A narrowband ultraviolet-B-emitting LiCaPO₄:Gd³⁺ phosphor with super-long persistent luminescence for over 100 h

Abstract

The past decades have witnessed a significant increase in interest in inorganic luminescent materials that emit in the narrowband ultraviolet-B (NB-UVB; 310–313 nm) spectral region due to the growing need for applications in photochemistry and photomedicine. However, the majority of existing NB-UVB phosphors rely on photoluminescence, which requires constant external excitation. This common but inconvenient photoluminescence style significantly slows down the progress of NB-UVB luminescence technology. Herein, we report the design and synthesis of a new Gd³⁺-doped NB-UVB-emitting persistent phosphor, LiCaPO₄:Gd³⁺, which shows strong NB-UVB persistent luminescence peaking at 312 nm and a super-long persistence time of >100 h after ceasing X-ray excitation. Owing to the zero-background noise from the ambient light, a UVB camera can detect the NB-UVB light emission originating from the charged LiCaPO₄:Gd³⁺ phosphor in a bright indoor environment. Through spectroscopic investigations and first-principles calculations, the nature of energy traps and the persistent luminescence mechanism of Gd³⁺ in the LiCaPO₄ host have been thoroughly studied. Besides, remarkable photochromic behavior when irradiating the phosphor with X-rays is also observed, and the possible intrinsic point defects that contribute to the colorization are proposed. This NB-UVB persistent phosphor shows great potential in indoor optical tagging, optical information storage, and dermatological therapy.

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Introduction

Ultraviolet light with a wavelength between 280 and 320 nm is known as ultraviolet-B (UVB) light, which has exhibited considerable benefits in the phototherapy field for the treatment of various skin disorders such as vitiligo and psoriasis.^1,2 Previous spectroscopic studies on monochromatic UVB light suggest that the most effective waveband for phototherapy is located in the narrowband UVB (NB-UVB; 310–313 nm) spectral region.^3,4 These discoveries swiftly prompt the development and application of artificial NB-UVB light sources for dermatological therapy.^5–7 At present, phototherapy based on a Philips TL-01 fluorescent tube has become the mainstream treatment for a variety of skin disorders, affecting millions of patients worldwide.^8,9 However, the enormous potential of UVB light in other crucial fields has not been well probed. For instance, UVB light that can be perceived by the plant photoreceptor UVR8 affects plant growth and development and reduces disease and pest incidence.^10–12 Besides, owing to the avoidance of interference from ambient lighting and their visible-blind characteristics, UVB-emitting materials are promising for application in covert optical tagging in an indoor-lighting environment.^13–15 Nevertheless, the advancement of UVB luminescence technology is being hampered by the widespread but unhandy photoluminescence form, where constant external excitation is required. Thus, exploring different UVB luminescence forms for new technological applications is highly desirable.

Persistent luminescence as a unique luminescence form has been widely investigated, where initial research works are mainly centered on visible-light persistent phosphors^16,17 followed by longer-wavelength near-infrared persistent phosphors.^18–22 Currently, visible and infrared persistent luminescence materials have been extensively employed as glow-in-the-dark materials for a wide range of important applications, such as night-vision surveillance,^23–25 optical data storage,^26–28 solar energy utilization,^29–31 anti-counterfeiting,^32–35 and biomedical imaging/diagnosis.^36–40 While there have been significant strides in visible and infrared persistent luminescence, research focusing on the shorter-wavelength ultraviolet region remains limited. Considering the potential applications of UVB light, conducting thorough investigations into UVB persistent luminescence as an alternative form of luminescence holds significant importance.

For UVB luminescence, trivalent gadolinium ions (Gd³⁺) are promising luminescence centers in solids due to their narrow emission line at around 311 nm originating from the ⁶P_7/2 → ⁸S_7/2 transition.⁴¹ However, Gd³⁺ has no tendency to be oxidized or reduced in the inorganic matrix because of its stabilized 4f⁷ electron configuration, resulting in no direct electron transfer between the Gd³⁺ and energy traps in the host. Therefore, Gd³⁺ ions usually cannot act as persistent luminescence centers directly. Recently, intense NB-UVB persistent luminescence of Gd³⁺ has been successfully achieved by utilizing persistent energy transfer from the Pr³⁺, Ce³⁺, Bi³⁺ or Pb²⁺ sensitizer to Gd³⁺. This is because their high-energy excited levels (i.e., the 4f5d excited state for Pr³⁺ and Ce³⁺ and the ³P₁ emitting state for Bi³⁺ and Pb²⁺) can well resonate with the emitting levels of Gd³⁺ (⁶P_J/⁶I_J excited states).^42–46 For instance, Yan and coworkers proved up-converted NB-UVB persistent luminescence in Pr³⁺-Gd³⁺ co-doped Lu₂Al₂Ga₃O₁₂ phosphors upon 450 nm blue LED irradiation, in which Pr³⁺ ions absorb two blue photons to fill electron traps and then transfer energy to Gd³⁺ emitters efficiently.¹⁴ To take it forward, the emergence of high-energy X-rays as an alternative charging source recently paved the way for Gd³⁺ ions as direct persistent luminescence centers in some Gd³⁺ single-doped phosphors,^28,47,48 because high-energy X-ray irradiation facilitates the formation of various defects as energy traps in these large bandgap materials.⁴⁹ For example, the Liu group first reported an X-ray charged LaMgAl₁₁O₁₉:Gd³⁺ persistent phosphor, which emits NB-UVB light peaking at 312 nm.⁵⁰ Two other Gd³⁺-activated NB-UVB persistent phosphors, ScPO₄:Gd³⁺ and Sr₂P₂O₇:Gd³⁺, were subsequently reported by Yang's group and Liang's group.^51–53 However, the pursuit of new X-ray-activated Gd³⁺ single-doped NB-UVB persistent phosphors, characterized by high afterglow brightness and extended persistence durations, remains a considerable challenge.

Here, we report a new NB-UVB persistent phosphor by doping Gd³⁺ into the LiCaPO₄ lattice. The synthesized LiCaPO₄:Gd³⁺ phosphor exhibits intense and super-long NB-UVB persistent luminescence with a peak wavelength of 312 nm and a persistent luminescence duration of >100 h after ceasing X-ray excitation, which is by far the best NB-UVB persistent phosphor to the best of our knowledge. Detailed thermoluminescence measurements were carried out to further study the de-trapping process during the NB-UVB afterglow process in this LiCaPO₄:Gd³⁺ phosphor. Through spectroscopic analysis and first-principles calculations, the nature of energy traps, the persistent luminescence mechanism, and the photochromic behavior have been elucidated.

Results and discussion

Crystal structure and phase identification

Fig. 1a depicts the simulated crystal structure of the LiCaPO₄ compound, which crystallizes in the trigonal system with a space group of P31c (159) and the lattice parameters of a = b = 7.5247 Å, c = 9.9657 Å, and V = 488.67 ų. The vertex-sharing [LiO₄] and [PO₄] tetrahedra that make up the framework of the LiCaPO₄ lattice provide broad, five-sided channels that run parallel to the [0001] direction, where the Ca²⁺ ions are located. There is just one type of Ca²⁺ site in the LiCaPO₄ lattice, yet the coordination surrounding the Ca site is wildly uneven. Six contacts (2.327–2.546 Å) and two longer ones (2.782 Å and 2.909 Å) between Ca²⁺ ions and oxygen constitute an irregular 8-fold coordination.^54,55

Fig. 1 (a) Crystal structure diagram of the LiCaPO₄ and the coordination environment of the Li, Ca, and P atoms. (b) XRD patterns of the LiCaPO₄:x%Gd³⁺ (x = 0, 0.2, 0.5, 1, 1.5, and 2) phosphors.

Fig. 1b shows the XRD patterns of the as-synthesized LiCaPO₄:x%Gd³⁺ (x = 0, 0.2, 0.5, 1, 1.5, and 2, x is relative to the Ca²⁺ ions) phosphors. The standard XRD pattern (JCPDS No. 79-1396) of LiCaPO₄ is taken as a reference. We confirm that the diffraction peaks of the as-synthesized LiCaPO₄:Gd³⁺ phosphors match the standard card of the LiCaPO₄ phase. The ionic radii of four-coordinated Li⁺ and P⁵⁺ and eight-coordinated Ca²⁺ are 0.59, 0.17, and 1.12 Å, respectively. As a result, Gd³⁺ ions (1.053 Å for eight coordination) prefer to occupy the Ca²⁺ site in the LiCaPO₄ host. Meanwhile, the mismatch in valence states and ionic radii between the Gd³⁺ and Ca²⁺ ions will introduce new trapping levels around Gd³⁺, which will be beneficial for the NB-UVB persistent luminescence of the LiCaPO₄:Gd³⁺ phosphor.

NB-UVB photoluminescence and persistent luminescence properties

Fig. 2a displays the normalized photoluminescence and excitation spectra of the LiCaPO₄:Gd³⁺ phosphor. Upon excitation at 274 nm, the phosphor shows intense UVB line emission peaking at 312 nm assigned to the Gd^{3+ 6}P_7/2 → ⁸S_7/2 transition. The excitation spectrum, monitored at 312 nm line emission, comprises several narrow excitation bands centered at 274, 253, and 247 nm, corresponding to the ⁸S_7/2 → ⁶I_J and ⁸S_7/2 → ⁶D_J transitions of Gd³⁺ ions, respectively. In addition, the LiCaPO₄:Gd³⁺ phosphor exhibits strong radioluminescence stemming from the characteristic ⁶P_7/2 → ⁸S_7/2 transition of Gd³⁺ upon high energy X-ray excitation, as depicted in Fig. 2b.

Fig. 2 (a) Normalized photoluminescence and excitation spectra of the LiCaPO₄:0.5%Gd³⁺ phosphor at room temperature. (b) Radioluminescence spectrum of the LiCaPO₄:0.5%Gd³⁺ phosphor. (c) Persistent luminescence decay curve when monitored at 312 nm emission in darkness at room temperature after irradiation with an X-ray beam. The upper inset displays the persistent luminescence emission spectrum obtained at 1 h decay after the cessation of X-ray irradiation. (d) Persistent luminescence decay curves of the LaMgAl₁₁O₁₉:Gd³⁺ phosphor monitored at 312 nm, the Sr₂P₂O₇:Gd³⁺ phosphor monitored at 312 nm, the ScPO₄:Gd³⁺ phosphor monitored at 313 nm and the LiCaPO₄:Gd³⁺ phosphor monitored at 312 nm after irradiation with an X-ray tube for 20 min.

Besides the strong NB-UVB radioluminescence, the irradiation of high-energy X-ray can also induce intense and super-long NB-UVB persistent luminescence in the LiCaPO₄:Gd³⁺ phosphor, while the irradiation of 254 nm UV light produces a negligible afterglow signal. As shown in Fig. S1,† the NB-UVB persistent luminescence decay curves and the corresponding afterglow emission spectra of the LiCaPO₄:Gd³⁺ phosphors with different Gd³⁺ doping concentrations were first recorded after irradiation with X-rays. It is observed that the NB-UVB persistent luminescence intensity monotonically declines with an increase in Gd³⁺ doping concentration within 5 min of natural decay. Nevertheless, as shown in the subsequent thermoluminescence (TL) measurements (Fig. S2†), the integral TL intensity of the LiCaPO₄:0.5%Gd³⁺ phosphor is the strongest, which suggests that the ideal doping concentration of Gd³⁺ is 0.5%. Furthermore, in order to study the relationship between the irradiation time and NB-UVB persistent luminescence performance, the NB-UVB afterglow curves of the LiCaPO₄:0.5%Gd³⁺ phosphor were measured by varying the excitation durations, as shown in Fig. S3.† The result indicates that 20 min of X-ray irradiation can induce the best NB-UVB afterglow performance.

Fig. 2c depicts the long-lasting NB-UVB persistent luminescence decay curve of the LiCaPO₄:0.5%Gd³⁺ phosphor monitored at 312 nm in darkness at room temperature after irradiation with an X-ray beam. As displayed in Fig. 2c, the NB-UVB afterglow intensity decreases rapidly within the initial hours before slowly diminishing over an extended period. After 100 h of decay, the persistent luminescence intensity is still more than two orders of magnitude higher than the background signal, suggesting that the NB-UVB afterglow from this phosphor can persist for much longer than 100 h. The inset of Fig. 2c presents the persistent luminescence emission spectrum measured at 1 h decay, which indicates that the doped Gd³⁺ emitters are the source of the NB-UVB afterglow. It is noted that in contrast to the X-ray-activated Gd³⁺-doped NB-UVB persistent phosphors developed thus far,^50,52,53 the as-synthesized LiCaPO₄:Gd³⁺ phosphor here displays the best NB-UVB persistent luminescence performance within 12 h natural decay under the same charging and decay conditions, as shown in Fig. 2d. That is, this is by far the best NB-UVB persistent phosphor to the best of our knowledge. Furthermore, the persistent NB-UVB luminescence power intensity of the charged phosphor in the first several minutes was also measured by using an optical power meter, as depicted in Table S1.† The initial NB-UVB afterglow power intensities at 30 s, 60 s and 300 s decay times are determined to be ∼11.79 mW m⁻², ∼9.21 mW m⁻² and ∼4.28 mW m⁻², respectively.

The influence of ambient light on NB-UVB afterglow performance of the LiCaPO₄:0.5%Gd³⁺ phosphor was also investigated under different light conditions. Fig. 3a displays the NB-UVB afterglow curves of the LiCaPO₄:0.5%Gd³⁺ phosphor monitored at 312 nm under different ambient illuminance (0, 150, and 300 lux) after irradiation by an X-ray tube. Compared with the persistent luminescence decay curve in the darkness, it is found that the NB-UVB afterglow intensity decays at a faster rate when the indoor white LED is turned on (Fig. S4† shows the emission spectrum of white LED.), which can be attributed to the increase of the de-trapping rate of the stored electrons in the energy traps under the continuous stimulation of white light. Meanwhile, the increase of ambient illuminance from 0 to 300 lux results in a much faster decrease rate. It is observed that the ambient-light-stimulated NB-UVB luminescence can still be detected easily after 12 h of decay in a bright indoor environment, as depicted in Fig. 3a. The afterglow emission spectra of the charged LiCaPO₄:0.5%Gd³⁺ phosphor obtained at different decay times in bright environment (150 lux) are given in Fig. 3b. As the decay time increases from 30 min to 12 h, the afterglow emission intensity gradually declines but the spectral shape and emission peak position remains constant, indicating that the ambient-light-stimulated NB-UVB luminescence in the bright environment is still ascribed to the Gd³⁺ emitters.

Fig. 3 (a) NB-UVB persistent luminescence decay curves of the LiCaPO₄:0.5%Gd³⁺ phosphor monitored at 312 nm under different ambient illuminance (0, 150, and 300 lux) after irradiation by an X-ray tube for 20 min. (b) NB-UVB persistent luminescence emission spectra at different decay times in a bright indoor environment (150 lux). (c and d) Thermoluminescence curves of the LiCaPO₄:0.5%Gd³⁺ phosphor measured at different decay instants in the dark and bright (300 lux) indoor environments after irradiation with an X-ray beam.

To study the de-trapping process during the NB-UVB persistent luminescence process in the LiCaPO₄:Gd³⁺ phosphor, the TL spectra of the charged LiCaPO₄:0.5%Gd³⁺ phosphor at different decay times were recorded in dark and bright (300 lux) indoor environments, respectively. All of the TL curves exhibit a comparatively wide emission band, demonstrating that continuous and wide distributions of trapping levels exist in this phosphor. Fig. 3c shows the variation of TL curves within 12 h of natural decay in the dark, which reflects that the trapped electrons are gradually released by room-temperature thermal stimulation. Before each TL curve measurement, the phosphor was irradiated using an X-ray tube for 20 min to fill the traps. The initial TL spectrum after irradiation by X-ray covers a broad temperature range over 300–500 K with the TL emission band peaking at 409 K. When the decay period is extended from 0 min to 12 h, the TL intensity gradually drops and the TL peak persistently shifts to the higher temperature side, showing that the shallow traps at low temperature are first emptied, followed by the slow release of deep traps at high temperature. The TL curves of a decaying LiCaPO₄:0.5%Gd³⁺ phosphor in a bright indoor environment (300 lux) at room temperature are depicted in Fig. 3d. Compared with the decay time-dependent TL spectra in Fig. 3c, a much more rapid decrease in TL intensity is observed as the illumination time of white LED lengthens. The initial rise method is used to estimate the trap depth of each curve in Fig. 3c and d. In this method, the initial fraction of the glow curve is fitted utilizing the Arrhenius equation:

I = C exp(–ΔE/kT) (1)

where T is the temperature, I represents the TL intensity, C is a fitting constant, k is the Boltzmann constant, and ΔE represents the trap depth. The fitting curves recorded in dark and bright (300 lux) indoor environments are plotted in ln(I) vs. 1/T coordinates, as given in Fig. S5a and c.† The original trap depth (immediately after stopping X-ray irradiation) is estimated to be 0.83 eV, suggesting that LiCaPO₄:Gd³⁺ is a suitable long NB-UVB persistent phosphor at room temperature. For these two cases in dark and bright (300 lux) indoor environments, the trap depth both increases as the decay time is prolonged from 0 min to 12 h, as depicted in Fig. S5b and d.† However, the calculated trap depth in the darkness is larger than that in the bright environment at the same decay instant. This is because the white LED light can induce the release of the trapped electrons into the conduction band from the deep traps. Part of these electrons will be re-captured by the depleted shallow traps in the LiCaPO₄:Gd³⁺ phosphor due to the continuous photostimulation of a polychromic white LED.

NB-UVB persistent luminescence mechanism

To further understand the NB-UVB persistent luminescence of the LiCaPO₄:Gd³⁺ phosphor upon X-ray irradiation, it is crucial to elucidate the nature of defects and the location of defect levels within the host band gap. For this, we have investigated possible intrinsic point defects in the LiCaPO₄ host by using the 2 × 2 × 1 supercell model (containing 168 atoms), including 11 vacancies (V_Li, V_Ca, V_P1−P3, and V_O1−O6), 10 antisites (Li_Ca, Ca_Li, Li_P1−P3, P_Li, Ca_P1−P3, and P_Ca), and four interstitial defects (Li_i, Ca_i, P_i, and O_i). Hybrid DFT in the PBE0 scheme with 30% HF exchange was employed for defect calculations. The calculated host band gap matches with the experimental value 8.26 eV as estimated from VUV excitation spectrum of LiCaPO₄:Ce^3+ 54 plus electron–hole binding energy.⁵⁶ Fig. S6† depicts the calculated defect formation energies (ΔE_f) as a function of the Fermi level (E_F), where three limiting cases for the atomic chemical potentials have been considered, corresponding to Li-, Ca-, and P-poor conditions during the synthesis. The results show that the point defects with low ΔE_f's are mainly associated with the Li element. This primarily stems from the raw material Li₂CO₃'s low melting point of approximately 720 °C, facilitating the diffusion of Li atoms. Although the defects with higher ΔE_f imply lower formation probabilities during the synthesis, their occurrence could not be neglected under X-ray irradiation owing to the elastic collision of the high-energy photons with the component atoms.

Fig. 4a shows the calculated thermodynamic charge transition levels of intrinsic point defects. The defect levels are dispersed within the host band gap, from the hole-trapping level Li_i (2+/+) with a depth of 0.07 eV to the electron-trapping level V_O6 (0/−) with a depth of 0.13 eV. Importantly, V_Li, V_Ca, and P_Ca provide hole-trapping levels, (+/0), (+/0), and (3+/+), with depths of 0.72, 0.82, and 0.69 eV, respectively, and Li_i offers an electron-trapping level (+/0) with a depth of 0.86 eV. These trap-depth values are close to that (0.83 eV) revealed by the TL glow curves (Fig. 3c and d).

Fig. 4 (a) Calculated thermodynamic charge transition levels of intrinsic point defects within the band gap of LiCaPO₄. The values in parentheses (in eV) indicate the energy separations of the levels with respect to the host VB maximum (set as 0 eV). (b) Calculated emission energies and wavelengths for excitons trapped at intrinsic point defects in LiCaPO₄. The values in parentheses (in nm) show the corresponding emission wavelengths.

The NB-UVB persistent luminescence of the LiCaPO₄:Gd³⁺ phosphor originates from the persistent energy-transfer from self-trapped excitons (STEs) or defect-trapped excitons (DTEs) to Gd³⁺, in which the electrons are promoted from the ⁸S_7/2 ground state to the excited states (e.g., ⁶P_J, ⁶I_J, ⁶D_J, and ⁶G_J in order of increasing energy), and then relax into the lowest ⁶P_7/2 excited state, resulting in the NB-UVB emission.⁵² Since the quantum-cutting emissions from the ⁶G_7/2 excited state were not observed, we suppose that this excited state was not populated by energy transfer and thus only the lower-energy ⁶P_J, ⁶I_J, and ⁶D_J states were reached. This means that only the excitons with emission energies in the range of 3.98–5.10 eV (ref. 57 and 58) probably act as effective sensitizers. As such, the STE with the calculated emission energy of 7.10 eV can be excluded. For DTEs, the calculated emission energies are displayed in Fig. 4b. It shows that the excitons trapped at V_Li, V_Ca, O_i, Li_Ca, and P_Li exhibit emissions within the above energy range, and thus may contribute to the NB-UVB persistent luminescence.

Photochromic properties of the LiCaPO₄:Gd³⁺ phosphor

Besides NB-UVB persistent luminescence, X-ray irradiation also induces a significant change in the body color of the LiCaPO₄:Gd³⁺ phosphor from white to dark brown, which can last for dozens of hours in indoor ambient light at room temperature. Fig. 5a shows the photographs of the photochromic LiCaPO₄:0.5%Gd³⁺ phosphor disc during the decoloration process within 1 h, which were taken in an indoor lighting environment (50 lux) at room temperature, along with the photograph of an unirradiated phosphor disc for comparison. The decoloration becomes faster by increasing the temperature to ∼100 °C. The decoloration process of a pre-irradiated LiCaPO₄:0.5%Gd³⁺ phosphor disc at varied temperatures is depicted in the inset of Fig. 5b, which shows an accelerated bleaching process with increasing ambient temperature. To quantitatively describe the body color change, the brightness of the LiCaPO₄:0.5%Gd³⁺ phosphor disc was recorded by a digital camera, which was extracted through the Image J software. The bleaching process of the pre-irradiated LiCaPO₄:0.5%Gd³⁺ phosphor disc at 336 K was taken as an example to illustrate the brightness extraction process (Fig. S7†). The brightness values were normalized (I_B) and then plotted against decay time, as shown in Fig. 5b. Obviously, the decoloration process was accelerated by raising the temperature from 336 to 364 K.

Fig. 5 (a) The photograph of an unirradiated LiCaPO₄:0.5%Gd³⁺ phosphor disc and the photographs of the decoloration process within 1 h of the LiCaPO₄:0.5%Gd³⁺ phosphor disc recorded in an indoor-lighting environment (50 lux) at room temperature after irradiation with an X-ray tube for 20 min. (b) The normalized brightness value, I_B, was plotted against time at varying temperatures. The inset shows the photographs of the decoloration process of a pre-irradiated LiCaPO₄:0.5%Gd³⁺ phosphor disc recorded at varying temperatures. (c) Comparison between the normalized darkness values (I_D) and the normalized integral TL intensities (I_TL) within 1 h of decay in an indoor lighting environment (50 lux) at room temperature. The inset displays the photographs of the decoloration process within 1 h. (d) Calculated absorption energies and wavelengths for excitons trapped at intrinsic point defects in LiCaPO₄, whose absorption wavelengths are in the 390–655 nm range. The values in parentheses (in nm) show the corresponding absorption wavelengths. (e) Schematic diagram of the NB-UVB persistent luminescence and photochromic mechanisms in the LiCaPO₄:Gd³⁺ phosphor.

The coloration phenomenon can be ascribed to the production of photochromic centers in the phosphor, which may result from the trapping of light-generated electrons via the lattice defects in LiCaPO₄:Gd³⁺.⁵⁹ The depth of the lattice defects associated with the photochromic centers can be derived by estimating the activation energy of thermal decoloration of the LiCaPO₄:Gd³⁺ phosphor. The activation energy (E_a) of thermal decoloration is accessible by fitting the Arrhenius equation:^60,61

ln(Δt) = E_a/RT + b (2)

where Δt is the interval of time corresponding to a fixed stage of transformation during the decolorization process, R is the molar gas constant (R = 8.314 J mol⁻¹ K⁻¹), and b is a constant. The activation energy can be determined from the slope of the fitting straight line obtained by plotting ln(Δt) versus 1/RT, as depicted in Fig. S8.† The obtained E_a values (≈ 0.70 eV) are lower than the initial smallest trap depth calculated using the initial rise method (0.83 eV). However, according to the previous research studies,^19,23 the photochromic centers are often formed by the deep traps in the lattice. To confirm the relationship between the energy traps contributing to the NB-UVB persistent luminescence and the lattice defects that are responsible for photochromic centers, we compared the variation of the normalized darkness values (I_D = 1 − I_B) and the normalized integral TL intensities (I_TL) of the pre-irradiated LiCaPO₄:0.5%Gd³⁺ phosphor disc within 1 h of decay in indoor lighting environment (50 lux) at room temperature, as shown in Fig. 5c. The integral TL intensity represents the total stored electrons contributing to the NB-UVB persistent luminescence, and the stored energy is released in such a way that shallow traps at low temperatures are first emptied, followed by the slow release of stored energy in deep traps at a higher temperature. Hence, the energy stored in deeper traps should show a slower decline over time. According to the Lambert–Beer law, the darkness of the LiCaPO₄:0.5%Gd³⁺ phosphor disc is proportional to the concentration of photochromic centers. Fig. 5c shows that the darkness of the LiCaPO₄:0.5%Gd³⁺ phosphor disc declines more slowly than the integral TL intensity, i.e., the decreasing trend of the concentration of photochromic centers is slower than the decay trend of the energy traps contributing to the NB-UVB persistent luminescence. Consequently, we demonstrate here that the average depth of the lattice defects related to the photochromism in this phosphor is higher than the mean depth of the traps reflected by the TL results and, thus, is larger than 0.83 eV. However, this conclusion contradicts the E_a value of 0.70 eV obtained by fitting the Arrhenius equation.

As shown in Fig. 3c and d, by comparing the variation of the TL spectra of a decaying LiCaPO₄:0.5%Gd³⁺ phosphor in dark and bright indoor environments, the contribution of ambient light illumination to the detrapping process is apparent. Therefore, focusing solely on the impact of fading temperature while disregarding the role of indoor ambient light in the fading process could explain the smaller E_a value obtained from the fitting. Based on this inference, a comparison experiment was conducted to verify the effect of indoor ambient light on the decoloration process of the LiCaPO₄:0.5%Gd³⁺ phosphor. As shown in Fig. S9,† the obvious contrast between the left (after decolorizing for 1 h in indoor ambient light environment at room temperature) and right (after bleaching for 1 h in dark at room temperature) of the LiCaPO₄:0.5%Gd³⁺ phosphor disc indicates that the effect of indoor ambient light on the bleaching process is significant and cannot be neglected, that is why the E_a value derived by fitting the Arrhenius equation is smaller than the real one.

Besides, we measured the diffuse reflectance spectra (DRS) of the LiCaPO₄:0.5%Gd³⁺ phosphor before and after X-ray irradiation, as depicted in Fig. S10a.† The DRS of the unirradiated LiCaPO₄:0.5%Gd³⁺ phosphor exhibits two absorption bands, i.e., 200–239 nm for host absorption and 239–380 nm assigned to the absorption of certain defects in the host, while several weak narrow absorption peaks of the Gd³⁺ ion are not visible due to the overlay with the strong absorption band.⁶² It is worth noting that the absorption of this phosphor at different wavelengths throughout the test range becomes stronger in different degrees after X-ray irradiation. As given in Fig. S10b,† variation in the reflectance (ΔR) of the LiCaPO₄:0.5%Gd³⁺ phosphor at various wavelengths is calculated by using the following equation:

ΔR = (R₀ − R_i)/R₀ × 100% (3)

where R₀ and R_i represent the reflectivity of the LiCaPO₄:0.5%Gd³⁺ phosphor before and after X-ray irradiation, respectively. The larger ΔR in the visible wavelength range, the greater the contribution of the photochromic centers that absorb light in this wavelength range to the photochromism. The wavelength range that has a larger ΔR (≥50%) is 340–655 nm after X-ray irradiation, whereas the wavelength range after excluding the non-visible wavelength is 390–655 nm. To clarify the possible origin of the photochromic centers, the absorption energies and wavelengths for excitons trapped at intrinsic point defects in LiCaPO₄ were calculated, as presented in Fig. S10c.† Among them, those with absorption wavelengths in the 390–655 nm range (1.89–3.18 eV) are shown in Fig. 5d. It suggests that the excitons trapped at V_P1, V_P2, Ca_i, Li_P1, Ca_P1, Ca_P2, and Ca_P3 have absorption wavelengths in the 390–655 nm wavelength range. As a result, the photochromic centers formed by these defects play a major role in photochromism. As can be seen from Fig. 4a, V_P1, V_P2, Ca_P1, and Ca_P2 provide hole-trapping levels (0/−) with depths of 1.04, 1.91, 2.87, and 3.26 eV, respectively, and Ca_i, Li_P1, and Ca_P3 offer electron-trapping levels (2+/−), (3−/4−), and (2−/3−) with depths of 1.30, 1.30, and 3.45 eV, respectively. All of these trap-depth values are higher than 0.83 eV, which is in line with the result demonstrated in Fig. 5c. This further indicates that the deep traps in the lattice should be responsible for forming photochromic centers.

Based on the above discussions, the NB-UVB persistent luminescence and photochromic mechanism of the LiCaPO₄:Gd³⁺ phosphor is proposed in Fig. 5e. The X-ray irradiation excites the electrons at the valence band of LiCaPO₄ host to the conduction band, leaving a hole in the valence band (process 1). The free electrons and holes may be trapped at lattice defects to form DTEs (process 2), from which emission occurs or the emission energy is transferred to Gd³⁺ 4f⁷ levels (process 3), resulting in the NB-UVB emission at 312 nm (process 4). These free electrons and holes may also be steadily captured by the traps contributing to the NB-UVB persistent luminescence (process 5) or certain defects related to the photochromic centers (process 6). Upon external thermal stimulation, the electrons and holes trapped by the traps that contribute to the NB-UVB persistent luminescence are released into the valence and conduction bands, respectively (process 7). The released carriers may be captured again by defects to produce DTEs, followed by persistent energy transfer to Gd³⁺. As a consequence, the NB-UVB persistent luminescence emission peaking at 312 nm occurs in the LiCaPO₄:Gd³⁺ phosphor. The coloring and bleaching processes are as follows: part of the photogenerated electrons and holes are captured by the defects to form F color centers and V color centers, respectively (process 6). Then the coloration LiCaPO₄:Gd³⁺ phosphor is observed. Under thermo- or photo-stimulation, the trapped electrons in the defects are released by absorbing sufficient energy and they recombine with the holes captured by the defects (process 8). As a result, the color of the LiCaPO₄:Gd³⁺ phosphor fades.

Application of the LiCaPO₄:Gd³⁺ phosphor in a bright indoor environment

The LiCaPO₄:Gd³⁺ phosphor exhibits the remarkable NB-UVB persistent luminescence that effectively evades interference from ambient light in a bright indoor environment, making it ideal for covert optical tagging in security and civilian contexts. To showcase its capability for indoor optical tagging, UVB imaging experiments were conducted under various indoor lighting conditions using a specialized UVB camera, as shown in Fig. 6a. Each image combines a UVB representation with a visible image, with the invisible UVB emission indicated by a purple pattern proportional to its intensity. Fig. 6a displays the NB-UVB persistent luminescence images of the charged LiCaPO₄:Gd³⁺ persistent phosphor taken at different decay times in dark and bright (150 lux) indoor environments, respectively. Despite the restricted sensitivity of our UVB camera, the detectable imaging time in the darkness can reach more than 6 h, which is much longer than other reported NB-UVB persistent phosphors. Compared with the NB-UVB persistent luminescence images recorded in the darkness, those images acquired in a bright indoor environment exhibit a larger pattern area within the first 1 h decay followed by a much faster decrease rate. Besides, the invisible optical tagging application of the LiCaPO₄:Gd³⁺ phosphor has been demonstrated under indoor-lighting conditions, as given in Fig. 6b and c. Fig. 6b depicts the static labelling application using a pre-irradiated LiCaPO₄:Gd³⁺ phosphor disc affixed to an individual in the hall. In a bright indoor-lighting environment, the UVB camera can clearly detect and image the strong NB-UVB light emission from the LiCaPO₄:Gd³⁺ phosphor. By virtue of the intense and super-long NB-UVB persistent luminescence of this phosphor, the application of dynamic tracking was also realized, as presented in Fig. 6c. A charged LiCaPO₄:Gd³⁺ phosphor disc was attached to a cartoon car on the floor of the lab. Utilizing the UVB camera, the movement of the cartoon car can be tracked by capturing UVB photon signals. The appealing self-sustained NB-UVB luminescence of the LiCaPO₄:Gd³⁺ phosphor in bright indoor environments underscores the significant potential of these deep UV persistent phosphors for optical tagging application.

Fig. 6 (a) NB-UVB persistent luminescence images of the charged LiCaPO₄:Gd³⁺ phosphor disc at different decay instants in dark and bright (150 lux) indoor environments. Invisible optical tagging application demonstration of the LiCaPO₄:Gd³⁺ phosphor under indoor-lighting conditions. (b) Static labelling application of a pre-irradiated LiCaPO₄:Gd³⁺ phosphor disc affixed to an individual in the hall. (c) Schematic diagram of the dynamic tracking application of a charged LiCaPO₄:Gd³⁺ phosphor disc attached to a cartoon car on the floor of the lab. An UVB camera was used to capture the position of the moving cartoon car in a two-dimensional plane.

Conclusions

In summary, we report a new LiCaPO₄:Gd³⁺ persistent phosphor that emits in the NB-UVB spectral region. This synthesized phosphor shows strong line emission at 312 nm and super-long persistent luminescence for >100 h after ceasing X-ray excitation, making it the best NB-UVB persistent phosphor by far to the best of our knowledge. Moreover, the NB-UVB persistent luminescence from the charged LiCaPO₄:Gd³⁺ phosphor can be distinctly detected using a UVB camera with high contrast, unaffected by indoor ambient light sources like artificial illumination and indoor sunlight. Even with the restricted sensitivity of our UVB camera, the detectable imaging time of the NB-UVB luminescence signal from the charged LiCaPO₄:Gd³⁺ phosphor disc can reach >6 h and >3 h in dark and bright environments, respectively, confirming the great potential of the LiCaPO₄:Gd³⁺ phosphor for covert optical tagging application. Furthermore, we figure out the nature of energy traps and the persistent luminescence mechanism of the LiCaPO₄:Gd³⁺ phosphor through thorough spectroscopic observations and theoretical analyses. Besides, remarkable photochromic behavior is observed when irradiating the LiCaPO₄:Gd³⁺ phosphor discs with X-rays and the photochromic centers formed by V_P1, V_P2, Ca_i, Li_P1, Ca_P1, Ca_P2, and Ca_P3 are demonstrated by spectroscopic analysis and first-principles calculations. The outcome of this work is expected to pave the way for the broad use of NB-UVB persistent phosphors in various research fields such as optical data storage, covert optical tagging, and dermatological 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

Y. L. acknowledges the financial support from the 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), the National Natural Science Foundation of China (Grant No. 51902184), and the “Qi-Lu Young Scholar Fund” from Shandong University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02407a

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