Persistent X-ray-activated phosphors: mechanisms and applications

Trivalent lanthanides in wide bandgap fluoride or phosphate hosts can present persistent luminescence between 200 nm and 1.7 µm after charging by X-rays. Mechanisms are reviewed and applications envisioned.

blue light for efficient charging through the bandgap and in that case, the so-called bandgap engineering could generally be applied 1,7,9 . On the contrary, a local model considers a direct electron transfer between activator and trapping centers 10,11 . Arguments for the two models are now well established and the energy required for the charging process, for instance, could control the process: high energy for global model while low energy is associated with local defect such as antisites, for instance, that are well established in garnets [12][13][14][15] , perovskite 16 , and spinel materials 17,18 . Calculations as proposed in ref. 19 further validate the so-called local model.
Very recently, X-rays have been used as charging light in various materials. In materials that could also be charged by UV or/and visible light such as ZnGa 2 O 4 :Cr, but also in wide bandgap materials which cannot be charged by usual lamps due to their large bandgap (>12 eV) 20 . These materials are mainly fluorides such as NaLnF 4 (Ln = Lu, Y, Gd, La) with bandgap values in the range 12-14 eV 21,22 . In that case, local trapping and detrapping model could well explain the observed persistent luminescence. Li et al. described the local defects in wide bandgap materials such as in NaYF 4 :Ln 3+ -doped materials with anions vacancies 22 created under X-ray charging. Notice that the efficiency of such charging can be very high in these nanomaterials as measured in Na (Gd,Lu)F 4 :Tb 3+ to be 2.8 × 10 16 photons/g 23 , which is surprisingly high for nano-sized particles and comparable to the commercial SrAl 2 O 4 :Eu 2+ ,Dy 3+ bulk persistent phosphor where the global model is the most relevant (de)trapping model. Another remarkable example of local

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Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. defect is presented in Nd, Ho, Tm, and Er:NaYF 4 @NaYF 4 core-shell nanoparticles in ref. 24 . Fluoride vacancies and/or fluoride Frenkel defects (vacancy-interstitial pairs) could be formed (see Fig. 2 in ref. 24 ). Some controversy still remains for these wide-bandgap hosts as in another recent work, Zhuang et al. 25 explain (see Fig. 2a, b in ref. 25 ) that the persistent luminescence mechanism occurs by global model but notice that there is some discrepancy in the bandgap value of NaYF 4 fluoride host, 13 eV reported in ref. 21 (see SI7 in ref. 21 ) in regard to 12 eV in 25 , while charge carriers are not easily delocalized in these compounds. Furthermore, as reported by Li et al. 22 , global model cannot well explain the persistent luminescence for the fluoride hosts doped with Gd 3+ , then efforts are still required to better model this amazing phenomenon taking into account either a Physics or material Chemistry point of view.
Through an appropriate selection of the Ln 3+ element in the synthesis of fluoride or phosphate hosts, a wide range of persistent luminescence emission, from 200 nm up to 1.7 µm, can be obtained after X-ray excitation (30 mA, 40 kV). Since the photon counts are much higher when materials are exposed to X-rays when compared to another light source, persistent phosphors developed by Li et al. should have many applications 22 among those listed in Fig. 1.
UV light irradiation in the 200-400 nm region may pose considerable phototoxicity to living cells. One way to solve this problem is to use materials that are excited at smaller wavelengths while emitting deep UV. Ce 3+ -or Gd 3+ -doped NaYF 4 have the capability of UV emission at 250 or 300 nm after being excited by X-rays. Such phosphors are likely to find applications, for example, in photoclick chemistry for which high energy over a long time is needed to create covalent bonds 26 . But other applications for which high energy is required could also be envisioned, such as sterilization and disinfection 27 .
When moving to the visible range, such presented Xray-activated phosphors should also find applications either in vitro for the development of biosensors 28 or in vivo for imaging 29 as well as in therapy 30 . Bioimaging applications can be improved when using probes emitting in BW-I (650-980 nm) or with nanoparticles emitting in BW-II (1000-1600 nm) since this allows imaging of deeper tissues and it gives access to images with better spatial resolution 24 . Here again, the luminescent phosphors developed by Li et al. could be particularly useful. In addition to these applications in biology, many more applications of luminescent phosphors excited by X-rays can be envisioned not only for anti-counterfeiting, information storage, and security 25 but also in cryopreservation and photocatalysis 31 . High-resolution luminescence imaging in NIR-II Fig. 1 Emission wavelength of Ln 3+ -doped persistent fluorides or phosphates after X-ray excitation and possible applications. Adapted from the original manuscript ref. 22