Abstract
During the past few decades, the research on persistent luminescent materials has focused mainly on Eu2+-doped compounds. However, the yearly number of publications on non-Eu2+-based materials has also increased steadily. By now, the number of known persistent phosphors has increased to over 200, of which over 80% are not based on Eu2+, but rather, on intrinsic host defects, transition metals (manganese, chromium, copper, etc.) or trivalent rare earths (cerium, terbium, dysprosium, etc.). In this review, we present an overview of these non-Eu2+-based persistent luminescent materials and their afterglow properties. We also take a closer look at some remaining challenges, such as the excitability with visible light and the possibility of energy transfer between multiple luminescent centers. Finally, we summarize the necessary elements for a complete description of a persistent luminescent material, in order to allow a more objective comparison of these phosphors.
Highlights
In most luminescent materials, the decay of the light emission lasts no longer than a few milliseconds after the end of the excitation
Since the discovery of SrAl2O4:Eu2+, Dy3+ in 1996 [4], many researchers and publications on persistent luminescent materials have focused on divalent europium as the activating ion
A TL-excitation mapping (Figure 6)—where the TL experiment is repeated for various excitation wavelengths—is especially useful, since it directly provides information on the trap filling probability of different wavelengths [258]; i.e., it shows which excitation wavelengths are suitable for inducing persistent luminescence
Summary
The decay of the light emission lasts no longer than a few milliseconds after the end of the excitation. CaS:Eu [7–9] and Ca2Si5N8:Eu [10,11], the choice is limited and the host lattices are chemically unstable or difficult to prepare This is especially unfortunate, since red afterglow phosphors are strongly desired for several applications, such as safety signage, paints and, more recently, as tracer particles for in vivo medical imaging [3,12–14]. Such a white emission is very difficult to obtain with only Eu2+ doping These ions often require a short (UV) excitation wavelength, making it impossible to charge these persistent phosphors using visible light. Until 1996, the majority of persistent luminescent applications was based on ZnS doped with copper and cobalt [4,16] This material emits a greenish broad-band spectrum centered around 540 nm (Figure 2), which remains visible for several hours after the end of the excitation. The most important ones (with the largest number of publications) to mention at this point are CaTiO3:Pr3+ (red), Y2O2S:Eu3+, Ti4+, Mg2+ (red) and CaS:Bi3+ (blue)
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