Persistent Luminescence in Eu2+-Doped Compounds: A Review

Abstract

In 1996, Matsuzawa et al. reported on the extremely long-lasting afterglow of SrAl2O4:Eu2+ codoped with Dy3+ ions, which was more than 10-times brighter than the previously widely used ZnS:Cu,Co. Since then, research for stable and efficient persistent phosphors has continuously gained popularity. However, even today - almost 15 years after the discovery of SrAl2O4:Eu2+, Dy3+ - the number of persistent luminescent materials is still relatively low. Furthermore, the mechanism behind this phenomenon is still unclear. Although most authors agree on the general features, such as the existence of long-lived trap levels, many details are still shrouded in mystery. In this review, we present an overview of the important classes of known persistent luminescent materials based on Eu2+-emission and how they were prepared, and we take a closer look at the models and mechanisms that have been suggested to explain bright afterglow in various compounds.

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

“…Although many new persistent phosphors have been reported since then [1,4], rare earth doped strontium aluminates are still by far the most studied and commercialized compounds. SrAl 2 O 4 :Eu,Dy is widely used in various glow-in-the-dark applications, ranging from emergency signage and watch dials to toys.…”

“…The discovery of europium and dysprosium codoped strontium aluminate (SrAl 2 O 4 :Eu,Dy) in 1994 marked the beginning of the second generation of persistent or afterglow phosphors, also called glow-in-the-dark materials [1][2][3]. Although many new persistent phosphors have been reported since then [1,4], rare earth doped strontium aluminates are still by far the most studied and commercialized compounds.…”

Revealing trap depth distributions in persistent phosphors

“…Although most of the general principles behind the afterglow mechanism are now understood, many of the details remain the subject of discussion. 7 A crucial role is played by energy levels in the band gap of the host material, introduced by defects in the crystal lattice (e.g., vacancies or codopants). These so-called "traps" are able to capture charge carriers originating from the luminescent centers (electrons in most cases, 8 although hole trapping has been suggested by some authors and for some materials 7 ).…”

“…7 A crucial role is played by energy levels in the band gap of the host material, introduced by defects in the crystal lattice (e.g., vacancies or codopants). These so-called "traps" are able to capture charge carriers originating from the luminescent centers (electrons in most cases, 8 although hole trapping has been suggested by some authors and for some materials 7 ). These charge carriers remain trapped until enough thermal energy is available to help them escape and recombine at a luminescent center.…”

Mechanoluminescence in BaSi2O2N2:Eu

“…Persistent luminescent materials are phosphors which are able to emit light for a long time after being excited [14]. This remarkable 'afterglow' of persistent luminescence originates in energy storage in the phosphor by trapping of charge carriers in long-lived energy levels inside the band gap.…”