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Abstract
Luminescence-based thermometry, especially the ratiometric temperature sensing technology, has attracted considerable attention recently due to its characteristics such as non-contact operating mode and strong capacity of resisting disturbance. Differing from the conventional strategy that usually needs continuous excitation, here an optical thermometry, which we have named the persistent luminescence intensity ratio (PLIR) thermometry, is proposed. The PLIR thermometry relies on the optical material SrF2:Pr3+ that could emit luminescence for several hours and even longer after being charged by X-ray. It has been demonstrated that the PLIR is sensitive to the variation of temperature and complies with the Boltzmann distribution. More importantly, the reliability of the proposed PLIR thermometry is verified. Our work may inspire others to develop more persistent luminescence thermometry.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Temperature plays a key role in a broad spectrum of fields such as the metabolism and catalysis processes [1–6]. Detection of temperature in an accurate and fast manner thus becomes an important thing. In general, temperature measurement methods can be roughly classified into contact type and non-contact type [7–9]. Compared with the conventional contact-type thermometers, such as the widely used thermal resistance and thermocouple, the non-contact temperature sensing plays an ever-increasingly significant role in more applications, for instance, monitoring the surface temperature of an object [10–13]. Among the numerous non-contact temperature sensing methods, the optical thermometry, especially the luminescence intensity ratio (LIR) strategy, is very promising [14–16]. For one thing, there is sound theory to support this thermometry, such as the Boltzmann distribution and energy transfer theory. For another, this strategy is able to avoid the influence of many surrounding factors, when compared to the method that is based on the emitting intensity of single transition [17,18]. For instance, the LIR of two transitions that originate from one pair of thermally coupled levels is immune to the instance between the luminescent centers and detector in reality. Thus, this type of thermometry could be easily shifted from one case to another, without the need of frequent calibrations [19]. However, each coin has two sides. The LIR methodology also has disadvantages. For example, the medium-induced optical distortions can significantly affect the LIR and thus the temperature readout, which should receive adequate attention [20].
Up to now, great effort has been made to explore more sensitive and accurate LIR thermometry [21,22]. Among the abundant luminescent centers, Pr3+ ion is very popular because of its abundant energy states and strong emissions over the visible wavelength range [23–27]. For example, Wang et al. recently showed the possibility of building up a sensitive LIR thermometry via phase transition [28]. They found that the Pr3+ doped LiYO2 phosphor could undergo the phase transition from β-type to α-type over the 323-330 K temperature range. Depending on this mechanism, the sensitivity of the constructed LIR thermometry could reach as high as 23% K−1 at 329 K. Undeniably, these works greatly enrich the optical thermometry family. It should be noted, however, that all these conventional LIR-type temperature measurement methods need continuous excitation. To enrich the family of LIR thermometry, developing new methods that do not require continuous excitation becomes meaningful but is still a challenge today.
Herein, an optical thermometry is proposed, depending on the as-prepared luminescent material of SrF2:Pr3+ that could emit persistent luminescence (PersL) for several hours without the need of continuous excitation. As this strategy relies on the persistent luminescence intensity ratio of SrF2:Pr3+, it is thus named as PLIR thermometry herein. As demonstrated in Fig. 1, the persistent luminescent material of SrF2:Pr3+ could store energy upon band-to-band excitation by X-ray. After the X-ray source is ceased, SrF2:Pr3+ could emit PersL for several hours. Due to the abundant energy levels of Pr3+, the LIR of multiple states could be used to reveal the variation of temperature. Therefore, the PLIR thermometry has been established with success. In addition, the reliability of PLIR thermometry has also been demonstrated.
2. Materials and methods
Synthesis of SrF2:Pr3+. SrF2:x% Pr3+ (x = 0.25, 0.5, 1, 3 and 5, molar ratio) was prepared by the commonly used high temperature solid state method. The raw materials are SrF2 (AR, Macklin), Pr6O11 (99.99%, Aladdin) and NH4F (99.99%, Aladdin). First, the stoichiometric raw powders of SrF2, Pr6O11 and NH4F were weighed and ground fully in mortar. Second, the homogeneous mixtures were put into the muffle furnace and sintered at 900 °C for 2 h. Finally, the powders were finely ground to obtain the target materials.
Characterization of SrF2:Pr3+. The powder XRD patterns of SrF2:x% Pr3+ (x = 0.25, 0.5, 1, 3 and 5, molar ratio) were obtained by the X-ray diffractometer (Bruker D8 ADVANCE, 10–90°, 40 kV and 40 mA). An X-ray source (JF-2000 W target, 40 kV, 30 mA) was used as the excitation source. The X-ray induced luminescence (RL) spectra of samples were measured using a spectrometer (Andor SR-500i). The temperature of samples was controlled by Linkam THMS-600 equipment (precision: 0.1 K). The particle size and elemental distribution were determined by transmission electron microscopy (TEM) (JEOL-2100Plus) and scanning electron microscopy (SEM) (Nova Nano SEM450, FEI).
3. Results and discussion
Figure 2(a) presents the comparison of the measured XRD patterns of SrF2:x% Pr3+ (x = 0.25, 0.5, 1, 3 and 5) as well as the standard references with no. PDF#01-0644 that represents the cubic crystal structure. The diffraction peaks’ positions of all prepared SrF2:Pr3+ could be well indexed with the reference data regardless of the doping concentration of Pr3+. The strong and sharp diffraction peaks are the indications of well crystallization. Moreover, no redundant diffraction peaks were collected for all samples. These results reveal that SrF2:x% Pr3+ (x = 0.25, 0.5, 1, 3 and 5) samples have been synthesized with the pure cubic crystal phase. The SEM images of the representative SrF2:0.5%Pr3+ shown in Fig. 2(b) indicate that the shape is irregular and the size is micro-scaled. The samples with other doping concentration of Pr3+ have the similar results and are not shown herein. The element mapping images of this sample reveal that F, Sr, and Pr elements are uniformly distributed in the as-prepared materials. The HR-TEM image of the representative SrF2:0.5%Pr3+, displayed in Fig. 2(d) which is obtained at the marginal area of Fig. 2(c), suggests that the lattice fringes are quite clear. The lattice spacing was measured to be 0.339 nm. This value is very close to the reference distance of 0.337 nm of the [111] lattice plane of SrF2. The image of selected area electron diffraction (SAED) of SrF2:0.5%Pr3+ is displayed in Fig. 2(e). The [422], [311] and [111] have been identified. It is a hard evidence that the sample was crystallized well. All these characteristics reveal that the persistent luminescence materials have been prepared with success.
Fig. 2. Characterization of SrF2:Pr3+. (a) XRD patterns of SrF2:x%Pr3+ (x = 0.25, 0.5, 1, 3 and 5). The bottom panel shows the partial enlarged detail. (b) SEM and elemental mapping images of F, Sr and Pr of SrF2:0.5%Pr3+. (c) TEM image, (d) HR-TEM image, and (e) SAED patterns of SrF2:0.5%Pr3+.
Figure 3(a) shows the RL spectrum of SrF2:0.5%Pr3+ under X-ray excitation. There are five obvious emission peaks at 483, 522, 539, 604 and 640 nm, which are ascribed to the 3P0-3H4, 3P1-3H5, 3P0-3H5, 1D2-3H4/3P0-3H6 and 3P0-3F2 transitions, respectively [27,29]. For easy discussions, these emission lines are labelled as ‘A’, ‘B’, ‘C’, ‘D’ and ‘E’, respectively. In general, the doping concentration of luminescent center has a great effect on the emitting intensity. So the RL spectra of SrF2:x%Pr3+ (x = 0.25, 0.5, 1, 3 and 5) under X-ray excitation have been collected. And the integrated emitting intensity of each RL spectrum has also been calculated, as presented in Fig. 3(b). Obviously, the emitting intensity increases first and then decreases. The optimal doping concentration of Pr3+ was found to be 0.5% in molar ratio. Therefore, the sample of SrF2:0.5%Pr3+ is selected as a representative to be mainly discussed in the following.
Fig. 3. LIR thermometry under X-ray excitation. (a) RL spectrum of SrF2:0.5% Pr3+ under X-ray excitation. (b) Integrated emitting intensity of SrF2:0.5% Pr3+ under X-ray excitation as well as the PersL intensity as a function of the doping concentration of Pr3+. (c) RL spectra of SrF2:0.5% Pr3+ in the 303–373 K temperature range under X-ray excitation. (d) LIRs of ‘B’/‘A’ and ‘B’/‘D’ of SrF2:0.5% Pr3+ as a function of temperature. (e) Relative sensitivity of the proposed LIR thermometry shown in (d).
Figure 3(c) depicts the RL spectra of SrF2:0.5% Pr3+ as a function of temperature over the 303-373 K temperature range, upon excitation of X-ray. With increasing temperature gradually, the whole emission intensity presents an upward trend, although these emission lines show slightly different response to the variation of temperature. The LIRs of ‘B’/‘A’, and ‘B’/‘D’ were then calculated, as they separately represent two different types of thermometry, the coupled and non-coupled states. The results are displayed in Fig. 3(d). All these two groups of LIRs increase gradually with the rise of temperature. Moreover, the LIRs depending on the ratio of ‘B’/‘A’ were found to obey well the Boltzmann distribution: [9]
For an optical thermometry, relative sensitivity (Sr) is one of the most important parameters used to characterize its performance. In general, Sr can be expressed as: [29]
Depending on this equation, the relative sensitivities of the two aforementioned LIR thermometry were calculated, as presented in Fig. 3(e). It can be observed that all relative sensitivities show a monotonic decline trend when the temperature is increased. At 303 K, the maximum relative sensitivities are 0.62% K−1 and 0.87% K−1 for the LIR thermometry relying on the ‘B’/‘A’ and ‘B’/‘D’ pairs, respectively.
After the as-prepared SrF2:0.5%Pr3+ was excited by X-ray for 5 min, the excitation source was stopped. SrF2:0.5%Pr3+ emits strong PersL, as presented in Fig. 4(a). It is obvious that there is no difference on peak positions between this PersL spectrum and the RL counterpart. The relative intensity of each emission band is slightly different upon two excitation models, which needs further investigation in the future. By monitoring the PersL intensity at 483 nm, the kinetic decay curve was collected, as displayed in Fig. 4(b). The PersL could be detected for over 30 hours, as demonstrated by the PersL spectrum in the inset of Fig. 4(b). Obviously, the signal of PersL is still recognizable. The long PersL is the base of PLIR thermometry. The thermoluminescence (TL) spectra of samples were also measured, as presented in Fig. 4(c). All these TL spectra own the similar profile with peak in the 280–400 K temperature range. For one thing, this result is consistent with that of RL and PersL shown in Fig. 3(b), indicating SrF2:0.5%Pr3+ has the maximum trap density. For another, the TL peak over the 280–400 K temperature range reveals that the depth of traps is relatively shallow [30]. At higher temperatures, the initial PersL intensity becomes stronger as higher temperature favors the release of trapped electrons. As a result, the duration time becomes shorter [30]. Therefore, our sample is more suitable for low temperatures in order to expand the phosphor working time interval that can monitor the temperature after X-ray radiation.
Fig. 4. Basis of PLIR thermometry. (a) PersL spectrum of SrF2:0.5%Pr3+ at the initial after ceasing X-ray immediately. (b) Kinetic decay curve of the PersL monitored at 483 nm. Inset shows the PersL spectrum of SrF2:0.5%Pr3+ at 30 h after ceasing X-ray. (c) Thermoluminescence results of samples. (d) PersL spectra at the first 20 minutes after the stoppage of X-ray. (e) LIRs of ‘B’/‘A’ and ‘B’/‘D’ as a function of time. The above experimental samples were all irradiated upon X-ray for 5 min before stopping the excitation source.
In the following, the feasibility of PLIR thermometry is further verified from another point of view. Figure 4(d) shows the room temperature PersL spectra collected uninterruptedly in the first 20 minutes after the stoppage of X-ray. It should be noticed that the PersL suffers from attenuation with the loss of time. And compared with the ‘A’ band, the ‘B’ band has a lower intensity of PersL. Having this fact in mind, it can be assumed that it will only be observable with a sufficiently good signal for a shorter time. After twenty minutes’ measurement, the signal to noise ratio of the ‘B’ band is no more than 100:1. Therefore, only the PersL spectra obtained during the first twenty minutes are investigated and analyzed. Depending on these original data, the LIRs of ‘B’/‘A’ and ‘B’/‘D’ were also calculated, as displayed in Fig. 4(e). The LIR of ‘B’/‘A’ fluctuates around a constant, which lays the foundation for the subsequent PLIR thermometry. In contrast, the LIR of ‘B’/‘D’ presents a downtrend, indicating this LIR is not such suitable for PLIR thermometry.
Figure 5(a) displays the PersL spectra of SrF2:0.5%Pr3+ at different temperatures. It can be observed that the spectral intensity first increases and then decreases with increasing the temperature. By integrating the emitting intensity of the ‘B’ and ‘A’ PersL bands, the LIR between them was obtained, as presented in Fig. 5(b). Obviously, the LIR increases gradually with the rise of temperature. These experimental values were found to accord well with Eq. (1), revealing that the populations at the 3P0 and 3P1 states are still under the control of the Boltzmann distribution even during the PersL process. This fact suggests that the PLIR thermometry could be realized.
Fig. 5. PLIR thermometry. (a) PersL spectra of SrF2:0.5%Pr3+ at the temperatures from 303 to 373 K after ceasing X-ray source. (b) LIR of ‘B’/‘A’ of SrF2:0.5%Pr3+ as a function of temperature.
Depending on Eq. (2), the relative sensitivity of this PLIR thermometry was calculated, as shown in Fig. 6(a). It can be observed that the relative sensitivity presents a monotonic decline trend with increasing temperature from 303 to 373 K. The maximum relative sensitivity was found to be 0.52% K−1 at 303 K. This result is similar with our previously published result [31]. Therefore, this work greatly expands the host matrix that can be used for Pr3+-based PLIR thermometry. In addition, the relative sensitivity of SrF2:0.5%Pr3+ is smaller than that of NaYF4:Er3+ as the 3P1 and 3P0 states of Pr3+ has a narrower gap than the 2H11/2 and 4S3/2 states of Er3+ [32]. After acquisition of relative sensitivity, the temperature resolution can be estimated. According to the definition, the temperature resolution is calculated to be ∼0.5 K at 303 K at initial time. And at 20 minutes after the stoppage of X-ray excitation, this parameter is reduced to be 2 K, due to the lower PersL and thus lower signal to noise ratio. Obviously, the temperature resolution becomes not that ideal with the loss of time, which is the disadvantage of PLIR thermometry. To solve this problem, great efforts must be made to enhance the intensity and duration of PersL [33]. Moreover, frequently charging is also a possible solution. However, considering the hazard and high-cost feature of X-ray source, developing the PersL material that could be efficiently excited by some cheap and commonly used light sources such as flashlight becomes very meaningful [34,35].
Fig. 6. Reliability of PersL thermometry. (a) Relative sensitive of the PLIR thermometry depending on the LIR of ‘B’/‘A’ of SrF2:0.5%Pr3+. (b) Continuously measured LIR of ‘B’/‘A’ of SrF2:0.5%Pr3+ by three times. (c) Heating-cooling repeatability test.
The reliability of PLIR thermometry is then demonstrated by Fig. 6(b) and (c). After the sample of SrF2:0.5%Pr3+ was exposed to X-ray irradiation for 5 minutes, X-ray source was removed. With gradually increasing the temperature from 303 to 373 K, the PersL spectra were collected and the LIR of ‘B’/‘A’ could be calculated. After the electrons in the traps were completely emptied, the sample of SrF2:0.5%Pr3+ was further charged by X-ray for another 5 minutes. Subsequently, the forementioned procedures were repeated twice, and the results are displayed in Fig. 6(b). Obviously, the three set of measured LIRs have an excellent overlap with each other. This result, together with the heating-cooling test shown in Fig. 6(c), demonstrates unquestionably that the PLIR thermometry is reliable.
4. Conclusions
In conclusion, a novel PLIR thermometry is proposed. It depends on the as-prepared Pr3+-doped SrF2 material which could emit PersL for several hours after being exposed to X-ray irradiation. During the PersL process, the populations at the 3P1 and 3P0 states response sensitively to the variation of temperature. Specifically, the LIR between these two PersL lines follows the Boltzmann distribution, which is increased gradually with the rise of temperature from 303 to 373 K. More importantly, the PLIR thermometry has been demonstrated to be reliable. Our work may inspire others to develop more persistent luminescence thermometry.
Funding
National Natural Science Foundation of China (11974097, 12104125); Advanced Talents Incubation Program of Hebei University (521100221006); Hebei Key Laboratory of Dielectric and Electrolyte Functional Material, Northeastern University at Qinhuangdao (HKDEFM2021302); Natural Science Foundation of Hebei Province (A2019201073).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. A. Bednarkiewicz, J. Drabik, K. Trejgis, D. Jaque, E. Ximendes, and L. Marciniak, “Luminescence based temperature bio-imaging: status, challenges, and perspectives,” Appl Phys. Rev. 8(1), 011317 (2021). [CrossRef]
2. R. G. Geitenbeek, A.-E. Nieuwelink, T. S. Jacobs, B. B. V. Salzmann, J. Goetze, A. Meijerink, and B. M. Weckhuysen, “In Situ Luminescence Thermometry To Locally Measure Temperature Gradients during Catalytic Reactions,” ACS Catal. 8(3), 2397–2401 (2018). [CrossRef]
3. J. Zhou, B. del Rosal, D. Jaque, S. Uchiyama, and D. Jin, “Advances and challenges for fluorescence nanothermometry,” Nat Methods 17(10), 967–980 (2020). [CrossRef]
4. T. Hartman, R. G. Geitenbeek, G. T. Whiting, and B. M. Weckhuysen, “Operando monitoring of temperature and active species at the single catalyst particle level,” Nat. Catal. 2(11), 986–996 (2019). [CrossRef]
5. C. D. S. Brites, S. Balabhadra, and L. D. Carlos, “Lanthanide-Based Thermometers: At the Cutting-Edge of Luminescence Thermometry,” Adv. Opt. Mater. 7(5), 1801239 (2019). [CrossRef]
6. D. Chrétien, P. Bénit, H.-H. Ha, S. Keipert, R. ElKhoury, Y.-T. Chang, M. Jastroch, H. T. Jacobs, P. Rustin, and M. Rak, “Mitochondria are physiologically maintained at close to 50 °C,” PLoS Biol. 16(1), e2003992 (2018). [CrossRef]
7. P. R. N. Childs, J. R. Greenwood, and C. A. Long, “Review of temperature measurement,” Rev. Sci. Instrum. 71(8), 2959–2978 (2000). [CrossRef]
8. S. W. Allison and G. T. Gillies, “Remote thermometry with thermographic phosphors: Instrumentation and applications,” Rev. Sci. Instrum. 68(7), 2615–2650 (1997). [CrossRef]
9. S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4756 (2003). [CrossRef]
10. D. A. Rothamer and J. Jordan, “Planar imaging thermometry in gaseous flows using upconversion excitation of thermographic phosphors,” Appl. Phys. B 106(2), 435–444 (2012). [CrossRef]
11. M. Aldén, A. Omrane, M. Richter, and G. Särner, “Thermographic phosphors for thermometry: A survey of combustion applications,” Prog. Energy Combust. Sci. 37(4), 422–461 (2011). [CrossRef]
12. K. Trejgis, A. Bednarkiewicz, and L. Marciniak, “Engineering excited state absorption based nanothermometry for temperature sensing and imaging,” Nanoscale 12(7), 4667–4675 (2020). [CrossRef]
13. J. Drabik and L. Marciniak, “Excited State Absorption for Ratiometric Thermal Imaging,” ACS Appl. Mater. Interfaces 13(1), 1261–1269 (2021). [CrossRef]
14. Y. Jiang, Y. Tong, S. Chen, W. Zhang, F. Hu, R. Wei, and H. Guo, “A three-mode self-referenced optical thermometry based on up-conversion luminescence of Ca2MgWO6:Er3+,Yb3+ phosphors,” Chem. Eng. J. 413, 127470 (2021). [CrossRef]
15. Q. Wang, M. Liao, Q. M. Lin, M. X. Xiong, Z. F. Mu, and F. G. Wu, “A review on fluorescence intensity ratio thermometer based on rare-earth and transition metal ions doped inorganic luminescent materials,” J. Alloys Compd. 850, 156744 (2021). [CrossRef]
16. L. Li, F. Qin, L. Li, H. Gao, and Z. Zhang, “Highly sensitive optical ratiometric thermal sensing based on the three-photon upconversion luminescence of Y2O3:Yb3+,Er3+ nano-thermometers,” J. Mater. Chem. C 7(24), 7378–7385 (2019). [CrossRef]
17. X. Huang, K. Huang, L. Chen, N. Chen, R. Lei, S. Zhao, and S. Xu, “Effect of Li+/Mg2+ co-doping and optical temperature sensing behavior in Y2Ti2O7: Er3+/Yb3+ upconverting phosphors,” Opt. Mater. 107, 110114 (2020). [CrossRef]
18. L. Zhao, J. Mao, B. Jiang, X. Wei, Y. Chen, and M. Yin, “Temperature-dependent persistent luminescence of SrAl2O4:Eu2+, Dy3+, Tb3+: a strategy of optical thermometry avoiding real-time excitation,” Opt. Lett. 43(16), 3882–3884 (2018). [CrossRef]
19. S. Balabhadra, M. L. Debasu, C. D. S. Brites, R. A. S. Ferreira, and L. D. Carlos, “Upconverting nanoparticles working as primary thermometers in different media,” J. Phys. Chem. C 121(25), 13962–13968 (2017). [CrossRef]
20. Y. Shen, J. Lifante, N. Fernandez, D. Jaque, and E. Ximendes, “In vivo spectral distortions of infrared luminescent nanothermometers compromise their reliability,” ACS Nano 14(4), 4122–4133 (2020). [CrossRef]
21. M. D. Dramićanin, “Trends in luminescence thermometry,” J. Appl. Phys. 128(4), 040902 (2020). [CrossRef]
22. M. D. Dramićanin, “Sensing temperature via downshifting emissions of lanthanide-doped metal oxides and salts. A review,” Methods Appl. Fluoresc. 4(4), 042001 (2016). [CrossRef]
23. S. Wang, S. Ma, G. Zhang, Z. Ye, and X. Cheng, “High-performance Pr3+-doped scandate optical thermometry: 200 K of sensing range with relative temperature sensitivity above 2%·K-1,” ACS Appl. Mater. Interfaces 11(45), 42330–42338 (2019). [CrossRef]
24. S. Zhou, G. Jiang, X. Wei, C. Duan, Y. Chen, and M. Yin, “Pr3+-Doped β-NaYF4 for Temperature Sensing with Fluorescence Intensity Ratio Technique,” J. Nanosci. Nanotechnol. 14(5), 3739–3742 (2014). [CrossRef]
25. E. Maurice, G. Monnom, G. W. Baxter, S. A. Wade, B. P. Petrski, and S. F. Collin, “Blue Light-Emitting-Diode-Pumped Point Temperature Sensor Based on a Fluorescence Intensity Ratio in Pr3+: ZBLAN Glass,” Opt. Rev. 4(1), A89–91 (1997). [CrossRef]
26. V. K. Rai, D. K. Rai, and S. B. Rai, “Pr3+ doped lithium tellurite glass as a temperature sensor,” Sensor Actuat A-Phys. 128(1), 14–17 (2006). [CrossRef]
27. C. D. S. Brites, K. Fiaczyk, J. F. C. B. Ramalho, M. Sójka, L. D. Carlos, and E. Zych, “Widening the Temperature Range of Luminescent Thermometers through the Intra- and Interconfigurational Transitions of Pr3+,” Adv. Optical Mater. 6(10), 1701318 (2018). [CrossRef]
28. S. Wang, J. Zhang, Z. Ye, H. Yu, and H. Zhang, “Exploiting novel optical thermometry near room temperature with a combination of phase-change host and luminescent Pr3+ ion,” Chem. Eng. J. 414(15), 128884 (2021). [CrossRef]
29. J. Stefanska and L. Marciniak, “Single-Band Ratiometric Luminescent Thermometry Using Pr3+ Ions Emitting in Yellow and Red Spectral Ranges,” Adv. Photonics Res. 2(7), 2100070 (2021). [CrossRef]
30. J. Xu and S. Tanabe, “Persistent luminescence instead of phosphorescence: History, mechanism, and perspective,” J. Lumin. 205, 581–620 (2019). [CrossRef]
31. L. Li, Z. Wu, C. Wang, X. Han, L. Marciniak, and Y. Yang, “Boltzmann-distribution-dominated persistent luminescence ratiometric thermometry in NaYF4:Pr3+,” Opt. Lett. 47(7), 1701–1704 (2022). [CrossRef]
32. Z. Wu, L. Li, X. Lv, H. Suo, C. Cai, P. Lv, M. Ma, X. Shi, Y. Yang, L. Marciniak, and J. Qiu, “Persistent luminescence ratiometric thermometry,” Chem. Eng. J. 438, 135573 (2022). [CrossRef]
33. L. Li, T. Li, Y. Hu, C. Cai, Y. Li, X. Zhang, B. Liang, Y. Yang, and J. Qiu, “Mechanism of the trivalent lanthanides’ persistent luminescence in wide bandgap materials,” Light: Sci. Appl. 11(1), 51 (2022). [CrossRef]
34. T. Maldiney, A. Bessière, J. Seguin, E. Teston, S. K. Sharma, B. Viana, A. J. J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, and C. Richard, “The in vivo activation of persistent nanophosphors for optical imaging of vascularization, tumours and grafted cells,” Nat. Mater. 13(4), 418–426 (2014). [CrossRef]
35. K. V. den Eeckhout, D. Poelman, and P. F. Smet, “Persistent Luminescence in non-Eu2+-doped compounds: A review,” Materials 6(7), 2789–2818 (2013). [CrossRef]
