A new insight into the mechanism of persistent luminescence phosphors SrS: Eu2+, Pr3+

The red persistent luminescence phosphors SrS: , (x = 0∼0.001, y = 0∼0.003) were synthesized by the solid-state reaction method in a weak reducing atmosphere of active carbon. The synthetic parameters, photoluminescence characteristics, and dynamic properties of thermoluminescence of the phosphors were studied systematically. According to the results of photoluminescence spectra of SrS: , , the energy level scheme of the phosphors was confirmed quantitatively. Based on the studies of the scanning electron microscope (SEM), UV-Visible absorbance spectra, afterglow decay curves, thermoluminescence(TL) spectra, and X-ray photoelectron spectroscopy (XPS) spectra, we found that the SrS: , (SSEP) phosphor presented the best persistent luminescence property, which mainly resulted from its outstanding absorbance characteristic, deeper depth of ( ) defects, and fewer defect concentration belonging to the higher-order kinetics. Those comprehensive results deduced that a complex defect ( )- should present in the Eu2+ doping phosphors, and the Pr3+ doping will produce defects, which can suppress the formation of ( ) defects and increase their energy depth.

Alkaline-earth sulfides (Ca, Sr, Mg)S have attracted much attention in the luminescent display industry resulting from their high luminous efficiency (3). They were also investigated as an important persistent luminescence materials host, which could be modulated to glow with different wavebands by doping proper dopants (3,22). The SrS: Eu 2+ phosphor is a potential candidate for being an excellent red persistent luminescence material, in which a great number of intrinsic defects are generated resulting from its soft base-hard acid character (20). Especially, these phosphors encapsulated with an oxide coating present better anti-moisture properties, which improves their applicability (23). Kong et al. (19) prepared SrS: Eu 2+ , Dy 3+ phosphors and explained the affection of microstructure on the persistent luminescence properties using a 'bridge-transmission' mechanism. Chen et al. (24) indicated that SrS: Eu 2+ , Sm 3+ phosphor presented nearinfrared photo-stimulated red luminescence, in which the Sm 3+ doping could significantly increase the densities of deep traps. Du et al. (12) found the red persistent luminescence phosphor CaS: Eu 2+ , Dy 3+ could be excited by red light, and in the excitation process the electrons transferred from 4f to 5d levels of Eu 2+ , then were trapped via the conduction band or localized states. Danilo et al. proposed that the Eu 2+ ions can be photo-oxidized under blue laser irradiation, and the excited electrons would transfer between Eu 2+ /Eu 3+ ions and intrinsic defects through the conduction band (20). The strontium sulfide phosphors were usually prepared by the solid-state reaction method (4,6,8,16,24), solvothermal synthesis method (7), and chemical co-deposition method (19), and the sintering temperatures in these references were all less than 1473 K. Yoshiyuki et al. (22) prepared SrS: Eu 2+ , Pr 3+ phosphors by a liquid phase reaction, and the optimum sample was achieved at the sintering temperature of 1673 K for 2 h. So the excellent persistent luminescence property seems closely related to the higher sintering temperature. Furthermore, the mechanism of persistent luminescence of alkaline-earth sulfide phosphors needs to be further studied. In this study, the SrS: Eu 2+ , Pr 3+ phosphors were prepared by solid-state reaction method in a reducing atmosphere of active carbon. The synthetic parameters, energy level scheme, persistent luminescence mechanism, and the defect types of the phosphors were systematically studied.

Experimental section
The phosphors are prepared by solid-state reaction. SrCO 3  Those luminescent powders' X-ray diffraction (XRD) data were collected on a Panalytical X'Pert PRO diffractometer using monochromated Cu Kα (λ = 1.5418 Å) radiation. The excitation and emission spectra were measured with a WFY-28 photoluminescence spectrometer to investigate the luminescent properties. A FJ427-A1 thermally stimulated spectrometer was utilized to reveal the afterglow behavior and analyze the defect energy levels. The SEM (JEOL JSM-6390LA) was used to characterize the microstructure of the phosphors. To estimate the components of the phosphors, the XPS spectra were carried out with a Kratos Amicus analyzer using Mg Kα radiation. UV-Visible absorbance spectra of the SS, SSP, SSE, and SSEP phosphors were carried out on a UV-Visible spectrophotometer (Thermo Scientific Evolution 220).

Crystal phase of the phosphors and morphological characterization
The effects of the doping amount of sulfur, annealed temperature and the annealed time on the crystal structures of SrS: Eu 2+ 0.0001 , Pr 3+ 0.003 phosphor were studied. As for the raw materials, the mole ratios of sulfur to SrCO 3 equaling 1, 2, 3, 4, and 5, are denoted as S1-S5, respectively. The XRD patterns of S1-S5 phosphors annealed at 1623 K for 1 h are shown in Figure 1(a), which indicates that SrS (JCPDS: 08-0489) as the main phase presents in all the XRD patterns of S1-S5, accompanying with slight impurity phases of SrO (JCPDS: 27-1304), SrCO 3 (05-0418), C (46-0944), and SrC 2 (03-0542). Figure 1(a, b) indicate that the impurity phases decrease gradually with sulfur doping and annealed temperature increasing. As shown in Figure 1(c), the XRD patterns of SrS: Eu 2+ 0.0001 , Pr 3+ 0.003 phosphor annealed at 1623 K for 3 and 4 h, can be indexed to the single phase of SrS with Fm-3 m space group. Annealed at 1623 K for 3 h, as shown in Figure 1(d), the samples with doping different amounts of Eu 2+ all present the single phase of SrS, and the XRD intensity enhances with increasing the Eu 2+ and Pr 3+ dopant. These results show that higher annealed temperature and longer annealed time are essential for the samples to crystallize into the single phase, and the rareearth ions doping is beneficial for enhancing the crystallinity. It has been reported that the dopant of some ions, including rare-earth ions, could improve the crystallization of the samples (9,(25)(26)(27). In the synthesized phosphors, we found the samples annealed at 1623 K for 3 h exhibited the best long afterglow properties (as shown in Figure S1), so in the following studies the samples annealed at 1623 K for 3 h were chosen as research specimens.
The morphological characteristics have vital effects on the luminescent properties of the samples (28). Figure 2 shows SEM micrographs of the SSE and SSEP with amplifying factors of 5.00 and 15.00 k. Figure 2 indicates that the grains of SSEP are much denser than those of SSE phosphor, which is consistent with the XRD results in Figure 1(d). Deduced from Figure 2(c) and (d), the distributions of grain sizes of the SSE and SSEP are 0.3-0.9 µm and 0.5-0.9 µm, respectively. It was reported that rare-earth doping can increase the density of phosphors (28). Figure 2(b,d) show that the SSEP is made up of tightly packed grains with smooth particle surfaces, which can reduce the non-radiation and scatter effectively (28).

UV-Visible absorbance spectra and XPS spectra analyses
The absorbance of phosphors has important effects on their luminescent property, such as the intensities of photoluminescence (PL), photoluminescence excitation (PLE), and afterglow. UV-Visible absorbance spectra of the SS, SSP, SSE, and SSEP are shown in Figure 3(a), and it shows that all the phosphors present obvious absorbance above 250 nm. The peak around 250 nm was also observed in UV-Visible absorbance spectra of SrS: Ce, Sm phosphor (29). The Eu 2+ and Pr 3+ doping can improve the absorbance above 250 nm, especially the SSP and SSEP phosphors can absorb the photons with long-wavelength efficiently. Suda et al. found the sulfur vacancy defects had significant effects on the long afterglow of CaS: Eu 2+ , Tm 3+ phosphor, and were sensitive to the temperature and the concentration of Eu 2+ (30). The intrinsic defects of SSEP usually consist of sulfur vacancy (V ·· S , V · S , V × S ), strontium vacancy (V Sr ), and substitution ion defects (Pr · Sr , Eu × Sr ). The chemical defect formulas can be written as (31): To study the sulfur vacancy defects of the SSE and SSEP, their XPS spectra were obtained. Figure 3(b) displays the XPS spectra of Sr 3d and S 2p, which are fitted with the Lorentzian-Gaussian type. The peak-differentiation results indicate that the spectra of Sr 3d consist of 3d 3/2 and 3d 5/2 components, and the spectra of S 2p consist of 2p 1/2 and  2p 3/2 components. The content of sulfur can be obtained by the following equation (26): where C y , I y , and S y are relative weight, the area of the resolved peak, and the sensitivity factor of the studied element, respectively, and the subscript i and y mean the ith element and the studied one. According to Equation (2), the calculated atomic ratios C S /C Sr are 0.598 and 0.703 for SSE and SSEP samples, respectively. So the sulfur vacancy defects should present in both samples, and the Pr 3+ doping suppresses the formation of the sulfur vacancy defects, which should be resulted from electropositivity for both sulfur vacancy defects (V · S or V ·· S ) and Pr · Sr defects, just as shown in Equation (1). For the ionic solid SrS, it is reasonable to create intrinsic defects, i.e. V Sr , to compensate for the charges of Pr · Sr and V · S defects. Due to the volatilization of sulfur during the synthesis procedure, the oxygen should incorporate for achieving charge neutrality, and the oxygen is also detected in XPS spectra. Figure 4 shows the PL and PLE spectra of the SS, SSE, SSP, and SSEP phosphors. Figure 4(a) indicates that a broad peak located at 475 nm appears in the PL spectra of SSE and SSEP phosphors. The Eu 2+ ions locate in octahedral symmetry in SrS: Eu 2+ and the 5d bands of Eu 2+ are split by the crystal field into e g and t 2g levels, with a separated energy spacing of 1.49 eV (32). The PLE peak mentioned above is assigned to the 4f 7 -4f 6 5d 1 (t 2g ) transition of Eu 2+ ions (3,8). The PLE spectra of SSEP at different monitoring wavelengths were obtained, and the peaks located at 290 and 330 nm were present in all the PLE spectra, as shown in Figure 4(b). The PLE peak located around 330 nm should be related to the charge transfer transition related to Pr 3+ ions, the transition of 4f 7 -4f 6 5d 1 (e g ) of the Eu 2+ ions, and intrinsic defects in SrS matrix (6,32,33). The half-bandwidth of the charge transfer bands in lanthanides has typical values of 0.6 ∼ 1.2 eV (34), while that of the peak around 330 nm are all less than 0.24 eV, so this PLE peak should result from Eu 2+ ions and the intrinsic defect. The bulk SrS has an indirect band gap of 4.3 eV (288 nm, at 300 K) (13,33), 4.4 eV (282 nm, at 300 K) (35), 4.49 eV (277 nm, at 14 K) (36). Thus the PLE peak located at 290 nm (4.28 eV) should be attributed to interband transitions of the SrS matrix (7).

Photoluminescence of the phosphors
The PL spectra of the SS, SSE, SSP, and SSEP monitored at 290 nm, and that of the SSEP monitored at 280, 290, 300, and 310 nm are shown in Figure 4(c) and (d), respectively. There are some PL peaks located at 345, 400, 473, and 499 nm in all the samples, and they should be related to the intrinsic defects and can be modulated by the doping ions of Eu 2+ and Pr 3+ . Considering the excited wavelength is 280 nm with a bandwidth of 2 nm, those peaks are irrelevant to the Xe lamp. Figure 4(c) indicates that the PL peak located at 499 nm is much more prominent in SSP, which can be assigned to the 3 P J (J = 0, 1, 2)-1 H 4 (4f -4f ) transition of Pr 3+ ion (33). The co-doping of Eu 2+ and Pr 3+ can suppress this peak located at 499 nm, which should be related to resonance energy transfer or energy relaxation between the Eu 2+ and Pr 3+ . The emission peak located at 610 nm along with the characteristic transitions 4f 7 ( 8 S 7/2 ) -4f 6 5d 1 (t 2g ) of Eu 2+ emerges in the Eu 2+ doped samples (8,19,22,23).
According to the results of the PL spectra, PLE spectra, and the band gap of the SrS matrix (4.28 eV), the energy level scheme of SSEP can be deduced, as shown in Figure 5. It was reported that the conduction band states of SrS hybridized with the 4f -5d excited states of Pr 3+ (37). The energy difference E of the first electric dipole allowed the 4f -5d transition of Pr 3+ in the SrS matrix, which can be expressed as: where the 49,340 cm −1 is the energy difference of the first 4f -5d transition of Ce 3+ as a free ion, and the D(SrS) is the crystal field depression in the SrS matrix, which is 26,084 cm −1 according to Ref. (34). The E(Pr 3+ , Ce 3+ ) means energy difference between E(Pr 3+ ) and E(Ce 3+ ) in SrS matrix, and it equals to 12240 ± 750 cm −1 (34). We can obtain the E(Pr 3+ , SrS), i.e. 35496 ± 750 cm −1 (4.32 ∼ 4.50 eV), therefore it means the ground state of Pr 3+ should locate in the valence band, as shown in Figure 5. According to the host referred 4f -electron binding energy (HRBE) curves of RE 2+ and RE 3+ ions in SrS (20), the ground state of Pr 3+ locates around the top of the valence band of SrS, which coincides with the deduced results above. In SrS matrix, the lowest excited states (4f 6 5d 1 ) of Eu 2+ locate at 0.40-0.70 eV below the conduction band edge (33,38). Considering the results of Figure 4(a), the ground states of Eu 2+ (4f 7 − 8 S 7/2 ) should locate at 1.10 eV above the valence band edge of SrS, as shown in Figure 5. Considering the analyses of the PLE peaks located at 330 nm, the deduced position of the intrinsic defect states of the SrS matrix should be at 0.52 eV above the valence band edge or/and below the conduction band edge of SrS. The electronegative defect V Sr usually locates above the top of the valence band, and the electropositive defects Pr · Sr , V ·· S , V · S locate below the bottom of the conduction band of SrS, as shown in Figure 5. For maintaining the balance of the local charge, V Sr defect should emerge around the Pr · Sr , V ·· S , and V · S defects (4).

TL spectra and afterglow decay curves
For understanding the defects existed in the phosphors SS, SSP, SSE, and SSEP, the TL spectra were studied. Before the TL tests, all the phosphors were heated to 673 K for photobleaching the samples. The phosphors were irradiated by a 15-watt Philips energysaving lamp for 5 min. Then the TL spectra were measured after the light source was withdrawn from the phosphors for 10 min. The information on the traps can be obtained by fitting the TL curve based on a TL Glow Analyzer (TLanal) Program (39), and all the deconvolution peaks of the TL glow curve were obtained according to the general order equation as follows (40,41): where I(T) is the TL intensity, i is the serial number of the TL peaks, s is the equivalent frequency factor, n 0 is the concentration of trapped charges at t = 0, E t is the trap energy level, k B is the Boltzmann's constant, b is the kinetic order, T 0 is the initial temperature of the TL and β is the heating rate, 1 K·s −1 in the experiment. The position of the TL glow peaks can be obtained using the T m -T stop method (33), and the corresponding results are shown in Figure 6(a). The results show that the TL curves of SSE and SSEP are both composed of four TL glow peaks. According to the deduced position of the TL glow peaks, the deconvolution results of the TL glow curve are shown in Figure  6(b) and Table 1. Figure 6(b) shows the SSE and SSEP present significant TL glow peaks in the temperature range of 300 ∼ 600 K, and they are both well fitted by four TL glow peaks. Pitale et al. (33) have indicated that the SrS: Pr 3+ exhibited a TL peak located around 370 K under UV light excitation. While the TL peak of SSP is too weak to be observed under the white light excitation in our test. Singh et al. (42) confirmed two types of defects existed in SrS: Ce 3+ phosphor at room temperature, and the defects located at 410 and 548 K were  assigned to the F + center (an electron trapped in an anion vacancy) and cationic or anionic vacancies combined with the doping ions respectively. The difference in the thermal activation energy between sulfur vacancies V ·· S and V · S equals 0.2 eV (43). Considering that the differences between E t1 and E t2 are both close to 0.2 eV for the SSE and SSEP, so the defect E t1 and E t2 should be ascribed to V · S (i.e. V ·· S + e) and V ·· S , respectively. The defect energy levels of V · S and V ·· S of the SSEP are consistent with the results in Figure 5 approximately. Due to Pr 3+ doping will produce Pr · Sr defect, which can suppress the formation of V · S and V ·· S defects. As shown in Table 1, the values of n 01 + n 02 decrease obviously as Pr 3+ doping. The defects of E t3 and E t4 may be related to the Pr · Sr (or Eu × Sr ) and V Sr , respectively. Furthermore, as shown in Table 1, the values of b 1 and b 2 are both larger than 2, which means most of the electrons released from E t1 and E t2 will be recaptured. The values of b 3 of SSE and SSEP are both larger than 1.5, which means the behavior of the E t3 defect tends to act as the second-order kinetics. These results indicate that the electronegative defects, such as V Sr , should be involved in the processes of electrons and holes being captured and recombination, as a result of reducing the recombination rate of the carriers. For further understanding the excitation and transfer process of the carriers, the long afterglow and TL curves of SSEP excited by 10-watt blue (emission wavelength 460 ∼ 470 nm), white, and violet LEDs are shown in Figure 6(c). It is impossible to excite electron-hole pairs over the band gap for the SSEP excited by the blue LED. While it can also present persistent luminescence, which means the electron-hole pairs are generated and transfer among Eu 2+ , the energy band of SrS, and the defect centers.
It is closely associated between afterglow decay curves and defects. Figure 6(d) shows the decay curves of afterglow phosphorescence of the SS, SSP, SSE, and SSEP. Before measuring the decay curves, the phosphors were irradiated by a 15-watt Philips energy-saving lamp for 5 min. The insets (1) and (2) in Figure 6(d) are the photographs of SSEP phosphor irradiated by the lamp and just after shutting down the lamp in the darkroom, respectively, and the afterglow of SSEP can also be observed after shutting down the lamp for 14 h. For understanding the mechanism of the afterglow properties, the afterglow decay curves of SSE and SSEP are fitted according to the following equation (8,19,25): where I represents the phosphorescence intensity, I 0 , I 1 , I 2 , and I 3 are constants, t is the time, and τ 1 , τ 2 , and τ 3 are the decay time constants. As shown in Figure 6(d), the afterglow decay curves of SSE and SSEP can be both well fitted according to Equation (5). The simulated parameters are obtained, as shown in Table 2. The results show that the τ 1 and τ 2 almost equal to each other for both SSE and SSEP, which indicates that the persistent luminescence of the two phosphors is related to two kinds of defects. It seems inconsistent with the analysis of TL, i.e. four kinds of defects existed in the two phosphors. It may result from the first-order kinetics property for the E t4 defect, and the second-order kinetics defects, i.e. E t1 and E t2 (E t3 ) defects, should take prominent roles in the persistent luminescence. The characteristic parameters τ 1 (τ 2 ) and τ 3 , are all enhanced significantly for the SSEP, which indicates that the depths of the defects in SSEP are both enhanced than those in SSE. Based on the analyses above, one proper mechanism of the long afterglow can be described as follows. The electron-hole pairs were generated when the SSEP was irradiated, and a part of electron-hole pairs belonging to Eu 2+ ions would recombine accompanied with light emission. Meanwhile, other electron-hole pairs would be captured by the traps directly or via conduction/valence band, described as the processes ➀-➄ and (i)-(iv) as shown in Figure 5. This direct transference of electrons between the Eu 2+ and Re 3+ had been revealed (44), just as the process ➄ in Figure 5. Due to thermal perturbation of the lattice, the trapped electrons and holes in the vicinity of Eu 2+ ions would be thermally stimulated, and then the Eu 2+ ion captures an electron from the conduction band (process ➀ in Figure 5) and captures a hole from the V Sr defect (process (ii) in Figure 5), thus accompanying the persistent luminescence when the electron and hole recombine. It is important to note that the appearance of the electronegative V Sr defect reduces the probability of capturing electrons released from the defects, i.e. V ·· S , V · S , and Pr · Sr , which should be responsible for the remarkable persistent luminescence.

Conclusions
SrS: Eu 2+ x , Pr 3+ y (x = 0 ∼ 0.001, y = 0 ∼ 0.003) phosphors were synthesized via the solidstate reaction in a reducing atmosphere of active carbon. The annealed temperature and time, the excess amount of sulfur, and Eu 2+ (Pr 3+ ) doping all have essential effects on the crystal phase of the phosphors. UV-Visible absorbance spectra and SEM of the phosphors indicate that the co-doping Eu 2+ and Pr 3+ can enhance the absorbance above 250 nm, and improve the crystallization. Based on analyzing the PL and PLE spectra, characteristic energy levels of Eu 2+ , Pr 3+ , and the intrinsic defects in SrS: Eu 2+ x , Pr 3+ y were confirmed quantitatively. The simulated results of the TL curves and XPS spectra demonstrate that the complex defects V · S (V ·· S )-V Sr should present in SSE and SSEP, and they play a vital role in the excitation, transition, and recombination process of the carriers. The Pr 3+ doping will introduce Pr · Sr defects, which can suppress the formation of V · S (V ·· S ) defects and increase its energy depth. The SSEP phosphor presents not only higher initial brightness, but also more outstanding long afterglow properties than those of SSE, which results from its excellent absorbance characteristics, deeper depth of V · S (V ·· S ) defects, and fewer higher-order kinetics defects.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Notes on contributors
Chao Xu received a Ph.D. degree in condensed matter physics from Shanghai University, Shanghai, China, in 2020. His research interests include inorganic luminescent materials, multiferroic materials, and photocatalytic materials.
Shenao Yu is currently working toward a bachelor's degree in new energy science and engineering from Jiangsu Ocean University, Lianyungang, China. He is mainly engaged in the research of luminescent materials.
Linxing Shi received a Ph.D. degree in optical engineering from Nanjing University of science and