Open Access
Add to BibSonomy
Abstract
Recently, single-band ratiometric (SBR) thermometry becomes a hot-spot in the research field of optical thermometry. Here we propose a new SBR thermometry by combining the temperature-induced red shift of charge transfer state (CTS) of W-O and Eu-O with the ground state absorption (GSA) and excited state absorption (ESA) of Eu3+. The emitting intensity of the 5D0-7F2 transition of Eu3+ is monitored under CTS, GSA and ESA excitations at different temperatures. It is found that the SBR thermometry, depending on the combination of [GSA + CTS] of Eu3+ doped calcium tungstate, has the highest relative sensitivity of 1.25% K−1 at 573 K, higher than conventional luminescent ratiometric thermometry such as the 2H11/2 and 4S3/2 thermally coupled states of Er3+.
© 2021 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Temperature is one of the most significant parameters in many fields, such as in industry, engineering, life science and so forth [1–3]. It thus makes temperature measurement become extremely important. In general, temperature measurement methods can be classified into contact and contactless types [4,5]. Compared to the conventional temperature sensors such as the representative thermocouple, thermal resistance and mercurial thermometer, optical temperature sensors, especially those based on the emission of luminescent materials, have gained ever-increasing attention over recent decades [6–10]. It is because these thermometers can detect temperature of object in a non-invasive mode, without the need of generating disturbance on the thermal distribution of object [11,12].
Up to now, it has been confirmed that several characteristics, mainly including intensity, lifetime, position, full width at half maximum, polarization, intensity ratio, etc., are all sensitive to the change of temperature [1,13–16]. Therefore, scientists have proposed a number of optical strategies for accurate and non-invasive temperature sensing. Among all these methods, luminescence intensity ratio (LIR)-based thermometry has emerged as a promising optical method [17–19]. As the LIR-based thermometry depends on the intensity ratio of two emission lines of luminescent materials, it has strong immunity to many factors that affect the accuracy of temperature sensing such as inhomogeneity of luminescent centres, distance between detector and emitting centres, fluctuation of excitation source, and so forth [16,20]. Thus, the LIR-based thermometry has been widely investigated across a broad range of domains. Conventionally, this type of thermometry relies on two emission lines that originate from one pair of thermally coupled states, typically, the 2H11/2 and 4S3/2 states of Er3+. For instance, Marciniak’s group recently disclosed the influence of covalency of bonds between rare earth ions and ligand such as O2− on the relative thermal sensitivity of [2H11/2,4S3/2] pair of Er3+ [21]. Over recent years, scientists have also developed novel thermometry depending on the emissions from two different luminescent centres such as Eu3+ and Tb3+. All these temperature measurement methods are on the basis of one excitation and two emissions.
Very recently, a new kind of method has been proposed to detect temperature, that is, the so-called single-band ratiometric (SBR) thermometry [20]. In regard to this method, a single emission band is measured twice under two excitations with different wavelength. As this single emission band usually has opposite response to the change of temperature, the SBR-based thermometry generally has a good sensitivity. Moreover, this type of optical thermometry could optimize the necessary experimental setup, when compared with the LIR-based strategy for sensing temperature. In 2016, Souza et al. successfully realized SBR-based thermometry via Y2O3:Eu3+ nanoparticles [22]. They demonstrated that the emission of Eu3+ upon ground state absorption (GSA) and that obtained upon excited state absorption (ESA) could be utilized for ratiometric temperature sensing. In 2020, Trejgis et al. designed a new SBR-based thermometry via LiLaP4O12:Eu3+, indicating that it is possible to use the red emission of Eu3+ obtained upon two different excitation conditions to detect temperature [20].
Different from these methods, here we proposed a new type of SBR-based thermometry by combining the charge transfer state (CTS) and GSA of Eu3+. It was found that the Eu3+ could emit bright red photoluminescence (PL) upon exciting the CTS band, the GSA band (7F0→5L6 transition), as well as the ESA band (7F1→5D1 transition). Moreover, the red emission of Eu3+ presented totally different temperature response upon various excitation conditions. Based on these interesting results, several SBR-based thermometry depending on [GSA + ESA], [GSA + CTS] and [CTS + CTS] were constructed with success. In addition, the relative sensitivity of these methods was further calculated and it was confirmed that the combination of [GSA + CTS] owned the best performance in consideration of thermal sensitivity.
2. Materials and methods
CaWO4:Eu3+ powder samples were prepared by high temperature solid state method. CaCO3 (AR), WO3 (99.99%) and Eu2O3 (99.99%) were used as raw materials. Firstly, these raw materials were weighted according to the calculated stoichiometric ratio, which was then blended completely in an agate mortar with the aid of moderate alcohol. Next, these uniform mixtures were put in a corundum crucible and sintered at 1523 K for eight hours to form the final samples.
A Bruker-AXS-D8 X-ray diffractometer (XRD), Cu Kα (λ=0.15406 nm) was employed to characterize the phase and crystal structure of CaWO4:1%Eu3+. The PL and photoluminescence excitation (PLE) spectra of CaWO4:1%Eu3+ were measured by the equipment of Hitachi F-4600 spectrophotometer equipped with a xenon lamp (450 W). The temperature-dependent PL and PLE were measured through the same spectrophotometer of Hitachi F-4600. The temperature of the sample was controlled by a home-made temperature heating and cooling stage with the operating range from room temperature to 573 K.
3. Results and discussion
The XRD patterns of CaWO4:x%Eu3+ (x=1, 10 and 20) are presented in Fig. 1(a). As can be observed, the sample’s diffraction peaks agree well with the reference data of the standard scheelite-type CaWO4 (ICSD #60548), both in the aspect of peak position or relative intensity of each diffraction peak. What’s more, there are no redundant diffraction peaks belonging to the secondary crystal phase. Thus, we have synthesized the pure-phase scheelite-type CaWO4:x%Eu3+ (x=1, 10 and 20) sample, and Eu3+ ions, as luminescent centres, had triumphantly entered into the [Ca2+] site in CaWO4. Moreover, the strong intensity of diffraction peaks suggests CaWO4:x%Eu3+ (x=1, 10 and 20) samples were crystallized well. Figure 1(b) shows the crystal structure of the as-prepared CaWO4:Eu3+. The [W6+] site is surrounded by four O2-, and the [Ca2+, Eu3+] site is coordinated by eight O2-.
Figure 2(a) depicts the PLE spectrum of CaWO4:1%Eu3+ monitored at 618 nm which corresponds to the central emitting wavelength of 5D0→7F2 transition. One can see from this figure that the PLE bands cover a quite broad wavelength range from 200 to 550 nm, which could be divided into three parts, i.e., CTS band [23], GSA bands and ESA band [24]. The GSA bands are located at 362, 382, 394, 416 and 465 nm, which separately correspond to the 7F0→5D4, 7F0→5G2, 7F0→5L6, 7F0→5D3 and 7F0→5D2 transitions [25]. The excitation line at 536 nm is known to be from the 7F1→5D1 transition. As the 7F1 state is higher than the 7F0 ground state, so the 536 nm excitation line is assigned to the ESA band. Moreover, there is a broad excitation band ranging from 200 to 350 nm, which is known to stem from the CTS between W-O and that between Eu-O. The position of this broad CTS excitation band is strongly associated with the change of temperature, which will be presented in the next part. The presence of these three types of excitation bands indicates that the red emission of Eu3+ can be observed upon CTS, GSA and ESA excitation modes. Moreover, there is also an energy transfer from CTS to Eu3+ [26].
Fig. 2. (a) PLE spectrum of CaWO4:1%Eu3+ monitored at 618 nm. (b) PL spectrum of CaWO4:1%Eu3+ excited at 394 nm. (c) Energy level diagram of Eu3+.
Upon excitation at 394 nm, CaWO4:1%Eu3+ mainly emitted red luminescence, and the PL spectrum is shown in Fig. 2(b). As can be observed, the spectrum is composed of four lines peaking at 595, 618, 657 and 705 nm, respectively. They are separately confirmed to be from the 5D0→7F1, 5D0→7F2, 5D0→7F3 and 5D0→7F4 transitions of Eu3+ [27,28]. The 618 nm emission line (5D0→7F2) has the strongest emitting intensity among all emission lines, revealing unquestionably that the Eu3+ site has no inversion symmetry. And all these transitions have been clearly presented in Fig. 2(c).
Figure 3(a) and 3(b) present the PLE spectra of CaWO4:1%Eu3+ at various temperatures from 303 to 573 K by monitoring the 618 nm emission line’s intensity, and Fig. 3(c) shows the 618 nm emission line’s intensity as a function of temperature at different excitations. As clearly depicted, the bands of CTS, GSA and ESA display various responses to the change of temperature. Specifically, the left part (200-300 nm) of CTS band decreased sharply while the right wing at around 304 nm kept nearly unchanged with the rise of temperature, due to the red shift of CTS [29]. By comparison, the GSA bands ranging from 300 to 500 nm showed severe attenuation with gradually increasing the temperature from 303 to 573 K, which also could be clearly observed from Fig. 3(c). This is likely to be ascribed to the well-known thermal quenching effect. Different from the CTS and GSA bands, the ESA line, which is located at ∼536 nm, could effectively resist the emission quenching aroused by the increment of temperature. Even at 573 K, the 536 nm excitation band is also more than 60% of its original emitting intensity at 303 K, which could be attributed to the competition between the Boltzmann distribution and thermal quenching effect. With gradually increasing the temperature from 303 to 573 K, more and more Eu3+ ions at the 7F0 ground state jumped to the adjacent upper 7F1 excited state [30,31]. It means that more Eu3+ ions were excited to the 5D0 emitting state to generate luminescence, which partly resisted the thermal quenching effect caused by the ever-increasing nonradiative transition. At this point, we can state, depending on the temperature-dependent PLE spectra, including the CTS, GSA and ESA lines, that the dependence of the red emission of Eu3+ on temperature is strongly related with the excitation wavelength, meaning that the SBR-based thermometry is expected to be established via a rational selection of excitations.
Fig. 3. (a) PLE spectra of CaWO4:1%Eu3+ at 303 K, 423 K and 573 K monitored at 618 nm. (b) Temperature-dependent PLE spectra of CaWO4:1%Eu3+. (c) PL intensity of 618 nm band under different excitation cases over the temperature range of 303-573 K.
Figure 4 shows the PL spectra of CaWO4:1%Eu3+ at various temperatures from 303 to 573 K upon excitation at 278, 304, 394 and 536 nm, respectively. As can be observed, the 618 nm emission band always has a relatively good signal to noise ratio under four different excitation circumstances. We noticed that Souza et al. proposed a similar SBR-based thermometry with the use of Y2O3:Eu3+ [22]. However, they monitored the emission intensity of the weak 5D0→7F4 transition, which is not favour of a high signal to noise ratio and thus an excellent measurement accuracy. Here, we monitored the 618 nm emission line that owns the strongest intensity among all emission lines of Eu3+, which is benefit for the final measurement of temperature. For the sake of discussion, the intensity of the 618 nm emission line obtained upon four typical excitations at 278, 304, 394 and 536 nm is labelled as I278nm, I304nm, I394nm and I536nm respectively. Note that these four excitation wavelengths separately correspond to the left and right wings of CTS band, GSA band and ESA band. One can see that upon excitation at the left wing of CTS band and the GSA band, the 618 nm emission line decreased quickly. While in Fig. 4(b), at temperatures below 483 K, the 618 nm emission band always increased. And at higher temperatures, this emission line showed a slight downtrend, which is assigned to the red-shift of CTS band and overlap of the right wing of CTS band. Upon excitation at 536 nm, the 618 nm emission line first kept a fluctuation trend over the 303-483 K range, and with the further rise of temperature, it presented a downtrend due to the dominated thermal quenching effect over the Boltzmann distribution between the 7F0 and 7F1 states [22].
Fig. 4. PL spectra of CaWO4:1%Eu3+ at the temperatures from 303 to 573 K upon excitation at (a) 278 nm, (b) 304 nm, (c) 394 nm and (d) 536 nm, respectively. Note that the inset in each figure shows the integral intensity of the 618 nm emission line as a function of temperature in the 303-573 K range.
The strong difference in the thermal dependence of 618 nm (5D0→7F2) emission band upon CTS, GSA and ESA excited enables creation of a single band ratiometric luminescent thermometer in which LIR is defined as follows:
As depicted in Fig. 5(a), all the defined LIRs had been normalized to their values at 303 K. Obviously, each set of LIRs increased gradually with increasing the temperature from room temperature to 573 K. As there is a one-to-one relationship between the LIR and temperature, all the three types of LIRs could be used to monitor the change of temperature.
Fig. 5. (a) Thermal evolution of normalized LIR1, LIR2 and LIR3 over the 303-573 K temperature range. (b) Relative sensitivity of S1, S2, S3 corresponding to the SBR-based thermometry based on LIR1, LIR2 and LIR3. As a comparison, the relative sensitivity of the conventional thermometry based on the 2H11/2 and 4S3/2 states was also presented (dotted line).
In order to quantify the ability of these three types of SBR thermometry, the relative sensitivity is generally compared, which is defined as:
Depending on this equation, we calculated the relative sensitivities of the proposed SBR thermometry, and the results are depicted in Fig. 5(b). For convenience, the relative sensitivities for LIR1, LIR2 and LIR3 are labelled as S1, S2 and S3, respectively. In addition, the mostly studied strategy based on the [2H11/2,4S3/2] pair of Er3+, denoted by S(Er3+), is also shown in Fig. 5(b) as a comparison [22,32,33]. One can see that at the temperatures lower than 450 K, there is no obvious difference among S1, S2 and S3. With further increasing temperature from ∼450 K, both S1 and S2 show upward trend and are higher than S(Er3+). At 573 K, both S1 and S2 reach to the maximum relative sensitivity of 1.25% K−1 and 1.04% K−1, which are separately four- and three-fold of S(Er3+), demonstrating the feasibility and superiority of our proposed SBR thermometry.
Dopant ion concentration plays a key role for the relative sensitivity of the luminescent thermometry depending on SBR strategy. Figure 6 presents the comparison of relative sensitivity of three samples doped with different Eu3+ concentration (1%, 10% and 20% in molar ratio), depending on [GSA + ESA] strategy. As can be observed, at temperatures below ∼500 K, the higher the doping concentration of Eu3+, the lower the relative sensitivity. This phenomenon is likely to stem from the cross relaxation effect that has been intensively studied before. Specifically, as shown in Fig. 2(c), the cross relaxations of (5D1, 7F0)-(5D0, 7F3) and (5D2, 7F0)-(5D0, 7F5) may exist in the case of high doping Eu3+. These possible cross relaxations are expected to enhance the populations of the 5D0 state. Therefore, the rate of decay of the 5D0 state becomes slower upon the GSA excitation. The relative thermal sensitivity of the high-doping sample is thus smaller than that of the low-doping case. While at temperatures higher than ∼500 K, a higher doping concentration of Eu3+ is in favor of a better thermal sensitivity, contrary to the former case. This is probably an indirect evidence that the cross relaxation not only depends heavily on the dopant concentration, but also is dependent of temperature, which deserves further investigation in the future. Finally, aiming to make the reader to realize the current state of the art, a summary of various luminescent thermometers that are on the basis of SBR mechanism has been presented in Table 1. It can be concluded that in comparison with other work, our strategy presents superiority at relatively high temperatures.
Fig. 6. Relative sensitivity of S3, S3’, S3’’ corresponding to 1%, 10% and 20% Eu3+ doped CaWO4, depending on [GSA + ESA] strategy.
Table 1. Summary of various luminescent thermometers using the SBR strategy.
4. Conclusions
In this work, we demonstrate the totally different temperature response of the GSA, ESA, and CTS excited mechanisms when monitoring the red emission (5D0→7F2) of Eu3+. Depending on these unique properties, we successfully design three types of SBR thermometry. They separately rely on the combination of [GSA + ESA], [GSA + CTS] and [CTS + CTS] emission bands. The relative sensitivities of these three types of SBR thermometry are calculated and compared. It is shown that both the SBR of [GSA + CTS] and that of [CTS + CTS] have superiority over the mostly studied [2H11/2,4S3/2] pair of Er3+. The relative sensitivity of the SBR thermometry on the basis of [GSA + CTS] is as high as 1.25% K−1 at 573 K, which is fourfold higher than that of the [2H11/2,4S3/2] pair of Er3+. Our findings are expected to inspire more scientists in the related fields to develop more sensitive and advanced optical temperature measurement methods.
Funding
Advanced Talents Incubation Program of Hebei University (521100221006, 521000981342); National Natural Science Foundation of China (12004093); Science and Technology Project of Hebei Education Department (QN2021018).
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. F. E. Maturi, C. D. S. Brites, E. C. Ximendes, C. Mills, B. Olsen, D. Jaque, S. J. L. Ribeiro, and L. D. Carlos, “Going Above and Beyond: A Tenfold Gain in the Performance of Luminescence Thermometers Joining Multiparametric Sensing and Multiple Regression,” Laser Photonics Rev. 15(11), 2100301 (2021). [CrossRef]
2. M. D. Dramicanin, “Sensing temperature via downshifting emissions of lanthanide-doped metal oxides and salts. A review,” Methods Appl. Fluoresc. 4(4), 042001 (2016). [CrossRef]
3. M. Suta, Z. Antic, V. Ethordevic, S. Kuzman, M. D. Dramicanin, and A. Meijerink, “Making Nd3+ a Sensitive Luminescent Thermometer for Physiological Temperatures-An Account of Pitfalls in Boltzmann Thermometry,” Nanomaterials 10(3), 543 (2020). [CrossRef]
4. I. E. Kolesnikov, A. A. Kalinichev, M. A. Kurochkin, D. V. Mamonova, E. Y. Kolesnikov, and E. Lähderanta, “Ratiometric Optical Thermometry Based on Emission and Excitation Spectra of YVO4:Eu3+ Nanophosphors,” J. Phys. Chem. C 123(8), 5136–5143 (2019). [CrossRef]
5. H. Suo, X. Zhao, Z. Zhang, and C. Guo, “Ultra-sensitive optical nano-thermometer LaPO4:Yb3+ /Nd3+ based on thermo-enhanced NIR-to-NIR emissions,” Chem. Eng. J. 389, 124506 (2020). [CrossRef]
6. S. Chen, W. Song, J. Cao, F. Hu, and H. Guo, “Highly sensitive optical thermometer based on FIR technique of transparent NaY2F7:Tm3+ /Yb3+ glass ceramic,” J. Alloys Compd. 825, 154011 (2020). [CrossRef]
7. C. D. Brites, X. Xie, M. L. Debasu, X. Qin, R. Chen, W. Huang, J. Rocha, X. Liu, and L. D. Carlos, “Instantaneous ballistic velocity of suspended Brownian nanocrystals measured by upconversion nanothermometry,” Nat. Nanotechnol. 11(10), 851–856 (2016). [CrossRef]
8. J. C. Martins, A. R. N. Bastos, R. A. S. Ferreira, X. Wang, G. Chen, and L. D. Carlos, “Primary Luminescent Nanothermometers for Temperature Measurements Reliability Assessment,” Adv. Photonics Res. 2(5), 2000169 (2021). [CrossRef]
9. K. Elzbieciak-Piecka, M. Suta, and L. Marciniak, “Structurally induced tuning of the relative sensitivity of LaScO3:Cr3+ luminescent thermometers by co-doping lanthanide ions,” Chem. Eng. J. 421, 129757 (2021). [CrossRef]
10. P. Cortelletti, A. Skripka, C. Facciotti, M. Pedroni, G. Caputo, N. Pinna, M. Quintanilla, A. Benayas, F. Vetrone, and A. Speghini, “Tuning the sensitivity of lanthanide-activated NIR nanothermometers in the biological windows,” Nanoscale 10(5), 2568–2576 (2018). [CrossRef]
11. Y. Gao, Y. Cheng, T. Hu, Z. Ji, H. Lin, J. Xu, and Y. Wang, “Broadening the valid temperature range of optical thermometry through dual-mode design,” J. Mater. Chem. C 6(41), 11178–11183 (2018). [CrossRef]
12. M. Sójka, J. F. C. B. Ramalho, C. D. S. Brites, K. Fiaczyk, L. D. Carlos, and E. Zych, “Bandgap Engineering and Excitation Energy Alteration to Manage Luminescence Thermometer Performance. The Case of Sr2(Ge,Si)O4 :Pr3+,” Adv. Opt. Mater. 7(23), 1901102 (2019). [CrossRef]
13. 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]
14. C. D. Brites, P. P. Lima, N. J. Silva, A. Millan, V. S. Amaral, F. Palacio, and L. D. Carlos, “Thermometry at the nanoscale,” Nanoscale 4(16), 4799–4829 (2012). [CrossRef]
15. 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]
16. D. Jaque and F. Vetrone, “Luminescence nanothermometry,” Nanoscale 4(15), 4301–4326 (2012). [CrossRef]
17. A. Ćirić, S. Stojadinović, and M. D. Dramićanin, “Luminescence temperature sensing using thin-films of undoped Gd2O3 and doped with Ho3+, Eu3+ and Er3+ prepared by plasma electrolytic oxidation,” Ceram. Int. 46(14), 23223–23231 (2020). [CrossRef]
18. L. Tong, X. Li, J. Zhang, S. Xu, J. Sun, H. Zheng, Y. Zhang, X. Zhang, R. Hua, H. Xia, and B. Chen, “NaYF4:Sm3+ /Yb3+@NaYF4:Er3+ /Yb3+ core-shell structured nanocalorifier with optical temperature probe,” Opt. Express 25(14), 16047–16058 (2017). [CrossRef]
19. M. Jia, Z. Sun, F. Lin, B. Hou, X. Li, M. Zhang, H. Wang, Y. Xu, and Z. Fu, “Prediction of Thermal-Coupled Thermometric Performance of Er3+,” J. Phys. Chem. Lett. 10(19), 5786–5790 (2019). [CrossRef]
20. 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]
21. K. Maciejewska, A. Bednarkiewicz, A. Meijerink, and L. Marciniak, “Correlation between the Covalency and the Thermometric Properties of Yb3+ /Er3+ Codoped Nanocrystalline Orthophosphates,” J. Phys. Chem. C 125(4), 2659–2665 (2021). [CrossRef]
22. A. S. Souza, L. A. Nunes, I. G. Silva, F. A. Oliveira, L. L. da Luz, H. F. Brito, M. C. Felinto, R. A. Ferreira, S. A. Junior, L. D. Carlos, and O. L. Malta, “Highly-sensitive Eu3+ ratiometric thermometers based on excited state absorption with predictable calibration,” Nanoscale 8(9), 5327–5333 (2016). [CrossRef]
23. Z. Sun, M. Jia, Y. Wei, J. Cheng, T. Sheng, and Z. Fu, “Constructing new thermally coupled levels based on different emitting centers for high sensitive optical thermometer,” Chem. Eng. J. 381, 122654 (2020). [CrossRef]
24. Q. Chang, X. Zhou, X. Zhou, L. Chen, G. Xiang, S. Jiang, L. Li, Y. Li, and X. Tang, “Strategy for optical thermometry based on temperature-dependent charge transfer to the Eu3 + 4f-4f excitation intensity ratio in Sr3Lu(VO4)3:Eu3+ and CaWO4:Nd3+,” Opt. Lett. 45(13), 3637–3640 (2020). [CrossRef]
25. L. Li, G. Tian, Y. Deng, Y. Wang, Z. Cao, F. Ling, Y. Li, S. Jiang, G. Xiang, and X. Zhou, “Constructing ultra-sensitive dual-mode optical thermometers: Utilizing FIR of Mn4+ /Eu3+ and lifetime of Mn4+ based on double perovskite tellurite phosphor,” Opt. Express 28(22), 33747–33757 (2020). [CrossRef]
26. S. Zhou, C. Duan, and M. Wang, “Origin of the temperature-induced redshift of the charge transfer band of GdVO4,” Opt. Lett. 42(22), 4703–4706 (2017). [CrossRef]
27. T. Xia, Y. Cui, Y. Yang, and G. Qian, “A luminescent ratiometric thermometer based on thermally coupled levels of a Dy-MOF,” J. Mater. Chem. C 5(21), 5044–5047 (2017). [CrossRef]
28. G.-H. Pan, L. Zhang, H. Wu, X. Qu, H. Wu, Z. Hao, L. Zhang, X. Zhang, and J. Zhang, “On the luminescence of Ti4+ and Eu3+ in monoclinic ZrO2: high performance optical thermometry derived from energy transfer,” J. Mater. Chem. C 8(13), 4518–4533 (2020). [CrossRef]
29. S. Zhou, C. Duan, M. Yin, X. Liu, S. Han, S. Zhang, and X. Li, “Optical thermometry based on cooperation of temperature-induced shift of charge transfer band edge and thermal coupling,” Opt. Express 26(21), 27339–27345 (2018). [CrossRef]
30. C. Sheng, X. Li, Y. Tian, X. Wang, S. Xu, H. Yu, Y. Cao, and B. Chen, “Temperature dependence of up-conversion luminescence and sensing properties of LaNbO4:Nd3+ /Yb3+ /Ho3+ phosphor under 808 nm excitation,” Spectrochim. Acta, Part A 244, 118846 (2021). [CrossRef]
31. 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]
32. X. Wang, Q. Liu, P. Cai, J. Wang, L. Qin, T. Vu, and H. J. Seo, “Excitation powder dependent optical temperature behavior of Er3+ doped transparent Sr0.69La0.31F2.31 glass ceramics,” Opt. Express 24(16), 17792–17804 (2016). [CrossRef]
33. H. Zhang, J. Ye, X. Wang, S. Zhao, R. Lei, L. Huang, and S. Xu, “Highly reliable all-fiber temperature sensor based on the fluorescence intensity ratio (FIR) technique in Er3+ /Yb3+ co-doped NaYF4 phosphors,” J. Mater. Chem. C 7(48), 15269–15275 (2019). [CrossRef]
34. J. Drabik, R. Kowalski, and L. Marciniak, “Enhancement of the sensitivity of single band ratiometric luminescent nanothermometers based on Tb3+ ions through activation of the cross relaxation process,” Sci. Rep. 10(1), 11190 (2020). [CrossRef]
35. K. Trejgis, R. Lisiecki, A. Bednarkiewicz, and L. Marciniak, “Nd3+ doped TZPN glasses for NIR operating single band ratiometric approach of contactless temperature readout,” J. Lumin. 224, 117295 (2020). [CrossRef]
36. W. Piotrowski, L. Dalipi, K. Elzbieciak-Piecka, A. Bednarkiewicz, B. Fond, and L. Marciniak, “Self-Referenced Temperature Imaging with Dual Light Emitting Diode Excitation and Single-Band Emission of AVO4:Eu3+ (A = Y, La, Lu, Gd) Nanophosphors,” Adv. Photonics Res. 2100139 (2021). [CrossRef]
37. L. Li, F. Qin, L. Li, H. Gao, and Z. Zhang, “New Strategy for Circumventing the Limitation of Thermally Linked States and Boosting the Relative Thermal Sensitivity of Luminescence Ratiometric Thermometry,” J. Phys. Chem. C 123(10), 6176–6181 (2019). [CrossRef]
38. J. Stefanska, M. Chrunik, and L. Marciniak, “Sensitivity Enhancement of the Tb3+-Based Single Band Ratiometric Luminescent Thermometry by the Metal-to-Metal Charge Transfer Process,” J. Phys. Chem. C 125(9), 5226–5232 (2021). [CrossRef]
39. J. Stefanska, K. Maciejewska, and L. Marciniak, “Blue-emitting single band ratiometric luminescent thermometry based on LaF3:Pr3+,” New J. Chem. 45(27), 11898–11904 (2021). [CrossRef]
