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Abstract
Optical temperature sensing of the non-thermally coupled energy levels (N-TCLs) based on fluorescence intensity ratio (FIR) technologies has excellent temperature sensitivity and signal recognition properties. In this study, a novel strategy is established to enhance the low-temperature sensing properties by controlling photochromic reaction process in Na0.5Bi2.5Ta2O9: Er/Yb samples. The maximum relative sensitivity reaches up to 5.99% K−1 at cryogenic temperature of 153 K. After irradiation with commercial laser of 405 nm for 30 s, the relative sensitivity is increased to 6.81% K−1. The improvement is verified to originate from the coupling of optical thermometric and photochromic behaviour at the elevated temperatures. The strategy may open up a new avenue to improve the thermometric sensitivity in photo-stimuli response photochromic materials.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Nowadays, the need for high spatial resolution and non-intrusive thermometric in biomedicine, industrial production and harsh environments (corrosive environments, high magnetic fields and strong radiation, etc.) has increased. Non-invasive thermometric techniques are gradually attracting attention [1–5]. As one of the non-contact thermometric strategies, fluorescence intensity ratio (FIR)-based luminescence thermometry is considered to be the most promising method for large-scale applications due to its fast response, high spatial resolution and non-invasiveness [6].
In past few years, the thermally coupled energy levels (TCLs) of rare-earth (RE) ion-doped upconversion luminescent materials have been used extensively in FIR-based temperature sensing and have driven the development of this technology [7,8]. However, the limited distance (about 200-2000 cm−1) between the two thermally coupled energy levels results in significant spectral overlap and low sensitivity [9]. At the same time, the non-thermally coupled energy levels (N-TCLs) can avoid the disadvantages of TCLs due to their large energy gap. Based on this consideration, many groups investigated temperature sensing behavior by constructing the N-TCLs of RE doped luminescent materials [10,11], allowing for a further increase in sensitivity. In these studies, main methods to improve the sensitivity of the N-TCLs focused on the selection of suitable luminescent ions and hosts, or structure design [12–14]. For example, Suo et al. investigated the effect of local symmetry distortion of RE2O3 (RE = Lu, Y and La):Er3+/Yb3+ on luminescence thermometry [15]. The symmetric distortion of the local site can effectively improve the temperature sensitivity. In addition, Liu et al. investigated the temperature sensitivity of Ba3Y4O9 doped with different RE ions Ln3+/Tm3+/Yb3+ (Ln3+ = Ho3+/Er3+) [16]. As a result, the design of FIR-based temperature sensing is still confined to trial-and-error experimentation. In addition, there are few reports on cryogenic sensing. Therefore, it is essential to search for new strategies to further improve the sensitivity of cryogenic optical thermometry.
In our previous studies, it is found that the emission intensity of these N-TCLs or TCLs in upconversion luminescent materials could be precisely modulated by the photochromic (PC) reaction [17–19], and reverses by alternating light irradiation and thermal stimulus. When the materials are irradiated with UV light, and vacancy-related defects could trap electrons, forming colour centers. The presence of colour centers can well capture the photogenerated electrons or holes from rare-earth ions, and affect the Boltzmann distribution of local electrons in different wavelength regions, thus altering the luminescent emission. Finally, the irradiated materials would show different temperature dependence of emission intensity located at the N-TCLs or TCLs, compared to the materials without irradiation, as shown in Fig. 1(a). These results indicate that the PC processes can modulate the temperature sensing behavior of upconversion luminescent materials. However, regulating the temperature sensing properties is limited for TCLs by PC reactions, due to the small difference of luminescent modulation contrast between two coupled energy levels, as reported in recently reported results [20]. With regard to N-TCLs, the temperature sensing properties are not restricted, based on the larger difference of luminescent modulation contrast between two energy levels. Therefore, it is proposed that the temperature sensing can be further improved by controlling PC process.
Fig. 1. Mechanism diagram of PC regulate of temperature sensor and samples morphology characterization. (a) Schematic diagram of the temperature dependence of the emission intensity at different wavelengths before and after light irradiation. (b) Schematic diagram of the coupling of optical thermometry and PC response of NBTa: Er/Yb materials. (c) XRD patterns and refined XRD peaks of the NBTa: 0.02Er/xYb powder samples with 2θ=20-80. (d) Schematic of crystal structure for NBTa: Er/Yb samples. (e) SEM image of the representative sample (x = 0.14). (f)-(k) Elemental mappings of O, Bi, Na, Er, Yb and Ta of the sample (x = 0.14), respectively.
In this work, we have successfully coupled PC reaction with optical thermometry in a novel multicolor upconversion materials: Na0.5Bi2.5Ta2O9: Er3+/Yb3+ (NBTa: Er/Yb), as shown in Fig. 1(b). The maximum sensitivity was increased from 5.99% K−1 (Sr), 0.0167 K−1 (Sa) to 6.81% K−1 (Sr), 0.0194 K−1 (Sa) after 405 nm irradiation for 30 s. The results show that dynamically increasing sensitivity through PC reaction is ideal for future developing high-performance temperature sensing materials.
2. Experimental section
2.1 Materials synthesis
All samples were prepared by the high temperature solid phase method. First, the standard chemical formula for this experiment was designed: Na0.5Bi2.48-xTa2O9: 0.02 mol Er3+/ x Yb3+ (x = 0.05, 0.08, 0.11, 0.14, 0.17, 0.2 mol). High purity metal oxides were used as raw materials: Ta2O5 (99.85%, Alfa Aesar), Bi2O3 (99.975%, Alfa Aesar), Na2CO3 (99.5%, Alfa Aesar), Er2O3 (99.9%, Alfa Aesar) and Yb2O3 (99.9%, Alfa Aesar). The powders were weighed according to stoichiometric ratios, mixed with anhydrous ethanol in an agate mortar and manually ground 3 times, then calcined in air at 1000°C for 2 h. The calcined powder was again added to the agate mortar and ground twice with anhydrous ethanol, then 8 wt% of PVA was dropped into the powder for granulation. Samples with a diameter of 12 mm and a thickness of 1 mm were manufactured in a mould at a pressure of 8 MPa. The prepared samples were kept at 550°C for 5 h to completely eliminate the PVA. Finally, all samples were sintered in air at 1200°C for 2 h.
2.2 Performance characterization
The phase structure of the samples was determined using a Cu target Kα radiation powder X-ray diffractometer (XRD, D8 Advanced, Bruker, Germany) with a scanning step of 0.02°. The surface morphology, microstructure and elemental composition of the samples were analysed by field emission scanning electron microscopy (SEM, Quanta 400, CENESIS) and energy spectrometry (EDS). Ultraviolet/Visible spectrophotometer (U-3900, HITACHI, Japan) was used to measure the diffuse reflectance spectrum. A fluorescence spectrophotometer (F-4600, HITACHI, Japan) equipped with a heating stage was used to measure upconversion (UC) and photoluminescence spectra at room temperature. The 980 nm excitation source used in the measurements was excited by a diode laser (100, Zolix, China). The PC reaction was performed by continuous irradiation of 405 nm (200 mW) laser for 30 s. The SL18 thermoluminescence meter was used to detect the thermoluminescence (TL) spectrum, and the detection temperature range was 298 K to 523 K.
3. Results and discussion
3.1 Phase and microstructure
The X-ray diffraction patterns of the prepared NBTa: Er/Yb ceramic samples are shown in Fig. 1(c). The main diffraction peak positions can be indexed to standard PDF cards (PDF#49-0609). In previous studies, NBTa has been shown to have the same crystal structure as SrBi2Ta2O9 (orthorhombic, space group A21am) [21,22], as shown in Fig. 1(d). As the concentration of doped Yb3+ ions increased, the secondary phase of YbTaO4 (PDF#24-1416) was gradually generated (Fig. 1(c)). All samples exhibit excellent crystallinity and density (Fig. S1(a)-(f), Supporting Information). As shown in Fig. 1(e), the SEM photograph of the representative sample with x = 0.14 shows a typical plate-like microstructure. The elemental mapping confirms well that all the elements are uniformly distributed in the host, as shown in Fig. 1(f)-(k) From the elemental content of the energy dispersive spectrometer (EDS) analysis (Fig. S1(g)), it can be inferred that the prepared material is consistent with the XRD analysis.
3.2 UC luminescence, photochromic and UC luminescence modulation
Figure 2(a) shows the UC emission spectra of all the samples under 980 nm excitation. Three representative emission peaks can be seen in the emission spectra, the peaks at 525 nm, 550 nm and 659 nm are coming from the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions of the Er3+ ion, respectively, as shown in the energy level diagram (Fig. 2(e)). As the concentration of Yb3+ ions increases, the intensity of green light gradually decreases, while the intensity of red light increases and then decreases, and an optimized concentration occurs at x = 0.14, as shown in Fig. 2(b). In fact, the enhanced UC emission is attributed to the increased concentration of Yb3+ ions, and the large absorption cross section of Yb3+ ions overlaps with the Er3+ ions energy level, facilitating energy transfer (Yb3+ → Er3+) [23,24]. However, the excessive Yb3+ ions concentration leads to the energy back-transfer (EBT) [25], which reduces the intensity of the UC emission. In order to reveal the process of photon transitions during UC, the UC emission spectra were examined at different pump powers for a typical sample (x = 0.14). As can be seen in Fig. 2(c), the luminous intensity (I) has a strong dependence on the pump power (P). The relationship between LnI and LnP is shown in Fig. 2(d). Based on the relationship between I and P (${I_{up}} \propto {P^n}$) [26], n values can be calculated for the green emission bands of n = 1.92 (525 nm) and n = 1.85 (550 nm) and the red emission band of n = 2.00 (659 nm). This suggests that two-photon adsorption processes dominate the UC emission of the NBTa host material.
Fig. 2. UC luminescence properties of NBTa: Er/Yb ceramic materials. (a) Room temperature UC emission spectra of NBTa: 0.02Er/xYb ceramic samples upon 980 nm. (b) The dependence of the UC emission intensity located at 550 and 659 nm on Yb3+ concentrations. (c) Pumping power dependent UC emission spectra of the samples (x = 0.14). (d) The pumping power dependence of green (525 nm and 550 nm) and red UC (659 nm) emission intensity for the sample (x = 0.14). (e) The energy level diagram and energy transition mechanism of Yb3+ and Er3+ ions.
Figure 3(a) shows the original and irradiated photographs of all samples in the dark field under 980 nm excitation. As the concentration of Yb3+ ions increased, the colour of the sample UC luminescence changes from green to yellow, corresponding chromaticity diagram and colour coordinate values are shown in Fig. S2 and Table S1. The colour change of UC emission can be well verified by measuring the UV-Vis absorption and UC luminescent spectra, as shown in Fig. 3(b) and (d). Figure 3(b) and Fig. S3 show the diffuse reflectance spectra and the corresponding photographs of all samples before and after irradiation at 405 nm for 30 s. After irradiation, the colour of the NBTa: Er/Yb ceramics changed from pale yellow to dark brown (the inset of Fig. 3(b)), accompanying a significant decrease in the reflectance intensity. And, the colour variation is reversible by alternating light and thermal stimulus, showing typical photochromic (PC) behavior. A parameter of ΔAbs represents the degree of light absorption as shown Eq. (1):
where Ab and Aa represent the reflection ratio before and after irradiation (405 nm, 30 s), respectively. The relationship between the maximum ΔAbs and the Yb3+ ion content are shown in Fig. 3(c). The absorption peaks are located at about 562 nm, and the maximum absorbance reached 18.8% when the Yb3+ ions concentration was 0.08 mol. As the Yb3+ ions concentration continues to increase, the absorbance gradually decreases. Notably, the ΔAbs value can be preserved after 10 reversible cycles under 405 nm irradiation and heating (200 °C, 5 min) (Fig. S4), demonstrating excellent reproducibility.Fig. 3. UC luminescent photographs and UC emission spectra of samples. (a) UC luminescence photographs of NBTa: 0.02Er/xYb ceramic samples before and after irradiation (405 nm) in dark field at room temperature. (b) Reflection-spectral changes of the sample (x = 0.14) before and after 405 nm irradiation for 30 s; the inset is photograph of colour changes before (left) and after (right) irradiation. (c) ΔAbs values located at 562 nm for the sample (x = 0.14). (d) UC emission spectral changes before and after 405 nm irradiation of the sample (x = 0.14); the inset shows the photographs of colour changes before and after irradiation in the dark field. (e) ΔRt variation (525, 550 and 659 nm) of the sample (x = 0.14) upon alternating 405 nm (30 s) irradiation and thermal bleaching (200 °C, 5 min) under 10 reversible cycles.
Figure 3(d) shows UC emission spectra of the sample with x = 0.14 before and after 405 nm irradiation for 30 s. It is found that the UC emission intensity decreased obviously, and the brightness weakened (the inset of Fig. 3(d)). The luminescence-modulation degree (ΔRt) can be calculated by the following formula:
where Rb and Ra are the intensity of UC luminescence before and after irradiation (405 nm, 30 s), respectively. Detailed results of the degree of modulation at 525, 550 and 659 nm for all samples are listed in Table S2. Taking a typical sample (x = 0.14), the modulation at 550 nm (62.6%) is much greater than at 525 nm (56.2%) and 659 nm (45.5%). The reason for this phenomenon is that the degree of luminescence modulation depends on the PC degree (ΔAbs) [27], as shown in Fig. S5. Meanwhile, ΔRt values at different temperature points from low to high temperature were measured by in-situ measurement, as shown in Fig. S6. Importantly, the ΔRt values at low temperature are far more than that at room temperature (303 K), which may be helpful to tune temperature sensing behavior at low temperature. The incomplete recovery of luminescence intensity at 483 K is due to the short duration time (30 s) at each temperature points. Similarly, 10 reversible cycles of ΔRt values were performed by alternating PC and bleaching processes under 405 nm irradiation and heating (200 °C, 5 min), as shown in Fig. 3(e). The sample exhibits excellent good fatigue resistance, as well as ΔAbs.3.3 Optical temperature sensing
Figure 4 shows the temperature-dependent (153-483 K) UC spectra of NBTa: Er/Yb (x = 0.14) before and after irradiation. The dependence of 2H11/2 → 4I15/2 (525 nm) and 4F9/2 → 4I15/2 (659 nm) transitions on temperature is obviously different, which can be seen intuitively from Fig. 4(a) and (b). With the increase of temperature, the UC luminescence intensity at 525 nm gradually increased and reached the maximum at 483 K without thermal quenching. This can be attributed to the phenomenon of phonon-assisted population inversion for Er3+ in NBTa: Er/Yb sample, which increases the population of the 2H11/2 state as the temperature rises to 483 K [28]. However, the UC luminescence intensity at 659 nm reached a maximum at 243 K and then gradually decreased. The above trends are similar to that of the sample after irradiation. The mapping of the temperature dependence of the upconversion emission spectrum of NBTa: Er/xYb (x = 0.14), as shown in Fig. 4(c) and (d). The standout is that the luminescence stability is significantly enhanced after irradiation while this is realized at the cost of luminescence intensity. The temperature dependence of luminescent emission intensity located at 525 nm and 659 nm before and after irradiation can be seen in Fig. 4(e) and (f), respectively. This particular phenomenon is attributed to the gradual release of electrons from the traps of the colour centres during the heating process. The increase in temperature dominated the reversible process of PC bleaching. During this process, the released electrons compensate for the thermal quenching of UC luminescence and thus a new equilibrium is reached dynamically.
Fig. 4. Temperature-dependent upconversion luminescence spectra of the NBTa: Er/xYb (x = 0.14) sample. (a) and (c) before irradiation at 153-483 K, (b) and (d) after irradiation (405 nm, 30 s) at 153-483 K. (e) and (f) The temperature-dependent emission intensity before and after irradiation, respectively.
In NBTa: Er/Yb ceramic samples, the 2H11/2 → 4I15/2 (525 nm) and 4F9/2 → 4I15/2 (659 nm) transitions of the Er3+ ions are selected as the N-TCLs thermometry signal. The energy gap between these two energy levels is approximately 4000 cm−1, which far exceeds the value of the energy gap of the TCL (200-2000cm−1). From the temperature-dependent UC emission spectra collected above, the non-thermally coupled fluorescence intensity ratio (FIR) corresponding to 525 nm (2H11/2 →4I15/2) and 659 nm (4F9/2 →4I15/2) can be calculated by the following equation [29]:
Fig. 5. Temperature sensing properties of the NBTa: Er/xYb (x = 0.14) sample. (a) The dependence of FIR on temperature before and after irradiation. (b) The absolute sensitivities (Sa) before and after irradiation. (c) The relative sensitivities (Sr) before and after irradiation. (d) Temperature uncertainty (σT) of the sample under different temperatures ranging from 153 to 483 K before and after irradiation. (e) Comparison of the relative sensitivity (Sr) of NBTa: Er/Yb and some representative Er/Yb co-doped materials.
Correspondingly, Sa and Sr variations before and after irradiation are shown in Fig. 5(b) and (c). It can be seen that the maximum values of Sa and Sr are significantly increased after irradiation. For a typical sample (x = 0.14), the maximum values of Sa before and after irradiation are 0.0078 K−1 and 0.0088 K−1, respectively, and the maximum values of Sr are 5.99% K−1 and 6.22% K−1, respectively. At the same time, the lowest temperature uncertainty is 0.64 K (483 K), which can meet the requirements for accurate temperature measurement. The effect of temperature uncertainty on sensitivity can be seen more visually from the change in σT before and after irradiation, as shown in Fig. 5(d) and Fig, S8. After irradiation, the temperature uncertainty decreases, corresponding to an increase in Sr, as shown in Fig. S10. The Sa and Sr before and after irradiation for all samples are shown in Fig. S9 and S10, and the detailed calculations are listed in Table S3. Comparison of the relative sensitivity of NBTa: Er/Yb and some representative Er/Yb co-doped inorganic temperature sensing materials is shown in Fig. 5(e). The relative sensitivity (Sr) (before and after irradiation) of this work is superior to recently reported results, as shown in Table 1. These results indicate potential applications of PC-modulated NBTa: Er/Yb materials for wide temperature range in optical thermometry.
Table 1. Maximum Sa and Sr value of the optical thermometer based on Er/Yb co-doped luminescent materials.
3.4 Mechanisms of luminescence and temperature sensing modulation
The coupling relationship between PC and temperature sensing can be rationally explained using a simplified schematic, as shown in Fig. 6. In this work, during the high temperature sintering of NBTa: Er/Yb ceramic materials, a large amount of Na+ and Bi3+ ions would volatilize to form defects such as metal ions vacancies and oxygen vacancies. According to our previously reported results, the PC phenomenon is related to the above-mentioned vacancy-related defects [40]. To investigate the trap distribution of the samples, the thermoluminescence (TL) curves are shown in Fig. S11. According to the TL multi-peak fitting method [17,41], the TL curve of the sample can be fitted with three sub-peaks, representing three different defect energy levels. The intensity and position of the peaks represent the density and depth of the traps, respectively. The depth of the defect energy levels represented by the peaks of the subpeaks can be calculated by the Hoogenstraaten method [42]:
where ΔE and Tm (K) represent the depth of defect level and the temperature corresponding to the TL fitting peak, respectively. The specific values of trap depth (ΔE), density (D) and area (A) for the NBTa and NBTa: Er/Yb (x = 0.14) ceramic samples are listed in Table S4. Based on the depth of the traps, the fitted traps of types 1 and 2 are classified as shallow traps, and 3 is classified as deep traps. It is noteworthy that the proportion occupied by shallow traps (ST) is much larger than that of deep traps (DT) in the NBTa samples, while the proportion of deep traps increases after doping with RE ions, as shown in Fig. S11 (a) and (b). It shows that the doping of RE ions increases the DT density of the sample.Fig. 6. Schematic diagram of the coupling relationships between PC and temperature sensing performance in NBTa: Er/Yb samples.
When irradiated by a 405 nm laser, a large number of electrons are trapped by defects to form colour centres. This also resulted in apparent colour changes in the UV-Vis reflectance of the samples (Fig. 3). At the same time, the large absorption cross-section of Yb3+ ions under 980 nm laser excitation can effectively transfer energy to Er3+ ions to significantly enhance emission and form the luminescence centres. After irradiation of 405 nm, the overlap between the absorption band and the emission band of UC emission causes electron transitions from the luminescent centres to the colour centres via resonant energy transfer (RET) [43], resulting in the luminescent quenching. As the temperature increases, the electrons gradually escape from the traps. During this process, the reverse transfer of energy occurs through RET, which inhibits the temperature-induced quenching to a certain extent, thereby dynamically optimizing the stability of luminescence. The degree of luminescent regulation and different response to temperature stimuli of the non-thermally coupled energy levels (2H11/2, 4F9/2) result in a large difference in FIR before and after 405 nm irradiation. Based on the above results, PC can significantly enhance the fitting values of FIR, and also greatly improve the sensitivity of thermometric. It is worth mentioning that the reversible process of PC and bleaching does not affect the temperature-dependent UC emission properties of the activator. These features could lead to a wider range of applications for NBTa-based materials in some fields, such as optical temperature sensing, optical switching and optical information storage.
Conclusions
In this work, a series of Na0.5Bi2.5Ta2O9:Er/Yb PC ceramics with multicolour UC emission (green to yellow) were successfully prepared by a high temperature solid phase method. A new strategy to improve the temperature sensing performance was proposed by coupling the PC reaction with optical thermometry. The construction of a N-TCLs (2H11/2 → 4I15/2, 4F9/2 → 4I15/2) resulted in maximum relative sensitivities (Sr) of 5.99% K−1 (original) and 6.81% K−1 (irradiation). Meanwhile, the samples showed excellent fatigue resistance between PC (405 nm irradiation) and bleaching (thermal stimulation). The results confirm that the new strategy proposed in this paper provides a new way of further improving the sensitivity of optical thermometry.
Funding
National Natural Science Foundation of China (51802164, 52062042); the Natural Science Foundation of Inner Mongolia (2020MS05044).
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.
Supplemental document
See Supplement 1 for supporting content.
References
1. S. A. Hilderbrand, F. Shao, C. Salthouse, U. Mahmood, and R. Weissleder, “Upconverting luminescent nanomaterials: application to in vivo bioimaging,” Chem. Commun. 28(28), 4188–4190 (2009). [CrossRef]
2. Z. Li, H. Yuan, W. Yuan, Q. Su, and F. Li, “Upconversion nanoprobes for biodetections,” Coord. Chem. Rev. 354, 155–168 (2018). [CrossRef]
3. A. Siaï, P. Haro-González, K. Horchani-Naifer, and M. Férid, “La2O3: Tm, Yb, Er upconverting nano-oxides for sub-tissue lifetime thermal sensing,” Sens. Actuators, B 234, 541–548 (2016). [CrossRef]
4. O. S. Wolfbeis, “An overview of nanoparticles commonly used in fluorescent bioimaging,” Chem. Soc. Rev. 44(14), 4743–4768 (2015). [CrossRef]
5. P. Zhou, Q. Zhang, F. Peng, B. Sun, X. Dou, B. Liu, D. Han, Y. Xue, and K. Ding, “Optical properties of Nd3+ ions doped GdTaO4 for pressure and temperature sensing,” J. Rare Earths 40(6), 870–877 (2022). [CrossRef]
6. X. D. Wang, O. S. Wolfbeis, and R. J. Meier, “Luminescent probes and sensors for temperature,” Chem. Soc. Rev. 42(19), 7834 (2013). [CrossRef]
7. W. Ran, H. M. Noh, S. H. Park, B. R. Lee, J. H. Kim, J. H. Jeong, and J. Shi, “Application of thermally coupled energy levels in Er3+ doped CdMoO4 phosphors: Enhanced solid-state lighting and non-contact thermometry,” Mater. Res. Bull. 117, 63–71 (2019). [CrossRef]
8. G. Xiang, Q. Xia, X. Liu, and X. Wang, “Optical thermometry based on the thermally coupled energy levels of Er3+ in upconversion materials,” Dalton Trans. 49(47), 17115–17120 (2020). [CrossRef]
9. Y. Cheng, Y. Gao, H. Lin, F. Huang, and Y. Wang, “Strategy design for ratiometric luminescence thermometry: circumventing the limitation of thermally coupled levels,” J. Mater. Chem. C 6(28), 7462–7478 (2018). [CrossRef]
10. P. Du, Y. Hua, and J. S. Yu, “Energy transfer from VO43− group to Sm3+ ions in Ba3(VO4)2:3xSm3+ microparticles: A bifunctional platform for simultaneous optical thermometer and safety sign,” Chem. Eng. J. 352, 352–359 (2018). [CrossRef]
11. S. Liu, L. Yan, J. Huang, Q. Zhang, and B. Zhou, “Controlling upconversion in emerging multilayer core-shell nanostructures: from fundamentals to frontier applications,” Chem. Soc. Rev. 51(5), 1729–1765 (2022). [CrossRef]
12. Q. Xiao, X. Yin, L. Lv, X. Dong, N. Zhou, K. Liu, and X. Luo, White up-conversion luminescence and highly-sensitive optical temperature sensing in Na3La(VO4)2:Yb,Er,Tm, Ho phosphors, J. Rare Earths (2022).
13. Y. Wang, C. Zuo, C. Ma, W. Ye, C. Zhao, Z. Feng, Y. Li, Z. Wen, C. Wang, X. Shen, X. Yuan, and Y. Cao, “Effects of Sc3+ ions on local crystal structure and up-conversion luminescence of layered perovskite NaYTiO4: Yb3+/Er3+,” J. Alloys Compd. 876, 160166 (2021). [CrossRef]
14. Y. Wang, Y. Wang, C. Ma, Z. Feng, C. Zuo, W. Ye, C. Zhao, Y. Li, Z. Wen, Z. Cao, Z. Cao, X. Shen, C. Wang, Y. Li, X. Yuan, and Y. Cao, “Excellent cryogenic optical thermometry based on green up-conversion of Ho3+-doped perovskite Na0.5Bi0.5TiO3 ceramics,” J. Mater. Chem. C 9(4), 1353–1361 (2021). [CrossRef]
15. H. Suo, X. Zhao, Z. Zhang, R. Shi, Y. Wu, J. Xiang, and C. Guo, “Local symmetric distortion boosted photon up-conversion and thermometric sensitivity in lanthanum oxide nanospheres,” Nanoscale. 10(19), 9245–9251 (2018). [CrossRef]
16. S. Liu, J. Cui, J. Jia, J. Fu, W. You, Q. Zeng, Y. Yang, and X. Ye, “High sensitive Ln3+/Tm3+/Yb3+ (Ln3+ = Ho3+, Er3+) tri-doped Ba3Y4O9 upconverting optical thermometric materials based on diverse thermal response from non-thermally coupled energy levels,” Ceram. Int. 45(1), 1–10 (2019). [CrossRef]
17. Z. Zhang, L. Guo, H. Sun, D. Peng, H. Zou, N. Sun, Q. Zhang, and X. Hao, “Rare earth orthoniobate photochromics with self-activated upconversion emissions for high-performance optical storage applications,” J. Mater. Chem. C 9(39), 13841–13850 (2021). [CrossRef]
18. Y. Zhu, H. Sun, Q. Jia, L. Guan, D. Peng, Q. Zhang, and X. Hao, “Site-Selective Occupancy of Eu2+ toward High Luminescence Switching Contrast in BaMgSiO4-Based Photochromic Materials,” Adv. Opt. Mater. 9(6), 2001626 (2021). [CrossRef]
19. P. Li, Z. Zhang, X. Gao, H. Sun, D. Peng, H. Zou, Q. Zhang, and X. Hao, “Fast self-bleaching Nb2O5-based photochromics for high security dynamic anti-counterfeiting and optical storage applications,” Chem. Eng. J. 435, 134810 (2022). [CrossRef]
20. X. Li, L. Guan, Y. Li, H. Sun, Q. Zhang, and X. Hao, “Optical control of Er3+-doped M0.5Bi2.5Nb2O9 (M = Li, Na, K) materials for thermal stability and temperature sensing using photochromic reactions,” J. Mater. Chem. C 8(44), 15685–15696 (2020). [CrossRef]
21. H. Takeda, R. Aoyagi, M. Matsushita, and T. Shiosaki, “Crystal structure and electromechanical coupling properties of Na0.5Bi2.5Ta2O9 dense ceramics,” Appl. Phys. A: Mater. Sci. Process. 76(2), 295–297 (2003). [CrossRef]
22. Y. Shimakawa, Y. Kubo, Y. Nakagawa, T. Kamiyama, H. Asano, and F. Izumi, “Crystal structures and ferroelectric properties of SrBi2Ta2O9 and Sr0.8Bi2.2Ta2O9,” Appl. Phys. Lett. 74(13), 1904–1906 (1999). [CrossRef]
23. H. Q. Wang, M. Batentschuk, A. Osvet, L. Pinna, and C. J. Brabec, “Rare-earth ion doped up-conversion materials for photovoltaic applications,” Adv. Mater. 23(22-23), 2675–2680 (2011). [CrossRef]
24. N. Song, S. Liu, P. Zhang, J. He, Q. Zhang, F. Wang, and B. Zhou, “Enhancing upconversion of Nd3+ through Yb3+-mediated energy cycling towards temperature sensing,” J. Rare Earths 39(12), 1506–1511 (2021). [CrossRef]
25. Q. Zhang, S. Yue, H. Sun, X. Wang, X. Hao, and S. An, “Nondestructive up-conversion readout in Er/Yb co-doped Na0.5Bi2.5Nb2O9-based optical storage materials for optical data storage device applications,” J. Mater. Chem. C 5(15), 3838–3847 (2017). [CrossRef]
26. H. Sun, Y. Lv, Y. Zhu, J. Lin, X. Wu, Q. Zhang, and X. Hao, “Photochromism-induced light scattering and photoswithing in Er doped (K,Na)NbO3 transparent ceramics,” J. Am. Ceram. Soc. 102(11), 6732–6740 (2019). [CrossRef]
27. Z. Yang, J. Du, L. I. D. J. Martin, A. Feng, E. Cosaert, B. Zhao, W. Liu, R. Van Deun, H. Vrielinck, and D. Poelman, “Designing Photochromic Materials with Large Luminescence Modulation and Strong Photochromic Efficiency for Dual-Mode Rewritable Optical Storage,” Adv. Opt. Mater. 9(20), 2100669 (2021). [CrossRef]
28. Y. Liu, G. Bai, Y. Lyu, Y. Hua, R. Ye, J. Zhang, L. Chen, S. Xu, and J. Hao, “Ultrabroadband Tuning and Fine Structure of Emission Spectra in Lanthanide Er-Doped ZnSe Nanosheets for Display and Temperature Sensing,” ACS Nano 14(11), 16003–16012 (2020). [CrossRef]
29. Y. Wang, L. Lei, R. Ye, G. Jia, Y. Hua, D. Deng, and S. Xu, “Integrating Positive and Negative Thermal Quenching Effect for Ultrasensitive Ratiometric Temperature Sensing and Anti-counterfeiting,” ACS Appl. Mater. Interfaces 13(20), 23951–23959 (2021). [CrossRef]
30. Y. Gao, F. Huang, H. Lin, J. Zhou, J. Xu, and Y. Wang, “A Novel Optical Thermometry Strategy Based on Diverse Thermal Response from Two Intervalence Charge Transfer States,” Adv. Funct. Mater. 26(18), 3139–3145 (2016). [CrossRef]
31. M. Wu, D. Deng, F. Ruan, B. Chen, and S. Xu, “A spatial/temporal dual-mode optical thermometry based on double-sites dependent luminescence of Li4SrCa(SiO4)2: Eu2+ phosphors with highly sensitive luminescent thermometer,” Chem. Eng. J. 396, 125178 (2020). [CrossRef]
32. J. Tang, P. Du, W. Li, and L. Luo, “Boosted thermometric performance in NaGdF4:Er3+/Yb3+ upconverting nanorods by Fe3+ ions doping for contactless nanothermometer based on thermally and non-thermally coupled levels,” J. Lumin. 224, 117296 (2020). [CrossRef]
33. F. Huang, T. Yang, S. Wang, L. Lin, T. Hu, and D. Chen, “Temperature sensitive cross relaxation between Er3+ ions in laminated hosts: a novel mechanism for thermochromic upconversion and high performance thermometry,” J. Mater. Chem. C 6(45), 12364–12370 (2018). [CrossRef]
34. J. Zhang, Y. Zhang, X. Li, Z. Zhai, and Z. Hua, “KBaYSi2O7:Yb3+-Er3+/Ho3+ phosphors: Optical temperature sensing materials of high sensitivity,” J. Lumin. 227, 117562 (2020). [CrossRef]
35. W. Liu, X. Wang, Q. Zhu, X. Li, X. Sun, and J. G. Li, “Upconversion luminescence and favorable temperature sensing performance of eulytite-type Sr3Y(PO4)3:Yb3+/Ln3+ phosphors (Ln = Ho, Er, Tm),” Sci. Technol. Adv. Mater. 20(1), 949–963 (2019). [CrossRef]
36. Y. K. Kshetri, B. Chaudhary, T.-H. Kim, H. S. Kim, and S. W. Lee, “Yb/Er/Ho-α-SiAlON ceramics for high-temperature optical thermometry,” J. Eur. Ceram. Soc. 41(4), 2400–2406 (2021). [CrossRef]
37. M. Vega, I. R. Martin, E. Cortés-Adasme, and J. Llanos, “Enhanced red up-conversion emission in Er3+/Yb3+ co-doped SrSnO3 for optical temperature sensing based on thermally and non-thermally coupled levels,” J. Lumin. 244, 118687 (2022). [CrossRef]
38. A. A. Kalinichev, M. A. Kurochkin, A. Y. Kolomytsev, R. S. Khasbieva, E. Y. Kolesnikov, E. Lähderanta, and I. E. Kolesnikov, “Yb3+/Er3+−codoped GeO2–PbO–PbF2 glass ceramics for ratiometric upconversion temperature sensing based on thermally and non-thermally coupled levels,” Opt. Mater. 90, 200–207 (2019). [CrossRef]
39. P. Du, X. Sun, Q. Zhu, and J.-G. Li, “Garnet-structured Li6CaLa2Nb2O12:Yb/Er new phosphor showing superior performance of optical thermometry,” Scr. Mater. 185, 140–145 (2020). [CrossRef]
40. Q. Zhang, H. Sun, X. Wang, X. Hao, and S. An, “Reversible Luminescence Modulation upon Photochromic Reactions in Rare-Earth Doped Ferroelectric Oxides by in Situ Photoluminescence Spectroscopy,” ACS Appl. Mater. Interfaces 7(45), 25289–25297 (2015). [CrossRef]
41. Y. Zhuang, D. Tu, C. Chen, L. Wang, H. Zhang, H. Xue, C. Yuan, G. Chen, C. Pan, L. Dai, and R. J. Xie, “Force-induced charge carrier storage: a new route for stress recording,” Light: Sci. Appl. 9(1), 182 (2020). [CrossRef]
42. G. Kitis, J. M. Gomez-Ros, and J. W. N. Tuyn, “Thermoluminescence glow-curve deconvolution functions for first, second and general orders of kinetics,” J. Phys. D: Appl. Phys. 31(19), 2636–2641 (1998). [CrossRef]
43. C. Zhou, Y. Zhao, L. Shang, Y. Cao, L. Z. Wu, C. H. Tung, and T. Zhang, “Facile preparation of black Nb4+ self-doped K4Nb6O17 microspheres with high solar absorption and enhanced photocatalytic activity,” Chem. Commun. 50(67), 9554 (2014). [CrossRef]
