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This research investigated the crystal chemical principles related to the incorporation of incompatible atoms into the garnet host lattice. These considerations might be used to guide and drive the design...

Recent developments in lighting and display technologies have led to an increased focus on materials and phosphors with high efficiency, chemical stability, and eco-friendliness. Mechanoluminescence (ML) is a promising technology for new lighting devices, specifically in pressure sensors and displays. CaZnOS has been identified as an efficient ML material, with potential applications as a stress sensor. This study focuses on optimizing the mechanoluminescent properties of CaZnOS:Tb through microwave-assisted synthesis. We successfully synthesized CaZnOS doped with Tb3+ using this method and compared it with samples obtained through conventional solid-state methods. We analyzed the material’s characteristics using various techniques to investigate their structural, morphological, and optical properties. We then studied the material’s mechanoluminescent properties through single impacts with varying energies. Our results show that materials synthesized through microwave methods exhibit similar optical and, primarily, mechanoluminescent properties, making them suitable for use in photonics applications. The comparison of the microwave and conventional solid-state synthesis methods highlights the potential of microwave-assisted methods to optimize the properties of mechanoluminescent materials for practical applications.

A good control over the valence state of dopants in luminescent materials or phosphors is important for the develop-ment of highly efficient phosphors for white light-emitting diodes (LEDs). Detailed spectroscopic studies allow us to reveal optically induced charge transfer processes and elucidate the underlying mechanisms in phosphors with additional functionalities such as photochromism or persistent luminescence. However, the spectro-scopic study of the valence switching of europium has scarcely been reported. Here, we report on the Sr3SiO5:Eu phosphor, in which photo-reduction (Eu3+-* Eu2+) and photo-or thermal-oxidation (Eu2+-* Eu3+) reactions are demonstrated. The variation of the illumination wavelength influences the efficiency of both the photo-reduction/oxidation and the accompanying dynamic process, especially when the two opposite reactions occur simultaneously. Temperature-dependent annealing indicates a large trap depth for the electron trapped by Eu3+. The good stability of Eu2+ obtained by photo-reduction and the repeatability of the Eu2+/Eu3+ valence switching are confirmed as well. Furthermore, the application of optical information storage is demonstrated based on this phosphor. The results of this work may not only improve the understanding of Eu2+/Eu3+ valence change during illumination but also allow the development of new functional luminescent materials.

Glow-in-the-dark materials have been around for a long time. While formerly materials had to be mixed with radioactive elements to achieve a sufficiently long and bright afterglow, these have now been replaced by much safer alternatives. Notably strontium aluminate, SrAl2O4, doped with europium and dysprosium, has been discovered over two decades ago and since then the phosphor has transcended its popular use in watch dials, safety signage, or toys with more niche applications such as stress sensing, photocatalysis, medical imaging, or flicker-free light-emitting diodes. A lot of research efforts are focused on further improving the storage capacity of SrAl2O4:Eu2+,Dy3+, including in nanosized particles, and on finding the underlying physical mechanism to fully explain the afterglow in this material and related compounds. Here an overview of the most important results from the research on SrAl2O4:Eu2+,Dy3+ is presented and different models and the underlying physics are discussed to explain the trapping mechanism at play in these materials.

Acoustically produced luminescence (APL) can be used for fast and easy mapping of ultrasound pressure fields, allowing quantitative investigation of these fields for a wide range of acoustic frequencies and pressures. APL offers a fast and inexpensive alternative for the conventional point-by-point hydrophone scanning. This can benefit industrial and medical ultrasound applications that experience stringent certification and safety requirements on pressure field characterization. APL was shown to originate from absorption-mediated heating by ultrasound irradiation of a membrane material, which consists of a polymer binder and a luminescent material (or phosphor). This heating induces local thermoluminescence emission, which is proportional to the ultrasound pressure. However, a precise framework describing the physics of the APL process, allowing the retrieval of acoustic field information from the measured light emission has been lacking. Here, we present a full theoretical model of the APL phenomenon, allowing the reconstruction of both the pressure and temperature fields from the measured luminescence. The developed theoretical model is verified using finite-element modeling and experimental validation. We then demonstrate how APL can be used to obtain a 3D reconstruction of an ultrasound pressure field, in a fast and easy way. Finally, the general model demonstrated here can also prove useful for other applications, e.g. in luminescence-based thermometry using persistent phosphors.

Advanced functional materials x, 2109635 (1-9) (2021). doi:10.1002/adfm.202109635, Light sensors are widely used to monitor light intensities, for instance in medical applications, in agriculture or for conservation of art. Most of these sensors are electronic devices that record continuously but applications that only require information of integrated intensities, measured over a long time, could greatly benefit from an integrating dosimeter that does not require a power supply. In this work a wireless and quantitative light dosimeter is presented based on SrAl$_2$O$_4$:Eu$^{2+}$,Sm$^{3+}$, a phosphor that exhibits stable energy storage upon exposure to blue and ultraviolet light. It is shown that a forward electron transfer from europium to samarium can be induced under illumination with blue or ultraviolet light while the reverse electron transfer can be achieved by illuminating the phosphors with green to infrared light. This reverse transfer is accomplished through excitation of the divalent samarium and results in bright, green optically stimulated luminescence. The stable energy storage, in combination with the possibility for optical read-out, makes SrAl$_2$O$_4$:Eu$^{2+}$,Sm$^{3+}$ ideally suited to be used as an integrating light dosimeter for monochromatic to broadband light, from the ultraviolet to the near infrared. To demonstrate this, a proof of concept dosimeter was developed in which this phosphor was successfully used to measure average daylight intensities., Published by Wiley-VCH, Weinheim

Flexible X-ray storage phosphor sheets are regarded as promising alternatives to conventional electronic flat-panel X-ray detectors, enabling X-ray imaging and dosimetry in less accessible situations. However, it is a challenge to develop phosphor-based detectors with high memory stability and convenient readout processes. Here, an approach to realize this using radiation-induced photoluminescence tuning in (Ba1-xSrx)(2)SiO4:Eu phosphors is demonstrated, ascribed to the reduction of Eu3+ toward Eu2+. The associated photoluminescence spectral change and the accompanying color contrast in response to the radiation dose are exploited for simultaneous X-ray dosimetry and imaging. The recorded image can be read out conveniently by a regular photo camera upon UV illumination and the radiation dose can be extracted during the imaging process in a ratiometric way via the green to red pixel intensity, avoiding the need for absolute intensity measurement. Moreover, the imaged information can be maintained for longer than 28 days and the plate can be reused for X-ray detection after bleaching upon 420 nm illumination, exhibiting superior memory retention ability and good cycling resistance. These results reveal the great potential of (Ba1-xSrx)(2)SiO4:Eu for X-ray-based microbeam radiation therapy and nondestructive inspection and are expected to stimulate research on phosphor-based photoluminescence modulated detectors.

Trivalent praseodymium exhibits a wide range of luminescent phenomena when doped into a variety of different materials. Herein, radioluminescent NaLuF4:20%Pr3+ nanoparticles are studied. Four different samples of this composition were prepared ranging from 400-70 nm in size. Kinetic studies of radioluminescence as a function of X-ray irradiation time revealed a decrease in the emissions originating from the 1S0 level, due to the formation or optical activation of defects during excitation, and a simultaneous increase in the visible emissions resulting from the lower optical levels. Thermoluminescence measurements elucidated that a local de-trapping mechanism was responsible for the increase in steady state emission and persistent luminescence originating from the lower optical levels. The results and mechanism described through this study serve to provide a novel nanoparticle composition with versatile luminescent properties and provides experimental evidence in favor of a local trapping model.

Traditional light sources, e.g., incandescent and fluorescent lamps, are currently being replaced by white light-emitting diodes (wLEDs) because of their improved efficiency, prolonged lifetime, and environmental friendliness. Much effort has recently been spent to the development of Mn

Colloidal QDs are hybrid objects that feature an inorganic, crystalline core surrounded by an organic ligand shell. The ligands are required for stabilising QD dispersions and, by passivating under-coordinated surface atoms, reducing the density of localized surface states that would quench their photoluminescence (PL). The integration of QDs in optoelectronic devices typically requires embedding of the QDs in a host material. Atomic layer deposition (ALD) is often considered as a deposition technique for such purpose. However, the ALD process chemistry can affect the surface of the QDs and their ligand shell, thus potentially leading to a reduction of the PL efficiency after embedding of the QDs. Hence, a fundamental understanding of the interplay between the ALD process chemistry and the QD photoluminescence stands out as a key challenge for turning ALD into a viable encapsulation method for QDs. In this work, a reactor for plasma enhanced ALD was equipped with the possibility to conduct in situ photoluminescence measurements during ALD encapsulation of QDs. This setup was used to investigate the influence of plasma treatment, precursor and reactant exposure on the PL of CdSe/CdS/ZnS core/shell/shell QDs during ALD of Al2O3, TiO2, HfO2, and ZnS. The in situ PL approach proved a versatile technique to screen suitable precursors, reactants and ALD processes for QD embedding and to investigate the interaction between QDs and reactive gaseous species in general.

The presented work gives an overview of recent advances in research and development of novel phosphors based on Eu2+-doped KxNa1-xLuS2 ternary sulfides which may be used as phosphors in white LED sources with tunable color correlated temperature. Tunability is achieved by using two different kinds of excitation diodes (nearUV and blue) in combination with the proper K/Na ratio in KxNa1-xLuS2:Eu2+ structure. Intense photoluminescence due to the Eu2+ 5d – 4f allowed transition covers a broad emission region in visible range under both nearUV (~390 nm) and blue (~450 nm) excitations. The possibility to change the intensity ratio of the excitation diodes yields the changes of the output color correlated temperature. These changes are clearly demonstrated by shifts in the CIE color space; light quality is also quantified and discussed using CRI (color rendering index) and CQS (color quality scale) metrics [1]. However, to fully understand the Eu2+ emission mechanisms in these sulfidic hosts and to be able to optimize phosphor preparation and its performance, additional experimental techniques are applied to characterize these KxNa1-xLuS2:Eu2+ phosphors. When a focused electron beam impinges on a cathodoluminescent (CL) material in a scanning electron microscope (SEM), secondary electrons, characteristic x-rays and visible photons are emitted. These signals can be used to construct a two-dimensional topographical, chemical and luminescence map, respectively [2]. Furthermore, thermal quenching and quantum efficiency measurements of KxNa1-xLuS2:Eu2+ phosphors will be discussed for the first time. V. Jarý, L. Havlák, J. Bárta, M. Rejman, A. Bystřický, Ch. Dujardin, G. Ledoux, M. Nikl. Circadian Light Source Based on KxNa1-xLuS2:Eu2+ Phosphor. ECS Journal of Solid State Science and Technology, 7 (1), R3182-R3188, (2018). doi: 10.1149/2.0231801jss. L.I.D.J. Martin, D. Poelman, P.F. Smet, J.J. Joos, Microscopic Study of Dopant Distribution in Europium Doped SrGa2S4: Impact on Thermal Quenching and Phosphor Performance. ECS Journal of Solid State Science and Technology, 7 (1) R3052-R3056 (2018). doi: 10.1149/2.0341709jss.

Inorganic luminescent materials or phosphors are currently applied on a large scale in white light-emitting diodes (LEDs) for lighting and display applications. These technologies rely on high-performance phosphors and require subtle fine-tuning of the material composition and the device manufacturing to achieve white LED devices with the desired characteristics1. Synthesis conditions strongly affect phosphor properties, often in an unpredictable and hard-to-control way. This can be attributed to an incomplete understanding of the effects of local variations in composition, morphology and emission properties on the overall performance of the phosphor. Since luminescent characterization methods are generally performed on a macroscopic scale, local variations are averaged out, and the detailed information on the interplay between structure and luminescence is lost. To gain more insight in how performance-determining mechanisms are influenced by synthesis conditions and sample composition, we combine cathodoluminescence (CL) spectroscopy in a scanning electron microscope (SEM) with an energy-dispersive X-ray (EDX) analysis. In SEM-CL-EDX, simultaneous identification of the morphology, the chemical composition and the spectral distribution of a phosphor with a spatial resolution down to 1 µm or less is achieved. For this work a heating stage was added to the setup, allowing to determine thermal quenching profiles on sub-micrometer sized areas. Temperature-dependent SEM-CL-EDX mappings were performed on SrGa2S4:Eu2+, a material which is known as a saturated green phosphor, especially suitable for display applications2 . This case study revealed that samples with ill-distributed dopant ions show a broad range of local quenching temperatures, as can be seen in Fig. 1. For the associated activation energy an upper limit of 0.61 eV was identified, corresponding to the intrinsic thermal quenching of isolated europium ions. Furthermore, the results confirm a previously suggested thermal quenching model which involves the presence of both isolated and clustered dopant ions3. References Smet, P.F.; Parmentier, A.B.; Poelman, D. J. Electrochem. Soc., 2011. 158(6): p. R37-R54. Joos, J.J.; Meert, K.W.; Parmentier, A.B.; Poelman, D.; Smet, P.F. Opt. Mater., 2012. 34(11): p. 1902-1907. Martin, L.I.D.J.; Poelman, D.; Smet, P.F.; Joos, J.J. ECS J. Solid State Sci. Technol., 2018. 7 (1) p. R3052-R3056. Figure 1: Distribution of local Eu concentration related to the distribution of local T 0.5 within a single phosphor grain for Sr1-x Ga2S4:Eu x with x = 0.01, 0.03 and 0.07. Figure 1

Ever since the introduction of the high-brightness blue LED in 1994 [1], research on photoluminescent materials has gained tremendous interest. The combined emission from a blue emitting LED chip and a yellow emitting phosphor yields white light. The first white LEDs (wLEDs) were based on an (In,Ga)N LED chip and the efficient, chemically stable yellow phosphor Y3Al5O12:Ce3+. Despite the high luminous efficacy of the radiation (LER) attainable with this single phosphor wLED, the performance in terms of color rendering is low due to a lack of red emission. Improvement of the color rendering index (CRI) requires an additional emitter in the red spectral region. Therefore, tremendous effort has been spent on the development of red emitting phosphor materials. Red sulfide phosphors such as (Ca,Sr)S:Eu2+ [2] were first investigated. However, the poor chemical stability and strong thermal quenching associated with sulfides led research to new phosphor classes. Due to their high chemical stability and high quantum efficiency, Eu2+ nitride phosphors [3] such as CaAlSiN3:Eu2+ are currently considered as the most suited candidate for wLED applications. Nevertheless, even the Eu2+ doped nitrides possess serious drawbacks. Apart from their higher production cost, they often suffer from broad excitation- and emission bands. Excitation bands extending beyond 500 nm cause reabsorption issues when used in a phosphor blend together with green or yellow phosphors. On the other hand, broad emission bands partially reaching above 650 nm are inefficient as the human eye sensitivity becomes negligible in this spectral region. Several red fluoride phosphors show a saturated red emission band below 650 nm based on the photoluminescence of Mn4+ in an octahedral fluorine coordination. In addition to their optimal optical performance, the low-cost production by means of wet chemical synthesis is an additional benefit. However, trace impurities in the form of impurity phases and secondary Mn valences often occur after synthesis. The present study reveals a strong correlation between purity after synthesis and stability of K2SiF6:Mn4+, a benchmark red fluoride phosphor [4-6]. Recently a two-step precipitation method has gained interest to improve the Mn valence related issue. K2MnF6, is first synthesized as a precursor material, incorporating Mn4+ in a [MnF6]2- coordination complex. In a second step, these complexes are incorporated in a fluoride host by dropwise addition of K2MnF6/KF in 40% HF and SiO2 in 40% HF. Although the chemical yield is higher compared to other chemical routes, such as wet chemical etching, and some improvements in stabilizing the Mn4+ valence state can be expected, careful examination of the purity proves necessary. Combined feedback from in- and ex-situ X-ray diffraction, diffuse reflection spectroscopy, X-ray absorption near edge spectroscopy, thermal gravimetric and differential thermal analysis revealed the presence of impurities after synthesis of K2SiF6:Mn4+. On the one hand, hydrated secondary crystalline phases, such as KMnF4•H2O, K2MnF5•H2O and similar hydrated structures, are easily formed during synthesis. As a result, parasitic absorption due to Mn3+ occurs which limits the attainable quantum efficiency of the as-synthesized impure phosphor. On the other hand, impurities such as KHF2 severely increase the hygroscopic behavior of the phosphor after synthesis. KHF2 is easily hydrolyzed, leading to hydrated KF, an impurity hardly detectable by XRD at room temperature. As a result of water absorption in the impure phosphor, a further reduction of Mn4+ to Mn3+ is observed when the phosphor is placed in high humidity. Nevertheless, detection, identification and elimination of these impurities lead to efficient and high purity K2SiF6:Mn4+, showing good chemical and thermal stability. To further enhance stability, thin protective Al2O3, TiO2 or SiO2 shells were coated using thermally assisted, plasma enhanced or ozone enhanced Atomic Layer Deposition (ALD) and subsequently evaluated. Nakamura, S., M. Senoh, and T. Mukai, High‐power InGaN/GaN double‐heterostructure violet light emitting diodes. Applied Physics Letters, 1993. 62(19): p. 2390-2392. Hu, Y., et al., Preparation and luminescent properties of (Ca1-x,Srx)S:Eu2+ red-emitting phosphor for white LED. Journal of Luminescence, 2005. 111(3): p. 139-145. Li, S., et al., Critical Review—Narrow-Band Nitride Phosphors for Wide Color-Gamut White LED Backlighting. ECS Journal of Solid State Science and Technology, 2017. 7(1): p. R3064-R3078. Sijbom, H.F., et al., K2SiF6:Mn4+ as a red phosphor for displays and warm-white LEDs: a review of properties and perspectives. Optical Materials Express, 2017. 7(9): p. 3332-3365. Murphy, J.E., et al., PFS, K2SiF6:Mn4+: the Red-line Emitting LED Phosphor behind GE's TriGain Technology™ Platform. SID Symposium Digest of Technical Papers, 2015. 46(1): p. 927-930. Paulusz, A.G., Efficient Mn(IV) Emission in Fluorine Coordination. Journal of The Electrochemical Society, 1973. 120(7): p. 942-947.

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.

Persistent phosphors are a specific type of luminescent materials having the unique ability to emit light long after the excitation has ended. They are commonly used as emergency signage in near ideal, isothermal indoor situations. Recently, their energy storage capacity was relied on for outdoor situations, e.g. for glow-in-the-dark road marks and in combination with solar cells and photo catalytic processes. In this work the influence of temperature, illumination intensity and the duration of the night is critically evaluated on the performance of afterglow phosphors. The persistent luminescence of SrAl2O4:Eu,Dy green emitting phosphors is studied under realistic and idealized conditions. It is found that the light output profile is hardly influenced by the ambient temperature in a wide range. This is due to the presence of a broad trap depth distribution, which is beneficial to cover the longer and colder winter nights. Temperature drops during the night are however detrimental. For traffic applications, the total light output of glow-in-the-dark road marks at the end of the night is not sufficient for the studied compound, although re-charging by the car's headlamps partially alleviates this. For energy storage applications, the trap density should be improved and tunneling recombination processes might be needed to overcome overnight temperature drops.

In this paper the luminescence of the scheelite-based CaGd₂(₁-x)Eu₂x(WO₄)₄ solid solutions is investigated as a function of the Eu content and temperature. All phosphors show intense red luminescence due to the ⁵D₀ - ⁷F₂ transition in Eu³⁺, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. For high Eu³⁺ concentrations the intensity ratio of the emission originating from the ⁵D₁ and ⁵D₀ levels has a non-conventional temperature dependence, which could be explained by a phonon-assisted cross-relaxation process. It is demonstrated that this intensity ratio can be used as a measure of temperature with high spatial resolution, allowing the use of these scheelites as thermographic phosphor. The main disadvantage of many thermographic phosphors, a decreasing signal for increasing temperature, is absent.

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.

Persistent phosphors, also called glow-in-the-dark materials, are a specific type of luminescent materials. They can emit light long after the excitation ended, which is realized by temporarily storing energy in the crystal lattice. Ambient heat can release the trapped charge carriers, after which recombination and light emission can occur. For many applications, such as in emergency signage, the storage capacity of persistent phosphors should further be increased. This would open new application areas, such as glowing road marks [1]. We show that the excitation of the europium center in the blue emitting Sr2MgSi2O7:Eu,Dy by near-UV light not only leads to charge trapping – essential to the persistent luminescence - but also to optically stimulated release of previously trapped charges and subsequent luminescence (OSL) [2]. This is substantiated by specific charging-OSL experiments making use of the presence of a thermal barrier for trapping at low temperature. Furthermore, the optical detrapping is observed to be significantly more important when a larger fraction of the traps is already filled, suggesting OSL is the limiting factor in the storage capacity of persistent phosphors. Not only photons can lead to detrapping, also pressure can in certain (mechanoluminescent (ML)) phosphors lead to light emission. Consequently, these materials can be used as pressure gauges or stress indicators [3]. Here we focus on the use of ML phosphors as a visualisation tool for ultrasound pressure fields [4]. In the case of BaSi2O2N2:Eu, a bluish green ML signal is observed proportional to the ultrasound intensity. An approach is presented to construct a 3D representation of the ultrasound pressure field. The obtained results are compared to numerical simulations, showing an excellent match [4]. [1] J. Botterman and P. F. Smet, "Persistent phosphor SrAl2O4:Eu,Dy in outdoor conditions: saved by the trap distribution," Opt. Express 23, A868-A881 (2015) [2] C. Tydtgat et al., "Optically stimulated detrapping during charging of persistent phosphors," Opt. Mater. Express 6, 844-858 (2016). [3] C.N. Xu et al., "Direct view of stress distribution in solid by mechanoluminescence, " Appl. Phys. Lett. 74, 2414-2416 (1999) [4] M. Kersemans et al., "Fast reconstruction of a bounded ultrasonic beam using acoustically induced piezo-luminescence, " Appl. Phys. Lett. 107, 234102 (2015)

Seven novel heterometallic coordination polymers [CeAg(Hbidc)(2)(H(2)O)(2)] (1) and [LnAg(Hbidc)(2)]·3(H(2)O)[Ln = Sm (2), Gd (3), Tb (4), Dy (5), Ho (6), Er (7), H(3)bidc = 1H-benzimidazole-5,6-dicarboxylic acid] have been synthesized under hydrothermal conditions and characterized by elemental analysis, FT-IR, TG analysis, and single crystal X-ray diffraction. X-Ray analysis revealed that the seven complexes present two different types of three-dimensional (3D) structures. Complex 1 crystallized in an orthorhombic manner having a Pna2(1) space group, consisting of a 3D framework with a 1D heterometallic chain. Polymers 2-7 are isostructural and crystallized in an orthorhombic fashion having a Pccn space group existing of a two-fold interpenetrating 3D framework. The luminescence properties and the magnetic properties of all polymers were investigated.

Persistent luminescent materials or glow-in-the-dark compounds have been known for centuries, but due to their limited performance in terms of total light output, they were never used for large scale applications. It is only since about 20 years ago, when SrAl2O4:Eu,Dy was introduced as a new persistent phosphor [1], that a leap forward in both brightness and decay time was achieved. This paper has initiated substantial research, which has led to an entire family of materials, showing an appreciable afterglow for more than 24 hours [2,3]. These developments have opened up a whole new series of applications, ranging from emergency illumination, decoration and toys to medical imaging. For the latter application, materials are needed with deep red to near infrared emission, which falls in the transparency window of biological tissues. This, together with the need for red emitters for indicating emergency situations, has led to an interest in long wavelength persistent luminescence. Eu2+ is by far the most common dopant in persistent luminescent compounds. However, only very few chemically stable hosts are known, notably a number of nitrides, that allow to shift the Eu2+ emission to the deep red. Current research therefore focusses on Mn2+ and Cr3+ as valuable alternative dopants [4], as they allow to reach longer wavelengths than possible with Eu2+. Fig. 1: Principle of in vivo imaging using persistent luminescent materials. In this work, we will focus on Cr-doped ZnGa2O4 (ZGO), which is one of the most promising dopant-host combinations for near infrared emission [4]. ZGO has a normal spinel structure, where the Zn2+ and Ga3+ ions are located in octahedral and tetrahedral voids respectively. Due to their similarity with Ga3+ in charge and size, the Cr3+ ions occupy Ga-sites in the host. It has been shown that the trap states in ZGO:Cr, which are necessary to store the energy, liberated during the afterglow, are related to antisite defects: a small fraction of Ga-ions occupying Zn-sites or vice versa [5]. In addition, authors have tried to increase the number of traps by introducing different codopants and/or elements to form solid solutions with ZGO. This presentation will discuss different methods to synthesize ZGO:Cr nanoparticles which are potentially useful for in vivo medical imaging. In addition, the effects of different types of dopants on the defects and thus on the persistent luminescence will be presented. Finally, some alternative hosts for emission from Cr3+ - SrAl2O4 (ubiquitous in visible light persistent luminescence) and YAGG (yttrium aluminium gallium garnet) - are evaluated, and the toxicity of the materials, which is obviously very important for biocompatibility, is discussed. [1] T. Matsuzawa, Y. Aoki, N. Takeuchi and Y.A. Murayama, J. Electrochem. Soc. 143, 2670–2673 (1996). [2] K. Van den Eeckhout, P.F. Smet, D. Poelman, Materials 3, 2536-2566 (2010) [3] K. Van den Eeckhout, D. Poelman, P.F. Smet, Materials 6, 2789-2818 (2013) [4] Y.X. Zhuang, Y. Katayama, J. Ueda and S. Tanabe, Optical Materials 36, 1907-1912 (2014) [5] T. Maldiney, A. Bessière, J. Seguin, E. Teston, S.K. Sharma, B. Viana, A.J.J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, C. Richard, Nature Materials 13, 418-426 (2014) Figure 1