Preparation and Study of Bright Orange-Yellow Long Persistent Luminescent Ca₂LuScGa₂Ge₂O₁₂:Pr³⁺ Phosphor

Abstract

Long persistent phosphors are widely used in many fields, such as LED, bioimaging, urgent lighting, temperature sensors, etc. Although green and blue long persistent phosphors are well developed, efficient orange-yellow long persistent phosphors are still relatively rare. In this work, a novel orange-yellow long-persistent phosphors Ca₂LuScGa₂Ge₂O₁₂:xPr³⁺ (CLSGGO:xPr³⁺, x = 0.003, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) are prepared and systematically investigated through its crystal structural information, photoluminescence, and persistent luminescence properties. Under ultraviolet light excitation, these phosphors exhibit orange-yellow emission stemming from the ³P₀ and ¹D₂ multiple electron transitions in the 4f level of Pr³⁺ ion. In addition, the material exhibits bright persistent luminescence. The complex garnet matrix structure of Ca₂LuScGa₂Ge₂O₁₂ provides excellent conditions for the formation of traps. Through the testing of thermoluminescence curve and function fitting, the density and depth of traps are studied; also, the storage and release process of carriers in the material are calculated in detail. A reasonable persistent luminescence mechanism is proposed for CLSGGO:0.01Pr³⁺. This work enriches the research content of photoluminescence and long persistent luminescence of Pr³⁺-doped garnet-based phosphors and paves the way for the future research of long persistent luminescent materials doped with rare earth ions.

1. Introduction

Long persistent luminescence (LPL) is a peculiar optical phenomenon suggesting that continuous luminescence exists after ceasing the excitation process. Since Matsuzawa et al. first reported aluminates green LPL materials (SrAl₂O₄:Eu³⁺, Dy³⁺) in 1996, LPL materials have attracted tremendous attention and been widely used in various fields such as night lighting, optical storage, and biomedical sciences [1]. Although significant progress has been achieved in green and blue LPL materials, orange-yellow LPL phosphors emitting at longer wavelength (550–600 nm) are relatively lacking [2,3,4,5]. To date, most orange-yellow LPL materials are based on complex co-doping (e.g., Eu²⁺/Dy³⁺, Ce³⁺/Mn²⁺) systems, which resulted in problems of poor color purity, higher cost, and more complex preparation process [6,7,8,9,10]. Single-doped system is a potential option, but orange-yellow LPL phosphors based on it are still scarce, and are suffering from defects of short persistent luminescent duration and insufficient thermal stability (quenching severely when temperature > 400 K) [11,12]. Therefore, preparation of orange-yellow LPL phosphors with high performances is still important.

To achieve excellent performance of the LPL phosphors, several key factors should be taken into account, such as dopped activators, hosts, and defects. Many f-f transitions in the Pr³⁺ ions and wide range of emission peaks could be achieved from the Pr³⁺ ions, ³P₀→³H₅ (green light), ¹D₂→³H₄ (red light) [13,14]. Potential cross-relaxation processes could also happen in Pr³⁺-doped phosphors. The emission spectra of the Pr³⁺-dopped phosphors could be further manipulated via defect engineering methods [15,16]. In the conventional hosts, the emission of Pr³⁺-dopped phosphors locates predominantly in the red spectral region, while that in the orange-yellow region is relatively rare and highly dependent on energy transfer or host sensitization processes [17,18,19]. Emission peak broadening and chromaticity coordinate deviation (ΔCIE > 0.05) occurs in the Pr³⁺-based LPL phosphors at high temperatures due to the thermal perturbation of 4f→5d energy levels [20]. Hosts with garnet structures (A₃B₅O₁₂) are widely applied in the LPL phosphors because of their advantages of controllable crystal field strength [21,22,23]. When it is used as the host of rare earth-dopped system, strong crystal field splitting (~104 cm⁻¹) could be achieved, and correspondingly suppress the non-radiative transitions in the system, leading to higher luminescence performances [24,25,26,27]. Although intensive research works based on the garnets LPL have been reported, most of them focused on the Eu²⁺/Cr³⁺ doping systems, and the LPL phosphors based on the Pr³⁺-dopped garnets are relatively rare [28,29,30]. It still needs to be investigated further how to achieve the orange-yellow sharp peak emission of Pr³⁺ through the manipulation of garnet lattice and how the defect types (such as oxygen vacancies, cation vacancies, etc.) affect the persistent luminescent duration and color stability.

In this work, a series of Pr³⁺ single-doped orange-yellow LPL phosphors Ca₂LuScGa₂Ge₂O₁₂:xPr³⁺ (CLSGGO:xPr³⁺, x = 0.003, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) are prepared. The structure and persistent luminescent properties of the phosphors were systematically characterized. The density and depth of traps in the samples with different Pr³⁺-doping concentrations are studied in detail through thermoluminescence curve tests and first-order kinetic function fitting, revealing the influence of Pr³⁺-doping concentration on the trap properties in the material. In addition, the thermoluminescence curve results of CLSGGO:0.01Pr³⁺ under different excitation times, different decay times, and different excitation temperatures revealed the carrier capture and release processes in the material. The relationship between the LPL decay curve and the decay time proved that the carrier energy transfer process between traps and luminescent centers mainly originated from the tunneling effect. Finally, based on the above photoluminescence and persistent luminescence characterizations, an electron capture model is proposed to describe the mechanism of photoluminescence and persistent luminescence phenomena in CLSGGO:xPr³⁺.

2. Materials and Methods

2.1. Preparation of Ca₂LuScGa₂Ge₂O₁₂:xPr³⁺ (x = 0.003, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05)

A series of Ca₂LuScGa₂Ge₂O₁₂:xPr³⁺ (x = 0.003, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) are prepared through a conventional high-temperature solid-state reaction method. Stoichiometric amounts of the raw materials CaCO₃ (99.99%, Shanghai, China, Aladdin), Lu₂O₃ (99.99%, Shanghai, China, Aladdin), Sc₂O₃ (99.99%, Shanghai, China, Aladdin), Ga₂O₃ (99.99%, Shanghai, China, Aladdin), GeO₂ (99.99%, Shanghai, China, Aladdin), and Pr₆O₁₁ (99.99%, Shanghai, China, Aladdin) are homogeneously mixed in an agate mortar and ground for 30 min. Then, the mixture is put into an alumina crucible and sintered at 1400 °C for 360 min in air. The heating protocol is initiated at a ramp rate of 3 °C/min until reaching 300 °C, followed by an increased heating rate of 5 °C/min up to 1400 °C. The temperature is maintained at 1400 °C for 360 min, subsequently cooled at a controlled rate of 5 °C/min to 800 °C, and finally allowed to cool naturally to ambient temperature. Post-cooling, the samples are ground into fine powders for subsequent characterization analyses.

2.2. Characterizations

The crystalline structures of the phosphor powders are identified by X-ray diffraction (XRD) using a D8 Focus diffractometer (Bruker Corporation, Germany) at 40 kV and 40 mA with graphite-monochromated Cu Kα radiation (λ = 0.15405 nm). Diffusion reflectance (DR) spectra are measured by RG-5000J UV-vis-NIR spectrophotometer (Zhong Tian Rui Guang, Tianjin, China). The morphology, micro-structures, and element mappings of the samples are characterized by using a spherical aberration corrected transmission electron microscope (Cs-TEM, Themis Z, ThermoFisher, Waltham, MA, USA) equipped with high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM, ThermoFisher, Waltham, MA, USA) detector and super-X energy dispersive spectrometer (EDS, ThermoFisher, Waltham, MA, USA). The photoluminescence excitation and photoluminescence spectra are recorded with FLS-920 (Edinburgh Instruments, Livingston, UK). The Photoluminescence Quantum Yield (PLQY) conducted measurements using the Edinburgh FLS1000 (Edinburgh Instruments, Livingston, UK). The TOSL-3D06A type thermoluminescence three-channel spectrometer (Guangzhou Rongfan Technology Co., Ltd, Guangzhou, China) is used to test the thermoluminescence curves of the samples, with the excitation light source being the equipped mercury lamp. Before conducting the thermoluminescence curve tests, the samples are annealed at 300 °C for 2 h to remove the charge carriers stored in the sample traps, and the samples are stored in the dark to avoid the influence of light on the materials.

3. Results

3.1. Crystal Structure

The XRD patterns of CLSGGO:xPr³⁺ (x = 0.003, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05) are depicted in Figure 1, where all the diffraction peaks of the samples match well with the standard card PDF#77-1888, implying that the CLSGGO:xPr³⁺ samples are successfully prepared with pure phase, and the Pr³⁺ ions are incorporated into the host [31].

In the CLSGGO host, Ca and Lu coordinate with O to form an octahedral dodecahedron; Ga, Ge, and Sc coordinate with O to form an octahedral hexahedron; and Ga and Ge coordinate with O to form a tetrahedral tetracoordinate. Based on the ionic valence and coordination number, the occupancy of Pr³⁺ can be roughly determined. Generally, the radius difference between the doped ion and the host ion can be judged by the following formula [32,33]:

$$D_r = \left|{\displaystyle \frac{R_m\left(CN\right) - R_d\left(CN\right)}{R_m\left(CN\right)}}\right| \times 100\%$$

(1)

Among them, D_r is the percentage difference in radius between the host ion and the doped ion. When D_r is less than 30%, it indicates that the doped ion can occupy the site of the host ion. R_m represents the host ion, R_d represents the doped ion, and CN is the coordination number. When CN is 8, the ionic radii of Pr³⁺, Lu³⁺, and Ca²⁺ are 1.126 Å, 0.977 Å, and 1.12 Å, respectively. According to the calculation, the D_r values of Lu³⁺ and Ca²⁺ with respect to Pr³⁺ are 13.3% and 0.53%, respectively, which indicates that, theoretically, Pr³⁺ can occupy the sites of Lu³⁺ and Ca²⁺ in the matrix. Additionally, Lu³⁺ and Pr³⁺ have the same valence. Therefore, Lu³⁺ is more likely to be occupied by Pr³⁺.

The micro-structures of the sample are characterized further by using Cs-TEM. Figure 2 and Figure 3 show the TEM, HAADF-STEM images, EDS mapping images, EDS spectrum, and elemental compositions of the sample CLSGGO:0.05Pr³⁺. Particles with different shapes and sizes (Figure 2a) are observed, indicating that the samples have no uniform morphology. Atomic resolution HAADF-STEM images (Figure 2b,c) of the as-prepared sample are taken along the [001] direction, which are well matched with the theoretical structure of the host (Figure 2d), proving the doped Pr³⁺ ions did not change the phase structure of the host further. Atomic resolution EDS mappings are achieved in Figure 2e. The occupation of Sc, Ca, Ga, Lu, Ge are clearly visible, while that of Pr is not so clear, which is ascribed to the low concentration of the doped Pr³⁺ ions. The EDS spectrum and the elemental compositions semiquantitative analysis results of the sample are depicted in Figure 3, which gives out the direct proof of the success doping of Pr³⁺ ion, although its fraction in the sample is low.

3.2. Property of Luminescence

The excitation spectrum and emission spectrum of CLSGGO:0.01Pr³⁺ have been tested and presented in Figure 4a,b. It can be observed that when the emission wavelength is fixed at 486 nm, CLSGGO:0.01Pr³⁺ has a broad excitation peak at 276 nm, which is attributed to the 4f²-4f¹5d¹ transition of Pr³⁺ [34,35]. Additionally, a sharp excitation peak at 425–475 nm comes from the characteristic excitation of Pr³⁺. Based on the above test results, 276 nm ultraviolet light is selected as the excitation wavelength and used to test the emission spectra under different Pr³⁺ doping concentrations, as shown in Figure 4b. The representative emission peaks of CLSGGO:Pr³⁺ are mainly located at 486 nm, 562 nm, 609 nm, 620 nm, 657 nm, 714 nm, and 740 nm, which are, respectively, attributed to the ³P₀→³H₄, ³P₀→³H₅, ¹D₂→³H₄, ³P₀→³H₆, ³P₀→³F₂, ³P₀→³F₃, and ³P₀→³F₄ transitions of Pr³⁺ ions [36,37,38]. As the Pr³⁺ doping concentration in the samples increases, the intensity of the emission spectrum shows a trend of first increasing and then decreasing. When the doping concentration reaches 0.01, the luminescence intensity reaches the maximum. When the doping concentration is greater than 0.01, the increase in Pr³⁺-doping concentration will exacerbate the concentration quenching effect between the luminescence centers, resulting in a continuous decrease in luminescence intensity [39,40]. Therefore, the optimal doping concentration of Pr³⁺ in this phosphor is confirmed to be 0.01.

Figure 5a shows the diffuse reflection spectrum of CLSGGO:0.01Pr³⁺ phosphor. The strongest absorption peak in the range of 230–350 nm is attributed to the 4f-5d transition of Pr³⁺ [41]. Additionally, in the ranges of 430–500 nm and 550–650 nm, there are also relatively weak peaks, which are attributed to the transition absorption of Pr³⁺ at ³H₄→³P_0,1,2 (430–500 nm), ³H₄→¹D₂, and ³H₄→³H₆ (550–650 nm), respectively [42,43]. Furthermore, the optical band gap (Eg) of CLSGGO:0.01Pr³⁺ is calculated using the following Kubelka–Munk equation [44,45]:

$$[\mathrm{F}\left({\mathrm{R}}_{\infty }\right)\mathrm{h}\mathsf{\nu }{]}^n=\mathrm{C}(\mathrm{h}\mathsf{\nu }-E_g)$$

(2)

$$F\left(R_{\infty }\right)={\displaystyle \frac{(1-R{)}^2}{2R}}={\displaystyle \frac{K}{S}}$$

(3)

Among them, F(R_∞) is calculated based on the measured reflectivity R and represents the Kubelka–Munk absorption coefficient; h is the Planck constant; ν is the frequency of light; n represents the transition coefficient and the direct energy band gap = 2; C is the absorption constant; E_g is the band gap [46,47]. Through calculation, the E_g of CLSGGO:0.01Pr³⁺ phosphor is approximately 4.83 eV, which is shown in the inset of Figure 5a. In addition, under excitation at the optimal excitation peak, the PLQY of CLSGGP:0.01Pr³⁺ is 49.87% in Figure 5b, indicating that CLSGGO:0.01Pr³⁺ has a relatively high luminous efficiency.

3.3. LPL Property and Trap Analysis

After the excitation light source turned off, the long persistent luminescence (LPL) materials can still maintain luminescence for a certain period of time [48,49]. The LPL spectrum and the thermoluminescence (TL) curve are often used to characterize the LPL properties of the samples and to infer the emission mechanism [50]. CLSGGO:Pr³⁺ still exhibits luminescence after being excited by 254 nm ultraviolet light for a period of time, which indicates that it has the property of LPL. To explore the intrinsic reason for the influence of Pr³⁺-doping concentration on LPL emission spectra, the TL curves at different doping concentrations are tested, mainly to study the influence of Pr³⁺ doping concentration on the trap density and trap distribution in Figure 6a. It can be found that changing the Pr³⁺-doping concentration will affect the shape of the TL curves, which means that the doping concentration is an important factor affecting the trap distribution and trap density. When the Pr³⁺-doping concentration in CLSGGO is 0.01, the intensity of the TL curve is the highest, and the peak is located at 197 °C. Generally, the intensity of the TL curve is related to the trap density, and the position of the peak in the TL curve is related to the trap depth; the greater the intensity, the greater the trap density, and the higher the temperature where the peak is located, the deeper the trap [51]. Therefore, the trap density of CLSGGO:0.01Pr³⁺ is the highest, and more traps are distributed in the deep region. The normalized results of different Pr³⁺-doping TL curves are shown in Figure 6b. It can be found that the shapes of the TL curves under different doping concentrations are roughly similar, which indicates that the trap distribution range and proportion of Pr³⁺-doped CLSGGO are approximately the same, mainly distributed in the range of 100 °C–300 °C, which means that there are more deep traps in these phosphors.

To conduct a detailed study on the trap states under different doping concentrations, the TL curves of different Pr³⁺ doping phosphors are fitted using the TL2000 1.0 version with first-order kinetic simulation, which are shown in Figure 6c–i [52]. The TL curves of all samples are fitted as the superposition of three fitting curves, indicating that there are three effective carrier trap types in these phosphors, which are represented in the figure as green trap A, yellow trap B, blue trap C, and the combined curve after the three traps are superimposed is blue. The temperature distribution range of trap A and trap B is significantly lower than that of trap C, indicating that the trap depth of trap A and trap B is shallower than that of trap C. The intensity of trap C is significantly higher than that of trap A and trap B, suggesting that trap C is the main trap type and has a larger trap density. When the doping concentration increases from 0.003 to 0.01, the proportion of trap A and trap B in the total TL curve area decreases and the proportion of trap C increases, indicating that the proportion of shallow traps reduces and the proportion of deep traps raises. When the doping concentration continues to increase from 0.01 to 0.05, the proportion of shallow traps of trap A and trap B increases and the proportion of deep traps of trap C decreases. In addition, when the doping concentration of Pr³⁺ is higher than 0.01, the intensity of the TL curve gradually decreases, indicating that continuously increasing the doping concentration will inhibit the generation of traps. Combining the results of the peak fitting of the TL curves, it can be found that a higher Pr³⁺ doping concentration changes the distribution of traps, inhibiting the formation of deep traps of trap C and promoting the formation of shallow traps trap A and trap C. The carriers existing in trap A and trap B can be slowly released at room temperature (RT), so CLSGGO:0.01Pr³⁺ exhibits a bright LPL phenomenon at RT. The carriers in the deep traps of trap C need to be stimulated by higher temperature or light to be released. In summary, the doping concentration of Pr³⁺ affects the generation and distribution of traps, which can effectively regulate the LPL performance.

In order to quantify the trap density and depth in the CLSGGO:xPr³⁺ phosphors, first-order kinetic simulation is employed to calculate the trap parameters under different doping concentrations. This theory has been widely applied in the study of trap properties of various LPL materials. Generally, the following equations are used to calculate the trap parameters [53]:

$$E_t = \left[2.52 + 10.2 \times \left({\mu }_g - 0.42\right)\right] \times {\displaystyle \frac{k_B{T}_m^2}{\omega }} - 2k_B{T}_m$$

(4)

$$n_0={\displaystyle \frac{\omega I_m}{\beta \times [2.52+10.2\times \left({\mu }_g-0.42\right)]}}$$

(5)

$${\mu }_g={\displaystyle \frac{T_2-T_m}{T_2-T_1}}$$

(6)

Herein, E_t represents the trap depth, n₀ represents the trap density, k_B is the Boltzmann constant; β is the heating rate, which is 3 K/s in this paper, μ_g represents the asymmetry parameter, T_m is the temperature corresponding to the TL peak, ω is the full width at half maximum (FWHM) of the peak, and I_m is the maximum intensity value of the peak. Since the LPL phenomenon at room temperature mainly originates from the release of carriers from shallow traps, the relevant parameters of shallow trap 1 (yellow line) are calculated and are shown in

Table 1. The calculated TL parameters of trap 1 (yellow line) of the CLSGGO:Pr³⁺ (x = 0.003–0.05) phosphors.

Sample	Tm/°C	T1/°C	T2/°C	μg	Im	E/eV	n0
x = 0.003	74	48	105	0.54	255,946	0.694	1,262,805
x = 0.005	77	50	106	0.52	228,600	0.700	1,212,912
x = 0.01	77	52	107	0.55	211,481	0.700	1,001,847
x = 0.02	75	52	100	0.52	133,740	0.696	615,589
x = 0.03	83	54	113	0.51	125,653	0.712	722,050
x = 0.04	76	51	103	0.52	126,010	0.698	618,368
x = 0.05	74	50	98	0.50	95,399	0.694	448,016

. It can be observed that the trap 1 depth of CLSGGO:xPr³⁺ varies from 0.694 eV to 0.712 eV. As the doping concentration increases, the trap depth E_t shows a trend of first increasing and then decreasing. When the Pr³⁺ doping concentration is at a lower value, such as 0.003, 0.005, or 0.01, the n₀ of traps in trap 1 is relatively high. As the doping concentration increases, the value of n₀ decreases, which means that too high a doping concentration is not conducive to the formation of trap 1, thereby affecting the LPL phenomenon at room temperature.

3.4. LPL Mechanism

In order to deeply explore the mechanism of LPL, TL curves with different charging times, different excitation temperatures, and different decay times are tested. These test results can be used to analyze the storage and release process of charge carriers in CLSGGO:0.01Pr³⁺ traps [54]. The CLSGGO:0.01Pr³⁺ with the strongest thermal luminescence curve intensity is selected as the representative for the above tests. Firstly, charging with 254 nm ultraviolet light for different times, the TL curves and their normalized results are shown in Figure 7a,b, including 30 s, 60 s, 180 s, 5 min, 10 min, 20 min, and 30 min. In Figure 7a, it can be observed that the intensity of the TL curves grows rapidly as the charging time increases. When the charging time is 10 min or longer, the increase rate of the thermal luminescence curve intensity slows down, indicating that the traps have been basically filled with charge carriers after charging with 254 nm ultraviolet light for 10 min [55]. In the low-temperature region of Figure 7a, the shallow traps intensity of shorter charging time is higher than that of longer charging time, which means that as the charging time increases, the charge carriers in the shallow traps tend to move to the deep traps [56]. In addition, by normalizing the TL curves under different charging times in Figure 7b, it is found that when the charging time is shorter, the distribution ratio of charge carriers in the shallow traps is higher than that of longer charging time, which means that the charge carriers tend to preferentially distribute in the shallow traps when the charging time is short [57]. When the charging time reaches 10 min or more, the shape of the normalized thermal luminescence curve remains basically unchanged, indicating that the distribution ratio of charge carriers in traps remains basically unchanged and longer charging times do not affect the distribution ratio of charge carriers [58]. Therefore, the charging time has an influence on the density and distribution ratio of charge carriers in the traps. When the charging time reaches 10 min or longer, the influence of the charging time on the density and distribution ratio of charge carriers in the traps is relatively small. Therefore, 10 min with 254 nm ultraviolet light is determined as the subsequent test condition.

Under the condition of being charged with ultraviolet light of 254 nm for 10 min, the test results of the TL curves under different decay time are presented in Figure 7c,d. As the decay time increases, the intensity of the TL curves continuously decreases. This is because as the decay time prolongs, the trapped carriers are constantly released. However, when the decay time is extended from 30 s to 120 min, the intensity of the TL curve only decreases to 88% of that at 30 s, indicating that the carriers stored in the traps are relatively stable and are not easily released at RT. The results after normalizing the decay curves are shown in Figure 7d, in which it can be observed that the FWHM of the TL curve gradually decreases as the decay time grows, and the position of the peak of the TL cross-section gradually moves towards the high-temperature side. This is mainly due to the disappearance of the TL curve in the low-temperature region, which indicates that the carriers in different traps are not released simultaneously. The carriers in the shallow traps are released first, and those in the deep traps are slowly released at RT [59]. Therefore, after being decayed for 120 min, the CLSGGO:0.01Pr³⁺ can still measure a strong TL curve in the high-temperature region. Moreover, the extension of the decay time causes the movement of the TL peak, which indicates that there is a series of continuous distributed traps in CLSGGO:0.01Pr³⁺. As the decay time increases, the decay part of the TL curves is mainly concentrated in the section before 100 °C, indicating that the persistent luminescence at room temperature is mainly due to shallow traps. Figure 7e–f shows the LPL spectra and TL curves of CLSGGO:0.01Pr³⁺ after being excited by 254 nm ultraviolet light for 10 min at different excitation temperatures. Figure 7e presents the LPL spectra of CLSGGO:0.01Pr³⁺ at different temperatures ranging from 298 K to 523 K with a step of 25 K. It can be observed that the peak shapes of the LPL spectra at different excitation temperatures are similar, but the peak intensities are different, indicating that the excitation temperature affects the LPL emission of CLSGGO:0.01Pr³⁺. To further explore the mechanism and reasons for the influence of excitation temperature on the LPL properties, the TL curves after being excited by 254 nm for 10 min at different temperatures are shown in Figure 7f [60]. As the excitation temperature increases, the peak intensity of TL curve shows a trend of first increasing and then decreasing. However, the intensity of the deep traps at higher temperatures above 275 °C is lower than that at RT. This is because temperature increasing can promote the release of carriers in the deep trap, enter the shallow traps, and be stored [61]. Therefore, at the beginning of the temperature increase, the strongest peak intensity of the CLSGGO:0.01Pr³⁺ TL curve increases. As the excitation temperature continues to increase, the stored carriers in the traps are continuously excited and released, so the number of carriers that can be stored in the traps decreases, resulting in a continuous decrease in the intensity of the TL curves. Therefore, the TL curve shows a trend of first increasing and then decreasing with the increase in temperature.

Figure 8a shows the variation pattern of the LPL intensity of CLSGGO:xPr³⁺ with the decay time. After the excitation light source is turned off 10 min after excitation, the LPL decay curves within 10 min are recorded. As shown in Figure 8a, within the first 100 s, the luminescence intensity decreased exponentially rapidly, because the carriers in the shallow traps can be released easily and produce luminescence at RT. Subsequently, the LPL intensity tended to stabilize and remained for a relatively long time, because the release rate of carriers in the deep traps is slower, so the LPL signal can be detected for a long time. After decaying for 10 min, the luminescence intensity of these phosphors is still higher than the background signal, which indicates that the LPL time of these phosphors can last for more than 10 min. Comparing the LPL decay curves of CLSGGO:xPr³⁺, it can be found that the LPL decay intensities of these phosphors are different. Compared to the LPL decay curve intensity of different doping concentrations, samples of 0.003, 0.005, and 0.01 are larger than those of 0.02, 0.03, 0.04, and 0.05, and the decay rate is slower, which is because the doping concentration of Pr³⁺ affects the distribution and density of traps, thereby affecting the LPL property of CLSGGO:xPr³⁺. In addition, studying the relationship between LPL intensity and decay time can reflect the carrier energy transfer process between traps and luminescence centers [62]. When electrons and holes undergo tunneling processes to recombine, the residual luminescence intensity and the reciprocal of time of the material will show a linear relationship. On the contrary, it will be achieved through the conduction band [32]. Therefore, the reciprocal of residual luminescence intensity is shown to be related to the decay time in Figure 8b. It can be found that within the 600 s decay time, the reciprocal of residual luminescence intensity (I⁻¹) is approximately proportional to the decay time, indicating that the tunneling effect is the main process of carrier energy transfer between traps and luminescence centers, and this process plays a major role in the LPL process.

To determine the LPL mechanism of CLSGGO:Pr³⁺, the LPL excitation spectra of CLSGGO:0.01Pr³⁺ at 488 nm, 612 nm, and 621 nm are supplemented, respectively, as shown in Figure 9a. It can be found that the LPL excitation peaks at the three wavelengths are located at 220 nm and 270 nm in the ultraviolet region, which indicates that the LPL of CLSGGO:0.01Pr³⁺ can only be produced by irradiation with higher energy light in the 200–300 nm range. This result becomes an important basis to propose the electron capture model to explain the LPL mechanism. Regarding the properties of CLSGGO:0.01Pr³⁺, a reasonable TL emission mechanism has been proposed and described in Figure 9b. The phenomenon of TL in CLSGGO:0.01Pr³⁺ is caused by the release of carriers that are trapped after ultraviolet light excitation, leading to the release of energy. Under the excitation of 254 nm, the carriers in the valence band (VB) absorb the excitation light and enter the conduction band (CB), generating holes in the valence band. The carriers excited to the conduction band undergo energy level transitions according to the energy level structure of Pr³⁺, emitting specific orange-red light emission peaks of Pr³⁺. Additionally, while being carried back to the ground state, they will also be caught by traps in the material, and continuous illumination will cause the traps in these phosphors to gradually be filled. When the excitation light source is turned off, the carriers in the shallow traps gradually release at RT, thereby back to the ground state and re-combining with the holes. Therefore, for a period of time after the excitation light source removed, CLSGGO:xPr³⁺ exhibits the phenomenon of initial relatively strong LPL. The carriers stored in the deeper traps can be relatively stable stored and affected by the high-energy stimulation, such as high temperature and laser. These carriers can return to the excited state energy level by a short distance and then go back to the ground state to re-combine with the holes, resulting in long-lasting but faint LPL phenomenon.

4. Discussion

In this study, we discuss the photoluminescence and persistent luminescence properties of Pr³⁺ doped in CLSGGO garnet host and thoroughly investigate the intrinsic connection between the carriers’ movement in CLSGGO:Pr³⁺ and the luminescence phenomenon, establishing a bridge between macroscopic phenomena and microscopic mechanism. This research innovatively studies the persistent luminescence of the material under different excitation and decay conditions, infers the carrier movement in the material, and proposes a physical electron capture model, which is consistent with the experimental phenomena, enriching the research achievements of rare earth doped garnet systems in luminescence. However, in this paper, the characterization of traps in the material is limited to the use of thermoluminescence curves within the range of 298 K–573 K. Supplementing the thermoluminescence curves at low temperatures and the characterization of the carrier movement state can provide a more comprehensive study of the persistent luminescent properties of the material. Regarding the conjecture that deep traps can be stimulated by light, the laser of 980 nm or 1064 nm can be used to verify this assumption. In conclusion, more comprehensive characterization methods can provide an accurate and detailed understanding of the luminescence properties of phosphors.

5. Conclusions

In conclusion, a series of Pr³⁺ doped garnet-type phosphors (CLSGGO:xPr³⁺) are successfully prepared by high-temperature solid-phase method. Firstly, XRD and EDS mapping provide detailed evidence for the pure phase of the material and the uniform distribution of elements. Cs-TEM explored the lattice structure and atomic arrangement of the garnet matrix from the atomic level deeply, proving the superior lattice structure of the material and further confirming the successful synthesis of the phosphor. The excitation and emission spectra of CLSGGO:xPr³⁺ are characterized, and the optimal doping concentration of Pr³⁺ for photoluminescence is determined to be 0.01. In addition, the diffuse reflectance spectrum of CLSGGO:0.01Pr³⁺ combined with the Kubelka–Munk equation calculated its optical band gap to be 4.83 eV. To study the influence of Pr³⁺-doping concentration on the LPL performance of the material, the TL curves of samples with different Pr³⁺-doping concentrations are measured and fitted with the first-order kinetic peak fitting software TL2000 1.0 version. These results indicated that the doping concentration affected the trap density and distribution in the material. Furthermore, the first-order kinetic simulation equation calculated the trap depth of the samples to be between 0.38 eV and 0.40 eV, and the trap density to be within the range of 8.80 × 10⁶–1.62 × 10⁷. The TL curves under different excitation times, different decay times, and different excitation temperatures studied the storage and release process of carriers in the traps. The relationship between the LPL decay curve and the decay time proved that the energy transfer process between carriers and the luminescence center mainly originated from the tunneling effect. Finally, based on the above characterization of the LPL luminescence phenomenon and mechanism, an electron capture physical model is proposed to describe the mechanism of the photoluminescence and LPL luminescence phenomena of CLSGGO:xPr³⁺.