floating zone furnace and consisted of a single phase with a monoclinic structure. The maximum intensity of the photoexcitation spectra (PLE) of all samples peaked at 421 nm. Under excitation at 421 nm, the photoluminescence (PL) spectra showed a broad emission in the visible region centered on 522 nm, which corresponds to the Eu²⁺ 4f⁶5d¹ → 4f⁷ transition, and three additional emissions in the near infrared region, corresponding to the ⁴F_3/2 → ⁴I_9/2 (880/907 nm), ⁴F_3/2 → ⁴I_11/2 (1062/1114 nm) and ⁴F_3/2 → ⁴I_13/2 (1332 nm) transitions from Nd³⁺. Thermoluminescence (TL) spectra showed that traps in the conduction band are shallower than that of the singly Eu-doped sample, and their concentration gradually increased with increasing Nd³⁺ content, reaching a maximum in the crystal prepared with 2.5 mol% Nd₂O₃. The lifetime of the afterglow decay curve also increased with increasing Nd³⁺ content of the crystals, reaching a maximum value of almost 35 seconds with 2.5 mol% Nd₂O₃. A mechanism for the long afterglow is proposed in which Eu²⁺ electrons are first excited, then transported through the conduction band to defect centers which function as traps (produced by Nd³⁺ doping), and the thermal energy at room temperature is sufficient to reverse this process, so that equilibrium is established and a long residual luminescence results.