Focus issue introduction : persistent phosphors

Persistent phosphors can emit light long after the excitation has ended. The field of persistent luminescence has strongly matured during the past decade, with considerable progress having been made in synthesis and characterization methods, the understanding of trapping and de-trapping mechanisms, and the application of these materials. This focus issue “Persistent Phosphors,” within the April 2012 issue of Optical Materials Express, features papers presented at the first International Workshop on Persistent Phosphors (Phosphoros 2011) held in Gent, Belgium. ©2012 Optical Society of America OCIS codes: (160.2540) Fluorescent and luminescent materials; (160.5690) Rare-earth-doped materials; (160.2900) Optical storage materials; (260.3800) Luminescence; (300.6280) Spectroscopy, fluorescence and luminescence; (250.5230) Photoluminescence. References and links 1. H. F. Brito, J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, and L. C. V. Rodrigues, “Persistent luminescence mechanisms: human imagination at work,” Opt. Mater. Express 2(4), 371–381 (2012). 2. T. Matsuzawa, Y. Aoki, N. Takeuchi, and Y. Murayama, “New long phosphorescent phosphor with high brightness, SrAl2O4:Eu ,Dy,” J. Electrochem. Soc. 143(8), 2670–2673 (1996). 3. K. Van den Eeckhout, P. F. Smet, and D. Poelman, “Persistent luminescence in Eu-doped compounds: a review,” Materials 3(4), 2536–2566 (2010). 4. D. Poelman, P. F. Smet, J. Hadermann, J. Botterman, and K. Van den Eeckhout, Phosphoros International Workshop on Persistent Phosphors (Academia Press, Gent, Belgium, 2011). 5. N. Avci, K. Korthout, M. A. Newton, P. F. Smet, and D. Poelman, “Valence states of europium in CaAl2O4:Eu phosphors,” Opt. Mater. Express 2(3), 321–330 (2012). 6. H. F. Brito, J. Hölsä, H. Jungner, T. Laamanen, M. Lastusaari, M. Malkamäki, and L. C. V. Rodrigues, “Persistent luminescence fading in Sr2MgSi2O7:Eu ,R materials: a thermoluminescence study,” Opt. Mater. Express 2(3), 287–293 (2012). 7. L. C. V. Rodrigues, H. F. Brito, J. Hölsä, and M. Lastusaari, “Persistent luminescence behavior of materials doped with Eu and Tb,” Opt. Mater. Express 2(4), 382–390 (2012). 8. J. Botterman, K. Van den Eeckhout, A. J. J. Bos, P. Dorenbos, and P. F. Smet, “Persistent luminescence in MSi2O2N2:Eu phosphors,” Opt. Mater. Express 2(3), 341–349 (2012). 9. T. Maldiney, G. Sraiki, B. Viana, D. Gourier, C. Richard, D. Scherman, M. Bessodes, K. Van den Eeckhout, D. Poelman, and P. F. Smet, “In vivo optical imaging with rare earth doped Ca2Si5N8 persistent luminescence nanoparticles,” Opt. Mater. Express 2(3), 261–268 (2012). 10. H. F. Brito, M. C. F. C. Felinto, J. Hölsä, T. Laamanen, M. Lastusaari, M. Malkamäki, P. Novák, L. C. V. Rodrigues, and R. Stefani, “DFT and synchrotron radiation study of Eu doped BaAl2O4,” Opt. Mater. Express 2(4), 420–431 (2012). 11. J. M. Carvalho, L. C. V. Rodrigues, J. Hölsä, M. Lastusaari, L. A. O. Nunes, M. C. F. C. Felinto, O. L. Malta, and H. F. Brito, “Influence of titanium and lutetium on the persistent luminescence of ZrO2,” Opt. Mater. Express 2(3), 331–340 (2012). 12. E. H. Otal, A. E. Maegli, N. Vogel-Schäuble, B. Walfort, H. Hagemann, S. Yoon, A. Zeller, and A. Weidenkaff, “The influence of defects formed by Ca excess and thermal post-treatments on the persistent luminescence of CaTiO3:Pr,” Opt. Mater. Express 2(4), 405–412 (2012). 13. M. Yamaga, Y. Ohsumi, T. Nakayama, and T. P. J. Han, “Persistent phosphorescence in Ce-doped Lu2SiO5,” Opt. Mater. Express 2(4), 413–419 (2012). #164975 $15.00 USD Received 16 Mar 2012; published 19 Mar 2012 (C) 2012 OSA 1 April 2012 / Vol. 2, No. 4 / OPTICAL MATERIALS EXPRESS 452


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 SrAl 2 O 4 :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 (Eu 2+ -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.

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 Eu 2+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.
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 Eu 2+ 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) Eu 2+ -doped compounds having sufficiently large redshift is limited, although new hosts, such as Ca 2 Si 5 N 8 :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 Ca 2 Si 5 N 8 :Eu,Tm phosphors [9].
Several papers dealt with non-Eu 2+ -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 ZrO 2 [11] .The red persistent luminescence in CaTiO 3 :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 CdSiO 3 :Tb, with a suggested mechanism for trapping and detrapping similar to that commonly reported for Eu 2+ -doped phosphors [7].Yamaga et al. reported the persistent luminescence in Lu 2 SiO 5 :Ce scintillator crystals [13].For Eu 3+ -doped persistent phosphors, such as in the well-known Y 2 O 2 S: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.

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