Role of defect states in persistent luminescence materials
Introduction
The persistent luminescent phosphors introduced in mid 1990s are the most recent commercial application where the luminescence of rare earth (R) ions is employed [1]. These Eu2+ doped alkaline earth aluminates, MAl2O4:Eu2+ (M = Ca and Sr) show intense broad band emission in the blue/green visible range [2], [3]. The old materials based on the Cu and Co doped zinc sulfides, ZnS:Cu, Co, will be replaced because of their low chemical stability, low efficiency as well as being environmentally hazardous due to the radioactive materials (e.g. promethium) added as a continuous excitation source [4]. The persistent luminescent of the Eu2+ doped alkaline earth aluminates was noted already in late 1960s [2], [3] since they were rejected from commercial use, because of this usually disadvantageous property.
The long afterglow shown by the MAl2O4:Eu2+ materials is enhanced by introducing selected R3+ ions as Dy3+ and Nd3+ as co-dopants (e.g. [5], [6]). The luminescence lifetime of the persistent luminescence is very long, in the range of several hours, in contrast to conventional Eu2+ doped materials with lifetimes of several hundred nanoseconds to a few microseconds (at 4 K) [7], [8].
Despite the about one hundred papers published the overall mechanism of persistent luminescence is only schematically known. The persistent luminescence process involves the formation and subsequent thermal bleaching of traps and emission from the Eu2+ ion [9]. For the detailed mechanism(s), several interesting and even exciting ones have been proposed [10], [11], [12]. In the first years the problem seemed to be solved easily: the Eu2+ doped and Nd3+ (or Dy3+) co-doped persistent luminescence was explained by the simple “cross-relaxation” process involving the Eu2+–Eu+ and R3+–R4+ pairs. However, such energies are not available by the excitation by daylight or artificial incandescent lamp or fluorescence tube. Neither the lattice can offer such stabilisation energies since the only real choice for either divalent or trivalent rare earth ions is the divalent alkaline earth site. Finally, the persistent luminescence is not produced only by R3+ co-doping since MAl2O4:Eu2+ are efficient—though less than the co-doped ones—persistent luminescence materials, too. Accordingly, after the first euphoric attempts one should return in the investigations back to the original idea: in most cases the afterglow is due to lattice defects. In this report, the possible lattice defects affecting the luminescence properties of these phosphors are described based on the EPR, Mössbauer, thermoluminescence (TL), and persistent luminescence studies on the CaAl2O4:Eu2+, R3+ materials.
Section snippets
Experimental
The polycrystalline CaAl2O4:Eu2+, R3+ materials were prepared by a solid state reaction between calcium carbonate (CaCO3), aluminium oxide (Al2O3), europium oxide (Eu2O3, 1 mol%), and rare earth oxide (R2O3, 2 mol%) by heating the mixtures at 1250 °C for 6 h in a N2+12% H2 atmosphere. Boron oxide (B2O3, 1 mol%) was used as a flux. The phase and structural purity of the samples were verified by X-ray powder diffraction. Usually no impurities were found and the CaAl2O4:Eu2+ samples were obtained in
EPR investigations
The EPR spectra of the CaAl2O4 powder samples, both with and without (Fig. 1) Eu2+ doping show that even the non-doped, and, more importantly, the non-co-doped material contains lattice defects. As an indication of an electron paramagnetic defect, a signal at ca. 3500 G [14], was found. The signal corresponding to a gyromagnetic g value of ca. 2.0 originates most probably from an electron trapped in an anion vacancy since the vacancy is positively charged with respect to the environment. A
Conclusions
The present results show without doubt the importance of the different—both cation and anion—defects to the persistent luminescence of the CaAl2O4:Eu2+, R3+ materials. Although the identification of the defects is not easy, an electron trap formed by an anion vacancy was evidenced by EPR measurements. Less accurate information of the defects was obtained by the thermoluminescence method. In contrast to the preceding two methods, the Mössbauer spectroscopy is not found very well feasible to the
Acknowledgements
The authors thank the Academy of Finland (project #5066/2000), the European Union and the Marie Curie Fellowship program, and the Graduate School of Materials Research (Turku, Finland) for financial support. The authors are indebted to Prof. W. Strk (Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław) and Prof. J. Legendziewicz (Faculty of Chemistry, University of Wrocław, Wrocław, Poland) for the use of different scientific equipments.
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2023, Journal of Rare EarthsCitation Excerpt :Thus, they were used in the fabrication of various commodities, like luminous indicators and smart fibers.10,11 Long-lasting rare earth doped aluminates (REDA; SrAl2O4:Eu2+,Dy3+) have been particularly attractive due to their high brightness, long-persistent phosphorescence, non-toxicity, non-radioactivity, and recyclability.12,13 Persistent photo-responsive phosphors are typically composed of two major components, including a crystal and an energy trap.