Ratiometric optical thermometry was designed using temperature-induced shift of V-O charge transfer band (CTB) edge combined with temperature-induced variation of Tb3+ emission in YV1-xPxO4. P was introduced into YVO4 lattice to form YV1-xPxO4 solid solution successfully, with the purpose of enhancing Tb3+ emission. Under 352 nm excitation which locates in the tail of the V-O CTB, emission spectra of YV0.3P0.7O4:Tb3+, Eu3+/Sm3+ were recorded at a series of temperatures ranging from 300 to 440 K. It is demonstrated that Tb3+ and Eu3+/Sm3+ emissions exhibit opposite temperature dependences. The mechanisms for such opposite variations have been interpreted in detail. Based on the varied fluorescence intensity ratio of Eu3+/Sm3+ to Tb3+ with temperature, high relative sensitivity was obtained with a maximal value of 2.85% K−1 around 365 K. Our results imply that the proposed strategy is a promising candidate for high-sensitive optical temperature sensing.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In the past decade, optical temperature sensing based on the fluorescent response to temperature has attracted tremendous attention due to its outstanding merits, including contactless detection, quick response, and high spatial resolution [1–3]. Among the various approaches for optical thermometry, ratiometric method which uses temperature dependent fluorescence intensity ratio (FIR) as detecting parameter has been regarded as a promising technique benefiting from the immunity to some disturbances, for instance, fluorescence losses, unevenly distributed luminescent centers, and fluctuations of excitation intensity [4–8].
Recently, we reported a fluorescence intensity-based strategy taking advantage of temperature-induced shift of V-O charge transfer band (CTB) edge [9–10]. Drastic temperature dependence of the fluorescence was obtained under a certain excitation in the tail of the CTB caused by the shift of the CTB edge. However, such intensity-based method may suffer from some disturbances as we mentioned above, which may further influence the measurement accuracy. On the basis of this strategy, if another luminescence center whose fluorescence is immune to temperature or exhibits opposite temperature dependence is introduced as a reference, ratiometric method can be applied. Taking YVO4:Eu3+ for an example, we found that Eu3+ emission increases with the increase of temperature ranging from 300 to 480 K under the 358 nm excitation which locates in the tail of the V-O CTB . As for the additional luminescence center to be introduced serving as the reference, efficient f-f transition excitation should exist and be dominant around 358 nm. Otherwise, it may exhibit similar temperature dependence with Eu3+ emission.
Tb3+ may be a promising candidate for the reference due to the existence of several f-f transition excitations around the position of V-O CTB edge . But, unfortunately, in YVO4 lattice, both f-f transition excitations and CT state excitation are inefficient for Tb3+ emission due to the strong quenching effect . However, in YPO4 lattice which has the same crystal structure with YVO4, Tb3+ can emit strong fluorescence under the characteristic f-f excitations [14–15]. Based on this, we conclude that partial substitution of V by P in YVO4 lattice would be helpful to Tb3+ emission. Thus, both temperature-induced shift of the V-O CTB edge and Tb3+ emission originating from f-f transition excitation can be guaranteed. Therefore, we expect that the solid solution YV1-xPxO4 may be an ideal matrix for temperature dependent FIR of Eu3+ to Tb3+.
To verify the feasibility of this proposal, rare earth ions doped YV1-xPxO4 solid solutions were synthesized. Temperature dependent optical properties of the as-prepared samples were studied for ratiometric optical temperature sensing.
Rare earth doped YV1-xPxO4 (x=0, 0.3, 0.5, 0.7, and 1) powders were prepared via a high temperature solid state method . The crystal structures of the obtained products were identified by XRD (X-ray diffraction) measurements using a powder X-ray diffractometer (PANalytical X′Pert3). The fluorescence spectra and decay curves were acquired using a fluorescence spectrometer (HORIBA Florolog-3) equipped with a 450 W xenon lamp and a pulsed 370 nm spectraLED (HORIBA). The temperature control of the sample was realized through heat conduction with a copper plate whose temperature was controlled using a heating tube and a temperature controller (OMRON E5CC-800).
3. Results and discussion
The XRD patterns of YV1-xPxO4:3% Tb3+, 0.2% Eu3+ (x = 0, 0.3, 0.5, 0.7, and 1) powders are given in Fig. 1. The diffraction peaks of the as-prepared samples with x = 0 and 1 are in accord with the standard data of tetragonal YVO4 (JCPDS No. 17-341) and YPO4 (JCPDS No. 11-254), respectively, indicating that pure crystalline phase was obtained. What’s more, with the increase of P concentration from x = 0 to 1, the positions of diffraction peaks shift slightly toward larger angle direction because of the substitution of V5+ ions (ionic radius 0.59 Å) by smaller P5+ ions (ionic radius 0.34 Å) . No additional diffraction peak exits in all of the samples. These indicate that a well-formed solid solution of YV1-xPxO4 structure was obtained.
In order to study the fluorescent properties of Tb3+ and Eu3+ in the solid solutions, emission spectra of the as-prepared samples were recorded under the excitation of 352 nm as shown in Fig. 2. This excitation wavelength was selected in order to guarantee that both Tb3+ and Eu3+ can be excited simultaneously. Specifically, under 352 nm excitation, Tb3+ can be excited from ground state 7F6 to 5D2, 5L9, 5G4 states . In the case of Eu3+, because 352 nm locates in the tail of the V-O CTB, it can be excited due to the efficient energy transfer from the CT state.
From the normalized emission spectra in Fig. 2, we found that the introduction of P into YVO4 lattice is in favor of Tb3+ emission as expected, especially when the P concentration is larger than fifty percent. The detailed transitions corresponding to the emission peaks have been marked in Fig. 2 . Moreover, it is observed that the spectral profiles of Eu3+ around 593 and 620 nm also change with the increase of P concentration. This is another evidence for the formation of solid solution YV1-xPxO4. These variations of Tb3+ and Eu3+ emissions are ascribed to the variations of microstructures surrounding rare earth ions caused by the change in composition, which are irrelevant to the morphology of the as-prepared materials .
Excitation spectra of YVO4:3% Tb3+, 0.2% Eu3+ and YV0.3P0.7O4:3% Tb3+, 0.2% Eu3+ monitored at Tb3+ 544 nm emission and Eu3+ 620 nm emission are shown in Figs. 3(a) and 3(b), respectively. The broad excitation band is ascribed to the V-O CT process. The sharp lines are ascribed to the characteristic f-f transitions of Tb3+ and Eu3+. For Eu3+ emission, the CT process is always efficient whether P5+ ions are introduced. But, in the case of Tb3+ emission in YVO4:3% Tb3+, 0.2% Eu3+, no efficient excitation exists for both the CT process and the characteristic f-f transitions, which is due to the strong quenching of Tb3+ emission by Tb-V interaction . However, when V5+ ions are partly substituted by P5+ ions, the separation between Tb3+ and V5+ increases, leading to a weakened Tb-V interaction. As a result, the quenching decreases and thus Tb3+ emission can be observed as shown in Fig. 2. When sufficient P were introduced, both the CT process and the characteristic f-f transitions turn efficient, which can be seen from Fig. 3(a).
YV0.3P0.7O4:3% Tb3+, 0.2% Eu3+ sample was selected to study the temperature dependent fluorescence properties of Tb3+ and Eu3+ due to the strong emissions for both these two ions under 352 nm excitation. As shown in Fig. 4(a), Eu3+ emission increases dramatically when the temperature increases from 300 to 440 K. However, Tb3+ emission exhibits a contrary tendency. The insert in Fig. 4(a) shows such opposite variations quantificationally. I541-555 and I605-635 represent the integrated intensities for 5D4 → 7F5 transition (541-555 nm) of Tb3+ and 5D0 → 7F2 transition (605-635 nm) of Eu3+, respectively. The relationship between the ratio of I605-635 to I541-555 and temperature were phenomenologically fitted as shown in Fig. 4(b) using the following equation:
The relative sensitivity SR was obtained according to the fitting result which is shown in Fig. 4(c). The maximal SR value reaches up to 2.7% K−1 around 365 K.
To further confirm the feasibility of the as-proposed strategy, it was also performed using YV0.3P0.7O4:3% Tb3+, 0.05% Eu3+ and YV0.3P0.7O4:3% Tb3+, 0.1% Sm3+ samples. The temperature dependent emission spectra recorded under 352 nm excitation are shown in Fig. 5. Due to the decrease of Eu3+ concentration, the FIR of Tb3+ to Eu3+ is remarkably enhanced at a certain temperature, which can be seen from the comparison between Fig. 4(a) and Fig. 5(a).
In the case of YV0.3P0.7O4:3% Tb3+, 0.1% Sm3+ sample, four characteristic emission bands of Sm3+ centered at 564, 602, 646, and 704 nm can be observed, which are ascribed to 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, 4G5/2 → 6H9/2, and 4G5/2 → 6H11/2 transitions, respectively . The emission of Sm3+ exhibits similar temperature dependence with that of Eu3+ in the above studied materials, enhancing with the increase of temperature monotonously by virtue of the temperature-induced red shift of the CTB edge.
The integrated intensities I541-555, I605-635, and I560-572 are used to describe the temperature dependent variations of Tb3+, Eu3+, and Sm3+ emissions, respectively, as shown in the insert of Fig. 5. The temperature dependences of the ratio of I605-635 to I541-555 in Fig. 5(a) and that of I560-572 to I541-555 in Fig. 5(b) were phenomenologically fitted using the following Eqs. (2) and (3):
The SR values were calculated according to the formula in Fig. 4(c). The maximal SR values for YV0.3P0.7O4:3% Tb3+, 0.05% Eu3+ and YV0.3P0.7O4:3% Tb3+, 0.1% Sm3+ samples reach up to 2.6% K−1 around 380 K and 2.85% K−1 around 365 K, respectively.
The mechanisms for temperature dependent variations of Eu3+/Sm3+ and Tb3+ were discussed as follows based on YV0.3P0.7O4:3% Tb3+, 0.2% Eu3+ sample taken as an example. Temperature dependent excitation spectra of YV0.3P0.7O4:3% Tb3+, 0.2% Eu3+ sample monitored at Eu3+ 697.5 nm and Tb3+ 544 nm emissions were recorded from 300 to 440 K as shown in Fig. 6 and Fig. 7, respectively. In the case of Eu3+, the CTB edge shifts to longer wavelength side with increasing temperature as expected, resulting in a dramatically enhanced excitation intensity at 352 nm, which can be seen from the enlarged image in Fig. 6. As for Tb3+, it was found that the excitation intensity decreases with increasing temperature. This is consistent with the temperature-induced decrease of Tb3+ emission mentioned above. In order to study whether the decrease of Tb3+ emission is related to the co-doped ions, emission spectra of Tb3+ single-doped YV0.3P0.7O4 and Tb3+, Eu3+/Sm3+ co-doped YV0.3P0.7O4 samples were measured under 352 nm excitation. The variations of the integrated emission intensity I541-555 of Tb3+ with temperature are shown in Fig. 8 (the integrated intensities are normalized at 300 K). Obviously, the decreases of Tb3+ emission are similar for all the studied materials, indicating that the decrease is not related to the co-doped ions.
Figure 9 presents the temperature dependent luminescence decay curves of Tb3+ 544 nm, Eu3+ 620 nm and Sm3+ 602 nm emissions of the three samples. It was found that the decay curves do not change with increasing temperature for all the cases, indicating that no temperature quenching occurs in the whole studied temperature range. Therefore, the temperature-induced decrease of Tb3+ emission can not be ascribed to temperature quenching behavior of Tb3+. We suggest that the increase of temperature may weaken the absorption of Tb3+, which further causes the decrease of Tb3+ emission.
Based on the temperature-induced shift of the V-O CTB edge and temperature-induced variation of Tb3+ emission, ratiometric optical thermometry was designed in Tb3+, Eu3+/Sm3+ co-doped YV1-xPxO4 solid solutions. The partial substitution of V by P played a crucial role for achieving and enhancing Tb3+ emission. This is caused by the weakened V-Tb interaction resulted from the increased separation between V5+ and Tb3+. Opposite temperature dependences for Eu3+/Sm3+ and Tb3+ emissions were achieved in YV0.3P0.7O4:Tb3+, Eu3+/Sm3+ under 352 nm excitation in the range of 300-440 K. Specifically, Eu3+/Sm3+ emission increases dramatically with increasing temperature benefiting from the temperature-induced shift of the V-O CTB edge. While Tb3+ emission decreases monotonously. Using FIR of Eu3+/Sm3+ to Tb3+ as detecting parameter, high relative sensitivity was obtained, which is higher than 2% K−1 in the range of 340-440 K. We conclude that this work may provide some new guidance in the development of optical temperature sensing as well as the application of rare earth doped luminescent materials.
National Natural Science Foundation of China (11804188, 51902178); Natural Science Foundation of Shandong Province (ZR2017BA030).
The authors declare no conflicts of interest.
1. M. Quintanilla and L. M. Liz-Marzán, “Guiding Rules for Selecting a Nanothermometer,” Nano Today 19, 126–145 (2018). [CrossRef]
2. X. F. Wang, Q. Liu, Y. Y. Bu, C. S. Liu, T. Liu, and X. H. Yan, “Optical temperature sensing of rare-earth ion doped phosphors,” RSC Adv. 5(105), 86219–86236 (2015). [CrossRef]
3. L. P. Li, Y. Zhou, F. Qin, Y. D. Zheng, H. Zhao, and Z. G. Zhang, “Relative sensitivity variation law in the field of fluorescence intensity ratio thermometry,” Opt. Lett. 43(2), 186–189 (2018). [CrossRef]
4. Y. Cheng, Y. Gao, H. Lin, F. Huang, and Y. S. Wang, “Strategy design for ratiometric luminescence thermometry: circumventing the limitation of thermally coupled levels,” J. Mater. Chem. C 6(28), 7462–7478 (2018). [CrossRef]
5. Y. Zhou, L. P. Li, F. Qin, and Z. G. Zhang, “Highly sensitive fluorescence intensity ratio thermometry by breaking the thermal correlation of two emission centers,” Opt. Lett. 44(18), 4598–4601 (2019). [CrossRef]
6. M. M. A. Mazza and F. M. Raymo, “Structural designs for ratiometric temperature sensing with organic fluorophores,” J. Mater. Chem. C 7(18), 5333–5342 (2019). [CrossRef]
7. S. S. Zhou, X. T. Wei, X. Y. Li, Y. H. Chen, C. K. Duan, and M. Yin, “Temperature sensing based on the cooperation of Eu3+ and Nd3+ in Y2O3 nanoparticles,” Sens. Actuators, B 246, 352–357 (2017). [CrossRef]
8. X. J. Zhu, W. Feng, J. Chang, Y. W. Tan, J. C. Li, M. Chen, Y. Sun, and F. Y. Li, “Temperature-feedback upconversion nanocomposite for accurate photothermal therapy at facile temperature,” Nat. Commun. 7(1), 10437 (2016). [CrossRef]
9. S. S. Zhou, C. K. Duan, and S. Han, “A novel strategy for thermometry based on the temperature-induced red shift of the charge transfer band edge,” Dalton Trans. 47(5), 1599–1603 (2018). [CrossRef]
10. S. S. Zhou, C. K. Duan, M. Yin, X. L. Liu, S. Han, S. B. Zhang, and X. M. 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]
11. S. S. Zhou, C. K. Duan, M. Yin, S. B. Zhang, and C. Wang, “High-sensitive optical temperature sensing based on 5D1 emission of Eu3+ in YVO4,” J. Alloys Compd. 784, 970–974 (2019). [CrossRef]
12. I. Carrasco, F. Piccinelli, I. Romet, V. Nagirnyi, and M. Bettinelli, “Competition between Energy Transfer and Energy Migration Processes in Neat and Eu3+-Doped TbPO4,” J. Phys. Chem. C 122(12), 6858–6864 (2018). [CrossRef]
13. R. G. DeLosh, T. Y. Tien, E. F. Gibbons, P. J. Zacmanidis, and H. L. Stadler, “Strong Quenching of Tb3+ Emission by Tb-V Interaction in YPO4-YVO4,” J. Chem. Phys. 53(2), 681–685 (1970). [CrossRef]
14. H. L. Xiong, J. C. Dong, J. F. Yang, Y. L. Liu, H. B. Song, and S. C. Gan, “Facile hydrothermal synthesis and multicolor-tunable luminescence of YPO4:Ln3+ (Ln = Eu, Tb),” RSC Adv. 6(100), 98208–98215 (2016). [CrossRef]
15. H. L. Xiong, Y. Zhang, Y. L. Liu, T. N. Gao, L. L. Zhang, Z. A. Qiao, L. Zhang, S. C. Gan, and Q. S. Huo, “Self-template construction of honeycomb-like mesoporous YPO4:Ln3+ (Ln = Eu, Tb) phosphors with tuneable luminescent properties,” J. Alloys Compd. 782, 845–851 (2019). [CrossRef]
16. F. Wang, X. J. Xue, and X. G. Liu, “Multicolor tuning of (Ln, P)-doped YVO4 nanoparticles by single-wavelength excitation,” Angew. Chem., Int. Ed. 47(5), 906–909 (2008). [CrossRef]
17. G. H. Pan, H. W. Song, Q. L. Dai, R. F. Qin, X. Bai, B. Dong, L. B. Fan, and F. Wang, “Microstructure and optical properties of Eu3+ activated YV1-xPxO4 phosphors,” J. Appl. Phys. 104(8), 084910 (2008). [CrossRef]
18. Z. H. Wang, X. F. Shi, X. J. Wang, Q. Zhu, X. D. Li, B. N. Kim, X. D. Sun, and J. G. Li, “Enhanced hydrothermal crystallization and color tailorable photoluminescence of hexagonal structured YPO4:Sm/Tb nanorods,” CrystEngComm 20(17), 2357–2365 (2018). [CrossRef]