# Persistent Nanophosphors: Poised to Outperform Bulk Counterparts?

Victor Castaing, Simon Delacroix, Thierry Gacoin, Encarnación Arroyo, Manuel Romero, Gabriel Lozano

► To cite this version:

Victor Castaing, Simon Delacroix, Thierry Gacoin, Encarnación Arroyo, Manuel Romero, et al. Persistent Nanophosphors: Poised to Outperform Bulk Counterparts?. Advanced Optical Materials, 2026, 14 (5), pp.e03356. 10.1002/adom.202503356. hal-05530000

HAL Id: hal-05530000

https://hal.science/hal-05530000v1

Submitted on 27 Feb 2026

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materials. This perspective highlights how emerging applications, progress in nanoscale synthesis, surface engineering and integration into advanced architectures are opening the path toward multifunctional, application-ready materials. Altogether, the nanoscale offers a transformative avenue that could enable persistent nanophosphors to outperform their bulk counterparts.

**Introduction.**

In less than 30 years, persistent phosphors, *i.e.* materials that continue emitting light after ceasing the excitation, and in particular their star compound, europium and dysprosium-doped strontium aluminate (SrAl₂O₄:Eu²⁺, Dy³⁺, SAO:Eu,Dy), have journeyed from the laboratory trials of Matsuzawa *et al.* to widespread industrial use such as road marking in many countries.¹⁻³ Beyond the industrial enthusiasm, this discovery elevated research on persistent luminescent (PersL) materials to a new level: from defect engineering to the exploration of new phases,⁴⁻⁶ and from complex syntheses to advanced methods for performance quantification.⁷⁻⁹ The entire field of condensed matter physics and chemistry joined forces in the pursuit of ever brighter and longer-lasting persistent phosphors.¹⁰ Although this collective effort has led to the development of persistent phosphors with diverse compositions, structures, morphologies, and emission colours, the performance of SAO:Eu,Dy remains unmatched. Nevertheless, owing to its deep-red PersL, chromium-doped zinc gallate (ZnGa₂O₄:Cr³⁺, ZGO:Cr) has emerged as a milestone, and is now established as the most promising persistent phosphor for biomedical applications.¹¹,¹² Indeed, the deep-red emission located in one of the so-called biological windows, penetrates through biological tissues,¹³,¹⁴ while its characteristic long-lasting emission allows for a high signal-to-noise ratio due to the absence of tissue autofluorescence.¹⁵,¹⁶

To confirm this potential, extensive chemical engineering efforts have been done to produce ZGO:Cr nanoparticles (NPs), not only with favourable energy-level structures for PersL, but also with the required features for biomedicine such as uniform nanometric size and shape, colloidal stability in physiological media, and high biocompatibility. Meeting these challenges required significant advances in synthesis methods, to the point that, nowadays, according to Scopus, over 75 % of published articles on ZGO:Cr concern its nanoscale version.¹⁷

The optimization of this persistent phosphor at the nanoscale has opened the door to a new paradigm: leveraging the nanoscale as an opportunity to unlock novel functionalities. As outlined in this Perspective, application-driven challenges, innovative synthesis routes,
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surface engineering, and composite and photonic integration all together point toward a bright
future for persistent nanophosphors.

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# 1. The challenge of light management in PersL nanophosphors.

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Conventional light emitters are typically characterized by their emission intensity and spectral
characteristics. In inorganic phosphors, after absorption of an excitation photon (1), light
emission originates from the radiative de-excitation (2) from the excited state of a dopant ion
(see Figure 1). Spectral characteristics are determined primarily by the host-matrix-dopant ion
combination while the total light output is dictated by three factors: the number of absorbed
photons, the light outcoupling and the radiative efficiency. The latter is strongly affected by
defects that act as non-radiative decay channels.

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In PersL phosphors, the picture becomes more complex. Structural defects such as oxygen
vacancies, antisite defects, or specific dopant ions are not only tolerated but required, since
they generate the trapping levels capable of storing excitation energy and release it over
extended timescales.<sup>18,19</sup> Photon absorption (1) can thus lead to direct emission (PL, 2),
charge trapping (3), or release of stored charges, occurring either thermally (4a) or optically
(OSD, 4b, see Figure 1).<sup>7,20,21</sup> PersL phosphors are therefore defined not only by afterglow
intensity and spectral characteristics, but also by their charging and discharging kinetics.
Regarding afterglow intensity, the main difficulty is to balance trapping capacity and radiative
efficiency: more traps increase storage but also introduce non-radiative losses. Another
intrinsic limitation is the only partial filling of traps during excitation, since the number of
stored charges at any time is governed by a dynamic equilibrium between trapping and
release. In this context, the emergence of reliable methods to characterize the processes
involved in PersL and quantify the overall PersL efficiency are crucial. Yet, the most common
performance assessment remains a simple comparison of afterglow intensity under
experimental condition-dependent measurements. Increasing efforts are providing more
universal performance metrics such as reporting the afterglow intensity in real photometric
units,<sup>22</sup> in total number of emitted photons,<sup>23,24</sup> or assessing the overall efficiency of the
afterglow (PersLQY),<sup>8</sup> but still need to be more routinely adopted. Furthermore, combined
theoretical and experimental approaches are essential to elucidate physical processes
governing the afterglow. For instance, DFT calculations can reveal the defect landscape in
phosphors,<sup>25</sup> while modelling afterglow and thermoluminescence glow curves allows for the
quantification of key parameters as OSD and trap depth distribution.<sup>7,9,26,27</sup> A recent analogy
between persistent phosphors and batteries led to a frequency domain analysis capable of

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estimating the efficiency of all the processes involved in the afterglow,²⁸ highlighting that rethinking the way persistent phosphors are analysed offers bright perspectives for the field.

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While these advanced methods are beginning to provide the essential quantitative data, this knowledge has not yet translated into a general design strategy to actively enhance the afterglow output. Similarly, spectral features, such as excitation bands and afterglow colour, are typically tuned by host and/or dopant modification,⁴,²⁹ inevitably altering the trap landscape, making it difficult to adjust those spectral features without impacting storage capacity or kinetics. Again, while decay kinetics have been extensively studied, charging kinetics remain rarely addressed and no reliable strategy exists for their independent optimization (see Figure 1).

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At the nanoscale, these challenges become even more pronounced. Downsizing a phosphor introduces a large fraction of surface atoms, whose altered coordination and defect chemistry can strongly affect both radiative efficiency and trap characteristics. In bulk, traps are predominantly located in the ordered crystal volume; in nanophosphors, surface traps and their interaction with the surrounding medium add a new level of complexity, offering new pathways for controlled trapping and release. These considerations define four central objectives for the design of new efficient PersL nanomaterials:

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(i) to increase the total number of photons emitted in the afterglow regime;

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(ii) to tune the afterglow spectral characteristics (emission colour and activation energies);

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(iii) to control charging and discharging kinetics while preserving efficiency, intensity, and spectral characteristics;

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(iv) to manipulate afterglow light to access unique properties (directionality, spectral multiplexing, etc.).

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Addressing these points, particularly where surface effects exist, will be essential to advance

<figure><img src="image_2.png" /></figure>

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## 2. PersL nanophosphors: carried by applications.

Figure 1: Schematics of the PersL mechanism with the perspectives of nanoengineering to tune the optical properties compared to standard chemical modifications.
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## 2.1. Biomedical applications.

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Biomedical applications (see Figure 2) require materials with nanometric dimensions, colloidal stability in physiological media, and long and intense afterglow.

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**Optical biosensor.** Optical clinical analyses for the identification and quantification of (bio)molecules are the most common methods for fast results. PersL materials offer a decisive advantage over conventional fluorophores by eliminating the autofluorescence of the (bio)molecules or other biosensor's components which usually interfere throughout excitation during classic measurements. By using a time-gated detection window, PersL nanophosphors boost the signal-to-noise ratio, thereby enhancing biosensor sensitivity. In this context, autofluorescence-free optical biosensors based on various PersL NPs, such as Zn₂GeO₄:Mn²⁺,³⁰,³¹ and ZnGa₂O₄:Cr³⁺,³²,³³ have been developed. However, the sensitivity and the specificity of PersL-based biosensors must be significantly improved before they become truly competitive. On the one hand, the nanoscale appears as an exceptional opportunity to increase the sensitivity using external strategies, beyond traditional chemical engineering used to boost the afterglow. A promising strategy for sensitivity enhancement lies in synergistic approaches that leverage the unique properties of PersL. The integration of PersL nanophosphors with plasmonic NPs for Surface Enhanced Raman Scattering (SERS) has already demonstrated the potential to create autofluorescence-free biosensors with unprecedented sensitivity,³⁴ pointing towards new directions for functional biosensors. On the other hand, the selectivity of biosensors is a critical challenge. The specific recognition is ensured by the interaction between the optical transducer, here the PersL NPs, and the target (bio)molecules, requiring significant effort in surface engineering. This is especially relevant considering complex media, where several (bio)molecules coexist, challenging their detection and quantification. This requires a renewed focus on designing multiplexed biosensors, based on multiple PersL NPs, capable of detecting and quantifying several (bio)molecules in complex media.³⁵,³⁶

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**Optical bioimaging.** Optical imaging is also growing due to its simplicity, cost-effective, and fast acquisition time. Besides, to enhance the precision in diagnosis or treatment, this technique can be combined with standard X-ray and magnetic resonance imaging.³⁷,³⁸ PersL materials possess key advantages for optical imaging, as collecting in the afterglow regime allows removing the endogenous background from tissues and biological fluids, and avoiding tissue damage.¹²,³⁹,⁴⁰ For these probes to be clinically viable, however, they face three main challenges: spectral characteristics, signal intensity, and biocompatibility. First, the PersL probes for in vivo bioimaging require an afterglow emission within one of the “biological

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windows” (BW), where the absorption coefficient is minimum, to facilitate the light penetration along tissues and maximize detection.⁴¹ In this context, most PersL nanophosphors developed for optical imaging emit in the first BW (deep red range to near infrared),¹² while shifting to higher wavelength in the near infrared (NIR) to reach the second and the third BW appears as the next objective to obtain probes with even higher signal to noise ratio.¹⁴ Indeed, biological tissues show decreased scattering with increasing wavelength in the NIR,¹⁴,¹⁵ allowing both deeper penetration depth and improved bioimages. Most common strategies employed towards this goal involve chemical modifications such as activator modification, as doping ZnGa₂O₄ with Ni²⁺ instead of Cr³⁺ leads to PersL at 1275 nm instead of 696 nm,⁴² and host variation, as Cr³⁺ emission peaks at 696 nm in ZnGa₂O₄ compared to 750 nm in β-Ga₂O₃.⁴³ Another approach consists of co-doping standard PersL phosphors with trivalent lanthanide ions to activate their well-known NIR emission through persistent energy transfer processes.¹⁹ For instance, Ce³⁺ in Y₃Al₂Ga₃O₁₂ already demonstrated persistent activation of the NIR emission of Nd³⁺ and Er³⁺, while Cr³⁺ activated Yb³⁺ PersL emission in ZnGa₂O₄.⁴⁴⁻⁴⁶ Although they show promise for biomedical applications, NIR PersL emitters still face significant materials challenges. These relate to the difficulty of obtaining intense emitters while preserving their nanoscale properties (see Section 3 and 4). These challenges must be overcome before they can be tested clinically. This spectral fine tuning is also critical for theranostics, where the afterglow can activate photosensitizers for photodynamic and photothermal therapies, creating irradiation-free nano light sources.⁴⁷,⁴⁸

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A second significant hurdle is achieving sufficiently high afterglow intensity to overcome the scattering caused by biological systems and the lack of development in clinical luminescent imaging technologies, which are not as competitive as X-ray or magnetic resonance scanners. Here, synergetic approaches are promising, for instance combining already optimized PersL nanoprobes with metallic NPs has been shown to enhance their brightness via surface plasmon resonance.⁴⁹ Finally, surface engineering is a key challenge not only to ensure biocompatibility of the probes but also to endow them with new functionalities as active targeting or drug delivery. Overcoming these challenges will unlock the full potential of bright, multifunctional PersL nanoprobes, paving the way for major advances in nanomedicine, including real-time surgical guidance and drug delivery monitoring.

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## 2.2. Energy applications.
**Lighting.** In the LED industry, inorganic phosphors are widely used as colour-converting materials, notably due to their high conversion efficiency (photoluminescence quantum yield, PLQY), and high chemical and physical stability. In this application field, PersL materials can be used as an upgraded version of colour converters in which their afterglow can bridge AC-driven LEDs non-emitting periods, therefore significantly reducing flickering. To satisfy lighting needs, PersL materials with white chromaticity and fast persistent decay kinetics (in the flickering period range) are needed. Additionally, working temperatures high enough to withstand LED operation is necessary to avoid drop in performance. In this regard, blue light-activated and broadband emitting SrSi₂O₂N₂:Eu²⁺, Mn²⁺,⁵⁰ Mg₃Y₂Ge₂.₇Si₀.₃O₁₂:Ce³⁺,⁵¹ LuCaMg₂Si₁.₅Ge₁.₅O₁₂:Ce³⁺ and Gd₃Al₂Ga₃O₁₂:Ce³⁺,Cr³⁺ persistent phosphors⁵² have been proposed for this purpose. Besides, in the context of the general miniaturization of optoelectronics, micro-LEDs recently emerged as promising candidates to be integrated as independent light sources into high resolution displays or smart arrays.⁵³ Here, the development of efficient nanoscale colour converters PersL materials with suitable afterglow characteristics will allow the design of a new generation of micro-LEDs with reduced flickering.

**Photocatalysis.** Semiconductor (SC) heterogeneous photocatalysis is another important research field that can benefit from long-lasting emission. Briefly, a light excitation triggers charges separation in SCs which will further migrate towards material surface catalyzing chemical reactions. SCs with high generated chemical potential and low carriers' recombination like TiO₂ nanoparticles are generally used for photocatalysis. Here, persistent phosphors offer a way to extend the photocatalytic activity beyond direct illumination by enabling afterglow-driven charge separation. This can occur through two main mechanisms: (i) energy transfer, where photons emitted by PersL phosphors excite the SCs even after the external light source is removed, prolonging catalytic activity, and (ii) charge transfer, where appropriate band alignments and close contact between PersL materials and SCs enable electrons and holes migration at their interface, forming efficient heterojunctions that reduce recombination and sustain charge availability. After excitation stoppage, thermal release of charge carriers from PersL phosphors may again lead to continuous photocatalytic activity. At the nanoscale, these interactions are significantly enhanced. First, nanophosphors reduce light scattering compared to bulky micro-powders, ensuring more photons to reach the SC. Second, their small size promotes intimate contact with the photocatalyst surface, improving energy and charge transfer efficiency in the heterostructures. However, serious challenges remain: the phosphors must absorb light in a spectral region different from that of the SC, emit within its
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absorption range, and possess chemically stable surfaces, especially in aqueous media. In that
sense, UV-emitting silicate-based nanophosphors (Ca₂MgSi₂O₇:Ce³+, CdSiO₃:Gd³+,Bi³+)⁵⁴,⁵⁵
have already enhanced TiO₂ activity, while visible-emitting types (Sr₃Al₄SiO₁₁:Eu²+,Dy³+,
Sr₂MgSi₂O₇:Eu²+,Dy³+)⁵⁶,⁵⁷ show promise with N-doped TiO₂ and Ag₃PO₄.

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*Solar cells.* Colour-converting phosphors can enhance solar-cell efficiency through the conversion of high-energy UV photons, typically unusable by most photovoltaic systems, into visible photons via PL. Beyond these conventional downshifting materials, PersL phosphors have emerged as promising alternatives to further increase solar cell performance by simultaneously storing part of the energy in traps. After sunset, the delayed release of this stored energy as PersL can continue to feed photons into the solar cell, enabling prolonged electricity generation beyond daylight hours. Initial proof-of-concept studies have demonstrated great potential: for example, integrating PersL materials with dye-sensitized solar cells led to conversion efficiency increases of up to 150%.<sup>58-62</sup> The nanoscale also plays a critical role in this strategy. Unlike bulk powders, nanophosphors minimize light scattering losses, allowing the fabrication of transparent coatings that can be applied on top of solar cells without hampering incident light. Moreover, their solution processability facilitates integration into thin-film photovoltaic architectures. Nevertheless, balancing high trap density, spectral compatibility, and transparency will be essential to unlock the full potential of PersL nanophosphors in photovoltaic systems.

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**2.3.** *Information processing.*

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*Anticounterfeiting.* Luminescent materials play a key role in anticounterfeiting, being the fluorescent inks used in currencies a perfect illustration of their interest for security purpose.<sup>63,64</sup> Afterglow materials, and in particular PersL phosphors, demonstrate an added value for this purpose as their long-lasting luminescence gives an additional time-gated check mode. Indeed, in contrast to fluorescent inks or tags based on PL materials, PersL patterns will be revealed not only during but also after excitation. Hence, anticounterfeiting tags composed of PersL materials or their combination with PL materials are more difficult to reproduce and provide enhanced security.<sup>65</sup> The challenge of obtaining efficient PersL materials with nanoscale dimensions is again relevant for this application as it will provide transparent inks, coatings and patterns, invisible before excitation light exposure that reveal during and after excitation, typically provided by UV lamps. Up to now, transparent materials based on nano ZnGa₂O₄ and derivatives doped with various transition metals,<sup>65-67</sup> as well as rare earth-doped complex garnets<sup>68</sup> and fluorides<sup>69</sup> have been used for this purpose. Patterning
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PersL materials at the nanoscale already demonstrated unique time-dependent chromaticity.⁶⁸

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Going one-step further, using photonics would open bright avenues towards unique anticounterfeiting tags with an unprecedented amount of time-gated check modes.⁷⁰

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*Imaging screens.* In a similar manner, the energy stored by persistent phosphors featuring deeper traps have been used to develop information storage devices, and in particular imaging displays. In these cases, an external stimulation, as NIR light or heating is employed to empty the deeper traps and recover the information. Here, the nanoscale allows the miniaturization of information storage materials, a general challenge in the field. Recently, Ou et al. pioneered the design of flexible, non-diffusive X-ray imaging screens by combining deep trap colloidally stable PersL NPs with an elastomer.⁷¹ In particular, they took advantage of the well-established low-temperature synthesis method of fluorides to prepare colloidal core-shell RE³⁺-doped NaLuF₄ featuring intense PersL after X-ray irradiation, without involving high temperature sintering. At this point, the combination of X-ray triggered delayed luminescence with film flexibility allowed imaging curved and 3D conformal electronics with unprecedented spatial resolution.

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Altogether bright perspectives may be brought by devised delayed and long-lasting emission at the nanoscale. However, key challenges further addressed must be overcome to demonstrate persistent nanophosphors true potential to boost relevant application fields. With careful design of both material interfaces and emission properties, persistent nanophosphors could become central to next-generation for all the aforementioned applications.
<figure><img src="image_3.png" /><figcaption>Figure 2: Schematics of the main application domains of PersL nanophosphors.</figcaption></figure>

### 3. Synthesis efforts provide high rewards.

As previously discussed, to fit with technological applications, nanophosphors need to have a precise composition, a low amount of detrimental surface defects, a high crystalline quality, nanoscale dimensions, and eventually, colloidal stability and biocompatibility. Increasing crystalline quality and passivate surface defects to obtain efficient luminescence while keeping colloidal stability is a well-known challenge in nanophosphor research, notably due to the high crystallization temperature of these refractory oxides. Currently, the phase space of bulk persistent phosphors is usually explored by solid state reaction. Solid precursors are annealed at high temperature leading to large crystal and agglomerated particles with high crystallinity and intense afterglow. Using top-down strategies (mechanical milling, laser bombardment...), the characteristic size of the phosphors can be decreased while trying to preserve the desirable high-quality luminescence properties. This approach usually renders inhomogeneous size and shape distribution with poorly faceted particles and uncontrolled structural defects. Another strategy consists of using bottom-up strategies to synthesize homogenous shape and nanometric particles. Commonly, this strategy is carried out by wet chemistry methods based on homogeneous precipitation reaction from solutions of suitable precursors, induced by heat treatment at moderate temperature to induce the precipitation with
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precise kinetics. These NP synthesis techniques usually balance crystalline quality and colloidal stability, and are highly based on an Edisonian, i.e. trial and error, approach. In this context, innovative syntheses strategies must be implemented to accelerate the discovery and optimization of new persistent well-crystallized nanophosphors.

### 3.1. *In situ* mechanistic studies.

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Solvothermal syntheses are known to favour good crystalline quality and colloidal stability thanks to the relatively high temperature and pressure reached in autoclaves.¹² However, due to their structure, autoclaves are blind reactors, challenging the rationalization of NPs formation mechanisms. Inspired by geosciences, a novel generation of autoclaves has been engineered with optical windows, to monitor particles nucleation and growth kinetics through *in situ* PL, Dynamic Light Scattering, Raman, and even X-ray Absorption Near Edge Structure measurements, and demonstrating the potential of formation mechanism understanding on precise NPs synthesis.⁷²,⁷³

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### 3.2. Molten salt syntheses.

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Another strategy to explore the phase space of persistent nanophosphors is the use of molten salts syntheses. Indeed, molten salts as solvents allow the nucleation and growth at high temperature (usually between 350 and 1000 °C) in a liquid phase of NPs with complex compositions like multicationic refractory oxides.⁷⁴,⁷⁵ Working in a liquid medium is interesting to increase the shock frequency between the precursors, thus decreasing the reaction temperature and increasing the reaction yields. Moreover, in contrast to classical solid-state reactions, the molten salt synthesis may preserve metastable phases.⁷⁶ Despite being promising, only few examples of PersL materials fabricated through molten salts are reported in the literature at our knowledge.⁷⁷⁻⁸⁰

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### 3.3. Innovative annealing techniques.

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Once homogenous NPs are obtained, thermal treatments are typically applied to cure quenching defects. Prolonged and/or high-temperature thermal treatments inevitably favour undesired particle growth, agglomeration and sintering, highly detrimental for the technological applications. Several strategies can be implemented to overcome this difficult challenge. A first one is the already demonstrated protected annealing strategy. Here, NPs are covered with a protective, sacrificial, and inert shell (typically SiO₂) before classic high temperature calcination, which can eventually be removed afterwards.⁸¹,⁸² Note that for
nanophosphor particles, the inert shell can also be used to increase their efficiency by surface passivation. For fluoride up-conversion (UC) nanophosphors, the epitaxial growth of an undoped matrix around the primary particles is frequently used, keeping away the emitting ions from the surface defects. Finding appropriate synthetic conditions to favour individual shell growth for each NP without secondary nucleation is far from trivial and usually requires extensive studies. Another approach consists of using innovative annealing techniques and unconventional thermal treatments to quench defects and improve the crystalline quality of NPs while preventing particle sintering. In this regard, molten salt synthesis (see Section 3.3), which allows working in a liquid medium at high temperatures, appears to be a promising approach to enhancing PersL properties of colloidal nanoparticles.⁸³ Also, recent works demonstrate the advantages of rapid thermal annealing (RTA) to calcine nanophosphors while preventing particle growth and sintering.⁸⁴⁻⁸⁶ Although no demonstration has been reported on PersL nanophosphors up to now, flash annealing method demonstrated interesting PL enhancement of downshifting phosphors while preserving their size and transparency when processed as thin films. Even faster than RTA, laser annealing enables the use of extremely rapid heating and cooling ramps. These unusual conditions could also favour the discovery of new metastable compositions with enhanced PersL characteristics. For example, Pitcher *et al.* reported a highly non-stoichiometric $Y_{3+x}Al_{5-x}O_{12}$ compound with modified luminescence properties synthesized by laser irradiation.⁸⁷ Furthermore, the laser annealing is localized, patternable, and can be used on compounds deposited on thermally sensitive substrates.⁸⁸ This particularity could also be especially interesting because most external strategies to boost PersL properties of nanophosphors (like photonics, see section 4.3) cannot handle high temperatures for a prolonged time, being their combination with nanophosphors a major challenge. To overcome these issues, the field still requires improved syntheses of colloidally stable and efficient nanophosphors, controlled calcination processes and novel protocols to integrate them with external architectures. In this context, a recent example shows the use of a silver antenna array coated by a thin aluminium oxide layer to allow annealing above their melting point using RTA without affecting their shape.⁸⁹ Besides, metallic arrays have been embedded in a PDMS sticker to allow their easy deposition after nanophosphor coating.⁸⁶ In a more general manner, as many photonic architectures do not support high temperatures, protective strategies or ways to allow their deposition after thermal treatment should be designed.

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## 3.4. Data-driven synthesis.
The search for new PersL materials requires to explore a multidimensional phase space in terms not only of complex compositions (nature of the host material, substitution, co-doping...) but also of syntheses parameters (temperature, time, precursors...). Thus, data-driven synthesis is an ideal method to accelerate the discovery of new PersL, especially if coupled with rapid innovative annealing treatments such as RTA or laser annealing leading to high-throughput syntheses. Furthermore, the optimization of a particular feature such as afterglow intensity or chromaticity can also be achieved thanks to this strategy. Machine learning has already proven to be efficient to predict experimental conditions for the synthesis of bulk PL materials but everything still needs to be done for PersL nanomaterials.⁹⁰

## 4. Nanoscale engineering for next-generation PersL.

As previously discussed, composition engineering remains the most widely used strategy to

<figure><img src="image_4.png" /><figcaption>Figure 3: Schematics of the nanoscale engineering for next-generation PersL. a) Surface engineering, b) composite and multicomponent strategy and c) photonic and plasmonic structures.</figcaption></figure>

tune the properties of PersL phosphors. However, due to the strong interdependence between material composition and its spectral and trapping characteristics, combined with the challenges of achieving precise chemical control at the nanoscale, fine tuning the afterglow of PersL nanophosphors *via* conventional compositional approaches remains limited. To overcome these constraints, novel, transversal strategies are needed to advance the field. In
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this context, the nanoscale opens new opportunities: surface engineering as well as the integration of PersL nanophosphors with diverse material systems, from flexible polymers to photonic structures, enabling the design of multifunctional, light responsive materials with enhanced or entirely new properties. In the following section, we explore how the specific needs of PersL nanophosphors can be addressed through recent advances in surface chemistry, composite design and photonic structuring, and how these approaches offer bright perspectives to unlock their full potential.

## 4.1. The weight of surface.

Driven by medical and energy-related applications (see section 2), the development of novel nanoscale PersL materials often derives from composition optimized for bulk phosphors. However, at the nanoscale, the surface becomes an added material dimension. It not only introduces structural and chemical defects that impact radiative efficiency and trap states but also enables interaction with the surrounding medium (see Figure 3a). Generally, surfaces are known to introduce defects that are luminescence quenchers. For example, the surface can promote activator oxidation (e.g. Eu²⁺ and Ce³⁺),⁹¹,⁹² segregation,⁹³ or incorporation in different lattice site compared to bulk,⁹⁴ and proximity to non-radiative centres like oxygen vacancies, all of which reduce the luminescence efficiency. In this context, surface passivation has emerged as a promising strategy to enhance the brightness of PersL nanophosphors. Encapsulation by inert SiO₂ or undoped ZnGa₂O₄ shells significantly boosted the afterglow of Zn₁.₂Ga₁.₆Ge₀.₂O₄:Cr³⁺ NPs.⁶⁵ Beyond quenching mitigation, core-shell architectures offer additional benefits such as tailored morphologies,⁹⁵ and added functionalities including drug loading and delivery through porous shells.⁹⁶,⁹⁷ Following fluoride examples,⁹⁸ progresses in oxide NPs synthesis as discussed in section 3, are still needed to provide the necessary control over cations occupation in particle core and surface to perfectly engineer PersL NPs surface. Yet, rather than being solely detrimental, surface defects may also offer unique opportunities for tuning PersL properties. For example, Mandl *et al.* demonstrated that decreasing the particle size in NaLuF₄:Pr³⁺ led to an increased afterglow, suggesting that engineered surface states can create beneficial traps.⁹⁹ Similarly, specific surface treatments can be applied to modify the trap structure, as deeper traps related with oxygen vacancies were successfully created in ZGO:Cr NPs by etching with EDTA.¹⁰⁰ Moreover, the high surface-to-volume ratio brings the surrounding media into the play, opening avenues towards novel properties and functionalities using molecular and solvent engineering. Organic ligands with devised optical properties can be grafted onto PersL NPs to
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enhance their afterglow through a dye-sensitized approach, as demonstrated with ZGO:Cr.¹⁰¹

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In another example, Liu *et al*. showed that reactive oxygen species (•OH) generated by H₂O₂

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degradation around ZGO:Cr significantly enhanced their afterglow brightness.³³

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findings highlight that surface-environment interactions, far from being parasitic, can be

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harnessed to activate novel sensing functionalities. Responsive PersL nanomaterials thus

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represent a unique opportunity provided by the nanoscale for the development of chemical

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and biological sensors.¹⁰²,¹⁰³ Altogether, these advances show that surface engineering at the

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nanoscale is not merely a challenge to overcome but a powerful lever to enhance PersL and

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introduce environment-responsive functionalities.

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## 4.2. Multicomponent and composites.

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The unique attributes of PersL nanophosphors, and especially colloidal PersL NPs, make

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them exceptional candidates for advanced nano engineering strategies like multicomponent

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and composite designs (see Figure 3b).¹⁰⁴,¹⁰⁵

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facilitate superior interfacial contact with host materials, enhancing mechanical, thermal, and

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optical properties. Furthermore, colloidal PersL NPs allow for easy processing and integration

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into matrices like polymers or glasses, minimizing uncontrolled diffusion and preserving the

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transparency of coatings, which is crucial for many optical applications.

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*Expanding activation: electrical and mechanical stimuli.* Beyond traditional photoexcitation,

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composites are forging new paths to activate PersL, opening exciting opportunities for

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advanced applications. For instance, electrically driven PersL has been realized by dispersing

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PersL phosphor powder with a high dielectric constant shell in a PDMS polymer matrix,

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paving the way for AC-driven electroluminescent devices.¹⁰⁶

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extended to design conductive composites that allow for direct electrical charge carrier

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injection into traps, providing a new pathway for afterglow activation. Similarly, mechanical

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stimuli can be leveraged to activate afterglow materials. Indeed, PersL phosphors that

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generally support afterglow release through mechanical stimulation, have been incorporated

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into composites to create pressure-sensitive materials that reveal a history of stress.¹⁰⁷

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Recent work has even demonstrated direct mechanical charging in a composite of

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Ca₉NaMg(PO₄)₇:Tb³⁺ powder and PDMS, showcasing a path towards advanced devices.¹⁰⁸

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Moreover, engineering the mechanical properties of afterglow materials through a composite

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strategy allows for the creation of flexible films. These flexible films are ideal for advanced

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applications, from durable anticounterfeiting tags on curved objects to innovative flexible X-

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ray detectors for imaging complex 3D structures.⁷¹ Here, flexible films embedding deep-trap
PersL nanophosphors can be folded inside 3D objects, such as electronic boards, to record X-ray exposure. After being unwrapped and heated, the afterglow would reveal the inner structure of the object, opening up new possibilities for non-destructive imaging. Finally, thermoresponsive materials, that possess the unique property to generate local heat when exposed to specific stimuli (electric or magnetic field, light, friction), can be used to allow stimuli-assisted thermal release of trapped charge carriers, offering a unique external control over the afterglow read-out, of major interest for information applications.

*Expanding the afterglow spectral ranges.* The composite strategy offers a powerful means to transcend the intrinsic spectral limitations of single PersL phosphors. A critical challenge for many PersL materials, and particularly those doped with trivalent rare earth ions, is their low absorption cross-section. This can be strategically overcome by blending PersL nanophosphors with sensitizing components, such as organic dyes or quantum dots, directly into a glass or polymer matrix. This approach can significantly enhance photon absorption and promote charging across a broad range of wavelengths. A particularly promising approach involves combining PersL nanophosphors with UC NPs to expand charging to novel spectral ranges, such as NIR,<sup>109,110</sup> of major interest for optical data manipulation.<sup>111</sup> Composites are also promising for precise chromaticity control. Beyond simple afterglow colour shift, combining various transparent PersL nanophosphors with distinct spectra and kinetics recently brought novel functionalities, such as time-dependent chromaticity.<sup>68</sup> In this case, the perceived afterglow colour shifts over time, holding bright potential for advanced information and security applications.

*Physical and chemical stability.* Ultimately, the composite strategy is promising for isolating PersL nanophosphors from external perturbations, thereby increasing performance and long-term viability. Encapsulation in polymer or glass matrix, for example, is essential for preserving the performance of moisture-sensitive materials like the benchmark SAO:Eu,Dy, whose properties can be degraded by hydrolysis, a problem that is amplified at the nanoscale.<sup>112-114</sup> Furthermore, the external temperature is a major environmental factor that need to be considered while designing an afterglow material for a particular application. In general, the broad trap distribution allows for a large temperature working range. Still, the temperature drop at night has been identified as a major drawback for outdoor applications like road markings.<sup>115</sup> Encapsulation of PersL phosphors in insulating matrices may have significant benefits in this context. By embedding PersL nanophosphors into insulating and protective matrices, this strategy paves the way for a new generation of afterglow devices that can withstand real-world conditions while simultaneously delivering advanced functionalities.
### 4.3. Combining PersL nanophosphors with photonic and plasmonic systems.

Nanophotonic structuring and plasmonic systems, allowing strong local density of optical states (LDOS) variations at dimensions comparable (or below) the wavelength of light, have proven efficient to tailor the emission properties of a large variety of nanoscale emitters, as standard nanophosphors.¹¹⁶ Through devised alteration of the optical processes involved in the afterglow mechanism, these systems appear as ideal transversal approaches to tune the properties of PersL nanophosphors (see Figure 3c). In the following, the unprecedented modifications of PersL key characteristics and functionalities through the integration of PersL nanophosphors into nanophotonic and plasmonic designs will be discussed.

*A new frontier in PersL: master afterglow kinetics and efficiency.* Persistent phosphors are characterized by their charging and afterglow kinetics, which arise from a dynamic equilibrium among a series of interconnected photophysical processes: optical excitation, charge carrier storage, subsequent release (both thermal and optical) and radiative emission. While non-optical processes, such as charge carrier storage and thermal detrapping, are not directly influenced by the LDOS, the overall system's kinetics can be profoundly tuned by strategically modulating the rates of competing optical processes. During the optical charging phase, the kinetics are mainly governed by a competition between trap filling and OSD. This latter process, where the very light used for excitation also liberates trapped carriers, is highly dependent on the excitation intensity.⁷ Therefore, the final state of the loading curve is not a simple function of available traps but a complex outcome of the balance between these two opposing rates. This dynamic presents a unique opportunity for nanophotonics. By designing structures that can artificially modify the local optical field, such as a periodic or aperiodic multilayer cavity, we gain a powerful lever to influence the charging process. These architectures can be engineered to create a resonance that spectrally overlaps with the charging wavelength, thereby enhancing the absorption of the excitation light and consequently promoting the accumulation of trapped charges. Similarly, the controlled inclusion of scattering particles (either dielectric and metallic) provides another pathway for enhancing light-matter interactions. By increasing the scattering events, the photon's path length within the absorbing medium is extended, leading to enhanced absorption.¹¹⁷ This strategy has been successfully transferred to persistent nanophosphors to alter their charging kinetics.¹¹⁸ Crucially, nanophotonic structures also offer a unique capability to manipulate OSD. This is an additional, powerful lever for precisely tuning the system's overall kinetics and efficiency. The ability to spectrally engineer the optical environment allows for the design
of photonic crystals that can selectively filter out wavelengths from broadband sources, such as sunlight or white LEDs, which are highly efficient for OSD but inefficient for charging. This capability fundamentally accelerates and improves storage kinetics. To fully unlock the potential of these structures as a precise tool for altering afterglow kinetics, a more thorough understanding of the absorption properties (spectra and cross-sections) of the filled traps themselves is required.¹¹⁹ Finally, the spontaneous radiative decay rate of the luminescent centre itself, a critical parameter in the overall emission kinetics, can be finely controlled by nanophotonic architectures through the Purcell effect. This provides an additional degree of freedom to rigorously investigate the effect of emission rate on PersL kinetics and efficiency and to design strategies that truly master PersL kinetics through design.

Tailoring afterglow brightness. Compared to standard emitters, the brightness of PersL materials does not solely depend on the number of emitting centres, their quantum efficiency and outcoupling. Indeed, the core interplay between trapping and emitting levels should also be considered, as the trap density and the storage/release efficiencies play a major role in the perceived afterglow brightness. In this section, we elaborate on the potential of nanophotonic and plasmonic systems to tune emission characteristics that yield brighter sources, while their effect on storage properties will be discussed in a following paragraph. Outcoupling enhancement through design is one of the rare available demonstrations of nanophotonics benefits on afterglow properties. The random inclusion of large scattering spheres with high refractive index into a coating made of canonical ZGO:Cr PersL NPs to exhibit a Mie glass behaviour brought a significant 1.5 fold enhancement of the afterglow light outcoupling.¹¹⁸ This promising result encourages the generalization of this strategy with other PersL NP-based coatings as well as the development of other designs that favour the light outcoupling from afterglow coatings. Note that, as demonstrated by Van der Heggen et al.,¹²⁰ the advantage of outcoupling increase can be twofold in persistent phosphors as it would also decrease internal reflections and the associated harmful reabsorption. Random surface texturing is another widespread strategy to enhance outcoupling from emitting layers, of particular interest for afterglow transparent coatings. Plasmonic nanostructures and arrays, typically nanostructured noble metals, are other promising candidates. Indeed, through the generation of local surface plasmon resonance (LSPR) or surface plasmon polaritons (SPPs), they can promote efficient scattering to demonstrate a significant gain in light extraction efficiency from a thin emitting layer. As the LDOS is directly proportional to the radiative decay rate, according to Fermi's golden rule, nanophotonic structures and plasmonic systems can also be used to alter the intrinsic light generation process. A proof of concept
1

demonstration of the positive effect of LDOS modification on afterglow brightness could be
realized through refractive index modification.<sup>121,122</sup> Then, following the numerous examples
of other emitting materials, nanophotonic and plasmonic architectures can be specifically
designed to tune the LDOS and therefore the afterglow brightness. For instance, thin
transparent PersL layers could be introduced into photonic crystals (PCs) to form cavities, or
coupled to plasmonic arrays, to induce strong LDOS enhancement.<sup>123</sup>

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*Tailoring afterglow spectral characteristics.* Beyond intensity enhancement, introducing an emitting layer into a 1D PC to form a cavity can significantly influence its spectral characteristics. As demonstrated using standard nanophosphors,¹²⁴,¹²⁵ by designing the cavity mode to be resonant with specific wavelengths, such architectures can selectively enhance or suppress certain wavelength ranges to tune the perceived emission colour, an opportunity yet to be seized for persistent nanophosphors. In a similar manner, PCs can precisely filter excitation wavelength ranges while resonant cavities can enhance others, appearing as an external route complementary to the composite strategy to precisely shape the charging spectra of PersL nanophosphors.

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*Creating new photonic functionalities.* In general, complexification of the emission through additional functionalities obtained by precise manipulation of light emission characteristics, are highly sought for security and information-related applications. Nanophotonic designs can provide light emitting materials with such novel functionalities, a strategy yet to be explored for afterglow emission manipulation. For instance, light directionality manipulation has been attained, by combining light emitters with PCs,¹²⁶ plasmonic nanostructures,⁸⁶,¹²⁷ and optical metasurfaces,¹²⁸ or by direct photonic patterning of the light emitters.⁷⁰,¹²⁹ Nanophotonic and plasmonic structuring also offers the ability to control the polarization state of the emitted light,⁷⁰,¹²³ which would be of great interest for enhanced security features that rely on specific polarization signatures. Notice that recent advances in photonic and plasmonic structure preparation allows macroscopic patterning of the structure, providing a unique way to shape the afterglow for advanced information or security applications. In a similar manner, controlled OSD can also bring functional advantages. NIR reactivation, for instance, supports long-term *in vivo* imaging,¹³⁰,¹³¹ and selective stimulation enables advanced anticounterfeiting and optical memory applications.⁶⁷ In the latter case, photonic cavities can be engineered to enhance light intensity at specific OSD wavelengths, typically in the NIR, to selectively and efficiently empty desired traps.

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**Conclusions.**
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While PersL nanophosphors are not yet outperforming their bulk counterparts, the path toward bright and application-ready materials is clearer than ever. Their unique properties, including their small size and afterglow, are poised to set the stage for a new era of advanced applications. The historical challenge of PersL at the nanoscale should no longer be seen as an insurmountable hurdle, but rather as an exciting opportunity to engineer energy storage in an unprecedented manner.

The path forward is driven by sophisticated synthesis techniques, including data-driven and high-throughput approaches that promise to accelerate the discovery of novel and functional materials. Still, the scalability of many of these advanced laboratory-scale methods to mass production remains a significant challenge. Complementing these synthetic advances, the nanoscale offers promising opportunities to tailor the afterglow. Surface engineering shifts from a challenge to overcome to a powerful lever to tune the afterglow of nanophosphors and introduce environment-responsive functionalities. Furthermore, the integration of PersL nanophosphors with other materials to form multicomponents or composites offers another route to enable new functional PersL activation and push the boundaries of performance. On their side, nanophotonic and plasmonic architecture might allow an unprecedented control of light manipulation in PersL nanophosphors, and fundamentally alter afterglow kinetics, brightness as well as providing novel functionalities.

The area now needs bright scientists with fresh ideas to bridge the chemistry, material science and physics disciplines, and overcome the preparation and design challenges to attain exceptional afterglow properties specifically designed for applications. The ultimate goal is to move beyond materials that simply glow to creating those that are smartly "responsive" to multiple, distinct stimuli (light, strain, temperature, and chemical environment), thereby enabling dynamic and interactive applications from smart coatings to next-generation diagnostics.

## Acknowledgements

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This project has received funding from the BBVA Foundation through a Leonardo Grant for Physics Researchers 2023, and by Spanish Ministry of Science and Innovation under grant EUR2023-143467 funded by MICIU/AEI/10.13039/501100011033. TG and SD acknowledge the French National Research Agency (ANR) for ANR-22-CE08-0027-01 and NanoMOL project n°309250, respectively. MR acknowledges the grant FPU23/01435 funded by MCIN/AEI/10.13039/501100011033.
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