Mechanism of Phosphorescence Appropriate for the Long-Lasting Phosphors Eu²⁺-Doped SrAl₂O₄ with Codopants Dy³⁺ and B³⁺

Abstract

The existing mechanisms proposed to explain the phosphorescence of SrAl2O4:Eu2+,Dy3+ and related phosphors were found to be inconsistent with a number of important experimental and theoretical observations. We formulated a new mechanism of phosphorescence on the basis of the facts that the d orbitals of Eu2+ are located near the conduction band bottom of SrAl2O4, that the Eu2+ concentration decreases during UV excitation, and that trace amounts of Eu3+ are always present in these phosphors. In our mechanism, some Eu2+ ions are oxidized to Eu3+ under UV, and the released electrons are trapped at the oxygen vacancy levels located in the vicinity of the photogenerated Eu3+ cations. The phosphorescence arises from the recombination of these trapped electrons around the photogenerated Eu3+ sites with emission at 520 nm. The codopant Dy3+ enhances the phosphorescence by increasing the number and the depth of electron traps, and the codopant B3+ enhances the phosphorescence by increasing the depth of electron traps. We also probed the origin of another emission at 450 nm of SrAl2O4:Eu2+ that occurs at low temperatures. Our analysis indicates that this emission is caused by a charge transfer from oxygen to Eu3+ cations and is associated with a hole trapping.

Trapping and detrapping inSrAl2O4:Eu,Dypersistent phosphors: Influence of excitation wavelength and temperature

“…Upon lowering the temperature a second, blue, emission band becomes visible (λ max = 445 nm). Although less well studied as the green emission band, its presence has been reported earlier in several papers [16,17,26]. In the figure, the temperature dependency of both blue and green emission upon excitation with 370 nm is visible.…”

“…Different coordination for both sites is then expected to sufficiently change the crystal field strength and centroid shift to arrive at the observed spectral differences. Nevertheless, several authors discarded this hypothesis because both sites are seemingly too similar to explain the large energy difference when judging on the emission bands [15,16,18]. As an alternative explanation, Poort et al [26] suggested that the two emission bands result from a possible preferential orientation of the d orbitals of Eu 2+ on Sr sites that appear to line up.…”

“…Detailed knowledge of the underlying mechanism of persistent luminescence is lacking, more specifically on the trapping and detrapping processes. First of all, it is still unclear whether the traps are related to host defects, the trivalent codopant, or a combination of both [15][16][17]. Different opinions also exist on the trapping and detrapping routes, regarding the role of the conduction band and whether tunneling processes also play a role [18,19].…”

In Vivo Repeatedly Charging Near‐Infrared‐Emitting Mesoporous SiO2/ZnGa2O4:Cr3+ Persistent Luminescence Nanocomposites

“…[4–6] Until now, persistent luminescence has relied on short-wavelength excitation (e.g., ultraviolet light) which has rather limited tissue-penetration depth. [7–10] To address this problem, a NIR-light-stimulated PL mechanism was proposed in LiGa 5 O 8 :Cr 3+ , to release energy trapped in deeper energy levels of the phosphor, but in this case, the energy must be precharged by UV-light and the photostimulated emission continues to weaken after each cycle of photostimulation and will finally become extinguished. [3,11] Very recently, the PL phosphor, ZnGa 2 O 4 :Cr 3+ (ZGC), was found to be activatable by using tissue-penetrable red light, which means that energy can be recharged and NIR PL imaging is no longer limited by the luminescence-decay life-time of the phosphor.…”

“…To make such NIR-persistent phosphors bulk crystal requires temperatures >750 °C in traditional solid-state annealing reactions. [1,7,13] Moreover, to convert such bulk crystal into nanoparticles that are sufficiently disperse for biological applications, certain tedious physical treatments such as grinding [2,3,14,15] or laser ablation [16] must be utilized. The afforded products are generally highly heterogeneous and suffer from severe agglomeration.…”