1. Introduction

Persistent luminescence, i.e., light emission persisting for a long time after the excitation has ended, is a phenomenon that has intrigued people for hundreds of years. One of the first written accounts of its observation in the western world dates back to the beginning of the 17th century when the so-called Bologna stone was described [1]. This material, probably BaS containing some natural impurities such as copper, was synthesized accidently in an effort to make gold out of barium sulfate. Many other persistent luminescent materials were discovered later, but they remained somewhat mysterious substances, as nobody really knew how they could “store” light for such long time. Research activity was limited, and the materials found only few applications.

The situation drastically changed about 15 years ago when Matsuzawa et al. discovered bright and long-lasting luminescence in SrAl₂O₄:Eu,Dy [2]. This has led to a renewed research interest, and it promoted the use of these green-emitting persistent phosphors in signalization, glow-in-the-dark toys, and some niche applications. In parallel with experimental materials research, the first models of persistent luminescence were developed in an attempt to describe the underlying processes that govern the physical phenomena.

Strangely enough, until recently, neither books nor review papers on persistent luminescence were available. A first review paper on (Eu²⁺-based) persistent luminescence was published by Van den Eeckhout, Smet, and Poelman in 2010 [3]. Also, a conference on persistent luminescent materials was lacking, and the Ghent University group therefore decided to organize the first international workshop on persistent phosphors. The conference was called “Phosphoros 2011,” and was held in Ghent, Belgium, 19–20 September 2011 [4].

The terms “persistent luminescence” and “persistent phosphor” are somewhat arbitrary. Many different names are given to the process [1] such as “long-lasting,” “persistent,” “afterglow,” “phosphorescence,” “LLP” (long-lasting phosphorescence), and any combinations thereof. A poll on specific aspects of persistent phosphors was conducted during the workshop, and it included the choice of a suitable and preferably unique name for the phenomenon. About 75% of the participants (Fig. 1 ) favored the use of a single name, with clearly the most support for “persistent luminescence” (68%), followed by “long-lasting phosphorescence” (20%). Materials showing persistent luminescence are thus referred to as “persistent phosphors,” hence the title of this focus issue.

 

Fig. 1 Group picture of the participants at the Phosphoros 2011 workshop. The piece of rock in the front is not the Bologna Stone.

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2. Focus issue highlights

Reference [1] is an overview paper that includes relevant details on the history of persistent luminescence and takes a somewhat different view than the recent review paper on Eu²⁺-doped phosphors by Van den Eeckhout et al. [3]. The authors make the statement that using imagination for finding persistent luminescence models is fine as long as the models are subsequently thoroughly investigated from both a chemistry and a physics point of view. Also, several research areas where progress is needed are outlined in this paper.

A large fraction of the work presented at the workshop dealt with Eu²⁺-doped materials, which is a reflection of the recent research activities on the subject. This is also evident in the focus issue, which includes papers on Eu²⁺-doped CaAl₂O₄ (deep blue emission) [5], Sr₂MgSi₂O₇ (blue) [6], BaAl₂O₄ (blue-green) [7], MSi₂O₂N₂ [M = Ba (bluish green)], Sr (yellowish green), Ca (yellow) [8], and Ca₂Si₅N₈ (orange-red) [9].

The poll and the discussion sessions during the workshop revealed that there still exists strong disagreement on the exact trapping mechanism for the most studied type of persistent phosphors, i.e., materials with Eu²⁺ as the luminescence center. Several papers discuss advanced techniques for studying the storage and release mechanisms in persistent phosphors. These include thermoluminescence spectroscopy, where the influence of temperature and the excitation energy is studied on the trapping efficiency [8], synchrotron radiation to probe the band gap energy and the relevant energy levels [7], calculations with density functional theory (DFT) [10], and x-ray absorption spectroscopy to study the valence state of dopants [5]. Surely, an integrated approach that uses different and complementary types of experiments and calculations is required to make considerable progress in the understanding of the trapping mechanism.

From the application side, the poll showed an expected consolidation in the traditional application areas, such as emergency signage, indicators, and toys. The use of persistent phosphors for in vivo imaging was believed to be a credible emerging application area. In contrast, the use of persistent phosphors for energy storage (e.g., in the framework of solar energy conversion) was met with skepticism, presumably because the amount of energy that can ideally be stored in a certain phosphor volume is relatively low.

During the workshop, the need for long wavelength emitting phosphors (orange-to-red for emergency signage applications, red-to-infrared for in vivo imaging) was repeatedly outlined [1,9]. The number of (stable) Eu²⁺-doped compounds having sufficiently large redshift is limited, although new hosts, such as Ca₂Si₅N₈:Eu,Tm, have been explored. Maldiney et al. showed that this material can indeed be used for in vivo imaging, by reducing the particle size of bulk Ca₂Si₅N₈:Eu,Tm phosphors [9].

Several papers dealt with non-Eu²⁺-doped phosphors, partially motivated by the increasing cost of rare earth compounds [11] and also by the need for red emitting phosphors. Carvalho et al. studied the (persistent) luminescence of Ti as an impurity in ZrO₂ [11]_. The red persistent luminescence in CaTiO₃:Pr was shown to depend on two factors, namely, the thermal treatment temperature and the defect concentration in the lattice, the latter being controlled by a small excess of Ca [12]. Green persistent luminescence was observed in CdSiO₃:Tb, with a suggested mechanism for trapping and detrapping similar to that commonly reported for Eu²⁺-doped phosphors [7]. Yamaga et al. reported the persistent luminescence in Lu₂SiO₅:Ce scintillator crystals [13]. For Eu³⁺-doped persistent phosphors, such as in the well-known Y₂O₂S:Eu,Ti,Mg phosphor, more research is required to settle the discussion of the storage and release mechanisms [1].

In conclusion, this focus issue of Optical Materials Express gives a brief but interesting overview of the state-of-the-art in the research on persistent phosphors and offers many avenues for future research. Despite the relatively small size of the persistent phosphor research community, there is a strong drive toward the design and characterization of specific phosphors, the development of new application areas, and a more profound understanding of the trapping and release mechanisms.

We express our gratitude to all reviewers and authors for their efforts in improving the manuscripts during the review process. We also thank David Hagan, Editor-in-Chief of Optical Materials Express, for supporting this focus issue and the OSA journal staff for their excellent support during the review and production.