Long-lasting far-UVC persistent luminescence for solar-blind optical tagging

Abstract

Far ultraviolet-C (far-UVC; 200–230 nm) luminescent materials have garnered significant interest in recent years, driven by the growing demands for applications such as disinfection and solar-blind imaging due to their distinct wavelength features. However, the research and development of far-UVC persistent phosphors are lacking. Here, we report the realization of far-UVC persistent luminescence in CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ phosphors, which show emissions peaking at 220 nm and 230 nm with a long persistence time of >24 h after ceasing X-ray excitation. This is by far the shortest UVC afterglow emission to the best of our knowledge. The far-UVC afterglow from the charged phosphors can be readily detected using a solar-blind UV camera in both indoor-lighting and outdoor environments owing to the absence of background noise from ambient light. The continuous photostimulation of indoor white LED light and outdoor sunlight has different impacts on the far-UVC afterglow performance of CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ phosphors, which is elucidated by the decay time-dependent thermoluminescence (TL) curves under different light conditions. This study expands the field of persistent luminescence to the far-UVC spectral region and will inspire the discovery of more excellent far-UVC persistent phosphors.

---

Introduction

Ultraviolet-C (UVC) light with a wavelength range from 200 to 280 nm has been instrumental in a variety of important applications such as disinfection, phototherapy, confidential communication, optical information storage, and solar-blind optical tagging.^1–6 For example, UVC light can effectively eradicate bacteria, viruses, and other pathogens by disrupting their nucleic acids, thereby halting their reproduction.^7,8 In the realm of solar-blind optical tagging, the lack of UVC light on the Earth's surface results in zero background interference, enabling the UVC signal to be monitored and imaged with high contrast.⁹ Currently, the market for UVC radiation sources is dominated by mercury discharge lamps with an emission wavelength of 254 nm. However, these gaseous light sources have the drawbacks of being bulky, fragile, and inefficient.^10,11 Furthermore, these lamps are designed for operation exclusively in areas inaccessible to humans, as their emission spectra pose a risk to human eyes and skin.^12,13 The recently developed solid-state deep-UV LEDs can replace gaseous lamps in some ways, but there are still challenging issues with these AlGaN-based UV LEDs, such as expensive production costs, complex preparation procedures, and low external quantum efficiency.^14,15 On the other hand, extensive research efforts have focused on lanthanide-based inorganic luminescent materials for UVC luminescence through upconversion or downshifting processes, opening up new avenues for UVC technology.^16–19 Among them, UVC persistent phosphors, characterized by their unique delayed luminescence properties, have drawn an unprecedented amount of research and technological attention.²⁰

For UVC persistent luminescence, Pr³⁺, Bi³⁺ and Pb²⁺ stand out as promising luminescence centers in solids because of their efficient 4f5d → 4f inter-configurational transition or ³P₁ → ¹S₀ transition.^21–26 Recently, some Pr³⁺-doped and Bi³⁺-doped UVC persistent phosphors have been reported.^27–31 For example, Yang et al. reported a Cs₂NaYF₆:Pr³⁺ UVC persistent phosphor with an emission peak at 250 nm and an afterglow time of ∼2 h after X-ray irradiation.³² Wang and coworkers reported a series of Pr³⁺-doped silicate-based UVC persistent phosphors, which can be effectively charged using a standard 254 nm mercury lamp and exhibit UVC persistent luminescence peaking at 265–270 nm.⁹ Lv et al. reported an X-ray-activated YBO₃:Pr³⁺ UVC persistent phosphor with an emission peak at 261 nm, which exhibits a remarkable enhancement of UVC luminescence intensity in bright environments.³³ However, the majority of existing UVC persistent phosphors show dominant emission bands above 250 nm,^8,34–36 while persistent phosphors that emit in the far-UVC spectral range (200–230 nm) are lacking. Notably, the far-UVC persistent phosphors can potentially revolutionize the way that UVC light is used; for instance, these far-UVC materials can act as self-sustained glowing sources for air, surface, and water disinfection application in occupied public locations because of their stronger absorbance than regular 254 nm UVC radiation and limited penetration into human tissues.^37–40

In this study, we report two new far-UVC persistent phosphors, CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺, which can emit far-UVC persistent luminescence with a peak wavelength at 220 nm and 230 nm for >24 h after the cessation of X-ray excitation. To our knowledge, this represents the shortest UVC afterglow emission observed thus far. Because of zero UVC background on the surface of the Earth, the UVC emission from the charged persistent phosphors can be clearly detected using a solar-blind UV camera for more than 1 h in an indoor lighting or outdoor sunlight environment. This work enriches the persistent phosphors emitting in the far-UVC spectral range, paving the way for their versatile applications such as solar-blind optical tagging, disinfection, and photodynamic therapy.

Experimental

Synthesis

CaSO₄:x%Pr³⁺ (x = 0.1, 0.2, 0.5, and 1) phosphors were prepared by the high-temperature solid-state reaction method. Stoichiometric amounts of CaSO₄·2H₂O (99%, Aladdin), Pr₆O₁₁ (99.99%, Aladdin), and (NH₄)₂SO₄ (99.99%, Macklin) powders were mixed and ground homogeneously in an agate mortar. Then the obtained powders were pre-sintered at 200 °C in air for 2 h. The pre-fired powders were ground again and pressed into 11 mm diameter discs using a 30 T hydraulic press. Finally, the discs were sintered at 1100 °C in air for 4 h to form the final CaSO₄:Pr³⁺ solid ceramic discs. The processes for determining the sintering conditions of the CaSO₄:Pr³⁺ phosphors are shown in Fig. S1–S4 in the ESI.† The CaSO₄:Pb²⁺ phosphors in this work were fabricated following the same procedure with the only variation being the use of PbO (99.999%, Aladdin) as the dopant source powder.

Characterization

The crystal structure and phase composition were checked using a PANalytical X'Pert PRO powder X-ray diffractometer with Cu Kα₁ radiation (λ = 1.5406 Å). Structure refinement was processed using the general structure analysis system (GSAS) program. The SEM image and elemental maps were recorded using a JSM-7800F field-emission SEM. The spectral properties, including radioluminescence and persistent luminescence properties, were measured using an FLS1000 spectrofluorometer (Edinburgh Instruments) equipped with a photomultiplier tube detector (PMT, 200–900 nm). Thermoluminescence spectra were recorded using an SL18 thermoluminescence setup (Guangzhou Rongfan Science and Technology Co., Ltd; heating rate, 4 °C per s). Before all the measurements, the samples were thermally cleaned at 450 °C to empty the traps thoroughly, followed by irradiation with an X-ray tube (Moxtek, tungsten target). UVC afterglow images from the persistent phosphors were taken using a solar-blind ultraviolet camera (WNZW-01, Suzhou Micro-Nano Laser & Photonic Technology Co., Ltd) with dual UVC (240–280 nm)-visible channels, which superimpose the UVC images onto the visible images and show the invisible UVC signal as an area of red color. UVC persistent luminescence power intensities were obtained with a Newport 1936-R optical power meter and a Newport 918D-UV-OD3R UV-enhanced silicon photodetector.

First-principles calculations

The initial atomic positions and symmetry information of the host crystal were taken from the Inorganic Crystal Structure Database and the primitive cell of CaSO₄ containing 96 atoms (16 Ca atoms, 16 S atoms, and 64 O atoms) was used in the simulation. Using the Vienna Ab initio simulation package (VASP),^41,42 theoretical simulations were conducted based on density functional theory (DFT), the generalized gradient approximation (GGA)-Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional was used for the description of the exchange and correlation energy of the electrons and the PBE+U method was utilized to calculate Pr-4f electrons.⁴³ Ca (4s²), S (3s²3p⁴), O (2s²2p⁴), and Pr (4f³6s²) were treated as valence electrons, and their interactions with the respective cores were described by the projected augmented wave (PAW) method. The equilibrium structures were acquired by optimizing the atomic positions until the energy change was less than 10^–6 eV and the Hellmann−Feynman forces on atoms were less than 0.01 eV Å⁻¹. The plane-wave cut-off energy was set at 550 eV. For k-point integration within the first Brillouin zone, a 3 × 3 × 3 Monkhorst–Pack grid was chosen for self-consistent calculation, while the high symmetry k-paths were generated for band structure calculation.

Results and discussion

Crystal structure and phase identification

The CaSO₄ crystal belongs to the Bmmb (63) space group with an orthorhombic structure, which has the lattice parameters of a = 6.992 Å, b = 6.999 Å, c = 6.240 Å, α = β = γ = 90°, and V = 305.37 ų. The simulated crystal structure of CaSO₄ is shown in Fig. 1a. In the CaSO₄ lattice, the Ca²⁺ cations are bound by eight oxygen ions, whereas the S⁶⁺ ion is encircled by a tetrahedron of four oxygen ions. Under consideration of a similar ionic radius and valence state, the Pr³⁺ ion (1.126 Å, CN = 8) and Pb²⁺ ion (1.29 Å, CN = 8) most probably occupy the Ca²⁺ site (1.12 Å, CN = 8) rather than the S⁶⁺ site (0.12 Å, CN = 4) in the CaSO₄ lattice. Fig. 1b presents the XRD patterns of the as-synthesized CaSO₄:x%Pr³⁺ (x = 0.1, 0.2, 0.5, and 1) phosphors. All the diffraction peaks are consistent with the standard XRD pattern of the CaSO₄ crystal (JCPDS No. 70-0909), suggesting that introducing Pr³⁺ will not affect the crystal structure of the CaSO₄ compound. The calculated and experimental results as well as the differences in the Rietveld refinement of the CaSO₄:0.2%Pr³⁺ phosphor are shown in Fig. 1d. The detailed refined results are collected in Table S1.† The orthorhombic CaSO₄ crystal system was used as the initial model for refinement. The detected and calculated XRD patterns match well with each other and the reliability parameters of the refinement are R_wp = 9.75%, R_p = 6.72%, and χ² = 2.65, which further manifests that the as-synthesized CaSO₄:Pr³⁺ phosphor is a pure CaSO₄ phase. XRD patterns of the as-obtained CaSO₄:x%Pb²⁺ (x = 0.5–5) phosphors are displayed in Fig. 1c, and the diffraction peaks of all samples can also be well indexed to the standard card of CaSO₄. The results demonstrate that the pure phase is formed independent of the Pb²⁺ concentration. The Rietveld refinement results (Fig. 1e) also suggest that the CaSO₄:Pb²⁺ phosphors were synthesized with good crystallinity in a pure phase through satisfactory refinement values of R_wp, R_p, and χ² (Table S1†). In addition, to study the microstructure of the prepared CaSO₄:Pr³⁺ phosphor, SEM and EDS mapping images were also recorded in Fig. 1f. The phosphor particles show irregular shapes and the sizes of these particles are in the micrometer range. The EDS elemental mapping results demonstrate that Ca, S, O, and Pr elements are homogeneously distributed throughout a randomly selected phosphor particle, further confirming that the CaSO₄:Pr³⁺ phosphor has been successfully prepared.

Fig. 1 (a) Crystal structure diagram of CaSO₄ and the coordination environment of the Ca and S atoms. XRD patterns of (b) CaSO₄:x%Pr³⁺ (x = 0.1–1) phosphors and (c) CaSO₄:x%Pb²⁺ (x = 0.5–5) phosphors. Rietveld refinement of (d) CaSO₄:0.2%Pr³⁺ and (e) CaSO₄:3%Pb²⁺ phosphors. (f) SEM and EDS mapping images of the CaSO₄:0.2%Pr³⁺ phosphor. The inset shows the EDS spectrum of the CaSO₄:0.2%Pr³⁺ phosphor.

UVC photoluminescence properties

Taking into account the fact that the luminescence properties of Pr³⁺ are sensitive to the host material, the band structure and density of states (DOS) of the CaSO₄ host and Pr³⁺-doped CaSO₄ were calculated using density functional theory (DFT). As shown in Fig. S5a,† the valence band (VB) maximum and the conduction band (CB) minimum are located at the same point, which suggests that the CaSO₄ host has a direct bandgap (E_g). Based on the calculation, the bandgap of CaSO₄ is 5.96 eV, which allows for the possibility of deep UV and even far-UVC luminescence. Besides, the total and partial DOSs of the CaSO₄ host in Fig. S5b† show that the d orbital of Ca atoms mainly forms the DOS of the CB, while the VB is mostly derived from the p orbital of O atoms. Fig. 2b presents the total and partial DOSs of Pr³⁺-doped CaSO₄. After doping Pr³⁺ into the CaSO₄ host, some new energy levels overlapping with the CB are generated, primarily resulting from the d orbital of the Pr atoms. This is consistent with the changes in the band structure of Pr³⁺-doped CaSO₄ in Fig. 2a compared with that of the CaSO₄ host.

Fig. 2 (a) Band structure and (b) total DOS and partial DOS of Pr-doped CaSO₄ (Pr replaces Ca). (c) Energy level scheme of the Pr³⁺ 4f² → 4f¹5d¹ configuration and the possible optical transitions. The radioluminescence spectra of (d) CaSO₄:0.2%Pr³⁺ and (e) CaSO₄:3%Pb²⁺ phosphors.

The UVC emission of the Pr³⁺-activated phosphors is ascribed to parity allowed 4f5d → 4f² inter-configurational transitions. Two general requirements must be met for Pr³⁺ 4f5d → 4f² transitions to occur in a solid: an appropriate placement for the 4f5d level, meaning that it is energetically below the ¹S₀ level and suitably separated from the closest ³P_J level, and a very little Stokes shift (less than 3000 cm⁻¹),⁴⁴ as depicted in Fig. 2c. The radioluminescence spectrum of the CaSO₄:0.2%Pr³⁺ phosphor upon high-energy X-ray excitation is presented in Fig. 2d. It includes the UV emission dominated by four distinct UVC emission peaks at 220 nm, 231 nm, 247 nm, and 255 nm, which are attributed to the inter-configurational transitions from the lowest 4f5d state to different 4f² states (³H₄, ³H₅, ³H₆, and ³F₂) of Pr³⁺, and the visible and infrared emission bands originating from the 4f → 4f intra-configurational transitions. The effective inter-configurational and intra-configurational transitions corresponding to different emission peaks were also labeled in Fig. 2d. Fig. 2e shows the radioluminescence spectrum of the CaSO₄:3%Pb²⁺ phosphor upon X-ray excitation. The inset is the energy level structure of the Pb²⁺ ion. When excited with X-rays, the CaSO₄:Pb²⁺ phosphor exhibits an ultra-narrow emission band in the UVC spectral range with a maximum at 230 nm and a relatively weak broad UVA emission band ranging from 300 to 400 nm, which can be ascribed to the ³P_0,1 → ¹S₀ transition of Pb²⁺ and the radiative emission of Pb²⁺ pairs and/or clusters formed in the CaSO₄ host lattice, respectively.

Far-UVC persistent luminescence properties

In addition to UVC radioluminescence, irradiation with high-energy X-rays can also induce long-lasting UVC persistent luminescence in CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ phosphors. As shown in Fig. S6 and S7,† the UVC persistent luminescence decay curves, the corresponding persistent luminescence emission spectra, and the TL curves of the CaSO₄:Pr³⁺ phosphors with different Pr³⁺ doping concentrations were recorded in detail after irradiation with an X-ray beam, which suggests that the optimal doping concentration of Pr³⁺ is 0.2%. Moreover, to investigate the correlation between the irradiation time and UVC persistent luminescence performance, the persistent luminescence decay curves and the corresponding persistent luminescence emission spectra of the CaSO₄:0.2%Pr³⁺ phosphor were analyzed by varying the excitation durations with the same X-ray irradiation dose (208.20 mGy s⁻¹), as illustrated in Fig. S8.† The results indicate that the CaSO₄:0.2%Pr³⁺ phosphor can achieve full charging with 15 min of X-ray excitation. Additionally, the TL curves of the CaSO₄:0.2%Pr³⁺ phosphor with varying irradiation durations also show that 15 min of 208.20 mGy s⁻¹ X-ray exposure is sufficient for complete charging (Fig. S9†). Likewise, by varying Pb²⁺ doping concentrations, the UVC afterglow performance and TL spectra of the CaSO₄:Pb²⁺ phosphors were also examined (Fig. S10 and S11†), showing that 3% is the ideal Pb²⁺ doping concentration. Furthermore, we also recorded the afterglow decay curves, afterglow emission spectra, and TL curves of the CaSO₄:3%Pb²⁺ phosphor with different irradiation times (Fig. S12 and S13†), which show that 15 min of 290.30 mGy s⁻¹ X-ray excitation can fully charge this phosphor.

Fig. 3a depicts the far-UVC persistent luminescence decay curve of the CaSO₄:0.2%Pr³⁺ phosphor monitored at 220 nm in the dark at room temperature after X-ray irradiation for 15 min. The data were collected by tracking the afterglow intensity at 220 nm over time for a duration of 24 h. The far-UVC afterglow intensity experiences a rapid decrease within the first few hours (0–6 h), followed by a gradual decline until the end of the observation period. Even after 24 h of natural decay in darkness, the far-UVC afterglow intensity remains significantly higher than the background, indicating that the far-UVC afterglow of the CaSO₄:0.2%Pr³⁺ phosphor can persist for over 24 h. The persistent luminescence emission spectrum of the CaSO₄:0.2%Pr³⁺ phosphor in the upper inset of Fig. 3a shares an identical emission profile with the radioluminescence spectrum in the UVC region, suggesting that the persistent UVC emission originates from the 4f5d → 4f² transition of Pr³⁺. Fig. 3b presents the afterglow emission spectra of the CaSO₄:0.2%Pr³⁺ phosphor acquired at different decay times within 6 h after ceasing X-ray excitation. Upon prolonging the decay time, the relative emission intensity of persistent luminescence decreases, while the spectral profiles including the peak position and spectral shape remain constant, which further demonstrates that the UVC afterglow is attributed to the Pr³⁺ emitter. As given in Fig. S14a and Table S2,† the far-UVC persistent luminescence power density of the CaSO₄:0.2%Pr³⁺ phosphor at 30 s after the stoppage of X-ray irradiation was determined to be ∼12.43 mW m⁻², and the recording lasts for 30 min. In addition, we also measured the far-UVC afterglow decay curve monitored at 230 nm and persistent luminescence emission spectra at different decay times of the CaSO₄:3%Pb²⁺ phosphor after irradiation with an X-ray tube, as shown in Fig. 3c and d. The results suggest that the far-UVC afterglow of the CaSO₄:3%Pb²⁺ phosphor can also last for >24 h and does stem from the Pb²⁺ emitters. Moreover, the afterglow power density of the CaSO₄:3%Pb²⁺ phosphor at a decay time of 30 s was found to be ∼23.60 mW m⁻² using an optical power meter (Fig. S14b and Table S2†), which is about twice as high as that of the CaSO₄:0.2%Pr³⁺ phosphor. Additionally, the afterglow power intensities of these two phosphors are comparable with other reported UVC persistent phosphors, as shown in Table S3.†

Fig. 3 (a) Far-UVC persistent luminescence decay curve of the CaSO₄:0.2%Pr³⁺ phosphor monitored at 220 nm in the dark at room temperature after X-ray irradiation. The upper inset shows the persistent luminescence emission spectrum acquired at 10 min decay after the stoppage of the irradiation. (b) Persistent luminescence emission spectra of the pre-irradiated CaSO₄:0.2%Pr³⁺ phosphor at different decay times. (c) Far-UVC afterglow decay curve of the CaSO₄:3%Pb²⁺ phosphor monitored at 230 nm in the dark at room temperature after X-ray irradiation. The upper inset displays the afterglow emission spectrum measured at 10 min decay after ceasing X-ray irradiation. (d) Afterglow emission spectra of the pre-irradiated CaSO₄:3%Pb²⁺ phosphor at different decay times. (e) UVC persistent luminescence images of the charged CaSO₄:0.2%Pr³⁺ phosphor disc at different decay times in the dark, in an indoor LED lighting environment (300 lux) and in an outdoor sunlight environment (∼20 000 lux), respectively. The phosphor disc was pre-irradiated with X-rays for 15 min.

By taking advantage of the zero solar UVC background on the Earth's surface, we verify the unique capability of the CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ UVC persistent phosphors as solar-blind tags for high-contrast in bright indoor and outdoor environments using a solar-blind UV camera. Fig. 3e displays the UVC persistent luminescence images of the charged CaSO₄:0.2%Pr³⁺ phosphor disc at various decay times under different ambient light conditions. Each image combines a UVC emission depiction with a visible one. Note that the detected invisible UVC emission is shown by the red pattern, whose area is proportional to the UVC emission intensity. It is found that the UVC persistent luminescence signal from the CaSO₄:0.2%Pr³⁺ phosphor can be detected and imaged for more than 1 h in the dark, in an indoor LED lighting environment (300 lux) or in an outdoor sunlight environment (∼20 000 lux), and both indoor ambient light and outdoor sunlight have no significant effect on the UVC afterglow intensity and detectable imaging time. However, the UVC persistent luminescence performance of the CaSO₄:3%Pb²⁺ phosphor under the photostimulation of ambient light is very different. As shown in Fig. S15,† the UVC afterglow photographs of the CaSO₄:3%Pb²⁺ phosphor in darkness and in bright indoor and outdoor environments were also captured after the cessation of X-ray excitation. Compared to that in the dark, the UVC afterglow intensity obviously increases within 1 h in bright indoor ambient light, but it exhibits a more pronounced enhancement in the initial stage and decays at a much faster rate in outdoor sunlight, which can be attributed to the fact that natural sunlight with such a high illuminance value (∼10 000 lux) can dramatically accelerate the release of the stored energy in the CaSO₄:3%Pb²⁺ phosphor.

Thermoluminescence properties

To investigate the de-trapping processes of CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ phosphors in the dark and bright environments and figure out the reason why the far-UVC afterglow performance of these two phosphors is affected differently by the photostimulation of ambient light, detailed TL measurements were carried out. Fig. 4a depicts the time-dependent TL curves of the decaying CaSO₄:0.2%Pr³⁺ phosphor in darkness, which reflect the room-temperature thermal emptying of the stored energy in energy traps. For the case of 30 s short decay in the dark, the TL curve covers almost the entire test temperature range from 300 to 600 K, with a dominant peak at 377 K (trap I), a shoulder at 447 K (trap II), and a weak glow peak at 572 K (trap III). With the increase of decay time from 30 s to 24 h, the integral TL intensity gradually diminishes, accompanied by a shift of the TL peaks for traps I, II, and III towards higher temperatures. The TL intensity of trap I drops most quickly, which is followed by a slow decline of TL intensities of trap II and trap III, indicating a close relationship between room-temperature thermal stimulation and trap depth in the CaSO₄:Pr³⁺ phosphor. The TL curves of a decaying CaSO₄:0.2%Pr³⁺ phosphor under the illumination of 300 lux white LED light are shown in Fig. 4b. Compared with the TL spectra in Fig. 4a, there is no obvious difference in the variation of the TL intensity of trap I, while the TL intensities of trap II and trap III exhibit a faster and more pronounced decrease trend with prolonged exposure to white light. Notably, trap I remains dominant among all traps in the phosphor, and the release of energy stored in trap I is almost unaffected by indoor LED lighting illumination. This perverse finding aligns with the results in Fig. 3e, which suggests that the continuous photo-stimulation from ambient light has minimal impact on the UVC persistent luminescence of the CaSO₄:0.2%Pr³⁺ phosphor. It makes sense that ambient light illumination can substantially accelerate the release of the stored energy in deeper traps (trap II and trap III). However, the reason why it does not notably affect the release of the stored energy in shallow traps (trap I) is vague and worth investigating.

Fig. 4 TL curves of the CaSO₄:0.2%Pr³⁺ phosphor recorded at different decay times in the (a) dark and (b) indoor white LED lighting environments (300 lux) after irradiation with X-rays. TL curves of the CaSO₄:3%Pb²⁺ phosphor measured at different decay times in the (c) dark and (d) indoor white LED lighting environments (300 lux) after X-ray irradiation.

Based on the time-dependent TL curves of the CaSO₄:0.2%Pr³⁺ phosphor in dark and bright (300 lux) indoor environments, we estimated the depths of the shallowest occupied trap corresponding to various decay times using an initial rise analysis method, as depicted in Fig. S16a–c.† As the decay time is prolonged from 30 s to 24 h, the obtained trap depth in darkness increases continuously from 0.76 to 1.07 eV, which is smaller than that calculated using the improved peak position method.⁴⁵ However, the calculated trap depth in the bright environment is lower than that in darkness at the same decay time, and first increases from 0.74 to 0.98 eV and then declines to 0.91 eV with the extension of the illumination time of white light, which is because ambient light stimulation can trigger trap redistribution within the phosphor. Specifically, partially stored electrons can be promoted from these deep traps to shallow traps directly or deep traps can release trapped electrons into the conduction band when exposed to polychromic white LED light. In the latter case, some of these released electrons can be re-captured again by the depleted shallow traps. According to the TL results, the electron transfer from deep traps to shallow traps is almost in dynamic equilibrium with the process of releasing electrons stored in these shallow traps for UVC luminescence upon continuous photostimulation of ambient light; thus it seems that photo-stimulation has no obvious effect on the UVC persistent luminescence performance of the CaSO₄:Pr³⁺ phosphor.

To find out why the influence of ambient light stimulation on the UVC afterglow performance of the CaSO₄:Pb²⁺ phosphor is different from that of the CaSO₄:Pr³⁺ phosphor, the TL spectra of the charged CaSO₄:3%Pb²⁺ phosphor at different decay times were also recorded in dark and bright (300 lux) indoor environments, as given in Fig. 4c and d. Comparing the TL curve of the CaSO₄:Pb²⁺ phosphor after 30 s short decay in the dark with that of the CaSO₄:Pr³⁺ phosphor, it can be observed that the TL peak for trap I moves slightly towards a higher temperature and its distribution is narrower. The peak position of the TL peak corresponding to trap II hardly changes, but the TL peak for trap III disappears when Pb²⁺ replaces Pr³⁺ as the dopant ion, indicating that trap I and trap II should stem from the intrinsic defects of the CaSO₄ host, and trap III is believed to result from the Pr-related extrinsic defects due to the charge imbalance between Pr³⁺ and Ca²⁺. Fig. 4d shows the time-dependent TL spectra of the pre-irradiated CaSO₄:Pb²⁺ phosphor under the illumination of 300 lux white LED light. Compared to that of the CaSO₄:Pb²⁺ phosphor in the dark, the TL intensities of trap I and trap II drop much faster as the illumination time of white light is extended, which is different from the response of the CaSO₄:Pr³⁺ phosphor to ambient light stimulation. The trap depths of the CaSO₄:Pb²⁺ phosphor at various decay instants were also calculated using the initial rising method, as displayed in Fig. S16d–f.† As shown in Fig. S16f,† the process of re-capturing electrons released from the deep traps in the depleted shallow traps is also evident upon ambient light illumination. However, for these shallow traps in the CaSO₄:Pb²⁺ phosphor, the predominant mechanism involves the release of stored electrons in them for UVC luminescence, outweighing the electron transfer from deep traps to shallow traps induced by photostimulation. This dominance is attributed to the absence of deep traps (trap III) in this phosphor, according to the results in Fig. 4c and d.

Solar-blind optical tagging application

Considering the long-lasting UVC persistent luminescence of CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ phosphors and the absence of UVC light on the Earth's surface, the CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ phosphors can function as solar-blind optical taggants for identifying and tracking objects in bright environments. We illustrated their covert optical tagging application in bright environments by using the CaSO₄:Pr³⁺ phosphor. Taking advantage of the invisible UVC luminescence signal from the charged CaSO₄:Pr³⁺ phosphor, the information encryption application using the binary code was first designed and realized, as shown in Fig. 5a. The charged CaSO₄:Pr³⁺ phosphor discs and other non-glowing ceramic discs (same size but without UVC afterglow) were arranged in a specific order of three rows and eight columns, and represent the binary codes “1” and “0”, respectively. We can only see three rows of dots with the naked eye or a standard digital camera. With the help of a solar-blind UV camera, the two kinds of ceramic discs were distinguished, and the encrypted information “UVC” in the binary code was decoded based on the ASCII table. As displayed in Fig. 5b, by virtue of the strong and long-lasting UVC afterglow of the CaSO₄:Pr³⁺ phosphor, the dynamic tracking application has also been demonstrated in a bright outdoor environment. A pre-irradiated CaSO₄:Pr³⁺ phosphor disc was affixed to a cartoon car in the outdoor square. The movement of the cartoon car can be tracked by utilizing a solar-blind UV camera to capture the UVC luminescence signal. The self-sustaining UVC luminescence of CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ phosphors presents exciting prospects for information encryption and long-lasting optical tagging applications in bright environments.

Fig. 5 (a) Visible and UVC images of letters “U”, “V”, and “C” encrypted using the binary code. (b) Schematic diagram of dynamic tracking application of a pre-irradiated CaSO₄:Pr³⁺ phosphor disc affixed to a cartoon car in the outdoor square. A solar-blind UV camera was used to capture the location of the cartoon car in a two-dimensional plane, with a potted plant as a reference.

Conclusions

In summary, we report two new far-UVC persistent phosphors, CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺, capable of emitting intense far-UVC afterglow at 220 nm and 230 nm for >24 h after ceasing X-ray excitation. To our knowledge, this represents the shortest UVC persistent luminescence emission observed thus far. Benefitting from the strong persistent UVC light emission and zero-background noise from ambient light, the afterglow signal from the charged phosphors can be clearly monitored and imaged using a solar-blind UV camera in bright indoor and outdoor environments. Moreover, we found that both indoor ambient light and outdoor sunlight exert a negligible influence on the UVC afterglow intensity and detectable imaging duration of the CaSO₄:Pr³⁺ phosphor, while the UVC afterglow performance of the CaSO₄:Pb²⁺ phosphor has a rather different profile. This phenomenon is elucidated by the decay time-dependent TL spectra in darkness and under indoor white LED light exposure, and the different trap distribution caused by the substitution of Pr³⁺ and Pb²⁺ for Ca²⁺ in the CaSO₄ host is responsible for the distinct UVC afterglow performance of the CaSO₄:Pr³⁺ and CaSO₄:Pb²⁺ phosphors. The far-UVC persistent phosphors discovered here are expected to bring new solutions to some important applications where far-UVC light is needed, such as sterilization, water purification, solar-blind optical tagging, and photodynamic therapy.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 12474407 and 51902184), Key Research and Development Program of Shandong Province (Major Scientific and Technological Innovation Project) (Grant No. 2021CXGC011101), the State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences (Grant No. SITP-NLIST-YB-2022-10), and the “Qi-Lu Young Scholar Fund” of Shandong University.

References

1. X. Fan, R. Huang and H. Chen, Application of ultraviolet C technology for surface decontamination of fresh produce, Trends Food Sci. Technol., 2017, 70, 9–19.
2. G. Matafonova and V. Batoev, Recent advances in application of UV light-emitting diodes for degrading organic pollutants in water through advanced oxidation processes: A review, Water Res., 2018, 132, 177–189.
3. E. L. Cates, M. Cho and J.-H. Kim, Converting Visible Light into UVC: Microbial Inactivation by Pr³⁺-Activated Upconversion Materials, Environ. Sci. Technol., 2011, 45, 3680–3686.
4. S. Nayak, S.-E. O'Donnell, C. M. Sales and R. V. Tikekar, Fructose Accelerates UV-C Induced Photochemical Degradation of Pentachlorophenol in Low and High Salinity Water, J. Agric. Food Chem., 2016, 64, 4214–4219.
5. X. He, E. Xie, M. S. Islim, A. A. Purwita, J. J. D. McKendry, E. Gu, H. Haas and M. D. Dawson, 1 Gbps free-space deep-ultraviolet communications based on III-nitride micro-LEDs emitting at 262 nm, Photon. Res., 2019, 7, B41–B47.
6. X. Chen, F. Ren, S. Gu and J. Ye, Review of gallium-oxide-based solar-blind ultraviolet photodetectors, Photon. Res., 2019, 7, 381–415.
7. M. Raeiszadeh and B. Adeli, A Critical Review on Ultraviolet Disinfection Systems against COVID-19 Outbreak: Applicability, Validation, and Safety Considerations, ACS Photonics, 2020, 7, 2941–2951.
8. X. Zhou, J. Qiao, Y. Zhao, K. Han and Z. Xia, Multi-responsive deep-ultraviolet emission in praseodymium-doped phosphors for microbial sterilization, Sci. China Mater., 2022, 65, 1103–1111.
9. X. Wang, Y. Chen, F. Liu and Z. Pan, Solar-blind ultraviolet-C persistent luminescence phosphors, Nat. Commun., 2020, 11, 2040.
10. M. M. Afandi, H. Kang, T. Kang, J. Park and J. Kim, AC-Driven Ultraviolet-C Electroluminescence from an All-Solution-Processed CaSiO₃:Pr³⁺ Thin Film Based on a Metal-Oxide-Semiconductor Structure, Adv. Mater. Interfaces, 2022, 9, 2200248.
11. S.-Y. Song, K.-K. Liu, J.-Y. Wei, Q. Lou, Y. Shang and C.-X. Shan, Deep-Ultraviolet Emissive Carbon Nanodots, Nano Lett., 2019, 19, 5553–5561.
12. A. Trevisan, S. Piovesan, A. Leonardi, M. Bertocco, P. Nicolosi, M. G. Pelizzo and A. Angelini, Unusual High Exposure to Ultraviolet-C Radiation, Photochem. Photobiol., 2006, 82, 1077–1079.
13. S. Zaffina, V. Camisa, M. Lembo, M. R. Vinci, M. G. Tucci, M. Borra, A. Napolitano and V. Cannatà, Accidental Exposure to UV Radiation Produced by Germicidal Lamp: Case Report and Risk Assessment, Photochem. Photobiol., 2012, 88, 1001–1004.
14. M. Kneissl, T.-Y. Seong, J. Han and H. Amano, The emergence and prospects of deep-ultraviolet light-emitting diode technologies, Nat. Photonics, 2019, 13, 233–244.
15. S. Liang and W. Sun, Recent Advances in Packaging Technologies of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes, Adv. Mater. Technol., 2022, 7, 2101502.
16. V. A. Pustovarov, K. V. Ivanovskikh, Y. E. Khatchenko, Q. Shi and M. Bettinelli, Energy conversion in LiSrPO₄ doped with Pr³⁺ ions, Radiat. Meas., 2019, 123, 39–43.
17. E. L. Cates and F. Li, Balancing intermediate state decay rates for efficient Pr³⁺ visible-to-UVC upconversion: the case of β-Y₂Si₂O₇:Pr³⁺, RSC Adv., 2016, 6, 22791–22796.
18. T. A. Johnson, E. A. Rehak, S. P. Sahu, D. A. Ladner and E. L. Cates, Bacteria Inactivation via X-ray-Induced UVC Radioluminescence: Toward in Situ Biofouling Prevention in Membrane Modules, Environ. Sci. Technol., 2016, 50, 11912–11921.
19. L. Li, P. Li, X. Lv, C. Cai, T. Li, X. Shi, D. Peng and Y. Yang, Ultraviolet-C mechanoluminescence from NaYF₄:Pr³⁺, Appl. Phys. Lett., 2024, 124, 101108.
20. X. Wang and Y. Mao, Emerging Ultraviolet Persistent Luminescent Materials, Adv. Opt. Mater., 2022, 10, 2201466.
21. M. Laroche, A. Braud, S. Girard, J. L. Doualan, R. Moncorge, M. Thuau and L. D. Merkle, Spectroscopic investigations of the 4f5d energy levels of Pr³⁺ in fluoride crystals by excited-state absorption and two-step excitation measurements, J. Opt. Soc. Am. B, 1999, 16, 2269–2277.
22. P. A. Tanner, C. S. K. Mak, M. D. Faucher, W. M. Kwok, D. L. Phillips and V. Mikhailik, 4f-5d transitions of Pr³⁺ in elpasolite lattices, Phys. Rev. B, 2003, 67, 115102.
23. Y. Wei, G. Xing, K. Liu, G. Li, P. Dang, S. Liang, M. Liu, Z. Cheng, D. Jin and J. Lin, New strategy for designing orangish-red-emitting phosphor via oxygen-vacancy-induced electronic localization, Light:Sci. Appl., 2019, 8, 15.
24. X. Lv, Y. Liang, Y. Zhang, D. Chen, X. Shan and X.-J. Wang, Deep-trap ultraviolet persistent phosphor for advanced optical storage application in bright environments, Light:Sci. Appl., 2024, 13, 253.
25. A. A. Bol and A. Meijerink, Luminescence of nanocrystalline ZnS:Pb²⁺, Phys. Chem. Chem. Phys., 2001, 3, 2105–2112.
26. M. Mehnaoui, R. Ternane, G. Panczer, M. Trabelsi-Ayadi and G. Boulon, Structural and luminescent properties of new Pb²⁺-doped calcium chlorapatites Ca_10−xPb_x(PO₄)₆Cl₂ (0 ≤ x ≤ 10), J. Phys.:Condens. Matter, 2008, 20, 275227.
27. H. Li, Q. Liu, J.-P. Ma, Z.-Y. Feng, J.-D. Liu, Q. Zhao, Y. Kuroiwa, C. Moriyoshi, B.-J. Ye, J.-Y. Zhang, C.-K. Duan and H.-T. Sun, Theory-Guided Defect Tuning through Topochemical Reactions for Accelerated Discovery of UVC Persistent Phosphors, Adv. Opt. Mater., 2020, 8, 1901727.
28. X. Zhao, F. Liu, Z. Yu, X. Li, C. Wang, F. Chen and X.-j. Wang, Sunlight stimulated solar-blind ultraviolet phosphor, Phys. Rev. Res., 2022, 4, L012028.
29. Y. Zhang, S. Yan, F. Xiao, X. Shan, X. Lv, W. Wang and Y. Liang, Long-persistent far-UVC light emission in Pr³⁺-doped Sr₂P₂O₇ phosphor for microbial sterilization, Inorg. Chem. Front., 2023, 10, 5958–5968.
30. Q. Liu, Z.-Y. Feng, H. Li, Q. Zhao, N. Shirahata, Y. Kuroiwa, C. Moriyoshi, C.-K. Duan and H.-T. Sun, Non-Rare-Earth UVC Persistent Phosphors Enabled by Bismuth Doping, Adv. Opt. Mater., 2021, 9, 2002065.
31. L. Liu, S. Peng, Y. Guo, Y. Lin, X. Sun, L. Song, J. Shi and Y. Zhang, Designing X-ray-Excited UVC Persistent Luminescent Material via Band Gap Engineering and Its Application to Anti-Counterfeiting and Information Encryption, ACS Appl. Mater. Interfaces, 2022, 14, 41215–41224.
32. Y.-M. Yang, Z.-Y. Li, J.-Y. Zhang, Y. Lu, S.-Q. Guo, Q. Zhao, X. Wang, Z.-J. Yong, H. Li, J.-P. Ma, Y. Kuroiwa, C. Moriyoshi, L.-L. Hu, L.-Y. Zhang, L.-R. Zheng and H.-T. Sun, X-ray-activated long persistent phosphors featuring strong UVC afterglow emissions, Light:Sci. Appl., 2018, 7, 88.
33. X. Lv, X. Shan, Y. Zhang and Y. Liang, Ambient light stimulation enabling intense and long-lasting ultraviolet-C persistent luminescence from Pr³⁺-doped YBO₃ in bright environments, J. Mater. Chem. C, 2023, 11, 4492–4499.
34. S. Yan, Y. Liang, Y. Chen, J. Liu, D. Chen and Z. Pan, Ultraviolet-C persistent luminescence from the Lu₂SiO₅:Pr³⁺ persistent phosphor for solar-blind optical tagging, Dalton Trans., 2021, 50, 8457–8466.
35. S. Yan, Y. Liang, Y. Zhang, B. Lou, J. Liu, D. Chen, S. Miao and C. Ma, A considerable improvement of long-persistent luminescence in LiLuSiO₄:Pr³⁺ phosphors by Sm³⁺ co-doping for optical tagging applications, J. Mater. Chem. C, 2022, 10, 17343–17352.
36. Y. Zhang, X. Shan, X. Lv, D. Chen, S. Miao, W. Wang and Y. Liang, Multimodal luminescence in Pr³⁺ single-doped Li₂CaSiO₄ phosphor for optical information storage and anti-counterfeiting applications, Chem. Eng. J., 2023, 474, 145886.
37. M. H. Wang, H. H. Zhang, C. K. Chan, P. K. H. Lee and A. C. K. Lai, Experimental study of the disinfection performance of a 222-nm Far-UVC upper-room system on airborne microorganisms in a full-scale chamber, Build. Environ., 2023, 236, 110260.
38. Y. Lv, X. Chen, W. Wu, F. Wu, X. Wu, W. Yuan and C. Qu, Experimental and numerical study on upper-room Far-UVC system under different ventilation schemes to disinfect airborne microorganisms in indoor environments, Build. Environ., 2024, 266, 112108.
39. K. Guo, Y. Pan, H. F. R. Chan, K.-F. Ho and C. Chen, Far-UVC disinfection of airborne and surface virus in indoor environments: Laboratory experiments and numerical simulations, Build. Environ., 2023, 245, 110900.
40. J. Zhao, E. M. Payne, B. Liu, C. Shang, E. R. Blatchley, W. A. Mitch and R. Yin, Making waves: Opportunities and challenges of applying far-UVC radiation in controlling micropollutants in water, Water Res., 2023, 241, 120169.
41. G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B, 1996, 54, 11169–11186.
42. G. Kresse and J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B, 1993, 47, 558–561.
43. M. Cococcioni and S. de Gironcoli, Linear response approach to the calculation of the effective interaction parameters in the LDA+U method, Phys. Rev. B, 2005, 71, 035105.
44. A. M. Srivastava, Aspects of Pr³⁺ luminescence in solids, J. Lumin., 2016, 169, 445–449.
45. S. Zhang, F. Zhao, S. Liu, Z. Song and Q. Liu, An improved method to evaluate trap depth from thermoluminescence, J. Rare Earths, 2025, 43, 262–269.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00184f

---