Efficient multicolor X-ray excited persistent luminescence enabled by Gd-mediated trap clusters

Domain: chemistry
Published: 2026-01-21
Editorial outcome: major_revision

Abstract
Persistent luminescence materials are promising for night-vision displays, background-free medical diagnostics, and high-resolution radiography, yet achieving efficient violet, yellow, and red emission within a single robust and scalable host remains a longstanding challenging. Here, we overcame this limitation by constructing Gd3+-mediated cluster traps within alkaline-earth fluorochlorides to minimize energy loss during electron migration. These clusters serve as both intrinsic emitters and efficient energy transfer platforms for various activators, including Eu2+, Sm2+, Tb3+, and Mn2+, enabling bright and spectrally tunable multicolor persistent luminescence upon X-ray irradiation. The persistent luminescence intensity of Eu2+ is enhanced by up to 32.7-fold upon Gd3+ codoping. Moreover, violet persistent luminescence from Eu2+ is employed to excite perovskite quantum dots for full-color time-domain dynamic displays, while Sm2+ emission facilitates low-dose, high-resolution delayed X-ray imaging. These findings establish a generalizable strategy for designing efficient multicolor persistent materials for advanced multi-functional optical technologies.

Introduction
Persistent luminescence (PersL) is a fascinating optical phenomenon wherein materials continue to emit photons following the cessation of excitation1–4. This time-delayed photon emission holds great promise for a wide range of applications, including optical data storage5–7, night-vision displays8, background-free medical diagnostics9 and high-resolution radiography10,11. Despite being discovered in the early 17th century, efficient inorganic PersL materials remain largely confined to blue-(e.g., CaAl2O4:Eu/Nd)12,13 and green-emitting (e.g., SrAl2O4:Eu/Dy and Sr2MgSi2O7:Eu/Dy)14,15 systems. Longer-wavelength emissions, such as yellow and red, are typically achieved in sulfide-based materials16,17, which suffer from poor stability and low brightness, while efficient violet PersL yet to be demonstrated. Extending PersL into the violet, yellow, and red spectral regions with high efficiency, especially using environmentally robust and scalable hosts remains a formidable challenge, hindering progress toward full-color displays18–20, multiplexed bioimaging21, and multichannel information encryption22–24. More critically, realizing multicolor PersL within a single host is exceptionally difficult, yet essential for minimizing compatibility issues and simplifying integration into multi-functional optical devices.

Room-temperature PersL typically arises from the spontaneous release of charge carriers trapped in shallow energy states, followed by their migration through the host lattice and subsequent radiative recombination at luminescent centers25. However, long-range carrier migration is often accompanied by substantial non-radiative losses, including multi-phonon relaxation26, defect-induced quenching27, and inefficient energy transfer28,29, which significantly compromises overall luminescence efficiency. In upconversion luminescence, spatially confining sensitizers and activators into clusters has proven effective for suppressing energy loss by shortening energy transfer distances30. Drawing inspiration from this strategy, we propose that co-localizing traps and activators into confined clusters can significantly enhance PersL by minimizing migration-associated energy dissipation, which in turn provides a powerful approach to realize bright multicolor PersL within a single host lattice through doping with various activators.

As a proof-of-concept, we introduce a Gd3+-mediated cluster engineering strategy in alkaline-earth fluorochloride hosts to confine trap carrier migration (Fig. 1). Substituting divalent alkaline-earth with trivalent Gd3+ induces the formation of [Gd-F-]n clusters. Upon X-ray irradiation, fluoride ions coordinated to Gd3+ preferentially form Frenkel defects, resulting in Gd3+-associated trap clusters that produce intense PersL from Gd3+ itself. More importantly, these clusters serve as efficient energy transfer platforms to various activators, including Eu2+, Sm2+, Tb3+, and Mn2+, enabling bright and spectrally tunable multicolor PersL. Notably, emissions from Eu2+ and Sm2+, despite their narrower full widths at half maximum (FWHM), exhibit much higher brightness than the benchmark green-emitting SrAl2O4:Eu/Dy. Moreover, these fluorochloride-based systems demonstrate superior air stability compared to commercial aluminates. We also demonstrate that the violet PersL from Eu2+ can serve as an excitation source for perovskite quantum dots, enabling full-color dynamic displays. Meanwhile, the red PersL from Sm2+ facilitates low-dose, high-resolution delayed X-ray imaging.

Results
Characterization and benchmarking of lanthanide-doped BaFCl phosphors
Lanthanide-doped BaFCl phosphors were scalable synthesized via a conventional scalable solid-state reaction, but under significantly milder conditions (1073 K for 1 h, Supplementary Fig. 1) compared to those typically required for commercial SrAl2O4:Eu/Dy (>1573 K for over 4 h)31. X-ray diffraction (XRD) patterns of the products with and without Gd3+ doping matched well with the phase-pure tetragonal BaFCl structure (Supplementary Fig. 2), featuring an optical band gap of 5.73 eV (Supplementary Fig. 3). Atomic-resolution integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM, Fig. 2a) and high-resolution TEM (Supplementary Fig. 4) revealed a highly crystalline microstructure. Elemental mapping (Fig. 2b) and energy-dispersive X-ray (EDX) spectroscopy (Supplementary Fig. 5) demonstrated the presence of F, Cl, Ba, Gd and Eu elements. Inductively coupled plasma optical emission spectrometry (ICP-OES) further confirmed the Eu and Gd doping levels in BaFCl:2Eu/1Gd to be ~1.51 and 0.62 mol%, respectively, closely agreement with the nominal composition. X-ray photoelectron spectroscopy (XPS) of the Eu 3d states revealed the coexistence of Eu2+ (1125, 1155 eV) and Eu3+ (1135, 1165 eV)32, with the Eu3+/Eu2+ ratio remaining unaffected by Gd3+ codoping (Supplementary Fig. 6).

Upon cessation of X-ray irradiation, the BaFCl:Eu exhibited weak PersL, featuring a sharp peak at 365 nm (Eu2+: 6P7/2 → 8S7/2) and a broad emission centered at 387 nm (Eu2+: 4f65d1 → 8S7/2) (Supplementary Fig. 7)33,34. The optimal Eu doping concentration was determined to be ~2 mol% (Fig. S7b, c). Notably, characteristic Eu3+ emission was absent, likely due to its extremely low probability of capturing trapped carriers10. As shown in Fig. 2c, the PersL intensity of BaFCl:Eu is markedly enhanced upon Gd3+ codoping. At an optimal Gd3+ concentration of 1 mol%, the integrated PersL intensity increases by up to 32.7-fold. The PersL decay curves reveal that both the peak intensity and afterglow duration are substantially improved with Gd3+ incorporation (Fig. 2d). The BaFCl:Eu/Gd exhibits rapid charging kinetics, reaching saturation within ~30 s, and maintains detectable PersL for over 100 h at room temperature (Supplementary Fig. 8). Time-lapse photographic images further corroborate the enhanced and long-lasting brightness across various decay intervals (Fig. 2i). Elevating the X-ray tube voltage from 10 to 50 KV resulted in an additional 65.8-fold enhancement in PersL intensity (Fig. 2e), attributed to the increased secondary electron generation and more efficient trap filling at higher excitation energies.

To benchmark performance, we compared the BaFCl:Gd/Eu with several commercial persistent phosphors, including CaAl2O4:Eu/Nd, SrAl2O4:Eu/Dy, Sr13Al22Si10O66:Eu, ZnS:Cu/Co, and CaS:Eu. Remarkably, the BaFCl:Gd/Eu outperformed all references (Fig. 2f), exhibiting significantly stronger PersL across a wider range of X-ray tube voltages (Supplementary Fig. 9). Notably, it shows a much narrower FWHM (~30 nm) compared to 72 nm for SrAl2O4:Eu/Dy, while its integrated PersL intensity is ~3.5 times higher under identical conditions (Supplementary Fig. 10). More importantly, the PersL intensity of BaFCl:Gd/Eu decreased by only ~5% after 60 days of air exposure with repeated irradiation and daily measurements, markedly outperforming SrAl2O4:Eu/Dy and ZnS:Cu/Co (Fig. 2h, and Supplementary Fig. 11), highlighting its superior long-term durability. As shown in Supplementary Fig. 12, the PersL intensity and spectral profile remained nearly unchanged during 60 continuous irradiation cycles (1 min each), confirming that the decrease in PersL intensity of SrAl2O4:Eu/Dy is not associated with X-ray irradiation damage. Despite the extremely low sensitivity of the human eye to violet light, the violet PersL luminance from BaFCl:Gd/Eu still reaches about 0.16 cd m−2 at 2 s after ceasing X-ray irradiation. This outstanding performance is attributed to the high X-ray absorption coefficient and the effective trapping of charge carriers (Supplementary Fig. 13). Additionally, the BaFCl:Gd/Eu achieves a photoluminescence quantum yield (PLQY) of 92.7%, far exceeding that of SrAl2O4:Eu/Dy (54.7%) (Supplementary Fig. 14). It should be noted that the comparison of reported quantum yields may be affected by the fact that SrAl2O4:Eu/Dy exhibits trapping and detrapping under UV excitation, which can reduce the overall efficiency, whereas BaFCl:Eu does not.

Mechanistic investigations
To uncover the origin of the enhanced PersL, we investigated how heterovalent Gd3+ doping modifies the local structure, trap states, and energy migration dynamics. Extended X-ray absorption fine structure (EXAFS) spectroscopy was employed to probe the coordination environment of Gd3+ in the BaFCl lattice. As shown in Fig. 3a, Supplementary Fig. 15, EXAFS fitting reveals that Gd3+ is coordinated by an average of ~2.1 F− and 2.1 Cl− ions, notably lower than the corresponding values for native Ba2+ sites (4.2 for F− and 5.1 for Cl−, Fig. 3b). In addition, the Gd–F bond length is ~1.88 Å, much shorter than the Ba–F bond length of 2.54 Å (Supplementary Table 1), indicating evident local lattice contraction surrounding the Gd3+ dopants. Despite the Gd3+ doping level being below 1 mol%, a distinct Gd–Gd scattering signal is observed in the Fourier-transformed EXAFS spectrum (Fig. 3c). This unexpected signal, combined with the reduced coordination numbers and shortened bond lengths, manifesting the formation of locally aggregated [Gd-F-]n clusters35. In addition, Raman spectroscopic confirms the appearance of an additional Gd–F vibrational mode36,37, in addition to the characteristic phonon modes of the BaFCl host lattice (Supplementary Fig. 16).

To further elucidate the structure of the clusters, density functional theory (DFT) simulations were performed to model the Gd L-edge X-ray absorption near edge structure (XANES) spectra for different Gd3+ substitution scenarios. Among the constructed crystal models, the configuration in which six Gd3+ ions replace nine Ba2+ ions and create three Ba2+ vacancies to maintain charge neutrality, produced a calculated XANES spectrum that closely matches the experimental data (Supplementary Fig. 17a, d). In contrast, models with either a single Gd3+ substituting one Ba2+, or two Gd3+ ions replacing three Ba2+ ions yielded simulated XANES profiles that deviate significantly from the experimental spectrum (Supplementary Fig. 17b, c, e, f). These results indicate a strong tendency for Gd3+ ions to aggregate within the BaFCl lattice, forming Gd-rich clusters.

Upon X-ray irradiation, high-energy photons interact with heavy atoms primarily through the photoelectric effect, generating hot electrons and deep holes38. These primary carriers undergo multiple scattering and Auger processes, producing abundant low-energy secondary electrons. Concurrently, the F− related Frenkel defects are formed, acting as carrier traps and generating PersL after the cessation of X-rays39–43. As shown in Supplementary Fig. 18, the current gradually increases upon X-ray irradiation and decays slowly with a delay exceeding 7 s after the X-ray is turned off, while the peak current rises at higher tube voltages. These observations indicate that the measured conductivity predominantly arises from defect-related ionic transport under X-ray irradiation. DFT calculations reveal the formation energy (Ef) of F− Frenkel defects within the [Gd-F-]n clusters, is substantially lower than that of F− bound to Ba2+, indicating the traps preferentially form within the clusters (Fig. 3d). Especially, for surface F− ions within the clusters, the Ef for displacement toward the cluster is significantly lower than that for displacement away from the cluster. In this scenario, both the trapped carriers and the associated recombination energy remains spatially confined within the clusters. To probe the role of Ln3+ doping on trap states while avoiding interference from Gd3+-mediated energy transfer, the optically inert Lu3+ ion with an ionic radius similar to that of Gd3+ was employed as a control dopant. X-ray active thermoluminescence (TL) analysis reveals that Lu3+ doping results in similar shallow traps, with no significant changes in trap depth (~0.76 eV) or density (Supplementary Fig. 19)44. In contrast, codoping with Gd3+ leads to a substantial enhancement in TL intensity below 400 K (Fig. 3e, and Supplementary Fig. 20), which is attributed to efficient charge transfer from the trapped carriers to Gd3+ ions within the clusters, followed by subsequent transfer to Eu2+.

Unlike X-ray excited PersL, which arises from the thermal release and migration of trapped electrons, X-ray excited optical luminescence (XEOL) originates from the recombination of free-electrons in the conduction-band with holes45. As shown in Supplementary Fig. 21, in the absence of Eu2+, strong characteristic Gd3+: (6P5/2 → 8S7/2 at 311 nm and 6P7/2 → 8S7/2 at 316 nm) emissions are observed in the XEOL spectrum46. With increasing temperature or excitation power, the I316nm/I311nm intensity ratio increases, indicating enhanced electron–phonon coupling and elevated nonradiative relaxation via the 6P5/2 → 6P7/2 transition (Supplementary Fig. 22). Upon Eu2+ codoping, the XEOL intensity of Gd3+ is nearly fully quenched (Supplementary Fig. 23), suggesting a near-unity energy transfer efficiency from Gd3+ to Eu2+ (Supplementary Fig. 24). A similar suppression is observed in the PersL intensity (Fig. 3f, and Supplementary Fig. 25) and decay curves (Supplementary Fig. 26) of Gd3+, further supporting this efficient energy transfer pathway. This efficiency arises from both the mean Gd–Eu distance of ~0.81 nm (Supplementary Fig. 27a, b), which is far below the typical critical distance of 2 to 6 nm for Förster resonance energy transfer47,48, and the strong overlap between the Eu3+ excitation spectrum and the Gd3+ emissions spectrum (Supplementary Fig. 27c). Notably, the PersL from Gd3+ in BaFCl:Gd lasts for more than 168 h (Supplementary Fig. 28). However, the XEOL intensity and pulsed X-ray excited life-time of Eu2+ in the BaFCl:Eu/Gd are similar to those in BaFCl:Eu and BaFCl:Eu/Lu (Supplementary Fig. 29), indicating that Gd3+ codoping does not enhance the excitation efficiency of the Eu2+: 5d states via free-electron recombination. This behavior is consistent with the spectral overlap between the broad Eu2+ excitation band and the Gd3+ emission region, where Gd3+ acts as an alternate excitation route rather than enhancing the excitation probability. This interpretation is further supported by the comparable photoluminescence intensity of Eu2+ under various UV excitation wavelengths, irrespective of Gd3+ codoping (Supplementary Fig. 30). These results reveal that if trapped electrons are thermally released as free-electrons before captured by activators, Gd3+ exerts minimal influence on the PersL intensity of Eu2+. Therefore, the observed enhancement in PersL upon Gd3+ doping is attributed to the formation of Gd3+-mediated trap clusters, which localize the trapped electrons and enable their efficient transfer to nearby Eu2+ ions. Monte Carlos simulations30 further reveal that spatial clustering of traps and activators within the lattice significantly increases the activation probability of luminescent centers compared to random distribution (Fig. 3g, h), confirming that energy clustering effectively minimizes energy loss during long-range carrier migration.

Universal enhancement of PersL across diverse activators and hosts
To validate the universality of the Gd3+-mediated trap cluster enhancement mechanism and expand its applicability to multi-color PersL, we investigated additional activators, including divalent Sm2+, trivalent Tb3+ and transition metal Mn2+, as well as alternative fluorochloride hosts such as CaFCl and SrFCl. As shown in Fig. 4a–e and Supplementary Fig. 31, all three dopant systems in the BaFCl host exhibited pronounced increases in PersL intensity upon Gd3+ codoping. The enhancements reach up to 150.3-fold for Sm2+, 22.9-fold for Tb3+, and 20.2-fold for Mn2+ (Fig. 4f), attributed to efficient cascade energy transfer from the trap cluster to Gd3+, and subsequently to the activator (Supplementary Fig. 32). The corresponding decay profiles further confirm that Gd3+ incorporation significantly improves both the PersL intensity and duration (Fig. 4g–i). Time-lapse photographic images intuitively demonstrate enhanced brightness across various decay intervals (Fig. 4j–l). In particular, in the absence of Gd3+, the PresL from Sm2+ and Mn2+ was invisible to the naked eye within 1 s post irradiation, but display clear visible emission even after 60 s with Gd3+ codoping. The initial PersL luminance values of the Sm2+, Tb3+, and Mn2+-doped systems reached 3.28, 0.87, and 0.56 cd m−2, respectively. The BaFCl:Gd/Sm achieves a high PLQY of 84.2% (Supplementary Fig. 33), and its integrated red PresL intensity was ~2.2 times higher than that of green SrAl2O4:Eu/Dy and ~47.8 times greater than that of red CaS:Eu (Supplementary Fig. 34). Furthermore, the Gd3+ codoping strategy was extended to other hosts, including CaFCl:Eu and SrFCl:Eu, yielding enhancements in PersL intensity by 101.5 and 107.8 times, respectively (Fig. 4d–f). These results highlight the broad compatibility of the Gd3+-mediated trap cluster engineering approach across different activator ions and host lattices. Notably, the XEOL intensity of these systems does not increase upon Gd3+ codoping (Supplementary Fig. 35), further confirming that the enhanced PersL originates from the formation of Gd3+-mediated trap clusters.

Full color PersL dynamic displays and low-dose delayed X-ray imaging
The superior violet-range PersL performance of BaFCl:Eu/Gd enables its use as an excitation source for halide perovskite (CsPbX3, X=Cl, Br, I) quantum dots (QDs) (Supplementary Fig. 36), thereby achieving full-spectrum PersL across the visible range. The bandgap and corresponding emission wavelength of the QDs were tuned by varying the halide composition49. A flexible BaFCl:Eu/Gd-integrated flexible PDMS film was placed above dispersions of different QDs, irradiated with X-rays for 30 s, and the resulting PersL photographs were captured using a smartphone after the cessation of X-rays (Supplementary Fig. 37). As shown in Fig. 5a and Supplementary Fig. 38, varying the QD composition from CsPbCl2Br1 to CsPbBr3 and CsPbI3 shifts the persistent emission peaks from 433 to 508 and 686 nm, respectively, covering a continuous color range from blue through green to red. Notably, the QD dispersions alone exhibited no observable XEOL or PersL under X-ray excitation (Supplementary Fig. 39), indicating the full-color PersL arises from radiative energy transfer from the BaFCl:Eu/Gd PersL to the QDs via reabsorption50. To further demonstrate dynamic time-domain multicolor display capability, we designed a composite pattern consisting of BaFCl doped with Eu/Gd, Sm/Gd, and Mn/Gd, along with four selected QDs (CsPbCl2Br1, CsPbCl0.5Br2.5, CsPbBr1.3I1.7 and CsPbBr2I1) placed above the BaFCl:Eu/Gd (Fig. 5b). Upon X-ray irradiation, each region displayed distinct PersL colors corresponding to their respective emitters. As Eu2+ PersL decayed over time, the emission from the QDs faded sequentially: yellow (~10 s), followed by red (~60 s). After 120 s, the Sm2+ PersL became invisible due to its faster decay compared to Eu2+ and Mn2+, while the emission from CsPbCl0.5Br2.5 shifted from blue-green to blue.

Benefiting from its rapid charging capability, the BaFCl:Sm/Gd-integrated transparent PMMA film enables low-dose PersL imaging. As shown in Supplementary Fig. 40, after just 5 s of X-ray irradiation at a dose of 1.6 μGy, a time-delayed X-ray image of the standard Pb line-pair template demonstrates a resolution of up to 20 lp mm−1, as validated by gray-value contrast analysis. Further analysis of the modulation transfer function (MTF) (Fig. 5c) confirms a spatial resolution of ~25.7 lp mm−1 at an MTF value of 0.251. The dose rate (0.32 Gy/s) is an order of magnitude lower than that used in medical diagnostics (5.5 μGy/s)52,53, while the resolution outperforms most previously reported PersL systems (Supplementary Table 2) and conventional flat-panel X-ray detectors (typically lower than 5 lp mm−1)54. Moreover, time-delayed images reveal embedded structures in commercial circuit boards that are otherwise invisible (Fig. 5d), highlighting the potential of this system for ultralow-dose, high-resolution, and background-free radiographic imaging.

Methods
Chemicals and reagents
BaF2 (99.9%), BaCl2·2H2O (99.99%), EuCl3·6H2O (99.9%), GdCl3·6H2O (99.99%), SmCl3·6H2O (99.9%), MnCl2·4H2O (99.9%), TbCl3·6H2O (99.99%), CaF2 (99.5%), CaCl2 (99.9%), SrF2 (99.9%), SrCl2·6H2O (99.99%), Cs2CO3 (99.9%), C2H3ClSi (99%), C2H3BrSi (98%), C2H3SiI (97%), Pb(CH3COO)2·3H2O (99.99%), cyclohexane (99.9%), N,N-dimethylformamide (DMF, ≥98%), Poly(methyl methacrylate) (PMMA) were all purchased from Shanghai Aladdin Reagent Co. 1-Octadecene (ODE, 90%), Oleic acid (OA, 90%) and Oleylamine (OAm, ≥98%) were supplied by Sigma Aldrich Co. Commercial SrAl2O4:Eu/Dy, CaAl2O4:Eu/Nd, Sr13Al22Si10O66:Eu, ZnS:Cu/Co, and CaS:Eu phosphors were purchased from Shenzhen Looking Long Technology Co. Gd2O2S:Tb was purchased from Felles Photonic Instruments (China) Co. Polydimethylsiloxane (PDMS) was purchased from Shenzhen Song Shine Technology Co. All chemicals employed in the experiment were of analytical grade and used directly without further treatment.

Synthesis of BaFCl:2Eu/1Gd and other phosphors
BaFCl:2Eu/1Gd was synthesized via a high-temperature solid-state reaction. A stoichiometric mixture of 10 mmol BaF2, 9.4 mmol BaCl2·2H2O, 0.4 mmol EuCl3·6H2O and 0.2 mmol GdCl3·6H2O was thoroughly ground in a mortar for 15 min. The resulting powder was heated at a rate of 7°C/min to 800°C and sintered for 1 h under a flowing 5% H2/N2 atmosphere. After cooling to room temperature, the product was ground for 5 min to yield the BaFCl:2Eu/1Gd phosphor. Other BaFCl-based phosphors, including BaFCl:Eu, BaFCl:Sm/Gd, BaFCl:Mn/Gd, BaFCl:Tb/Gd were synthesized using the same procedure by substituting the corresponding dopant precursors.

Synthesis of flexible BaFCl:Eu/Gd-integrated PDMS film
BaFCl:Eu/Gd phosphor (0.5 g) was dispersed into 1.1 g of PDMS (prepolymer: crosslinker = 10:1 by weight) and stirred for 20 min at room temperature to achieve a homogeneous mixture. The resulting slurry was then cured at 70°C for 2 h, yielding a flexible BaFCl:Eu/Gd-integrated PDMS film.

Synthesis of cesium-oleate (Cs-OA) precursor
Cs2CO3 (0.409 g), ODE (20 mL), and OA (1.25 mL) were added into a 50 mL three-neck flask. The mixture was degassed under N2 at 120°C for 1 h, then heated to 150°C until a clear solution formed. The resulting Cs-oleate solution was transferred to a 10 mL volumetric flask and stored for subsequent use.

Synthesis of CsPbX3 quantum dots (QDs)
Pb(CH3COO)2·3H2O (0.0759 g) was loaded into a 50 mL three-neck flask, followed by the sequential addition of 5 mL ODE, 2.5 mL OA, 2.5 mL OAm, and 250 µL Cs-OA. Then mixture was heated to 110°C under a nitrogen atmosphere and maintained for 45 min. Subsequently, the temperature was rapidly increased to 220°C, at which point 200 µL of a halide precursor solution (Cl, Br, and I in the desired molar ratio) was quickly injected. After 10 s of reaction, the flask was immediately cooled in an ice-water bath to room temperature. The resulting solution was centrifuged at ~5700 × g for 5 min to remove the supernatant. The precipitate was redispersed in 6 mL cyclohexane and centrifuged again at ~4600 × g for 5 min.

Synthesis of composite pattern
BaFCl:Eu/Gd (1 g), BaFCl:Sm/Gd (0.3 g), and BaFCl:Mn/Gd (1 g) phosphors were each dispersed in 1.1 g, 0.33 g, and 1.1 g of PDMS, respectively. The mixtures were then cast into a silicone butterfly mold, with each phosphor occupying designated regions. The mold was then cured at 70°C for 2 h to form the composite pattern. The wings embedded with BaFCl:Eu/Gd phosphor were further decorated with various halide perovskite QDs.

Synthesis of BaFCl:Sm/Gd-integrated film
PMMA powder (0.65 g) was dissolved in 3 mL of DMF by heating to 70°C under stirring for 4 h to obtain a transparent solution. A 1 mL aliquot of this solution was then mixed with 50 mg of BaFCl:Sm/Gd and stirred for 2 h until homogeneous. The resulting mixture was ultrasonicated for 5 min, poured into a glass mold, and left to stand for 10 min to eliminate air bubbles. Finally, the film was cured at 70°C for 2 h to obtain the BaFCl:Sm/Gd-integrated film.

Characterizations
X-ray diffraction (XRD) analysis was collected using a powder diffractometer (Bruker D8 Advance) equipped with a Cu-Kα radiation (λ=1.5405 Å). Microstructure images and elemental compositions was obtained using a scanning electron microscope (SEM) (Japan Hitachi SU8010) equipped with an energy dispersive X-ray spectroscopy (EDS, Aztec X-Max 80T). X-ray photoelectron spectroscopy (XPS) analyzes were performed using Thermo Fisher Scientific K-Alpha with a tube voltage of 15 kV and a tube current of 10 mA. Photoluminescence (PL) and photoluminescence excitation (PLE), and photoluminescence quantum yield (PLQY) were carried out using Edinburgh FS-5 spectrofluorometer (Xenon Lamp, 150 W). X-ray excited optical luminescence (XEOL) spectra, X-ray excited persistent luminescence (PersL) spectra, and PersL decay curves were recorded using a spectrometer (OmniFluo 960 X-ray, Zolix, China) coupled with an X-ray tube and Hamamatsu CR131 Photomultiplier Tube. The afterglow curves reported in this work represent photon counts directly obtained from the spectrometer. The PersL spectra presented in Fig. 4a–c, Supplementary Figs. 8a, 22, 31, 34, 35a, b, and 38 were recorded using a fiber-coupled spectrometer (QEpro) linked to the X-ray source, with an acquisition time of ~5 s. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed using an Agilent 7700 instrument. Thermoluminescence (TL) curve was tested by LTTL-3DS type TL 3D spectrometer. Raman microscope (Gloucestershire, UK) was utilized to acquire Raman spectra. The temperature-dependent XEOL spectra were recorded by TAIZI Godzilla fiber spectrometer (Shanghai, China) with a copper heater. Atomic-resolution integrated differential phase contrast scanning transmission electron microscopy (iDPC-STEM) and high-resolution TEM images were conducted using a Double spherical aberration-corrected transmission electron microscope (Spectra 300, Thermofisher, US). UV-vis spectra were measured using a Craic Technologies micro-spectrophotometer. Pulsed X-ray excited decay curve was measured using an X-ray excitation luminescence lifetime test system (picoX 5084, Shanghai UPU Optoelectronic Technology co.). X-ray imaging experiments were conducted using a custom-built setup including a miniature X-ray tube and an ORCA-Fusion BT camera (C15440-20UP).

Measurement of PersL luminescence brightness: 100 mg of phosphor powder was evenly spread on a transparent plate and gently pressed with a glass plate to ensure uniform thickness. The prepared sample plate was then placed on the probe of a photometer (Xinbao Instrument, SM218E). The sample was irradiated with X-rays at 50 kV for 1 min, and the initial afterglow luminance was recorded immediately after the X-ray source was turned off.

First-principles DFT calculations were carried out using the Vienna Ab initio Simulation Package (VASP, version 5.4.4) with the projector augmented wave (PAW) method55. The exchange–correlation functional was treated within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) functional56. A plane-wave cutoff energy of 400 eV was employed. Geometry optimizations were performed until the residual forces on all atoms were less than 0.03 eV Å⁻¹. Brillouin-zone integrations were performed using a 3×3×3 Monkhorst-Pack k-point mesh for a 4×4×2 supercell. The self-consistent electronic convergence criterion was set to 10−4 eV. The defect formation energies (Ef) of three types of F− defects, F− located inside the Gd cluster, on the surface of the Gd cluster, and far from the Gd cluster, were calculated according to the following equation:
Ef = Edefect − Eperfect (1)
Where Eperfect and Edefect represent the total energies of the Gd-doped supercell without and with F− Frenkel defects, respectively. To elucidate the formation energy and migration tendency of F− Frenkel defects, the F− ions were systematically displaced from their original lattice sites by -0.2 Å, -0.4 Å, -0.6 Å, -0.8 Å, -1.0 Å, -1.5 Å, 0.2 Å, 0.4 Å, 0.6 Å, 0.8 Å, 1.0 Å, and 1.5 Å, respectively.

Gd L-edge XANES simulations were performed using the finite difference method for near-edge structure (FDMNES) code (version 2025.5.5), a DFT-based and fully relativistic program for ab initio X-ray spectroscopy calculations57,58. To identify the formation of Gd-rich clusters in BaFCl, structural models containing different amounts of Gd3+ ions were constructed and relaxed using DFT optimization to minimize manual bias prior to FDMNES simulations. Preliminary calculations of the Gd L2,3-edge were carried out in the multiple scattering mode (Green’s formalism) under the muffin-tin approximation using the energy-dependent Hedin–Lundqvist exchange potential to rapidly identify the structural configurations consistent with the experimental spectra. Subsequently, accurate simulations were conducted using the finite difference method with a self-consistent field cluster radius of 6 Å. The cluster radius (R) was progressively increased until spectral convergence was achieved, and in this case, R = 6.5 Å was found to be sufficient. To ensure optimal agreement with experimental results, convolution parameters were further refined by comparing the simulated and measured spectra.

Energy migration from traps to Gd3+ activators was modeled using Monte Carlo simulations (version 3.12.4)30,59. Specifically, the spatial distribution of traps and activators was constructed based on the EXAFS-derived structural model (Fig. S17a). In the clustered configuration, the actual Gd3+ doping concentration was set to 2%, and the lattice parameters of BaFCl were taken as a=b=4.418 Å, c=7.281 Å, and α=β=90°. A total of 40 clusters were generated, each modeled as a 1 nm-diameter sphere with Gd3+ ions randomly distributed inside, and randomly positioned within a 10 nm cubic crystal. Trap states were also randomly distributed within the clusters. To compare the differences in energy transfer between clustered and uniform configurations, a non-clustered model was constructed in which both traps and Gd3+ ions were randomly distributed throughout a 10 nm cubic crystal, while maintaining identical structural and compositional parameters. A Förster energy transfer framework with a Förster radius of ~1 nm was employed to describe the charge transfer dynamics between traps and Gd3+ activators. Each Monte Carlo simulation was repeated 1000 times to ensure statistically reliable results.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, Yang and coworkers employ a host-guest doping strategy, utilizing Gd³⁺-mediated cluster traps within the\nBaFCl lattice to achieve violet persistent luminescence (PersL). These platforms can efficiently transfer energy to activators\nlike Eu2+, Sm2+, Tb3+, and Mn2+, enabling tunable multicolor PersL under X-ray irradiation. The application of EXAFS in\nconjunction with DFT to characterize trap states demonstrates a certain degree of innovation. Nevertheless, some problems\nstill need to be addressed.\n1. It seems that Fig. S15b is not well-fitted. Is it necessary to refit it and conduct an analysis?\n2. From TL analysis (Fig. S17 a), it seems that the mechanistic explanations for the PersL within the two models, which may\ninvolve both shallow and deep trap states. you cannot selectively analyze only the TL spectra within the range of 300-400K\n(Fig. 3 e, Fig. S17 b-c). Furthermore, is there a phenomenon of trap-state electron tunneling that leads to long afterglow?\n3. The carriers trapped in deep traps can be gradually transferred to the activator (Eu2+, Sm2+, Tb3+, and Mn2+) through\ntunneling-mediated energy transfer, which constitutes a critical mechanism underlying energy transfer processes, and the\nauthors should explicitly clarify this point (RSC Advances, 2015, 5, 82704-82710)\n4. From Figs. 4g–l, the sole introduction of Eu²⁺, Sm²⁺, Tb³⁺, and Mn²⁺ dopants can also lead to the formation of new trap\nstates for PersL. Combining the trap states generated by Gd3+, it is crucial to consider whether the observed long afterglow\nphenomenon originates from the migration of charge carriers among these trap states, rather than being exclusively\nattributed to energy transfer mechanisms. The PersL mechanism still needs to be further elaborated.\n5. The existing systems are already very complex. Without a clear mechanism, why did the author choose to further conduct\nexperiments with using perovskite materials? Does the author want to illustrate something more meaningful? Moreover,\nlead-based perovskites also have instability and toxic issues.\n6. The spatial resolution is approximately 25.7 lp/mm, whereas the corresponding dose rate on the same material is only\n0.32 μGyair/s. Why does one performance indicator show a significantly higher value, whereas the other remains relatively\nlow? What factors contribute to this discrepancy?\n7. \"BaFCl:1Gd and BaFCl:2Eu/1Gd\"—a consistent and precise notation style should be employed to explicitly reflect the\ncompositional ratios, as determined by ICP-OES analysis, instead of utilizing a vague or generalized format.\n8. The fabrication of PersL materials and widely used doping strategy for light-emitting applications are hot topics in\nchemistry and materials. To arouse a broad interest from readership in this field, some strongly related works on the recent\nfabrications of PersL materials (Nat. Commun. 2024, 15, 5281; Angew. Chem. Int. Ed. 2025, 64, e202502782; CCS\nChemistry, 2023, 5, 2866), and the wide doping strategy for light-emitting applications (Nat. Commun. 2024, 15, 9491;\nAngew. Chem. Int. Ed. 2025, 64, e202505322; Chem. Soc. Rev. 2021, 50, 5564) could be included as references.\n9. As an excitation source, the X-ray irradiation dose must be explicitly stated in all figure captions; otherwise, it may lead to\nthe misconception that a high quantum yield was achieved using a large radiation dose.", "reviewer_2": "This work reports a new strategy for developing multicolor X-ray–excited\npersistent luminescence (PersL) materials, which can be further extended to design\nhighly efficient PersL systems and enable multifunctional optical applications. By\nconstructing Gd3+-mediated cluster traps within alkaline-earth fluorochloride hosts to\nminimize energy loss during electron migration, the authors achieve efficient emission\nacross the violet, yellow, and red spectral regions. Notably, bright PersL is realized\nwithin a single, robust, and scalable host, thereby minimizing compatibility issues and\nsimplifying integration into multifunctional optical devices. In particular, the X-ray–\nexcited PersL achieved through cluster construction is remarkably efficient.\nBaFCl:Gd/Eu exhibits an integrated PersL intensity 3.5 times higher than that of the\nbenchmark green-emitting SrAl₂O₄:Eu/Dy. The initial luminance of ≈4.53 cd m⁻² is\nexceptionally high for a violet PersL material. Furthermore, the authors demonstrate\nthe use of these PersL materials as excitation sources for perovskite quantum dots to\nachieve full-color time-domain dynamic displays, as well as for low-dose, high-\nresolution delayed X-ray imaging.\nOwing to the scalable preparation method, high performance, and excellent air\nstability, these materials show clear potential for commercialization. Overall, this is a\nhighly valuable contribution and suitable for publication. I recommend acceptance after\nthe authors address the following points to strengthen the manuscript.\n1. XPS spectra indicate the coexistence of Eu2+ and Eu3+ in the as-prepared\nBaFCl:Eu, whereas only PersL from Eu3+ was observed. Why was no PersL from Eu3+\ndetected? In addition, efficient Gd3+ → Eu3+ energy transfer is commonly reported, yet\nno Eu3+ PersL is observed in the BaFCl:Gd/Eu system. Could the authors clarify this\nphenomenon?\n2. Both the X-ray tube voltage and current influence the PersL intensity. The\nauthors reported that “Elevating the X-ray tube voltage from 10 to 50 KV resulted in\nan additional 65.8-fold enhancement in PersL intensity...”. Was the X-ray tube current\nkept fixed? If so, please provide the detailed value.\n3. The photograph in Figure 2i lacks a scale bar, which should be included for clarity.\n4. When comparing PersL intensity, the authors used SrAl O :Eu/Dy as a reference,\n2 4\nwhereas for luminance comparison, CaAl O :Eu/Nd was used. Why not use the same\n2 4\nreference material for consistency? As shown in Figure 2f, the PersL intensities of\nSrAl O :Eu/Dy and CaAl O :Eu/Nd appear similar.\n2 4 2 4\n5. The authors state that efficient emission was achieved across the violet, yellow,\nand red spectral regions. However, only the PLQY values of BaFCl:Eu/Gd and\nBaFCl:Sm/Gd are provided. I suggest that the PLQY of BaFCl:Eu/Mn should also be\nincluded for completeness.\n6. The authors attribute the full-color PersL to radiative energy transfer from\nBaFCl:Eu/Gd PersL to the QDs via reabsorption. It would strengthen the claim to\nprovide the photoluminescence excitation spectrum of at least one type of QD.\n7. CsPbX quantum dots have been widely investigated for X-ray detection and\n3\nimaging in previous studies, which suggests that efficient XEOL should be detectable\nfrom these QDs. However, the authors report that no XEOL was observed from the QDs.\nThis discrepancy should be clarified.", "reviewer_3": "This review report regards the manuscript “Ultrabright Multicolor X-ray Persistent Luminescence Enabled by Gd-mediated\nTrap Clusters” submitted to Nature Communications. The manuscript presents an investigation into the use of Gd as a\nbridging element to connect charge-trapping centers in BaFCl to luminescent centers. The central idea - that introducing Gd\ncreates clusters of defects and luminescent centers which can universally enhance persistent luminescence - is interesting\nand of potential importance to the field. Finally BaFCl:Eu/Gd is combined with perovskite quantum dots to obtain persistent\nluminescence over a wide range of colors and the materials is also tested for X-ray imaging applications.\nHowever, despite the promise of the concept, the manuscript suffers from several critical shortcomings in execution and\npresentation. The work lacks focus, the conclusions are not consistently supported by experimental data, and there is a lack\nof clarity in the methodology used to evaluate luminescence intensities which raises doubts about the reported intensities\n(one of the more interesting results of the manuscript). These concerns raise doubts about the reliability of the results and\nprevent me from recommending publication in its current form. Below I grouped the most important concerns per theme.\nConcerns regarding the proposed mechanism:\n1. Throughout the manuscript there is an ambiguity regarding the processes of charge transfer and energy transfer. There is\nno clear distinction between the part of the process driven by charge transfer (trapping and detrapping) and the part related\nto energy transfer (between Gd3+ ions and from Gd3+ to the activator). It is unclear what the recombination center in this\nmaterial is.\n2. It is proposed that Frenkel defects play an important role in the persistent luminescence mechanism. For this the authors\nrefer to references 7 and 33. In reference 7 the involvement of Frenkel defects is implied based on DFT calculations but\nnotably no Frenkel defects are observed in EPR. Similar conclusions can be drawn from reference 33, so the statement is\nnot supported by any experimental evidence.\n3. The mechanism proposed in Figure S22 implies the simultaneous detrapping of both electron and holes and subsequent\nrecombination at Gd3+. Based on results from numerous other persistent phosphor, it is highly improbable that this takes\nplace at the same temperature.\n4. Monte Carlo simulations are included in the manuscript but they are not coupled to any of the experimental results. The\nonly conclusion that follows from the ‘simulation’ is that the energy transfer is more probable is the donor and acceptor are\ncloser together which is a well known experimental observation.\nExperimental details:\nThere is a lot of information in the supplementary but key information is missing.\n1. How were persistent luminescence intensities in cd/m² obtained? Given that the intensity of persistent luminescence is\none of the ‘selling points’ of the manuscript this information is important.\n2. What was the excitation intensity or wavelength during quantum yield measurements? The quantum yield of persistent\nphosphors is known to be intensity dependent so this is important information.\n3. How were DFT calculations performed? DFT is used a lot throughout the manuscript but very little information about the\ncalculations or the results is available. Instead only the interpretation of the results is included in the manuscript.\n4. The stability of several materials in ‘air’ is recorded over 60 days ‘under repeated irradiation at one-day intervals’.\nSrAl2O4 is stable in air so presumably the decrease in intensity is due to radiation damage but there is no way to tell due to\nthe lack of details in the manuscript.\nQuantification of persistent luminescence intensities and comparison to benchmarks:\n1. The results of persistent luminescence intensities are scattered over several figures but none of the figures have tics or\nnumbers on the y-axis so it is impossible to compare intensities. I would have liked to see the afterglow and TL of the\nEu, Gd, Eu/Gd and Eu/Lu sample in one figure with comparable intensities.\n2. To quantify the performance of the BaFCl:Eu/Gd the material’s persistent luminescence is compared to other persistent\nluminescent materials such as SrAl2O4:Eu,Dy, CaAl2O4:Eu,Nd, … However, the comparison is flawed because these\nmaterials, benchmarks for UV/VIS excited persistent luminescence, contain much lighter elements so the X-ray attenuation\nwill be lower. The initial intensity for commercial SrAl2O4:Eu,Dy powder after excitation with UV light is in the order of 10\ncd/m² which is higher than the intensity reported here for BaFCl:Eu/Gd.\n3. The quantum yield is reported under UV excitation and the values obtained for SrAl2O4:Eu,Dy and BaFCl:Eu/Gd are\ncompared to each other. This is not a fair comparison as SrAl2O4:Eu,Dy exhibits trapping and detrapping under UV\nexcitation while BaFCl:Eu/Gd does not.\n4. For some of the BaFCl compositions the initial intensities (but not the decay curves) are reported in cd/m², a photometric\nunit taking into account the eye sensitivity. However, the emission of BaFCl:Eu,Gd is almost entirely (>80%) located under\n400 nm where the eye sensitivity is zero (< 0.01). This raises a question as to how this can give rise to such a high intensity\nof 4.53 cd/m². Especially considering that SrAl2O4:Eu,Dy which has an emission that almost perfectly coincides with\nmaximum eye sensitivity (~1) has an emission intensity of at most 10 cd/m² and is only 5 times weaker than the\nBaFCl:Eu/Gd phosphor (according to 2f).\n5. Afterglow times up to 168 hours are reported but this is highly dependent on the sensitivity of the equipment used to\nrecord the persistent luminescence. It is standard practice to report the time needed to reach a certain threshold intensity or\nat least report the intensity of the light after the stated persistent luminescence time (and the integration time used after\n168h).\n6. In the conclusions it is stressed that the Sm2+ intensity is increased by a factor 150.3. However, in Figure 5 the red\nluminescence isn’t visible anymore after 120 seconds. This raises more questions about how the results are presented in\nthe manuscript.\n7. The material is combined with quantum dots to obtain persistent luminescence over the entire spectrum. But the spectra\nare normalized so there is no information about how intense the luminescence of the quantum dots is in the end. Also, how\nefficient is the light from the persistent luminescence of BaFCl:Eu/Gd converted to visible quantum dot emission?"}, "author_response": "Response to reviewer’s comments\nWe greatly appreciate the reviewers’ insightful comments which are very helpful for\nimprovement of our manuscript. In response to the valuable comments raised by the\nreferees, we provide point-by-point responses along with the modifications (marked\nin blue) made in the revised manuscript.\nReviewer 1:\nComment 1: It seems that Fig. S15b is not well-fitted. Is it necessary to refit it and\nconduct an analysis?\nResponse 1: We sincerely appreciate the reviewer’s valuable comment. We have\nrefitted the EXAFS results of the Ba L-edge in k-space, as shown in the revised Fig.\nS15b. After refitting, the R-factor decreased from 0.0187 to 0.0063, demonstrating a\nsignificant improvement in fitting quality. The EXAFS analysis confirms the presence\nof three major scattering paths: Ba–F, Ba–Cl, and Ba–Ba. Owing to the larger\ncoordination environment around Ba compared with Gd, the fitting quality of Ba\nL-edge data is lower than that of Gd K-edge. Nevertheless, the obtained R-factor of\n0.0063 indicates that the fitting results are reliable and reasonable.\nThe revised Fig. S15b and the corresponding discussion have been incorporated into\nthe revised manuscript.\nThe fitting results are summarized in Table S1. The obtained R-factors (0.0063\nfor Ba and 0.0020 for Gd) demonstrate that both fittings are reliable and physically\nreasonable. The EXAFS analysis confirms the presence of three major scattering\npaths for Ba (Ba–F, Ba–Cl, and Ba–Ba) and Gd (Gd–F, Gd–Cl, and Gd–Gd),\nrespectively. These results indicate that Gd3+ ions successfully substitute Ba2+ sites in\nthe lattice, resulting in a reduced local coordination number and increased structural\ndistortion around Gd.\nc XANES spectra of Gd K-edge. d EXAFS fitting of Gd K-edge in k-space.\nComment 2: From TL analysis (Fig. S17 a), it seems that the mechanistic\nexplanations for the PersL within the two models, which may involve both shallow\nand deep trap states. you cannot selectively analyze only the TL spectra within the\nrange of 300-400K (Fig. 3 e, Fig. S17 b-c). Furthermore, is there a phenomenon of\ntrap-state electron tunneling that leads to long afterglow?\nResponse 2: We sincerely appreciate the reviewer’s insightful comment. In this work,\nwe mainly focus on the room-temperature persistent luminescence (PersL), which\nprimarily originates from the thermal release of electrons stored in shallow trap states.\nAs shown in Fig. R1, the population of deep traps is markedly reduced in Gd/Eu\nco-doped BaFCl compared with Eu-doped BaFCl, while the corresponding trap depth\nincreases slightly from 0.90 eV (450/500) to 0.952 eV (476/500). This observation\nsuggests that the bright PersL observed at room temperature is predominantly\ngoverned by shallow traps rather than deep ones.\nphosphors.\nPrevious studies have shown that shallow traps with depths around 0.69 eV are\noptimal for room-temperature PersL emission, whereas deeper traps (>0.9 eV) mainly\ncontribute to photostimulated luminescence under optical excitation [Adv. Mater.\n2020, 32, 2003881; Chem. Sci. 2019, 10, 5452–5460; Matter 2023, 6, 4261–4273].\nLiu et al. further demonstrated that short-range displacements of fluoride ions\ngenerate transient shallow traps, while long-range displacements form long-lived deep\ntraps. Electrons in shallow traps can spontaneously escape to the conduction band\nover time, accompanied by defect self-healing, whereas electrons in deep traps require\nexternal optical or thermal stimulation to migrate to luminescent centers (Nature 2021,\n590, 410–415).\nTherefore, we selectively analyzed the TL spectra within the 300–400 K range to\nhighlight the contribution of shallow traps that dominate PersL behavior at room\ntemperature. According to the reviewer’s helpful comment, Fig. S20 has been revised\naccordingly, and the following clarification has been added to the revised manuscript:\n“The deep-trap population is markedly reduced in Gd/Eu co-doped BaFCl compared\nwith Eu-doped BaFCl, and the corresponding trap depth increases from 0.9 eV to 0.95\neV, indicating that the bright PersL observed at room temperature is predominantly\ngoverned by shallow traps rather than deep ones.”\nband and subsequently captured by luminescent centers, or directly transferred to\nluminescent centers through quantum tunneling. Dellbecq et al. reported that, in\nKCl:AgCl and KCl:TlCl systems, the PersL decay resulting from tunneling\nrecombination between trapped holes and electrons follows a linear dependence of\nluminescence intensity (I) on the inverse of time (t-1) [J. Phys. Chem. C 2009, 113,\n20493]. This t-1 decay behavior has since been widely recognized as a characteristic\nsignature of tunneling-assisted recombination in persistent phosphors [Phys. Rev. B\n1974, 9, 4497; Nat. Mater. 2012, 11, 58; J. Alloys Compd. 2010, 504, 177]. Following\nthese precedents, we analyzed the PersL decay profiles of BaFCl:Eu, BaFCl:Eu/Gd,\nand BaFCl:Gd at room temperature, as well as the PersL spectra of BaFCl:Gd at\ndifferent temperatures (89, 129, 169, 209, 249K), to evaluate the possible contribution\nof tunneling. As shown in Fig. R2a–c, the PersL intensity of all examined samples\ndoes not exhibit a linear correlation with t-1, indicating that quantum tunneling is not\nthe dominant recombination mechanism in our system. Moreover, as the temperature\nincreases from 89 K to 249 K, the PersL intensity rises significantly, suggesting that\nthe release of trapped electrons is predominantly thermally activated rather than\ntunneling-assisted.\nintensity–time plot (red line), and fitted linear curve (blue line) for BaFCl:2Eu\nphosphor. (b) PersL decay curve at 387 nm emission (black line), inverse\nintensity–time plot (red line), and fitted linear curve (blue line) for BaFCl:2Eu/1Gd\nphosphor. (c) PersL decay curve at 311 nm emission (black line), inverse\nintensity–time plot (red line), and fitted linear curve (blue line) for BaFCl:1Gd\nphosphor. (d) PersL spectra of BaFCl:1Gd phosphor after X-ray irradiation at different\ntemperatures ranging from 89 K to 249 K.\nComment 3: The carriers trapped in deep traps can be gradually transferred to the\nactivator (Eu2+, Sm2+, Tb3+, and Mn2+) through tunneling-mediated energy transfer,\nwhich constitutes a critical mechanism underlying energy transfer processes, and the\nauthors should explicitly clarify this point (RSC Advances, 2015, 5, 82704-82710).\nResponse 3: We appreciate the reviewer’s insightful suggestion and the reference\nhighlighting the tunneling-mediated energy transfer mechanism. Following this, we\nanalyzed the PersL decay behavior of BaFCl:Eu and BaFCl:Gd to evaluate the\npossible contribution of tunneling, which is typically characterized by a linear\ndependence of luminescence intensity (I) on the inverse of time (t-1). It should be\nnoted that the PersL intensities of Sm2+-, Tb3+-, and Mn2+-doped samples were too\nweak and decayed too rapidly, resulting in large fitting errors; thus, they were\nunsuitable for analyzing tunneling recombination behavior. Following these\nprecedents, we analyzed the PersL decay profiles of BaFCl:Eu, BaFCl:Eu/Gd,\nand BaFCl:Gd at room temperature, as well as the PersL spectra of BaFCl:Gd at different\ntemperatures (89, 129, 169, 209, 249K) to evaluate the possible contribution of\ntunneling. As shown in Fig. R2a–c, the PersL intensity of all examined samples\ndoes not exhibit a linear correlation with t-1, indicating that quantum tunneling is not\nthe dominant recombination mechanism in our system. Moreover, as the temperature\nincreases from 89 K to 249 K, the PersL intensity rises significantly, suggesting that\nthe release of trapped electrons is predominantly thermally activated rather than\ntunneling-assisted.\ndopants can also lead to the formation of new trap states for PersL. Combining the\ntrap states generated by Gd3+, it is crucial to consider whether the observed long\nafterglow phenomenon originates from the migration of charge carriers among these\ntrap states, rather than being exclusively attributed to energy transfer mechanisms.\nThe PersL mechanism still needs to be further elaborated.\nResponse 4: Thanks for the reviewer’s kind comment. Liu et al. (Nature 2021, 590,\n410–415) reported that “through elastic collisions between high-momentum X-ray\nphotons and light fluoride ions, anion Frenkel defects are generated within the crystal\nlattice, which can trap thermalized low-energy electrons and enable long-lived photon\nstorage. Specifically, fluoride vacancies and interstitials act as electron and hole traps,\nrespectively. Displacements of fluoride ions over short and long distances lead to the\nformation of transient shallow and long-lived deep trap states, respectively.” In this\ncontext, the introduction of different activators does not alter the intrinsic nature of\nthese traps but instead modulates their depths and distributions. As shown in Fig. S18,\nthe deep-trap population is markedly reduced in Gd/Eu co-doped BaFCl compared\nwith Eu-doped BaFCl, while the corresponding trap depth increases from 0.90 eV\n(450/500) to 0.952 eV (476/500). In contrast, the shallow trap depth remains nearly\nunchanged, but its thermoluminescence (TL) intensity is significantly enhanced. The\nreduction in deep-trap population upon Gd incorporation indicates a decreased\nelectron-trapping efficiency, suggesting that charge migration from deep to shallow\ntraps is not promoted in Gd/Eu co-doped BaFCl.\nElectron migration among different trap states is highly temperature-dependent\nand is strongly suppressed at low temperatures. To examine the possible presence of\nsuch trap-to-trap migration, we measured the PersL spectra of BaFCl:Gd and\nBaFCl:Eu/Gd at 89 K. As shown in Fig. R3, the PersL intensity of Gd3+ is\nsubstantially reduced, consistent with the behavior observed at room temperature.\nThese results suggest that carrier migration among trap states plays a limited role in\nthe observed enhanced afterglow. Furthermore, the energy transfer processes from\nGd3+ to Eu2+, Sm2+, Tb3+, and Mn2+ have been extensively reported in previous studies\n6\n(refs: Acta Metallurgica Sinica-English Letters, 2015, 28, 487-491; Nature, 2021, 590,\n410–415;). Therefore, the markedly enhanced PersL intensity observed in Gd/Eu\nco-doped BaFCl is most likely dominated by efficient energy transfer rather than by\ncarrier migration among different trap states.\nGd\nEu/Gd\ny\nt\ni\ns T = 89 K\nn\ne\nt\nn\ni\nL\ns\nr\ne\nP\n300 350 400 450\nWavelength (nm)\nFig. R3 X-ray excited PersL spectra of BaFCl:Gd and BaFCl:Eu/Gd recorded at 89 K.\nComment 5: The existing systems are already very complex. Without a clear\nmechanism, why did the author choose to further conduct experiments with using\nperovskite materials? Does the author want to illustrate something more meaningful?\nMoreover, lead-based perovskites also have instability and toxic issues.\nResponse 5: We sincerely appreciate the reviewer’s valuable comment. The\nexperiments involving perovskite quantum dots (QDs) were not intended to further\ncomplicate the PersL mechanism but rather to demonstrate the exceptionally bright\nviolet afterglow of the Gd/Eu-codoped BaFCl system, which is sufficiently intense to\nexcite perovskite QDs. This experiment serves as an application-oriented\nproof-of-concept, independent of the underlying PersL mechanism.\nBecause purely inorganic afterglow systems often face challenges in achieving\nfine spectral tunability, we employed perovskite QDs as a color-tunable model system.\nTheir well-defined emission bands and high compatibility with violet excitation\nenabled the realization of bright, full-spectrum afterglow modulation through simple\n7\nphysical mixing of Gd/Eu:BaFCl and perovskite QDs. We can reasonably speculate\nthat, with appropriate heterostructure design and interface engineering, both the color\nand brightness of the afterglow emission could be further optimized.\nWe fully acknowledge that lead-based perovskites suffer from toxicity and\nstability issues. In future studies, such QDs can be replaced with environmentally\nbenign alternatives—such as organic fluorophores or carbon-based dots—which offer\ncomparable tunability and excitation compatibility. Therefore, the present results can\nserve as a guiding framework for research groups experienced with organic or\ninorganic carbon dots to further develop this concept.\nFinally, the observed color-tunable afterglow also implies the potential for\nmulti-color, time-resolved delayed X-ray imaging. However, realizing this prospect\nwill require a dedicated time-gated imaging setup and extensive additional\nexperiments, which are beyond the scope of the present study.\nComment 6: The spatial resolution is approximately 25.7 lp/mm, whereas the\ncorresponding dose rate on the same material is only 0.32 μGyair/s. Why does one\nperformance indicator show a significantly higher value, whereas the other remains\nrelatively low? What factors contribute to this discrepancy?\nResponse 6: We thank the reviewer for this insightful comment. In X-ray imaging,\nimage quality is primarily governed by the scintillation brightness and the\nsignal-to-noise ratio (SNR) of the detected photons. Under higher radiation doses, a\nlarger number of X-ray photons interact with the scintillator, generating stronger light\noutput and consequently improving image contrast and spatial resolution. Therefore,\nconventional X-ray imaging typically requires relatively high radiation doses to obtain\nclear and detailed images, which inevitably raises concerns regarding radiation safety.\nEnhancing the performance of X-ray detection materials is thus crucial for achieving\nhigh-quality imaging under reduced exposure conditions.\nIn this work, we employ X-ray–excited persistent luminescent (PersL) materials\nto enable time-delayed X-ray imaging, where the emitted afterglow contributes to\nimage formation even after the cessation of X-ray excitation. Theoretically, the\n8\nresolution of delayed images increases with higher radiation doses, as stronger\nexcitation produces brighter afterglow emission. As shown in Fig. R4, the image\nresolution indeed improves with increasing X-ray dose rate from 0.23 to 3.64 μGy\n(Adv. Funct. Mater. 2024, 34, 2409156). However, our objective is to balance image\nquality with radiation safety by maintaining high spatial resolution under low-dose\nconditions. Based on both literature reports and our experimental results, the\nGd/Eu-codoped BaFCl system exhibits remarkably high spatial resolution (~25.7\nlp·mm-1) even at a low dose rate of only 0.32 μGy·s-1. This exceptional performance\noriginates from its intense PersL emission, which preserves image contrast and\neffectively compensates for the reduced excitation intensity. Therefore, the high\nspatial resolution achieved under low radiation dose does not imply any inconsistency\nbut rather highlights the superior photon utilization efficiency of the developed\nX-ray–excited persistent luminescent material, enabling high-clarity imaging even\nunder minimal irradiation.\nFig. R4. X-ray images of a circuit board obtained under different X-ray irradiation\ndoses. (Adapted from Adv. Funct. Mater. 2024, 34, 2409156.)\nComment 7: \"BaFCl:1Gd and BaFCl:2Eu/1Gd\"—a consistent and precise notation\nstyle should be employed to explicitly reflect the compositional ratios, as determined\nby ICP-OES analysis, instead of utilizing a vague or generalized format.\nResponse 7: We appreciate the reviewer’s insightful comment. The actual doping\nconcentrations in BaFCl:1Gd and BaFCl:2Eu/1Gd were determined by ICP-OES\nanalysis. The results show that the Gd concentration in BaFCl:1Gd is approximately\nXX mol%, while the Eu and Gd concentrations in BaFCl:2Eu/1Gd are ~1.51 and 0.62\nmol%, respectively. These values indicate that the actual doping concentrations are\nclose to the nominal ones.\nother researchers, we have retained the nominal doping compositions (e.g.,\nBaFCl:1Gd and BaFCl:2Eu/1Gd) in the text, as is commonly adopted in the fluoride\nliterature (Nat. Photonics, 2024, 18, 440-449; J. Am. Chem. Soc. 2011, 133,\n17122–17125; Nat. Commun., 2020, 11, 1174; Adv. Mater. 2015, 27, 6418–6422).\nThe corresponding actual concentrations determined by ICP-OES have been added to\nthe revised manuscript accordingly.\nInductively coupled plasma optical emission spectrometry (ICP-OES) further\nconfirmed the Eu and Gd doping levels in BaFCl:2Eu/1Gd to be ~ 1.51 and 0.62\nmol%, respectively, closely agreement with the nominal composition.\nInductively coupled plasma optical emission spectrometry (ICP-OES) analysis\nconfirmed that the Gd doping levels in BaFCl:0.5Gd, BaFCl:1Gd, and BaFCl:1.5Gd\nwere approximately 0.65%, 1.08%, and 1.54 mol%, respectively, which are in close\nagreement with their nominal compositions.\nComment 8: The fabrication of PersL materials and widely used doping strategy for\nlight-emitting applications are hot topics in chemistry and materials. To arouse a\nbroad interest from readership in this field, some strongly related works on the recent\nfabrications of PersL materials (Nat. Commun. 2024, 15, 5281; Angew. Chem. Int. Ed.\n2025, 64, e202502782; CCS Chemistry, 2023, 5, 2866), and the wide doping strategy\nfor light-emitting applications (Nat. Commun. 2024, 15, 9491; Angew. Chem. Int. Ed.\n2025, 64, e202505322; Chem. Soc. Rev. 2021, 50, 5564) could be included as\nreferences.\nResponse 8: We appreciate the reviewer’s insightful comment. We have carefully\nreviewed the suggested references, which indeed provide valuable perspectives and\ncan attract broad interest from readers in this field. Accordingly, these references have\nbeen incorporated into the revised manuscript.\nComment 9: As an excitation source, the X-ray irradiation dose must be explicitly\nstated in all figure captions; otherwise, it may lead to the misconception that a high\n10\nquantum yield was achieved using a large radiation dose.\nResponse 9: We appreciate the reviewer’s insightful suggestion. In accordance with\nthis comment, the X-ray irradiation conditions have been explicitly added to all\nrelevant figure captions in the revised manuscript to avoid any potential\nmisunderstanding regarding the excitation dose. It should be noted that the X-ray\nirradiation dose depends strongly on both the tube power and the irradiation time.\nComment 1: XPS spectra indicate the coexistence of Eu2+ and Eu3+ in the as-prepared\nBaFCl:Eu, whereas only PersL from Eu3+ was observed. Why was no PersL from Eu3+\ndetected? In addition, efficient Gd3+ → Eu3+ energy transfer is commonly reported,\nyet no Eu3+ PersL is observed in the BaFCl:Gd/Eu system. Could the authors clarify\nthis phenomenon?\nResponse 1: We thank the reviewer for this insightful comment. Liu et al. reported\nthat no Eu3+ emission was observed because hole–Eu3+ centers could not form in\nEu3+-doped fluoride compounds, as supported by electronic structure calculations\n(Nature, 2021, 590, 410–415). Similarly, Chen et al. demonstrated that Eu3+ ions,\nwhose 4f ground states lie within the valence band, are unable to trap holes under\nirradiation, thereby preventing electron–hole recombination and persistent\nluminescence generation (Matter, 2023, 6, 4261–4273). Therefore, the absence of\nEu3+ PersL in our system is likely due to the inability of Eu3+ ions to effectively\ncapture trapped holes. In the BaFCl:Gd/Eu system, the excited Eu2+ ions exhibit a\nmuch larger absorption cross-section than Eu3+, which further suppresses the\nobservation of Eu3+ emission.\nThe following explanation has been added to the revised manuscript: Notably,\ncharacteristic Eu3+ emission was absent, likely due to its extremely low probability of\ncapturing trapped carriers.\nThe authors reported that “Elevating the X-ray tube voltage from 10 to 50 KV\nresulted in an additional 65.8-fold enhancement in PersL intensity...”. Was the X-ray\ntube current kept fixed? If so, please provide the detailed value.\nResponse 2: We agree that both the X-ray tube voltage and current influence the\nPersL intensity. According to the reviewer’s insightful comment, the X-ray tube\ncurrent was kept constant at 200μA, and this information has been added to the\nrevised manuscript accordingly.\nfor clarity.\nResponse 3: According to the reviewer’s insightful comment, a scale bar of 1 cm has\nbeen added to the photograph in Figure 2i for clarity.\nComment 4: When comparing PersL intensity, the authors used SrAl O :Eu/Dy as a\n2 4\nreference, whereas for luminance comparison, CaAl O :Eu/Nd was used. Why not use\n2 4\nthe same reference material for consistency? As shown in Figure 2f, the PersL\nintensities of SrAl O :Eu/Dy and CaAl O :Eu/Nd appear similar.\n2 4 2 4\nResponse 4: We appreciate the reviewer’s insightful comment. The commercial\nSrAl O :Eu/Dy phosphor exhibits the highest PersL intensity under X-ray excitation;\n2 4\ntherefore, it was selected as the reference to highlight the superior X-ray–excited\nPersL performance of our BaFCl:Eu/Gd system. In contrast, the luminance intensity is\nclosely related to the spectral sensitivity of the human eye, which is much higher in\nthe green region than in the blue or violet regions. To ensure a relative fair visual\ncomparison, we used CaAl O :Eu/Nd, a blue-emitting phosphor, as the reference for\n2 4\ncomparing the luminance intensity of BaFCl:Eu/Gd.\nComment 5: The authors state that efficient emission was achieved across the violet,\nyellow, and red spectral regions. However, only the PLQY values of BaFCl:Eu/Gd\nand BaFCl:Sm/Gd are provided. I suggest that the PLQY of BaFCl:Eu/Mn should also\nbe included for completeness.\nResponse 5: We appreciate the reviewer’s insightful comment. In our experiments,\nwe attempted to measure the PLQY of all studied systems, including BaFCl:Eu/Gd,\nBaFCl:Sm/Gd, and BaFCl:Eu/Mn. However, the BaFCl:Eu/Mn sample cannot be\neffectively excited under UV irradiation, making it impossible to obtain a reliable\nPLQY value for this composition.\nComment 6: The authors attribute the full-color PersL to radiative energy transfer\nfrom BaFCl:Eu/Gd PersL to the QDs via reabsorption. It would strengthen the claim\n13\nto provide the photoluminescence excitation spectrum of at least one type of QD.\nResponse 6: We appreciate the reviewer’s helpful comment. According to the\nsuggestion, the absorption spectra of the CsPbBr I , CsPbBr I and CsPbBr I QDs\n1 2 1.3 1.7 2 1\nhas been provided in the revised Supporting Information (Fig. S35). It can be clearly\nobserved that the excitation band of the QDs well overlaps with the PersL emission\nregion of BaFCl:Eu/Gd under X-ray irradiation, supporting the proposed radiative\nenergy transfer mechanism via reabsorption.\nFig. S35 Absorption spectra of CsPbBr I , CsPbBr I and CsPbBr I .\n1 2 1.3 1.7 2 1\nComment 7: CsPbX quantum dots have been widely investigated for X-ray\n3\ndetection and imaging in previous studies, which suggests that efficient XEOL should\nbe detectable from these QDs. However, the authors report that no XEOL was\nobserved from the QDs. This discrepancy should be clarified.\nResponse 7: Thanks for the reviewer’s kind comment.\nWe appreciate the reviewer’s insightful comment. The X-ray attenuation coefficient (μ)\ncan be calculated from the relationship between the incident intensity (I ) and the\n0\ntransmitted intensity (I) according to the equation , where x is the sample\nthickness (J. Phys.: Conf. Ser. 2020, 1567, 032069). Based on this attenuation law,\nX-ray absorption is strongly dependent on material thickness. Therefore, CsPbX QDs\n3\ndispersed in solution exhibit significantly weaker X-ray absorption compared with\nsolid thin films. As a result, when the QD solution was irradiated under X-ray source,\nneither XEOL nor PersL was detected. This result is consistent with previous reports,\nin which CsPbX QDs were used exclusively in solid-state configurations—such as\n3\nthin films or substrate coatings—for X-ray detection and imaging (Nature 2018, 561,\n88; Adv. Mater. 2019, 31, 1901644).\nConcerns regarding the proposed mechanism:\nComment 1: Throughout the manuscript there is an ambiguity regarding the\nprocesses of charge transfer and energy transfer. There is no clear distinction between\nthe part of the process driven by charge transfer (trapping and detrapping) and the part\nrelated to energy transfer (between Gd3+ ions and from Gd3+ to the activator). It is\nunclear what the recombination center in this material is.\nResponse 1: We greatly appreciate the reviewer’s insightful comment. In Figure 1\nand its caption, we have now explicitly distinguished between the charge-transfer (CT)\nand energy-transfer (ET) processes. Under X-ray irradiation, elastic collisions\nbetween high-momentum X-ray photons and light fluoride ions induce the formation\nof anion Frenkel defects within the crystal lattice, thereby enabling long-lived photon\nstorage (refs: Mod. Probl. Condens. Matter Sci. 1986, 13, 473–525; Laser Photonics\nRev., 2025, 2500092; Nature, 2021, 590, 410–415; Comment 2). These defects\ngenerate fluoride vacancies and interstitials that act as electron traps and hole traps,\nrespectively. The trapped charge carriers can subsequently migrate toward\nluminescence centers, where radiative recombination occurs. In the absence of Gd3+\ncodoping, mainly the CT process occurs—electrons are captured from the conduction\nband and later released to recombine with holes at luminescence centers, resulting in\nweak persistent luminescence (PersL). Upon Gd3+ codoping, the detrapping process\nprimarily occurs within Gd3+-rich clusters, where released electrons migrate toward\nGd3+ ions and recombine with localized holes, thereby populating the excited states of\nGd3+. This is subsequently followed by an ET process from Gd3+ to the activator ions,\nwhich markedly enhances the PersL intensity.\nConsidering that the primary distinction between the samples with and without\nGd3+ lies in the detrapping and subsequent energy-transfer processes, the\nelectron-trapping step from the conduction band was omitted from the schematic for\nclarity, but is illustrated in Fig. S24.\nillustration of structural evolution in MFCl (M = Ca, Sr, Ba) upon Gd3+ doping and\nthe corresponding charge transfer (CT) and energy transfer (ET) pathways. For clarity,\nCl- ions are omitted from the crystal structures. Two Gd3+ ions substitute for three\nBa2+ ions, generating one Ba2+ vacancy to maintain charge neutrality. The Gd3+\ndopants tend to aggregate within the crystal lattice, promoting the formation of\nX-ray-induced Frenkel defects in Gd-rich clusters. Upon Gd3+ codoping, the\ndetrapping process predominantly occurs within these clusters, where released\nelectrons migrate toward Gd3+ ions and recombine with captured holes, populating the\nexcited states of Gd3+. Subsequently, ET from Gd3+ to the activator ions gives rise to\nstrong multicolor PersL. In contrast, the absence of such clusters leads to long-range\ncarrier migration and increased non-radiative losses, yielding faint PersL.\nBased on the above analysis, we propose that upon Gd3+ codoping, the radiative\nrecombination centers are predominantly located at Gd3+ ions within Gd-rich clusters,\nas most X-ray-induced Frenkel defects are largely confined to these regions. In\ncontrast, in the absence of Gd3+ codoping, recombination mainly occurs at the\nluminescence centers (Nature, 2021, 590, 410–415). When electron detrapping takes\nplace within the clusters, carrier migration distances are minimized, thereby reducing\nnonradiative losses. Without such confinement, however, carriers undergo long-range\nmigration accompanied by substantial energy dissipation. Consequently, the formation\n16\nof Gd-rich clusters markedly enhances the persistent luminescence performance.\nTo more clearly distinguish the charge-transfer processes (trapping and\ndetrapping) from the energy-transfer processes (among Gd3+ ions and from Gd3+ to\nactivators), as well as to clarify the recombination centers, we have redrawn Fig. S24\nand added the corresponding discussion (see Comment 3).\nComment 2: It is proposed that Frenkel defects play an important role in the\npersistent luminescence mechanism. For this the authors refer to references 7 and 33.\nIn reference 7 the involvement of Frenkel defects is implied based on DFT\ncalculations but notably no Frenkel defects are observed in EPR. Similar conclusions\ncan be drawn from reference 33, so the statement is not supported by any\nexperimental evidence.\nResponse 2: We appreciate the reviewer’s valuable comment. The formation of\nFrenkel defects under X-ray irradiation is physically reasonable and has been widely\nreported in previous studies (Point defects in materials, Wiley, New York, 1988;\nDefects and defect processes in nonmetallic solids Wiley, New York, 1985; Laser\nPhotonics Rev., 2025, 2500092; Low Temperature Physics, 2018, 44, 269–277; Mod.\nProbl. Condens. Matter Sci., 1986, 13, 473–525; J. Phys. Chem. Solids, 1960, 17,\n88–92). Although direct observation of X-ray-induced Frenkel defects remains\nchallenging—mainly due to the lack of in-situ techniques compatible with\nhigh-energy X-rays and associated radiation safety concerns—numerous works have\nemployed EPR and optical methods to provide indirect yet compelling evidence of\ntheir presence.\nUpon X-ray irradiation, energetic photons interact with the crystal lattice,\ntransferring sufficient energy to displace ions from their equilibrium positions. This\nprocess generates Frenkel defects, consisting of a vacancy and an interstitial ion pair.\nIn fluoride-based hosts, the anion sublattice (F⁻) is particularly susceptible to such\ndisplacements due to its relatively low binding energy. The formation of Frenkel\ndefects introduces additional charge carriers within the bandgap, thereby altering the\nelectrical transport behavior. Consequently, the electrical conductivity measurement\n17\nserves as an effective probe to confirm defect generation. The increase in conductivity\nafter X-ray exposure arises from the elevated concentration of mobile charge carriers\n(such as F⁻ vacancies or interstitials). By comparing the conductivity before and after\nirradiation, one can quantitatively infer the formation and evolution of\nradiation-induced Frenkel defects in the lattice. From this view of point, we conducted\nthe electrical conductivity measurement for the BaFCl:Eu/Gd.\nFigure R5 shows the schematic of the device used to measure the X-ray\nvoltage-dependent current of the BaFCl:Eu/Gd film. The BaFCl:Eu/Gd film and FTO\nglass were clamped together using a probe-type alligator clip. The current was\nmeasured under a DC bias of 200 V, while the X-ray source operated in an on–off\nmode, being activated every 30 seconds and maintained for 30 seconds each time. The\nX-ray tube voltage was varied in the range of 15 - 60 kV. As shown in Fig. S18,\nthe current exhibits a pronounced rise when the X-ray is switched on, and its peak value\nincreases with higher X-ray tube voltages. Such a voltage-dependent enhancement in\nconductivity suggests the formation of Frenkel defects induced by X-ray irradiation.\nFig. R5 Photograph and schematic illustration of the setup used to detect the current\nsignals of BaFCl:Eu/Gd films under X-ray irradiation.\nBased on the above results and analysis, we have incorporated the following\nadditional content into the revised manuscript to strengthen the discussion and clarify\nthe experimental evidence.\nAs shown in Fig. S18, the current exhibits a pronounced rise when the X-ray is\nswitched on, and its peak value increases with higher X-ray tube voltages. This\nvoltage-dependent enhancement in conductivity further supports the formation of\nFig. S18 Current signal of the BaFCl:Eu/Gd film measured under X-ray irradiation at\ntube voltages of 15, 30, and 60 kV.\nComment 3: The mechanism proposed in Figure S22 implies the simultaneous\ndetrapping of both electron and holes and subsequent recombination at Gd3+. Based\non results from numerous other persistent phosphor, it is highly improbable that this\ntakes place at the same temperature.\nResponse 3: We greatly appreciate the reviewer’s insightful comment highlighting\nthat the simultaneous detrapping of both electrons and holes is highly improbable. We\napologize for the misleading depiction in the previous version of the figure. Under\nX-ray irradiation, elastic collisions between high-momentum X-ray photons and light\nfluoride ions generate anion Frenkel defects within the crystal lattice. These defects\ncreate fluoride vacancies and interstitials, which serve as electron and hole traps,\nrespectively (refs: Mod. Probl. Condens. Matter Sci. 1986, 13, 473–525; Laser\nPhotonics Rev., 2025, 2500092; Nature, 2021, 590, 410–415; Comment 2). Upon\nGd3+ codoping, these traps are predominantly confined within Gd-rich clusters.\nElectrons in shallow traps can be thermally released over time and migrate toward\n19\nluminescence centers. Separately, trapped holes may also migrate to these centers;\nhowever, these two detrapping events occur at different temperatures rather than\nsimultaneously. During recombination, the radiative energy released from these\ncharge carriers populates the excited states of Gd3+ ions, which subsequently transfer\nenergy to the activator ions. To clarify this sequence, we have redrawn Figure S24 and\nprovided revised mechanistic explanations in the manuscript.\nFig. 24 Schematic illustration of the charge transfer (CT, trapping and detrapping) and\nenergy transfer (ET) from Gd3+ to Eu2+.\nDuring the XEOL process, X-ray photons primarily interact with heavy atoms\nthrough the photoelectric effect, producing hot electrons and deep holes. Subsequent\nelectron–electron scattering and Auger relaxation generate numerous low-energy\nsecondary carriers. XEOL emission occurs when the excited states of activator ions\nare populated through the radiative recombination of these thermalized carriers. In\ncontrast, during the X-ray-excited PersL process, elastic collisions between\nhigh-momentum X-ray photons and light fluoride ions induce the formation of anion\nFrenkel defects, enabling long-lived photon storage. These defects act as electron and\nhole traps, and the trapped charge carriers migrate separately to luminescence centers\nover time. Upon Gd3+ codoping, the traps become spatially confined within Gd-rich\nclusters, promoting efficient radiative recombination through Gd3+ and subsequent\nenergy transfer to activator ions.\ncoupled to any of the experimental results. The only conclusion that follows from the\n‘simulation’ is that the energy transfer is more probable is the donor and acceptor are\ncloser together which is a well known experimental observation.\nResponse 4: We appreciate the reviewer’s insightful comment. We first\nexperimentally confirmed the presence of Gd-rich clusters in BaFCl:Gd/Eu. Since it is\nnot feasible to prepare two samples with and without Gd clustering while maintaining\nidentical chemical compositions, we employed Monte Carlo simulations to\ncomplement the experimental observations. These simulations were designed to\nelucidate the differences in electron migration from traps to activators between\nclustered and uniformly distributed configurations under equivalent compositional\nconditions. The direct correlation between the Monte Carlo simulations and our\nexperimental results lies in the use of the crystal structure model derived from the\nEXAFS analysis (Fig. S17a). Considering that the excited-state population of Gd3+\noriginates from the radiative recombination energy of electrons and holes, we adopted\na Förster energy transfer framework to simulate the charge transfer process between\ntraps and activators (Nature Nanotechnology, 2021, 16, 1011–1018).\nThe following methodological details have been added to the revised\nSupporting Information for clarity.\nEnergy migration from traps to Gd3+ activators was modeled using Monte Carlo\nsimulations[5]. Specifically, the spatial distribution of traps and activators was\nconstructed based on the EXAFS-derived structural model (Fig. S17a). In the\nclustered configuration, the actual Gd3+ doping concentration was set to 2%, and the\nlattice parameters of BaFCl were taken as a = b = 4.418 Å, c = 7.281 Å, and α = β =\n90°. A total of 40 clusters were generated, each modeled as a 1 nm-diameter sphere\nwith Gd3+ ions randomly distributed inside, and randomly positioned within a 10 nm\ncubic crystal. Trap states were also randomly distributed within the clusters. To\ncompare the differences in energy transfer between clustered and uniform\nconfigurations, a non-clustered model was constructed in which both traps and Gd3+\nions were randomly distributed throughout a 10 nm cubic crystal, while maintaining\n21\nidentical structural and compositional parameters. A Förster energy transfer\nframework with a Förster radius of approximately 1 nm was employed to describe the\ncharge transfer dynamics between traps and Gd3+ activators.[6,7] Each Monte Carlo\nsimulation was repeated 1000 times to ensure statistically reliable results.\nExperimental details:\nComment 1: How were persistent luminescence intensities in cd/m2 obtained? Given\nthat the intensity of persistent luminescence is one of the ‘selling points’ of the\nmanuscript this information is important.\nResponse 1: We appreciate the reviewer’s valuable comment. In accordance with the\nreviewer’s suggestion, detailed experimental information on the measurement of\npersistent luminescence intensities (in cd·m-2) has been added to the revised\nmanuscript.\nMeasurement of PersL luminescence brightness: 100 mg of phosphor powder was\nevenly spread on a transparent plate and gently pressed with a glass plate to ensure\nuniform thickness. The prepared sample plate was then placed on the probe of a\nphotometer (Xinbao Instrument, SM218E). The sample was irradiated with X-rays at\n50 kV for 1 min, and the initial afterglow luminance was recorded immediately after\nthe X-ray source was turned off.\nA schematic illustration of the luminescence brightness measurement setup is shown\nin Fig. R6.\nFig. R6 Schematic illustration of the luminescence brightness measurement setup.\nmeasurements? The quantum yield of persistent phosphors is known to be intensity\ndependent so this is important information.\nResponse 2: We appreciate the reviewer’s valuable comment. The photoluminescence\nquantum yield (PLQY) measurements were performed using an Edinburgh\nInstruments FS5 spectrofluorometer equipped with a 150 W continuous xenon lamp\nas the excitation source. The excitation wavelengths used for SrAl O :Eu/Dy,\n2 4\nBaFCl:Eu/Gd, and BaFCl:Sm/Gd were 365 nm, 323 nm, and 365 nm, respectively.\nThis information has been added to the corresponding figure captions in the revised\nSupporting Information.\nComment 3: How were DFT calculations performed? DFT is used a lot throughout\nthe manuscript but very little information about the calculations or the results is\navailable. Instead only the interpretation of the results is included in the manuscript.\nResponse 3: According to the reviewer’s helpful comment, we have incorporated\nadditional computational details into the revised Supporting Information.\nFirst-principles DFT calculations were carried out using the Vienna Ab initio\nSimulation Package (VASP) with the projector augmented wave (PAW) method.[1]\nThe exchange–correlation functional was treated within the generalized gradient\napproximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) functional.[2] A\nplane-wave cutoff energy of 400 eV was employed. Geometry optimizations were\nperformed until the residual forces on all atoms were less than 0.03 eV Å⁻¹.\nBrillouin-zone integrations were performed using a 3 × 3 × 3 Monkhorst–Pack\nk-point mesh for a 4 × 4 × 2 supercell. The self-consistent electronic convergence\ncriterion was set to 10-4 eV. The defect formation energies (E) of three types of F-\nf\ndefects, F- located inside the Gd cluster, on the surface of the Gd cluster, and far from\nthe Gd cluster, were calculated according to the following equation:\nWhere E and E represent the total energies of the Gd-doped supercell\nperfect defect\n23\nwithout and with F- Frenkel defects, respectively. To elucidate the formation energy\nand migration tendency of F- Frenkel defects, the F- ions were systematically\ndisplaced from their original lattice sites by -0.2 Å, -0.4 Å, -0.6 Å, -0.8 Å, -1.0 Å,\n-1.5 Å, 0.2 Å, 0.4 Å, 0.6 Å, 0.8 Å, 1.0 Å, and 1.5 Å, respectively.\nGd L-edge XANES simulations were performed using the finite difference\nmethod for near-edge structure (FDMNES) code, a DFT-based and fully relativistic\nprogram for ab initio X-ray spectroscopy calculations that determines the electronic\nstructure self-consistently in real space[3,4]. To identify the formation of Gd-rich\nclusters in BaFCl, structural models containing different amounts of Gd3+ ions were\nconstructed and relaxed using DFT optimization to minimize manual bias prior to\nFDMNES simulations. Preliminary calculations of the Gd L -edge were carried out\n2,3\nin the multiple scattering mode (Green’s formalism) under the muffin-tin\napproximation using the energy-dependent Hedin–Lundqvist exchange potential to\nrapidly identify the structural configurations consistent with the experimental spectra.\nSubsequently, accurate simulations were conducted using the finite difference method\nwith a self-consistent field cluster radius of 6 Å. The cluster radius (R) was\nprogressively increased until spectral convergence was achieved, and in this case, R =\n6.5 Å was found to be sufficient. To ensure optimal agreement with experimental\nresults, convolution parameters were further refined by comparing the simulated and\nmeasured spectra.\nComment 4: The stability of several materials in ‘air’ is recorded over 60 days ‘under\nrepeated irradiation at one-day intervals’. SrAl O is stable in air so presumably the\n2 4\ndecrease in intensity is due to radiation damage but there is no way to tell due to the\nlack of details in the manuscript.\nResponse 4: We appreciate the reviewer’s valuable comment. To evaluate the\nlong-term PersL stability of SrAl O :Eu/Dy under repeated irradiation, each\n2 4\nmeasurement in Fig. 2h was performed after X-ray exposure for 20 s at one-day\nintervals over a period of 60 days. To further assess whether radiation damage\ncontributed to the observed intensity decrease, we monitored the PersL behavior of\n2 4\neach exposure lasting 1 min. As shown in Fig. S12, both the PersL intensity and\nspectral profile remained nearly unchanged, confirming that the intensity reduction is\nnot caused by X-ray-induced damage.\nSince the SrAl O :Eu/Dy powder was stored in ambient air without any\n2 4\nprotective atmosphere, the gradual decrease in intensity is most likely due to moisture\nabsorption from the environment. This explanation aligns with previous reports\nshowing that the PersL performance of SrAl O :Eu/Dy deteriorates upon exposure to\n2 4\nhumid conditions or water, owing to its 3D AlO tetrahedral network that forms\n4\nchannel-like structures. These channels facilitate the penetration of water molecules,\nleading to O–Sr–O bond cleavage and hydrolysis, which severely degrade or\neliminate luminescent performance (Adv. Funct. Mater. 2025, e15541; Adv. Funct.\nMater. 2025, 35, 2414872).\nThe following text has been added to the revised manuscript: As shown in Figure S12,\nthe PersL intensity and spectral profile remained nearly unchanged during 60\ncontinuous irradiation cycles (1 min each), confirming that the decrease in PersL\nintensity of SrAl O :Eu/Dy is not associated with X-ray irradiation damage.\n2 4\nFig. S12. (a) Persistent luminescence (PersL) spectra of SrAl O :Eu/Dy recorded over\n2 4\ndifferent irradiation cycles. (b) Integrated PersL intensity of SrAl O :Eu/Dy as a\n2 4\nfunction of irradiation cycle number.\nQuantification of persistent luminescence intensities and comparison to benchmarks:\nseveral figures but none of the figures have tics or numbers on the y-axis so it is\nimpossible to compare intensities. I would have liked to see the afterglow and TL of\nthe Eu, Gd, Eu/Gd and Eu/Lu sample in one figure with comparable intensities.\nResponse 1: We sincerely appreciate the reviewer’s thoughtful comment. In the\nrevised manuscript, we have added the TL spectra of the BaFCl:Eu, BaFCl:Eu/Gd,\nand BaFCl:Eu/Lu samples in a single figure with comparable intensities (Fig. S20a).\nThese data were originally provided in Fig. S17a and Fig. S18 of the Supporting\nInformation. It should be noted that the emission bands of BaFCl:Gd are located at\n311 and 316 nm, which are below the detection limit (350–1000 nm) of the\nLTTL-3DS type TL 3D spectrometer equipped with X-ray excitation. Therefore, it is\nnot feasible to accurately record the TL spectra of BaFCl:Gd using this instrument,\nand the obtained data would not reliably represent the trap characteristics of this\nsample. Despite our efforts to contact several research groups, we were unable to\naccess suitable equipment capable of measuring in this wavelength range. We would\ngreatly appreciate the reviewer’s understanding of this limitation. In addition, we have\nincluded a new figure presenting the PersL spectra of the Eu-, Gd-, Eu/Gd-, and\nEu/Lu-doped samples with comparable intensities (Fig. S20b). These data were\npreviously shown separately in Fig. 2c and Fig. 3f of the original manuscript.\nFig. S20 a Comparison of X-ray active TL spectra of BaFCl:2Eu, BaFCl:2Eu/1Lu and\nBaFCl:2Eu/1Gd phosphors. b Comparison of X-ray active PersL spectra of\nBaFCl:2Eu, BaFCl:1Gd, BaFCl:2Eu/1Lu and BaFCl:2Eu/1Gd phosphors.\npersistent luminescence is compared to other persistent luminescent materials such as\nSrAl O :Eu,Dy, CaAl O :Eu,Nd, … However, the comparison is flawed because these\n2 4 2 4\nmaterials, benchmarks for UV/VIS excited persistent luminescence, contain much\nlighter elements so the X-ray attenuation will be lower. The initial intensity for\ncommercial SrAl O :Eu,Dy powder after excitation with UV light is in the order of 10\n2 4\ncd/m² which is higher than the intensity reported here for BaFCl:Eu/Gd.\nResponse 2: We appreciate the reviewer’s insightful comment. We agree that the\nemployed commercial persistent luminescent materials are benchmarks for UV/VIS\nexcitation. We also acknowledge that the initial PersL intensity of commercial\nSrAl O :Eu,Dy after UV excitation (on the order of 10 cd·m-2) is higher than that of\n2 4\nBaFCl:Eu/Gd. At present, however, there are no well-commercialized materials\nspecifically designed for X-ray-excited PersL. The primary objective of our work is\nprecisely to develop bright, stable, and long-lasting X-ray–excited persistent\nluminescent materials that could be potentially commercialized in the future. Under\nthis circumstance, we used these well-known UV-excited commercial phosphors as\nreferences for comparative evaluation. Upon X-ray irradiation, incoming photons\ninteract with the lattice atoms mainly through the photoelectric and Compton\nscattering effects, generating hot electrons and deep holes (ACS Energy Lett. 2021, 6,\n739–768). These primary charge carriers undergo electron–electron scattering and\nAuger processes, producing abundant secondary electrons with lower kinetic energy.\nThrough interactions with phonons, their energy is gradually dissipated, allowing\nlow-energy electrons and holes to be trapped at defect sites. Therefore, UV-excited\ncommercial persistent materials can also exhibit PersL under X-ray excitation.\nAs shown in Fig. R7, under identical excitation power (10 W), the PersL\nintensity of SrAl O :Eu,Dy under X-ray irradiation reaches approximately 65.7% of\n2 4\nthat obtained under UV excitation, confirming that its persistent luminescence can be\neffectively activated by X-rays. We agree with the reviewer that SrAl O :Eu,Dy\n2 4\ncontains lighter elements, leading to a lower X-ray attenuation coefficient. In contrast,\nthe superior X-ray–excited performance of BaFCl:Eu/Gd originates from its higher\nrevised manuscript.\n365nm UV\nX-Ray\ny\nt\ni\ns\nn\ne\nt\nn\ni\nL\ns\nr\ne\nP\n350 400 450 500 550 600 650 700\nWavelength (nm)\nFig. R7. Comparison of PersL spectra of SrAl O :Eu,Dy under X-ray irradiation and\n2 4\nUV excitation at the same excitation power of 10 W.\nConsidering that there are currently no well-established materials specifically\noptimized for X-ray–excited PersL, and that the PersL intensity of SrAl O :Eu,Dy\n2 4\nunder X-ray irradiation reaches about 65.7% of that under UV excitation at the same\nexcitation power, SrAl O :Eu,Dy remains the most suitable reference material\n2 4\navailable at present.\nComment 3: The quantum yield is reported under UV excitation and the values\nobtained for SrAl O :Eu,Dy and BaFCl:Eu/Gd are compared to each other. This is not\n2 4\na fair comparison as SrAl O :Eu,Dy exhibits trapping and detrapping under UV\n2 4\nexcitation while BaFCl:Eu/Gd does not.\nResponse 3: We appreciate the reviewer’s valuable comment and fully agree that\nSrAl O :Eu,Dy exhibits trapping and detrapping processes under UV excitation.\n2 4\nAccording to the equation TotalQY = n(total) / n(abs) = (n + n ) / n(abs), Victor\nPL PersL\nCastaing et al. reported that the total quantum yield (TotalQY) of a SrAl O :Eu,Dy\n2 4\nsingle crystal is approximately 53.2%, which is comparable to the PLQY estimated\nfrom steady-state measurements (Adv. Optical Mater. 2024, 12, 2401638). Therefore,\n28\nalthough SrAl O :Eu,Dy involves trapping and detrapping under UV excitation while\n2 4\nBaFCl:Eu/Gd does not, the total quantum yield of SrAl O :Eu,Dy remains lower than\n2 4\nthat of BaFCl:Eu/Gd.\nComment 4: For some of the BaFCl compositions the initial intensities (but not the\ndecay curves) are reported in cd/m2, a photometric unit taking into account the eye\nsensitivity. However, the emission of BaFCl:Eu,Gd is almost entirely (>80%) located\nunder 400 nm where the eye sensitivity is zero (< 0.01). This raises a question as to\nhow this can give rise to such a high intensity of 4.53 cd/m². Especially considering\nthat SrAl O :Eu,Dy which has an emission that almost perfectly coincides with\n2 4\nmaximum eye sensitivity (~1) has an emission intensity of at most 10 cd/m² and is\nonly 5 times weaker than the BaFCl:Eu/Gd phosphor (according to 2f).\nResponse 4: We appreciate the reviewer’s insightful comment. We agree that the\nhuman eye sensitivity is nearly zero (<0.01) below 400 nm. In our case, however, the\nPersL band of BaFCl:Eu,Gd extends to ~450 nm, with the integrated intensity ratio\n𝐼 /𝐼 = 18%. Therefore, a small but non-negligible portion of the\n(400–450) nm <400 nm\nemission lies within the region detectable by the employed photometer (Xinbao\nInstrument, SM218E). To confirm the reliability of our photometric measurement, we\ncalibrated the system using a commercial SrAl O :Eu,Dy phosphor. The UV-excited\n2 4\ninitial PersL brightness recorded by the same instrument was ~14.86 cd/m2 (Fig. R8a),\nwhich is slightly higher than the value (~12.1 cd/m2) reported in the literature (Optical\nMaterilas, 2016, 53, 94-100). Under X-ray excitation, the recorded initial brightness\nof SrAl O :Eu,Dy was ~11.09 cd/m2 (Fig. R8b), corresponding to about 74% of the\n2 4\nvalue measured under UV excitation. This ratio is consistent with their respective\nPersL spectra (see Comment 2, Fig. R), confirming that our photometric system and\nmethodology are reliable.\nAs shown in Fig. R8c, the steady-state luminescence brightness of BaFCl:Eu,Gd\nunder continuous X-ray irradiation reached ~13.9 cd/m2, mainly originating from the\n400–450 nm range. Immediately after ceasing the X-ray excitation, the initial\nrecorded brightness decreased to ~4.48 cd/m2 (Fig. R8d, re-measured value, slightly\n29\ndifferent from that in the original manuscript). This brightness further decayed to\n~0.16 cd/m2 (Fig. R8e) within 2 s, owing to the ultra-low human eye sensitivity. For\ncomparison, SrAl O :Eu,Dy maintained a brightness above 3 cd/m2 (Fig. R8f) under\n2 4\nthe same X-ray conditions. These results collectively validate the reasonableness of\nthe reported photometric values and the consistency of our measurement procedure.\nFig. R8 Photographs showing the measurement of luminescence brightness using the\nphotometer (Xinbao Instrument, SM218E).\nTo further clarify the luminescence brightness, we have added the following\nstatement to the revised manuscript: It should be noted that the violet PersL from\nBaFCl:Gd/Eu decays to approximately ~0.16 cd/m2 within 2 s, primarily due to the\nextremely low sensitivity of the human eye to violet emission.\nComment 5: Afterglow times up to 168 hours are reported but this is highly\ndependent on the sensitivity of the equipment used to record the persistent\nluminescence. It is standard practice to report the time needed to reach a certain\nthreshold intensity or at least report the intensity of the light after the stated persistent\nluminescence time (and the integration time used after 168h).\nResponse 5: We appreciate the reviewer’s insightful comment regarding the\ndependence of the afterglow duration on the detection sensitivity. The emission bands\nof BaFCl:Gd are centered at 311 and 316 nm, which are beyond the detectable range\nof the photometer used for visible brightness measurements. Therefore, we provide\nthe detailed detection parameters, including the detector type, integration time, and\nphoton counts. The PersL decay was recorded using an OmniFluo 960 X-ray\nspectrometer (Zolix, China) coupled with a Hamamatsu CR131 photomultiplier tube.\napproximately 5 photon counts. In our case, the photon counts at 316 nm remained\nabove 25 after 168 h, corresponding to a signal-to-noise ratio greater than 5.\nThe corresponding clarification has been added to the revised Experimental\nSection and figure captions for clarity: By using a Hamamatsu CR131 photomultiplier\ntube, the PersL intensity maintained a signal-to-noise ratio above 5 after 168 h. The\nintegration time for each 1 nm wavelength step was 0.1s.\nComment 6: In the conclusions it is stressed that the Sm2+ intensity is increased by a\nfactor 150.3. However, in Figure 5 the red luminescence isn’t visible anymore after\n120 seconds. This raises more questions about how the results are presented in the\nmanuscript.\nResponse 6: We sincerely appreciate the reviewer’s thoughtful comment. The apparent\ninconsistency arises from the different measurement methods used for\npersistent luminescence (PersL) spectra and photographic images. To minimize\ntemporal effects and enable accurate comparison of PersL intensities, the spectra of\nrapidly decaying samples were recorded using a fiber-coupled spectrometer (QEpro)\nlinked to the X-ray source, with a recording time of less than 1 s. Therefore, although\nthe red luminescence becomes invisible to the naked eye after approximately 120 s,\nthis does not affect the spectral comparison, which was conducted at much shorter\nacquisition times.\nIn the revised manuscript, we have clarified this point in the Experimental\nSection and explicitly indicated which figures were obtained using the fiber\nspectrometer: The PersL spectra presented in Figures 4a-c, S8a, S22, S30, S33, and\nS37 were recorded using a fiber-coupled spectrometer (QEpro) linked to the X-ray\nsource, with an acquisition time of ~ 5 s.\nComment 7: The material is combined with quantum dots to obtain persistent\nluminescence over the entire spectrum. But the spectra are normalized so there is no\ninformation about how intense the luminescence of the quantum dots is in the end.\nconverted to visible quantum dot emission?\nResponse 7: We sincerely thank the reviewer for this valuable comment. Following\nthe suggestion, we have now provided the unnormalized PersL spectra of\nBaFCl:Eu,Gd combined with various CsPbX quantum dots (QDs), as shown in Fig.\nBaFCl:Eu,Gd/CsPbBr I QD composite.\n1.3 1.7\nTo evaluate the light conversion efficiency, we defined the light conversion\nefficiency (𝜂) from BaFCl:Eu/Gd to CsPbX QDs as : 𝜂=(𝐼 −𝐼 )/𝐼 , where 𝐼 and 𝐼\n3 0 𝑋 0 0 𝑋\nrepresent the integrated PersL intensity of Eu2+ emission in the absence and presence\nof CsPbX QDs, respectively. The calculated 𝜂 values for CsPbCl Br , CsPbCl Br ,\n3 2 1 1 2\nCsPbCl Br , CsPbBr , CsPbBr I , CsPbBr I , CsPbBr I and CsPbI were\n0.5 2.5 3 1 2 1.3 1.7 2 1 3\napproximately 94.9%, 95.3%, 95.1%, 95.4%, 95.4%, 93.7%, 93.9% and 93.7%,\nrespectively.\nThese results demonstrate that the energy transfer from BaFCl:Eu,Gd to the QDs\nis highly efficient, effectively converting the persistent emission of BaFCl:Eu,Gd into\nbroadband visible luminescence covering the entire spectral range.\nThe additional Fig. S37 and corresponding discussion have been added to the\nrevised manuscript for clarity.\nFig. S37 PersL spectra of various CsPbX QDs sensitized by BaFCl:Eu/Gd.\n3\nX-ray source. The light conversion efficiency (𝜂) from BaFCl:Eu/Gd to CsPbX QDs\n3\nwas defined as 𝜂 = (𝐼 − 𝐼 )/𝐼 , where 𝐼 and 𝐼 correspond to the integrated Eu2+\n0 𝑋 0 0 𝑋\nPersL intensities in the absence and presence of CsPbX QDs, respectively. The\n3\ncalculated 𝜂 values for CsPbCl Br , CsPbCl Br , CsPbCl Br , CsPbBr , CsPbBr I ,\n2 1 1 2 0.5 2.5 3 1 2\nCsPbBr I , CsPbBr I and CsPbI were approximately 94.9%, 95.3%, 95.1%,\n1.3 1.7 2 1 3\n95.4%, 95.4%, 93.7%, 93.9% and 93.7%, respectively.\nWe sincerely appreciate all of the reviewers’ constructive comments, which have\ngreatly helped us to improve the overall quality of this work.\nWe greatly appreciate the reviewers’ insightful comments which are very helpful for\nimprovement of our manuscript. In response to the valuable comments raised by the\nreferee 3, we provide point-by-point responses along with the modifications (marked\nin blue) made in the revised manuscript.\nReviewer 3:\nComment 1: I agree with the authors that the creation of Frenkel defects is likely.\nHowever, as previously mentioned the research referenced in the manuscript does not\nconvincingly prove this point. I would suggest the authors add meaningful references\nto work that experimentally show the creation of Frenkel defects after X-ray\nirradiation.\nResponse 1: We sincerely appreciate the reviewer’s insightful comment. To\nstrengthen the discussion, we have added several references that provide direct\nexperimental evidence for the formation of Frenkel defects induced by X-ray or\nexciton irradiation. These studies support our interpretation regarding the creation of\nFrenkel defects under X-ray exposure. The newly cited references are as follows:\nRef 40-43:\nLow-temperature creation of Frenkel defects via hot electron-hole recombination\nin highly pure NaCl single crystals, Low Temperature Physics, 2016, 42, 547-551;\nRadiation creation of cation defects in alkali halide crystals: Review and today’s\nconcept, Low Temperature Physics, 2018, 44, 269–277;\nUnveiling the Dynamics of Frenkel Defects in Fluoride Materials for\nX-Ray-Induced Persistent Luminescence and Advanced Imaging Applications, Laser\nPhotonics Rev., 2025, 2500092;\nCreation of Frenkel Defect Pairs by Excitons in Alkali Halides, Mod. Probl.\nCondens. Matter Sci. 1986, 13, 473–525;\nComment 2: In the discussion of the mechanism it is stated that Frenkel defects\npreferentially form close to the Gd-clusters. In addition, the mechanism requires the\nbecomes too large to obtain efficient energy transfer. Both points are insufficiently\nmotivated/supported by the data in the manuscript.\nResponse 2: We sincerely appreciate the reviewer’s insightful comment. For a given\ncrystal structure under identical X-ray irradiation conditions, the formation of Frenkel\ndefects is primarily determined by their formation energies (E).\nf\nWe performed DFT\ncalculations to compare the E values of F– Frenkel defects associated with the\nf\n[-Gd-F-] clusters, including both surface and interior F– sites, as well as F– bound to\nn\nBa2+. As shown in Figure 3d, the calculated E of F– Frenkel defects within the\nf\n[-Gd-F-] clusters is substantially lower than that of F– bound to Ba2+, indicating that\nn\nthe defect-related traps preferentially form within the clusters. Moreover, for surface\nF– ions within the [-Gd-F-] clusters, the E for displacement toward the cluster is\nn f\nsignificantly lower than that for displacement away from the cluster, further\nconfirming that Frenkel defects tend to form and accumulate within the Gd clusters.\nBy employing the constructed model crystal structure shown in Fig. S17a, we\nrandomly distributed Eu ions into the lattice at a concentration of 2 mol% (Fig.\nS27a-b). The calculated mean nearest Gd–Eu distance was ~0.81 nm, which is far\nbelow the commonly reported critical distance (R ) of 2–6 nm for Förster resonance\n0\nenergy transfer (FRET) (refs: J. Am. Chem. Soc. 2016, 138, 15972–15979; Nat.\nBiotechnol. 2003, 21, 1387). This critical distance corresponds to a FRET efficiency\nof 50% (Schaufele et al., Molecular Imaging: FRET Microscopy and Spectroscopy,\nOxford University Press, 2005, pp. 72–94). As shown in Fig. R1, when the\ndonor–acceptor distance reaches 0.5 R₀, the energy transfer efficiency can be as high\nas 98.5% (ref: Joseph R. Lakowicz, Principles of Fluorescence Spectroscopy, Book,\nSpringer, P446, Figure 13.2). Furthermore, the excitation spectrum of Eu2+ exhibits\nstrong overlap with the emission spectrum of Gd3+ (Fig. S27c). Under these\nconditions, highly efficient energy transfer from Gd3+ to Eu2+ is expected and feasible.\n2\n)\nE\n1.0 r = 0.5 R\n( 0\ny E = 0.985\nc\n0.8\nn\ne\ni\nc\ni\nf\nf 0.6\ne\nr\ne\ns f 0.4\nn\na\nt r 0.2 r = 1.5 R 0\ny E = 0.117\ng\nr\ne\n0.0\nn\nE 0.0 0.5 1.0 1.5 2.0 2.5\nr / R\n0\ncritical distance for Förster resonance energy transfer.\nThe following contents have been added in the revised manuscript\nEspecially, for surface F- ions within the clusters, the E for displacement toward\nf\nthe cluster is significantly lower than that for displacement away from the cluster.\nThis efficiency arises from both the mean Gd–Eu distance of approximately 0.81\nnm (Figure S27a-b), which is far below the typical critical distance of 2 to 6 nm for\nFörster resonance energy transfer47,48, and the strong overlap between the Eu3+\nexcitation spectrum and the Gd3+ emission spectrum (Figure S27c).\nFig. S27 (a) Three-dimensional spatial distribution of Gd and Eu dopant ions within\nthe BaFCl host; (b) Corresponding histogram of the nearest Gd-Eu distance\nprobability; the red curve denotes the fitting trend; (c) Comparison of the normalized\nPersL spectrum of Gd3+ and the photoluminescence excitation spectrum of Eu2+.\nrandomly distributed Eu ions into the lattice at a concentration of 2 mol%.\nThe\ncalculated mean nearest Gd–Eu distance was ~0.81 nm.\nComment 3: Throughout the manuscript but also in the responses to questions raised\nby other reviewers a comparison is made between the TL peaks at low and high\ntemperatures. Was the data corrected for thermal quenching? Without a proper\ncorrection it is impossible to draw meaningful conclusions from the comparison of TL\nintensities at different temperatures.\nResponse 3: We sincerely appreciate the reviewer’s insightful comment. The\ntemperature-dependent PersL spectra mentioned were not included in the current\nmanuscript but were provided in the previous response letter to address the\nmechanism-related question raised by Reviewer 1. In that response, the PersL spectra\nof the BaFCl:Gd phosphor were measured after X-ray irradiation at temperatures\nranging from 89 K to 249 K to qualitatively illustrate the thermally assisted PersL\nprocess. Although the data were not corrected for thermal quenching, we believe that\nthe comparative analysis remains meaningful for the following reasons. First, the\nPersL spectra were mainly used, together with the fitting of PersL intensity as a\nfunction of t-1, to illustrate the possible PersL process. This analysis was not directly\nrelated to the quantitative comparison of absolute PersL intensities, and the approach\nhas been accepted by Reviewer 1. Second, thermal quenching of inorganic phosphors\ngenerally becomes more significant at elevated temperatures. However, in our\nmeasurements, the PersL intensity increased as the temperature rose from 89 K to 249\nK. Therefore, even if the data were corrected for thermal quenching, the observed\ntrend—an increase in PersL intensity with temperature—would remain unchanged.\nConsequently, the qualitative interpretation and conclusions drawn from these results\nremain valid.\nComment 4: The new version of Figure S24 and the accompanying text now suggest\nthat either holes or electrons are trapped on Gd3+, which would imply the creation of\nthe proposed model is not plausible.\nResponse 4: We sincerely appreciate the reviewer’s valuable comment. The\nmigration of trapped holes or electrons to lanthanide activators has indeed been\ndiscussed in several previous studies. For instance, it has been reported that “Electrons\nin shallow traps can spontaneously escape over time to the conduction band with a\nconcurrent defect self-healing process, whereas electrons in deep traps require extra\nenergy in the form of either optical or thermal stimulation to migrate to Tb3+ emitters.\nTrapped holes can also migrate towards Tb3+ emitters, forming hole–Tb3+ centres that\nradiatively recombine with captured electrons” (Nature, 2021, 590, 410–415).\nSimilarly, Light: Science & Applications (2022, 11, 51) reported that “The trivalent\nlanthanides are expected to attract an electron (or hole) first and then will draw a hole\n(or electron) due to the Coulomb force. In all cases, the trivalent lanthanides\neventually bind the excitons as this is the origin of energy. The excitons recombine\nand transfer the energy to the trivalent lanthanides that bind the excitons. The trivalent\nlanthanides are excited from the ground state to the excited state, which is followed\nby the non-radiative relaxation processes and final emitting of PersL.” These studies\nindicate that the capture of electrons or holes by lanthanide ions does not necessarily\nlead to the formation of divalent or tetravalent species (e.g., Gd2+ or Gd4+). Based on\nthis understanding, we have further clarified the related discussion in the revised\nSupporting Information.\nDuring the XEOL process, incident X-ray photons predominantly interact with\nheavy atoms via the photoelectric effect, generating hot electrons and deep holes.[8,9]\nSubsequent electron-electron scattering and Auger relaxation processes produce\nnumerous low-energy secondary carriers. XEOL emission arises when these\nthermalized carriers recombine radiatively, populating the excited states of activator\nions. In contrast, during X-ray-excited PersL, elastic collisions between\nhigh-momentum X-ray photons and light fluoride ions induce the formation of anion\nFrenkel defects, which serve as long-lived traps for electrons and holes. The trapped\ncarriers migrate separately to luminescent centers and recombine over time. Trivalent\n5\nlanthanides can sequentially attract an electron (or hole) and subsequently capture the\nopposite charge through Coulomb interaction, leading to exciton formation. The\nrecombination of these excitons transfers energy to the lanthanide ions, generating\nPersL emission. Upon Gd3+ codoping, the traps become spatially confined within\nGd-rich clusters, which enhances radiative recombination through Gd3+ and facilitates\nefficient energy transfer to the activator ions.\nComment 5: The authors have added conductivity measurements as a proof of the\nexistence of Frenkel defects but the analysis is lacking. Under X-ray irradiation a lot\nof free charge carriers are created (and this is also voltage dependent). How can you\ndiscriminate between the contribution of these free charge carriers and those of\nmobile species like F interstitials?\nResponse 5: We sincerely appreciate the reviewer’s valuable comment. X-ray\nphotons interact with the lattice atoms primarily through the photoelectric and\nCompton scattering effects, generating hot electrons and deep holes. Subsequent\nelectron–electron scattering and Auger relaxation processes produce a large number of\nsecondary electrons, resulting in abundant charge carriers with lower kinetic energies.\nThese carriers rapidly thermalize through phonon interactions and accumulate near\nthe conduction and valence bands within a subpicosecond timescale. Their transport\ntoward luminescence centers typically occurs within 10-12–10-8 s (ref: ACS Energy\nLett., 2021, 6, 739–768). Consequently, the current signal arising purely from these\nfree charge carriers should appear and vanish almost instantaneously upon switching\nthe X-ray on or off, as also demonstrated in Adv. Mater., 2019, 31, 1901644\n(“Flexible, Printable Soft-X-Ray Detectors Based on All-Inorganic Perovskite\nQuantum Dots”). As this figure cannot be used directly, it is not included here.\nIn contrast, as shown in our current–time measurements (Fig. S18),\nBaFCl:Eu/Gd exhibits a gradual current rise more than 7s after the X-ray is switched\non, and decays slowly with a delay exceeding 7 s after the X-ray is turned off. This\ndelayed response cannot be accounted for by free charge carriers and is instead\nconsistent with the behavior of mobile species such as F- interstitials. We therefore\n6\nattribute this slow component to the migration of mobile charged defects, specifically\nanion Frenkel defects induced under irradiation. Based on theoretical calculations,\nsimilar diffusion-controlled relaxation of F interstitials has been reported in fluoride\nmaterials (Nature, 2021, 590, 410–415), where interstitial F- ions gradually migrate\nback to their original vacancy sites. These observations reveal that the measured\nconductivity predominantly arises from defect-mediated ionic transport, providing\nstrong evidence for the formation of Frenkel defects under X-ray irradiation.\nThe following contents have been added in the revised manuscript and\nsupporting information.\nAs shown in Figure S18, the current gradually increases upon X-ray irradiation\nand decays slowly with a delay exceeding 7 s after the X-ray is turned off, while the\npeak current rises at higher tube voltages. These observations indicate that the\nmeasured conductivity predominantly arises from defect-related ionic transport under\nX-ray irradiation.\nFig. S18 Current signal of the BaFCl:Eu/Gd film measured under X-ray irradiation at\ntube voltages of 15, 30, and 60 kV.\nComment 6: The authors maintain that the initial afterglow intensity of the material\nis 4.48 cd/m². As a reaction to my previous comments they have however added that\nafter two seconds the intensity drops to 0.16 cd/m². I repeat that this high initial value\n7\nis not in line with the results shown in the paper. I calculated the luminous efficacy of\nradiation for SrAl2O4:Eu,Dy (477 lumen/watt) and estimated it for the BaFCl:Eu\nbased on the spectrum in the paper (< 1 lumen/watt). This implies that to have a\nsimilar intensity in cd/m², the intensity of the BaFCl:Eu should be at least 477 times\nhigher than that of SrAl2O4:Eu,Dy. This is not in line with the results shown in the\nmanuscript (e.g. Figure 2f). The result obtained after 2 seconds (0.16 cd/m²) on the\nother hand is compatible with the reported results. As the authors maintain the claim\nthat the initial intensity is 4.48 cd/m², I have the following questions in the hope of\nclarifying the issue:\na. Was the detector used to measure the spectra in Figure 2f intensity calibrated to\ncorrect for the wavelength dependency of the sensitivity?\nb. Was the detector used to measure TL and afterglow in the manuscript intensity\ncalibrated to correct for the wavelength dependency of the sensitivity?\nc. If the intensity drops from 4.48 cd/m² to 0.16 cd/m² this implies that there is\nsignificant TL intensity below room temperature. Please provide TL curves below\nroom temperature.\nResponse 6: We thank the reviewer for this insightful comment. The PersL spectra\nshown in Figure 2f were recorded using a spectrometer, which signals was not\ncaptured within less than 1 s. In contrast, the initial intensity of 4.48 cd/m2 was\nmeasured by a photometer (Xinbao Instrument, SM218E) in real time within <1 s.\nFrom this perspective, the initially reported value of 4.48 cd/m2 does not strictly\nreflect the luminescence enhancement degree, which is reasonable. As the reviewer\ncorrectly noted, the intensity measured after 2 s (0.16 cd/m²) is consistent with the\nreported results and is retained.\nWe note that the instruments used for TL, afterglow, and luminance (cd/m2)\nmeasurements were from different manufacturers, some measured at collaborating\nlaboratories. Detailed detector calibration parameters are not fully available, which\nlimits direct comparison. We appreciate the reviewer’s understanding of this\nlimitation. Furthermore, PersL intensity is highly temperature dependent. Higher\ntemperatures promote electron release but also induce significant thermal quenching.\ntemperature is much weaker than at room temperature, indicating that bright PersL\nrequires an appropriate measurement temperature. This highlights the inherent\ntrade-off between achieving high initial brightness and controlling electron release.\nTo avoid potential confusion, and following the reviewer’s recommendation, we\nhave removed the initial 4.48 cd/m2 intensity from the revised manuscript, while\nretaining the 0.16 cd/m2 value measured at 2s after the X-ray is turned off.\nComment 7: Many of the figures still have no values on the vertical axis. This does\nnot allow for a correct interpretation of the data and should be fixed.\nResponse 7: In response to the reviewer’s insightful comment, we have added values\non the vertical axes of all PersL spectra (Fig 2, Fig 3, Fig 4, Fig S1, Fig S7, Fig S8,\nFig S9, Fig S10, Fig S20, Fig S21, Fig S23, Fig S25, Fig S29, Fig S301, Fig S34, Fig\nS38) in the revised manuscript and Supporting Information.\nComment 8: It is not clear whether the afterglow curves that are shown are in\nphotometric or in radiometric units. This needs to be clarified in the manuscript.\nResponse 8: We thank the reviewer for this insightful comment. The afterglow curves\nreported in the manuscript represent photon counts directly obtained from the\nspectrometer. To avoid repetition across different figures, this clarification has been\nadded to the Experimental section of the revised Supporting Information to prevent\nany potential misunderstanding.\nThe afterglow curves reported in this work represent photon counts directly obtained\nfrom the spectrometer.\nComment 9: The information collected in Fig. S37 is very meaningful and allows for\na meaningful comparison of the intensities of the different CsPbX QDs sensitized by\nBaFCl:Eu/Gd. However, the conclusion of the figure is that a lot of the light is lost. Is\nthis due to the low quantum yield of the QDs? The authors have chosen to report\nenergy transfer efficiencies (all close to 90%) but as this doesn’t translate to a high\n9\nemission intensities of the QDs it would be more meaningful to report integrated\ninitial intensities. As an example: If I put a layer with a black powder on top of the\nBaFCl:Eu/Gd I will have a ET efficiency close to 100% using the formula you use but\nof course no light emission.\nResponse 9: We sincerely thank the reviewer for this insightful suggestion. In our\nexperiments, the BaFCl:Eu/Gd film was placed in direct contact with a quartz\ncontainer holding the QDs to minimize energy losses between the BaFCl:Eu/Gd layer\nand the QDs. Nevertheless, because the BaFCl:Eu/Gd film was physically attached to\nthe outer wall of the quartz container, a certain degree of energy loss is unavoidable.\nWhen neglecting this small loss, the PersL emission from Eu ions is either absorbed\nby the QDs or transmitted through them. As shown in Fig. S37, the transmitted Eu\nPersL signal remains clearly detectable, indicating that the Eu emission passes\neffectively through the QD layer.\nAlthough the calculated energy-transfer efficiencies are high (~90%), this does not\ntranslate into intense QD emission. The main limiting factor is the relatively low\nphotoluminescence quantum yield of the synthesized CsPbX QDs.\nWe acknowledge\nthis limitation and note that we are continuing to improve our synthesis procedures to\nobtain QDs with ultrahigh quantum yields.\nThe following contents have been added in the revised supporting information.\nFig. S38 PersL spectra of various CsPbX QDs sensitized by BaFCl:Eu/Gd (a), and\n3\nthe corresponding variations in integrated emission intensity (b).\nThe PersL spectra were recorded using a fiber-coupled spectrometer (QEpro)\n10\nlinked to the X-ray source. Because the BaFCl:Eu/Gd film was physically attached to\nthe outer wall of the quartz container holding the QDs, a certain degree of optical loss\nis unavoidable. When neglecting this small loss, the light conversion efficiency (𝜂)\nfrom BaFCl:Eu/Gd to CsPbX QDs was defined as 𝜂= (𝐼 − 𝐼 )/𝐼 , where 𝐼 and 𝐼\n3 0 𝑋 0 0 𝑋\ncorrespond to the integrated Eu2+ PersL intensities in the absence and presence of\nCsPbX QDs, respectively. The calculated 𝜂 values for CsPbCl Br , CsPbCl Br ,\n3 2 1 1 2\nCsPbCl Br , CsPbBr , CsPbBr I , CsPbBr I , CsPbBr I and CsPbI were\n0.5 2.5 3 1 2 1.3 1.7 2 1 3\napproximately 94.9%, 95.3%, 95.1%, 95.4%, 95.4%, 93.7%, 93.9% and 93.7%,\nrespectively. The substantial energy loss shown in Fig. S37b is mainly attributed to\nthe relatively low quantum yield of the prepared QDs.\nComment 10: It is now stressed that there is a 150-fold improvement in intensity for\nthe Sm-doped materials upon addition of the Gd, however I think the nuance that the\nintensity is still very low should be added more explicitly to the abstract and\nconclusion to add context. Alternatively the authors could report a more balanced\nvalue based on the average improvement observed in all of the measured materials.\nResponse 10: In accordance with the reviewer’s helpful suggestion, we have replaced\nthe discussion of Sm-doped samples with Eu-doped samples in the revised manuscript.\nThe contents of “The PersL of Eu²⁺ is enhanced by up to 32.7-fold upon Gd3+\ncodoping.” and “The red PersL from Eu2+ is remarkably enhanced, by up to 32.7-fold,\nthrough Gd3+ codoping.” have been added in the revised abstract and conclusion,\nrespectively.\nComment 11: I would like the authors to include explicitly in the manuscript that the\ncomparison of reported quantum yields might be influenced by the fact that the\nSrAl O phosphor displays trapping and detrapping under UV excitation (which might\n2 4\nlower the overall efficiency as pointed out by the authors in the introduction) while\nthe BaFCl:Eu does not.\nResponse 11: We appreciate the reviewer’s valuable comment. The following content\nhas been added in the revised manuscript.\n(PLQY) of 92.7%, far exceeding that of SrAl O : Eu/Dy (54.7%) (Figure S14). It\n2 4\nshould be noted that the comparison of reported quantum yields may be affected by\nthe fact that SrAl O : Eu/Dy exhibits trapping and detrapping under UV excitation,\n2 4\nwhich can reduce the overall efficiency, whereas BaFCl:Eu does not.\n12"}, {"round": 2, "reviewer_comments": {"reviewer_1": "In my view, the authors have answered all the questions and revised related points, and thus this revised work can be\npublished as it is.", "reviewer_2": "The authors have satisfactorily addressed all my comments. The revised manuscript is recommended for publication.", "reviewer_3": "I thank the authors for their detailed response and the modifications they have made to the manuscript. However, after\ncarefully reviewing the provided documents I have to conclude that several of my concerns were not fully addressed.\nAdditionally, I would like to argue for a more balanced reporting of the results with a meaningful and nuanced comparison to\nother materials. Below I express my remaining concerns.\nConcerns with regard to the proposed mechanism.\n1. I agree with the authors that the creation of Frenkel defects is likely. However, as previously mentioned the research\nreferenced in the manuscript does not convincingly prove this point. I would suggest the authors add meaningful references\nto work that experimentally show the creation of Frenkel defects after X-ray irradiation.\n2. In the discussion of the mechanism it is stated that Frenkel defects preferentially form close to the Gd-clusters. In addition,\nthe mechanism requires the Eu to be close to the Gd-clusters too, otherwise the separation between the Gd and Eu\nbecomes too large to obtain efficient energy transfer. Both points are insufficiently motivated/supported by the data in the\nmanuscript.\n3. Throughout the manuscript but also in the responses to questions raised by other reviewers a comparison is made\nbetween the TL peaks at low and high temperatures. Was the data corrected for thermal quenching? Without a proper\ncorrection it is impossible to draw meaningful conclusions from the comparison of TL intensities at different temperatures.\n4. The new version of Figure S24 and the accompanying text now suggest that either holes or electrons are trapped on\nGd3+, which would imply the creation of Gd2+ or Gd4+ both of which are very exotic species. Hence, I repeat my concern that\nthe proposed model is not plausible.\n5. The authors have added conductivity measurements as a proof of the existence of Frenkel defects but the analysis is\nlacking. Under X-ray irradiation a lot of free charge carriers are created (and this is also voltage dependent). How can you\ndiscriminate between the contribution of these free charge carriers and those of mobile species like F interstitials?\nConcerns with regard to the comparison of the afterglow intensities to other materials.\nI would like to start by saying that there is no doubt that the addition of Gd3+ results in a remarkable improvement of the\npersistent luminescence intensity. However, I have several concerns about the comparison of the performance of different\n(commercial) materials and the reported values. And frankly, I must say that the choice of which values are reported in the\nabstract and conclusions comes very close to cherry picking. I noticed that the authors partly agree with some of my\ncomments but unfortunately this did not translate to any meaningful changes to the manuscript text.\n6.The authors maintain that the initial afterglow intensity of the material is 4.48 cd/m². As a reaction to my previous comments\nthey have however added that after two seconds the intensity drops to 0.16 cd/m². I repeat that this high initial value is not in\nline with the results shown in the paper. I calculated the luminous efficacy of radiation for SrAl2O4:Eu,Dy (477 lumen/watt) and\nestimated it for the BaFCl:Eu based on the spectrum in the paper (< 1 lumen/watt). This implies that to have a similar\nintensity in cd/m², the intensity of the BaFCl:Eu should be at least 477 times higher than that of SrAl2O4:Eu,Dy. This is not in\nline with the results shown in the manuscript (e.g. Figure 2f). Th result obtained after 2 seconds (0.16 cd/m²) on the other\nhand is compatible with the reported results. As the authos maintain the claim that the initial intensity is 4.48 cd/m², I have\nthe following questions in the hope of clarifying the issue:\na. Was the detector used to measure the spectra in Figure 2f intensity calibrated to correct for the wavelength dependency of the\nsensitivity?\nb. Was the detector used to measure TL and afterglow in the manuscript intensity calibrated to correct for the wavelength\ndependency of the sensitivity?\nc. If the intensity drops from 4.48 cd/m² to 0.16 cd/m² this implies that there is significant TL intensity below room\ntemperature. Please provide TL curves below room temperature.\n7. Many of the figures still have no values on the vertical axis. This does not allow for a correct interpretation of the data and\nshould be fixed.\n8. It is not clear whether the afterglow curves that are shown are in photometric or in radiometric units. This needs to be\nclarified in the manuscript.\n9. The information collected in Fig. S37 is very meaningful and allows for a meaningful comparison of the intensities of the\ndifferent CsPbX QDs sensitized by BaFCl:Eu/Gd. However, the conclusion of the figure is that a lot of the light is lost. Is this\ndue to the low quantum yield of the QDs? The authors have chosen to report energy transfer efficiencies (all close to 90%)\nbut as this doesn’t translate to a high emission intensities of the QDs it would be more meaningful to report integrated initial\nintensities. As an example: If I put a layer with a black powder on top of the BaFCl:Eu/Gd I will have a ET efficiency close to\n100% using the formula you use but of course no light emission.\n10. It is now stressed that there is a 150-fold improvement in intensity for the Sm-doped materials upon addition of the Gd,\nhowever I think the nuance that the intensity is still very low should be added more explicitly to the abstract and conclusion\nto add context. Alternatively the authors could report a more balanced value based on the average improvement observed in\nall of the measured materials.\n11. I would like the authors to include explicitly in the manuscript that the comparison of reported quantum yields might be\ninfluenced by the fact that the SrAl2O4 phosphor displays trapping and detrapping under UV excitation (which might lower\nthe overall efficiency as pointed out by the authors in the introduction) while the BaFCl:Eu does not."}, "author_response": "Response to reviewer’s comments\nWe greatly appreciate the reviewers’ insightful comments which are very helpful for\nimprovement of our manuscript. In response to the valuable comments raised by the\nreferee 3, we provide point-by-point responses along with the modifications (marked\nin blue) made in the revised manuscript.\nReviewer 3:\nComment 1: I agree with the authors that the creation of Frenkel defects is likely.\nHowever, as previously mentioned the research referenced in the manuscript does not\nconvincingly prove this point. I would suggest the authors add meaningful references\nto work that experimentally show the creation of Frenkel defects after X-ray\nirradiation.\nResponse 1: We sincerely appreciate the reviewer’s insightful comment. To\nstrengthen the discussion, we have added several references that provide direct\nexperimental evidence for the formation of Frenkel defects induced by X-ray or\nexciton irradiation. These studies support our interpretation regarding the creation of\nFrenkel defects under X-ray exposure. The newly cited references are as follows:\nRef 40-43:\nLow-temperature creation of Frenkel defects via hot electron-hole recombination\nin highly pure NaCl single crystals, Low Temperature Physics, 2016, 42, 547-551;\nRadiation creation of cation defects in alkali halide crystals: Review and today’s\nconcept, Low Temperature Physics, 2018, 44, 269–277;\nUnveiling the Dynamics of Frenkel Defects in Fluoride Materials for\nX-Ray-Induced Persistent Luminescence and Advanced Imaging Applications, Laser\nPhotonics Rev., 2025, 2500092;\nCreation of Frenkel Defect Pairs by Excitons in Alkali Halides, Mod. Probl.\nCondens. Matter Sci. 1986, 13, 473–525;\nComment 2: In the discussion of the mechanism it is stated that Frenkel defects\npreferentially form close to the Gd-clusters. In addition, the mechanism requires the\nbecomes too large to obtain efficient energy transfer. Both points are insufficiently\nmotivated/supported by the data in the manuscript.\nResponse 2: We sincerely appreciate the reviewer’s insightful comment. For a given\ncrystal structure under identical X-ray irradiation conditions, the formation of Frenkel\ndefects is primarily determined by their formation energies (E).\nf\nWe performed DFT\ncalculations to compare the E values of F– Frenkel defects associated with the\nf\n[-Gd-F-] clusters, including both surface and interior F– sites, as well as F– bound to\nn\nBa2+. As shown in Figure 3d, the calculated E of F– Frenkel defects within the\nf\n[-Gd-F-] clusters is substantially lower than that of F– bound to Ba2+, indicating that\nn\nthe defect-related traps preferentially form within the clusters. Moreover, for surface\nF– ions within the [-Gd-F-] clusters, the E for displacement toward the cluster is\nn f\nsignificantly lower than that for displacement away from the cluster, further\nconfirming that Frenkel defects tend to form and accumulate within the Gd clusters.\nBy employing the constructed model crystal structure shown in Fig. S17a, we\nrandomly distributed Eu ions into the lattice at a concentration of 2 mol% (Fig.\nS27a-b). The calculated mean nearest Gd–Eu distance was ~0.81 nm, which is far\nbelow the commonly reported critical distance (R ) of 2–6 nm for Förster resonance\n0\nenergy transfer (FRET) (refs: J. Am. Chem. Soc. 2016, 138, 15972–15979; Nat.\nBiotechnol. 2003, 21, 1387). This critical distance corresponds to a FRET efficiency\nof 50% (Schaufele et al., Molecular Imaging: FRET Microscopy and Spectroscopy,\nOxford University Press, 2005, pp. 72–94). As shown in Fig. R1, when the\ndonor–acceptor distance reaches 0.5 R₀, the energy transfer efficiency can be as high\nas 98.5% (ref: Joseph R. Lakowicz, Principles of Fluorescence Spectroscopy, Book,\nSpringer, P446, Figure 13.2). Furthermore, the excitation spectrum of Eu2+ exhibits\nstrong overlap with the emission spectrum of Gd3+ (Fig. S27c). Under these\nconditions, highly efficient energy transfer from Gd3+ to Eu2+ is expected and feasible.\n2\n)\nE\n1.0 r = 0.5 R\n( 0\ny E = 0.985\nc\n0.8\nn\ne\ni\nc\ni\nf\nf 0.6\ne\nr\ne\ns f 0.4\nn\na\nt r 0.2 r = 1.5 R 0\ny E = 0.117\ng\nr\ne\n0.0\nn\nE 0.0 0.5 1.0 1.5 2.0 2.5\nr / R\n0\ncritical distance for Förster resonance energy transfer.\nThe following contents have been added in the revised manuscript\nEspecially, for surface F- ions within the clusters, the E for displacement toward\nf\nthe cluster is significantly lower than that for displacement away from the cluster.\nThis efficiency arises from both the mean Gd–Eu distance of approximately 0.81\nnm (Figure S27a-b), which is far below the typical critical distance of 2 to 6 nm for\nFörster resonance energy transfer47,48, and the strong overlap between the Eu3+\nexcitation spectrum and the Gd3+ emission spectrum (Figure S27c).\nFig. S27 (a) Three-dimensional spatial distribution of Gd and Eu dopant ions within\nthe BaFCl host; (b) Corresponding histogram of the nearest Gd-Eu distance\nprobability; the red curve denotes the fitting trend; (c) Comparison of the normalized\nPersL spectrum of Gd3+ and the photoluminescence excitation spectrum of Eu2+.\nrandomly distributed Eu ions into the lattice at a concentration of 2 mol%.\nThe\ncalculated mean nearest Gd–Eu distance was ~0.81 nm.\nComment 3: Throughout the manuscript but also in the responses to questions raised\nby other reviewers a comparison is made between the TL peaks at low and high\ntemperatures. Was the data corrected for thermal quenching? Without a proper\ncorrection it is impossible to draw meaningful conclusions from the comparison of TL\nintensities at different temperatures.\nResponse 3: We sincerely appreciate the reviewer’s insightful comment. The\ntemperature-dependent PersL spectra mentioned were not included in the current\nmanuscript but were provided in the previous response letter to address the\nmechanism-related question raised by Reviewer 1. In that response, the PersL spectra\nof the BaFCl:Gd phosphor were measured after X-ray irradiation at temperatures\nranging from 89 K to 249 K to qualitatively illustrate the thermally assisted PersL\nprocess. Although the data were not corrected for thermal quenching, we believe that\nthe comparative analysis remains meaningful for the following reasons. First, the\nPersL spectra were mainly used, together with the fitting of PersL intensity as a\nfunction of t-1, to illustrate the possible PersL process. This analysis was not directly\nrelated to the quantitative comparison of absolute PersL intensities, and the approach\nhas been accepted by Reviewer 1. Second, thermal quenching of inorganic phosphors\ngenerally becomes more significant at elevated temperatures. However, in our\nmeasurements, the PersL intensity increased as the temperature rose from 89 K to 249\nK. Therefore, even if the data were corrected for thermal quenching, the observed\ntrend—an increase in PersL intensity with temperature—would remain unchanged.\nConsequently, the qualitative interpretation and conclusions drawn from these results\nremain valid.\nComment 4: The new version of Figure S24 and the accompanying text now suggest\nthat either holes or electrons are trapped on Gd3+, which would imply the creation of\nthe proposed model is not plausible.\nResponse 4: We sincerely appreciate the reviewer’s valuable comment. The\nmigration of trapped holes or electrons to lanthanide activators has indeed been\ndiscussed in several previous studies. For instance, it has been reported that “Electrons\nin shallow traps can spontaneously escape over time to the conduction band with a\nconcurrent defect self-healing process, whereas electrons in deep traps require extra\nenergy in the form of either optical or thermal stimulation to migrate to Tb3+ emitters.\nTrapped holes can also migrate towards Tb3+ emitters, forming hole–Tb3+ centres that\nradiatively recombine with captured electrons” (Nature, 2021, 590, 410–415).\nSimilarly, Light: Science & Applications (2022, 11, 51) reported that “The trivalent\nlanthanides are expected to attract an electron (or hole) first and then will draw a hole\n(or electron) due to the Coulomb force. In all cases, the trivalent lanthanides\neventually bind the excitons as this is the origin of energy. The excitons recombine\nand transfer the energy to the trivalent lanthanides that bind the excitons. The trivalent\nlanthanides are excited from the ground state to the excited state, which is followed\nby the non-radiative relaxation processes and final emitting of PersL.” These studies\nindicate that the capture of electrons or holes by lanthanide ions does not necessarily\nlead to the formation of divalent or tetravalent species (e.g., Gd2+ or Gd4+). Based on\nthis understanding, we have further clarified the related discussion in the revised\nSupporting Information.\nDuring the XEOL process, incident X-ray photons predominantly interact with\nheavy atoms via the photoelectric effect, generating hot electrons and deep holes.[8,9]\nSubsequent electron-electron scattering and Auger relaxation processes produce\nnumerous low-energy secondary carriers. XEOL emission arises when these\nthermalized carriers recombine radiatively, populating the excited states of activator\nions. In contrast, during X-ray-excited PersL, elastic collisions between\nhigh-momentum X-ray photons and light fluoride ions induce the formation of anion\nFrenkel defects, which serve as long-lived traps for electrons and holes. The trapped\ncarriers migrate separately to luminescent centers and recombine over time. Trivalent\n5\nlanthanides can sequentially attract an electron (or hole) and subsequently capture the\nopposite charge through Coulomb interaction, leading to exciton formation. The\nrecombination of these excitons transfers energy to the lanthanide ions, generating\nPersL emission. Upon Gd3+ codoping, the traps become spatially confined within\nGd-rich clusters, which enhances radiative recombination through Gd3+ and facilitates\nefficient energy transfer to the activator ions.\nComment 5: The authors have added conductivity measurements as a proof of the\nexistence of Frenkel defects but the analysis is lacking. Under X-ray irradiation a lot\nof free charge carriers are created (and this is also voltage dependent). How can you\ndiscriminate between the contribution of these free charge carriers and those of\nmobile species like F interstitials?\nResponse 5: We sincerely appreciate the reviewer’s valuable comment. X-ray\nphotons interact with the lattice atoms primarily through the photoelectric and\nCompton scattering effects, generating hot electrons and deep holes. Subsequent\nelectron–electron scattering and Auger relaxation processes produce a large number of\nsecondary electrons, resulting in abundant charge carriers with lower kinetic energies.\nThese carriers rapidly thermalize through phonon interactions and accumulate near\nthe conduction and valence bands within a subpicosecond timescale. Their transport\ntoward luminescence centers typically occurs within 10-12–10-8 s (ref: ACS Energy\nLett., 2021, 6, 739–768). Consequently, the current signal arising purely from these\nfree charge carriers should appear and vanish almost instantaneously upon switching\nthe X-ray on or off, as also demonstrated in Adv. Mater., 2019, 31, 1901644\n(“Flexible, Printable Soft-X-Ray Detectors Based on All-Inorganic Perovskite\nQuantum Dots”). As this figure cannot be used directly, it is not included here.\nIn contrast, as shown in our current–time measurements (Fig. S18),\nBaFCl:Eu/Gd exhibits a gradual current rise more than 7s after the X-ray is switched\non, and decays slowly with a delay exceeding 7 s after the X-ray is turned off. This\ndelayed response cannot be accounted for by free charge carriers and is instead\nconsistent with the behavior of mobile species such as F- interstitials. We therefore\n6\nattribute this slow component to the migration of mobile charged defects, specifically\nanion Frenkel defects induced under irradiation. Based on theoretical calculations,\nsimilar diffusion-controlled relaxation of F interstitials has been reported in fluoride\nmaterials (Nature, 2021, 590, 410–415), where interstitial F- ions gradually migrate\nback to their original vacancy sites. These observations reveal that the measured\nconductivity predominantly arises from defect-mediated ionic transport, providing\nstrong evidence for the formation of Frenkel defects under X-ray irradiation.\nThe following contents have been added in the revised manuscript and\nsupporting information.\nAs shown in Figure S18, the current gradually increases upon X-ray irradiation\nand decays slowly with a delay exceeding 7 s after the X-ray is turned off, while the\npeak current rises at higher tube voltages. These observations indicate that the\nmeasured conductivity predominantly arises from defect-related ionic transport under\nX-ray irradiation.\nFig. S18 Current signal of the BaFCl:Eu/Gd film measured under X-ray irradiation at\ntube voltages of 15, 30, and 60 kV.\nComment 6: The authors maintain that the initial afterglow intensity of the material\nis 4.48 cd/m². As a reaction to my previous comments they have however added that\nafter two seconds the intensity drops to 0.16 cd/m². I repeat that this high initial value\n7\nis not in line with the results shown in the paper. I calculated the luminous efficacy of\nradiation for SrAl2O4:Eu,Dy (477 lumen/watt) and estimated it for the BaFCl:Eu\nbased on the spectrum in the paper (< 1 lumen/watt). This implies that to have a\nsimilar intensity in cd/m², the intensity of the BaFCl:Eu should be at least 477 times\nhigher than that of SrAl2O4:Eu,Dy. This is not in line with the results shown in the\nmanuscript (e.g. Figure 2f). The result obtained after 2 seconds (0.16 cd/m²) on the\nother hand is compatible with the reported results. As the authors maintain the claim\nthat the initial intensity is 4.48 cd/m², I have the following questions in the hope of\nclarifying the issue:\na. Was the detector used to measure the spectra in Figure 2f intensity calibrated to\ncorrect for the wavelength dependency of the sensitivity?\nb. Was the detector used to measure TL and afterglow in the manuscript intensity\ncalibrated to correct for the wavelength dependency of the sensitivity?\nc. If the intensity drops from 4.48 cd/m² to 0.16 cd/m² this implies that there is\nsignificant TL intensity below room temperature. Please provide TL curves below\nroom temperature.\nResponse 6: We thank the reviewer for this insightful comment. The PersL spectra\nshown in Figure 2f were recorded using a spectrometer, which signals was not\ncaptured within less than 1 s. In contrast, the initial intensity of 4.48 cd/m2 was\nmeasured by a photometer (Xinbao Instrument, SM218E) in real time within <1 s.\nFrom this perspective, the initially reported value of 4.48 cd/m2 does not strictly\nreflect the luminescence enhancement degree, which is reasonable. As the reviewer\ncorrectly noted, the intensity measured after 2 s (0.16 cd/m²) is consistent with the\nreported results and is retained.\nWe note that the instruments used for TL, afterglow, and luminance (cd/m2)\nmeasurements were from different manufacturers, some measured at collaborating\nlaboratories. Detailed detector calibration parameters are not fully available, which\nlimits direct comparison. We appreciate the reviewer’s understanding of this\nlimitation. Furthermore, PersL intensity is highly temperature dependent. Higher\ntemperatures promote electron release but also induce significant thermal quenching.\ntemperature is much weaker than at room temperature, indicating that bright PersL\nrequires an appropriate measurement temperature. This highlights the inherent\ntrade-off between achieving high initial brightness and controlling electron release.\nTo avoid potential confusion, and following the reviewer’s recommendation, we\nhave removed the initial 4.48 cd/m2 intensity from the revised manuscript, while\nretaining the 0.16 cd/m2 value measured at 2s after the X-ray is turned off.\nComment 7: Many of the figures still have no values on the vertical axis. This does\nnot allow for a correct interpretation of the data and should be fixed.\nResponse 7: In response to the reviewer’s insightful comment, we have added values\non the vertical axes of all PersL spectra (Fig 2, Fig 3, Fig 4, Fig S1, Fig S7, Fig S8,\nFig S9, Fig S10, Fig S20, Fig S21, Fig S23, Fig S25, Fig S29, Fig S301, Fig S34, Fig\nS38) in the revised manuscript and Supporting Information.\nComment 8: It is not clear whether the afterglow curves that are shown are in\nphotometric or in radiometric units. This needs to be clarified in the manuscript.\nResponse 8: We thank the reviewer for this insightful comment. The afterglow curves\nreported in the manuscript represent photon counts directly obtained from the\nspectrometer. To avoid repetition across different figures, this clarification has been\nadded to the Experimental section of the revised Supporting Information to prevent\nany potential misunderstanding.\nThe afterglow curves reported in this work represent photon counts directly obtained\nfrom the spectrometer.\nComment 9: The information collected in Fig. S37 is very meaningful and allows for\na meaningful comparison of the intensities of the different CsPbX QDs sensitized by\nBaFCl:Eu/Gd. However, the conclusion of the figure is that a lot of the light is lost. Is\nthis due to the low quantum yield of the QDs? The authors have chosen to report\nenergy transfer efficiencies (all close to 90%) but as this doesn’t translate to a high\n9\nemission intensities of the QDs it would be more meaningful to report integrated\ninitial intensities. As an example: If I put a layer with a black powder on top of the\nBaFCl:Eu/Gd I will have a ET efficiency close to 100% using the formula you use but\nof course no light emission.\nResponse 9: We sincerely thank the reviewer for this insightful suggestion. In our\nexperiments, the BaFCl:Eu/Gd film was placed in direct contact with a quartz\ncontainer holding the QDs to minimize energy losses between the BaFCl:Eu/Gd layer\nand the QDs. Nevertheless, because the BaFCl:Eu/Gd film was physically attached to\nthe outer wall of the quartz container, a certain degree of energy loss is unavoidable.\nWhen neglecting this small loss, the PersL emission from Eu ions is either absorbed\nby the QDs or transmitted through them. As shown in Fig. S37, the transmitted Eu\nPersL signal remains clearly detectable, indicating that the Eu emission passes\neffectively through the QD layer.\nAlthough the calculated energy-transfer efficiencies are high (~90%), this does not\ntranslate into intense QD emission. The main limiting factor is the relatively low\nphotoluminescence quantum yield of the synthesized CsPbX QDs.\nWe acknowledge\nthis limitation and note that we are continuing to improve our synthesis procedures to\nobtain QDs with ultrahigh quantum yields.\nThe following contents have been added in the revised supporting information.\nFig. S38 PersL spectra of various CsPbX QDs sensitized by BaFCl:Eu/Gd (a), and\n3\nthe corresponding variations in integrated emission intensity (b).\nThe PersL spectra were recorded using a fiber-coupled spectrometer (QEpro)\n10\nlinked to the X-ray source. Because the BaFCl:Eu/Gd film was physically attached to\nthe outer wall of the quartz container holding the QDs, a certain degree of optical loss\nis unavoidable. When neglecting this small loss, the light conversion efficiency (𝜂)\nfrom BaFCl:Eu/Gd to CsPbX QDs was defined as 𝜂= (𝐼 − 𝐼 )/𝐼 , where 𝐼 and 𝐼\n3 0 𝑋 0 0 𝑋\ncorrespond to the integrated Eu2+ PersL intensities in the absence and presence of\nCsPbX QDs, respectively. The calculated 𝜂 values for CsPbCl Br , CsPbCl Br ,\n3 2 1 1 2\nCsPbCl Br , CsPbBr , CsPbBr I , CsPbBr I , CsPbBr I and CsPbI were\n0.5 2.5 3 1 2 1.3 1.7 2 1 3\napproximately 94.9%, 95.3%, 95.1%, 95.4%, 95.4%, 93.7%, 93.9% and 93.7%,\nrespectively. The substantial energy loss shown in Fig. S37b is mainly attributed to\nthe relatively low quantum yield of the prepared QDs.\nComment 10: It is now stressed that there is a 150-fold improvement in intensity for\nthe Sm-doped materials upon addition of the Gd, however I think the nuance that the\nintensity is still very low should be added more explicitly to the abstract and\nconclusion to add context. Alternatively the authors could report a more balanced\nvalue based on the average improvement observed in all of the measured materials.\nResponse 10: In accordance with the reviewer’s helpful suggestion, we have replaced\nthe discussion of Sm-doped samples with Eu-doped samples in the revised manuscript.\nThe contents of “The PersL of Eu²⁺ is enhanced by up to 32.7-fold upon Gd3+\ncodoping.” and “The red PersL from Eu2+ is remarkably enhanced, by up to 32.7-fold,\nthrough Gd3+ codoping.” have been added in the revised abstract and conclusion,\nrespectively.\nComment 11: I would like the authors to include explicitly in the manuscript that the\ncomparison of reported quantum yields might be influenced by the fact that the\nSrAl O phosphor displays trapping and detrapping under UV excitation (which might\n2 4\nlower the overall efficiency as pointed out by the authors in the introduction) while\nthe BaFCl:Eu does not.\nResponse 11: We appreciate the reviewer’s valuable comment. The following content\nhas been added in the revised manuscript.\n(PLQY) of 92.7%, far exceeding that of SrAl O : Eu/Dy (54.7%) (Figure S14). It\n2 4\nshould be noted that the comparison of reported quantum yields may be affected by\nthe fact that SrAl O : Eu/Dy exhibits trapping and detrapping under UV excitation,\n2 4\nwhich can reduce the overall efficiency, whereas BaFCl:Eu does not.\n12"}, {"round": 3, "reviewer_comments": {"reviewer_3": "I would like to thank the authors for the revisions. I have carefully reviewed the rebuttal and the revised manuscript, and I\nconclude that my concerns have been properly addressed."}, "author_response": ""}]

Decision letter