December 1, 2014 / Vol. 39, No. 23 / OPTICS LETTERS
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Strategy for thermometry via Tm3+-doped NaYF4 core-shell nanoparticles Shaoshuai Zhou,1 Guicheng Jiang,2 Xinyue Li,1 Sha Jiang,3 Xiantao Wei,1 Yonghu Chen,1 Min Yin,1 and Changkui Duan1,* 1 2
Department of Physics, University of Science and Technology of China, Hefei 230026, China
Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin 150001, China 3 School of Science, Chongqing University of Post and Telecommunications, Chongqing 400065, China *Corresponding author: [email protected] Received October 14, 2014; accepted October 23, 2014; posted October 29, 2014 (Doc. ID 224808); published November 24, 2014
Optical thermometers usually make use of the fluorescence intensity ratio of two thermally coupled energy levels, with the relative sensitivity constrained by the limited energy gap. Here we develop a strategy by using the upconversion (UC) emissions originating from two multiplets with opposite temperature dependences to achieve higher relative temperature sensitivity. We show that the intensity ratio of the two UC emissions, 3 F2;3 and 1 G4 , of Tm3 in β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell nanoparticles under 980 nm laser excitation exhibits high relative temperature sensitivity between 350 and 510 K, with a maximum of 1.53% K−1 at 417 K. This demonstrates the validity of the strategy, and that the studied material has the potential for high-performance optical thermometry. © 2014 Optical Society of America OCIS codes: (160.5690) Rare-earth-doped materials; (280.4788) Optical sensing and sensors; (300.6280) Spectroscopy, fluorescence and luminescence. http://dx.doi.org/10.1364/OL.39.006687
Recently, optical thermometers based on lanthanide ionactivated luminescent materials have attracted great attention due to their noninvasive mode and quick response [1–12]. In particular, the fluorescence intensity ratio (FIR) method, exploiting the temperature-dependent luminescence of the thermally coupled energy levels (TCELs), has been regarded as a very promising approach for optical temperature sensing by virtue of its reduced dependence on the measurement conditions, such as fluorescence loss and fluctuations of exciting light [13]. Rare earth ions possessing TCELs have been explored in regard to this FIR method during the past several years. For instance, temperature measurement inside an individual HeLa cell was performed using 2 H11∕2 and 4 S3∕2 states of Er3 in NaYF4 :Yb3 ∕Er3 nanoparticles (NPs) by Vetrone et al. [4]. Recently, Ho3 ion doped material was verified to be another promising candidate with higher relative sensitivity [5,6]. TCELs in other lanthanide ions such as Pr3 [7], Nd3 [1,8], Gd3 [9], Dy3 [10], and Tm3 [11,12] were also investigated for optical thermometers. According to the principle of a TCEL-based temperature sensor, the FIR is governed by the Boltzmann distribution ratio [13]. Hence, the relative sensitivity, which is a key parameter of the temperature sensing capability, is proportional to the energy gap of the corresponding TCELs [5]. However, the gap of the TCELs is upper bounded by thermal coupling conditions at about 2000 cm−1 [13]. A survey of the thermometry using the TCEL-based FIR technique reveals that Ho3 -doped NaLuF4 has the largest relative sensitivity in the operational temperature interval from 390 to 780 K [5]. However, further improvement of the sensing sensitivity is difficult to achieve with this TCEL-based FIR technique due to the restriction mentioned above. Herein, we report a strategy for FIR-based temperature sensing in β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 0146-9592/14/236687-04$15.00/0
core-shell NPs by virtue of the upconversion (UC) luminescence of Tm3 under 980 nm laser excitation. Pr3 is deliberately doped into the inert shell to test the laser heating effect. The thermal behaviors of the emissions originating from 3 F2;3 and 1 G4 states were investigated from 302 to 510 K. A high relative sensitivity was achieved and the maximum value reached 1.53% K−1 at 417 K, which is higher than all the previously reported rare earth ions doped temperature sensors using TCELs-based FIR technique. β-NaYF4 :20%Yb3 , 0.5%Tm3 core NPs and β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell NPs were synthesized in high boiling solvents oleic acid and octadecene according to the previously reported protocol [14,15]. The size and morphology of the NPs were acquired using high-resolution transmission electron microscopy (HRTEM, Model JEOL-2010). X-ray diffraction (XRD) patterns were obtained with an x-ray diffractometer (Rigaku-TTR-III) with Cu Kα radiation (λ 0.15418 nm). The emission spectra were obtained by a Jobin–Yvon HRD-1 double monochromator equipped with a Hamamatsu R928 photomultiplier. The signal was analyzed by an EG&G 7265 DSP Lock-in Amplifier. The temperature of the nanopowder was controlled by an attached copper post, whose temperature was controlled by a temperature controller (OMRON E5CC800) with a type-K thermocouple and a heating tube. The morphologies of the core and core-shell NPs are shown in Figs. 1(a) and 1(b), respectively. The core NPs are of uniform size and shape with an average diameter of 24 nm. After the epitaxial growth of the shell, the morphologies of the particles change from isotropic nanospheres into short nanorods with an average size of 27 nmdiameter × 39 nmlength. Both the core and core-shell NPs are monodispersed and can be easily dispersed in cyclohexane. Figure 1(c) shows the XRD patterns of the core and core-shell NPs, which agree well © 2014 Optical Society of America
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Fig. 1. TEM images of (a) β-NaYF4 :20%Yb3 , 0.5%Tm3 core and (b) β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell NPs. (c) XRD patterns of the core and core-shell NPs as well as the standard data of β-NaYF4 (JCPDS No. 16-0334) as a reference.
with the standard data of β-NaYF4 (JCPDS No. 16-0334), indicating a pure crystalline hexagonal phase. The β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 coreshell structure is designed to enhance the UC luminescence of Tm3 , as well as to cut off the energy transfer from Pr3 to Yb3 [16], as depicted in Fig. 2(a). Figure 2(b) shows the comparison of UC luminescence spectra of the core and core-shell NPs (powder) acquired under the same measurement conditions at room temperature. Characteristic emission peaks of Tm3 located at 361, 450, 473, 646, and 696 nm originate from 1 D2 → 3 H6 , 1 D → 3 F , 1 G → 3 H , 1 G → 3 F , and 3 F 3 2 4 4 6 4 4 2;3 → H6 transitions, respectively [17,18]. And the UC emissions are
Fig. 2. (a) Schematic diagram of the core-shell NPs. (b) UC emission spectra of the core and core-shell NPs (powder) under 980 nm laser excitation with power of 47 mW. The inserts are photographs of the UC luminescence for the core and core-shell NPs dispersed in cyclohexane.
Fig. 3. Schematic illustration of the UC processes for the asprepared core-shell NPs under 980 nm laser excitation. The dashed–dotted, dashed, curved, and full arrows indicate photon excitation, energy transfer, multiphonon relaxation, and emission processes, respectively.
dramatically enhanced for the core-shell NPs. As shown in the insert of Fig. 2(b), the core-shell NPs in cyclohexane exhibit bright blue emission under 980 nm laser excitation. It is concluded that a high UC efficiency is achieved in the core-shell NPs. Detailed UC processes for the luminescence of Tm3 are illustrated in Fig. 3 [18]. As for UC materials doped with Yb3 ions for temperature sensing, the heating effect caused by strong absorption of 980 nm excitation should be given more attention, which may lead to an enhanced temperature of the material’s surface compared with the surroundings [19]. Therefore, it is necessary to check and to reduce or even to eliminate this deviation for accurate temperature sensing. Pr3 -doped materials have been investigated for temperature sensing based on its TCELs 3 P1 and 3 P0 , in which the FIR varies with temperature according to a theoretical equation governed by the Boltzmann-type distribution ratio [7]. There is no evident heating effect when excited directly by the blue laser. In this work, before the investigation of the temperature-dependent UC luminescent properties, Pr3 ions doped in the shell were adopted to detect the heating effect caused by a 980 nm laser. Specifically, the sample was irradiated by a 980 nm laser for UC luminescence and a 447 nm laser to excite Pr3 for temperature sensing at the same time. The spot size on the sample illuminated by the 980 nm laser was estimated to be 2 mm × 3 mm. The power was adjusted through neutral density filters. It is demonstrated that the emission spectrum ranging from 510 to 570 nm of Pr3 under excitation of 980 nm with power of 47 mW or below coincides with that obtained when the 980 nm laser was turned off. So we conclude that the heating effect of 980 nm laser can be ignored by using a laser power not exceeding 47 mW. The UC emission spectra of the core-shell NPs in the range from 620 to 740 nm were measured at a series of temperatures between 302 and 510 K under 980 nm excitation of 47 mW. As shown in the normalized spectra in Fig. 4, the emission peak located at 696 nm increases gradually with the enhancement of temperature. What is more, the intensity increase becomes more and more pronounced in the same temperature interval with increasing temperature.
December 1, 2014 / Vol. 39, No. 23 / OPTICS LETTERS
Fig. 4. UC spectra in the region from 620 to 740 nm for the core-shell NPs at various temperatures from 302 to 510 K. The intensities are normalized to the peak around 646 nm.
The integral emission intensities of the two emission peaks at 646 and 696 nm, originating from the 1 G4 → 3 F4 and 3 F2;3 → 3 H6 transitions, respectively, exhibit remarkably different temperature-dependent behaviors. As shown in Fig. 5(a), the intensity of 3 F2;3 → 3 H6 transition (696 nm) increases monotonously with temperature in the whole investigated temperature range. The main reason for this is the increasing population of 3F 3 2;3 states from the lower H4 state with temperature due to thermal population, which further promotes the radiative transition from 3 F2;3 states [11,12,20]. However,
Fig. 5. (a) Integral emission intensities of 3 F2;3 → 3 H6 and 1 G4 → 3 F4 transitions varied with temperature under 980 nm excitation. (b) Temperature dependence of the ratio R between the integral intensities of 3 F2;3 → 3 H6 and 1 G4 → 3 F4 transitions for the core-shell NPs.
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the intensity of 646 nm first increases with the increase of temperature until 390 K and then decreases. From the energy level diagram, there is an obvious mismatch in the energy levels between Yb3 and Tm3 ; thus the upconverted population of 1 G4 can be affected by the participation of phonons, which is temperature dependent. As a result, the UC luminescence is enhanced due to the phonons’ assistance as temperature rises in a certain temperature range. On the other hand, as the temperature rises further, the population of 1 G4 may also decrease via phonon-assisted cross relaxation, leading to the decrease of UC luminescence. Furthermore, due to the fact that the 1 G4 state is populated from 3 H4 state through the energy transfer ET 3 shown in Fig. 3, the decreased population in 3 H4 state because of the thermal population of 3 F2;3 will also reduce the population of 1 G4 state. To sum up, the dominance of thermal population or phonon-assisted processes results in the different temperature dependences of the two UC emissions. Figure 5(b) is the temperature dependence of the ratio R between the integral emission intensities of 3 F2;3 → 3 H6 and 1 G4 → 3 F4 transitions. The experimental data are well fitted with the following equation: 3499 0.82: R 11485 × exp − T
(1)
Also notable, the slope of the fitting curve, namely the absolute change of the ratio R with temperature, increases gradually as the temperature rises in the range from 300 to 510 K. This remarkable variation of the ratio R relative to the temperature indicates that this material can be used as an optical temperature sensor under near infrared irradiation. Relative sensitivity S R , the relative change of the ratio R with respect to temperature variation, is a key parameter to characterize sensing ability of sensors. For the current material, the relative sensitivity in the range from 300 to 510 K is depicted in Fig. 6, which shows a relatively high temperature sensitivity in a wide temperature range from 350 to 510 K, with the maximal S R value reaching 1.53% K−1 at 417 K. The S R values of Er3 [4]
Fig. 6. Relative sensitivity based on 3 F2;3 → 3 H6 and 1 G4 → 3 F4 transitions of Tm3 as a function of temperature. The S R values of temperature sensors using TCELs in Er3 and Ho3 are presented for comparison.
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and Ho3 [5] doped temperature sensors using a TCELbased FIR technique were also presented in Fig. 6 for comparison. It is noted that the S R values are given only for temperatures above 390 K for Ho3 , due to poor thermal coupling of the two related levels, 5 F1 ∕5 G6 and 5 F ∕5 K , and extremely poorly resolved luminescence 2;3 8 from the upper energy levels 5 F1 ∕5 G6 at lower temperatures. The temperature sensitivities obtained here are superior to those based on TCELs in a wide temperature range. In conclusion, a strategy for FIR-based optical temperature sensing is put forward here to achieve higher relative sensitivity than those based on TCELs. By using the different thermal behaviors of the two UC emissions originating from 3 F2;3 and 1 G4 states of Tm3 in β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell NPs, high relative sensitivities are achieved in a wide temperature range from 350 to 510 K, with the maximum reaching 1.53% K−1 at 417 K. This is superior to other previously reported rare-earth ions doped temperature sensors using a TCEL-based FIR technique in the same temperature range. We conclude that the β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell NPs are potentially excellent optical thermometers with a high relative sensitivity. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11374291, 11274299, 11204292, 11404321, and 11311120047). References 1. W. Xu, Q. Song, L. Zheng, Z. Zhang, and W. Cao, Opt. Lett. 39, 4635 (2014). 2. S. Zheng, W. Chen, D. Tan, J. Zhou, Q. Guo, W. Jiang, C. Xu, X. Liu, and J. Qiu, Nanoscale 6, 5675 (2014).
3. X. Wang, O. S. Wolfbeis, and R. J. Meier, Chem. Soc. Rev. 42, 7834 (2013). 4. F. Vetrone, R. Naccache, A. Zamarron, A. J. de la Fuente, F. Sanz-Rodriguez, L. M. Maestro, E. M. Rodriguez, D. Jaque, J. G. Sole, and J. A. Capobianco, ACS Nano 4, 3254 (2010). 5. S. Zhou, S. Jiang, X. Wei, Y. Chen, C. Duan, and M. Yin, J. Alloy Compd. 588, 654 (2014). 6. W. Xu, H. Zhao, Y. Li, L. Zheng, Z. Zhang, and W. Cao, Sens. Actuators B 188, 1096 (2013). 7. S. Zhou, G. Jiang, X. Wei, C. Duan, Y. Chen, and M. Yin, J. Nanosci. Nanotechnol. 14, 3739 (2014). 8. D. Wawrzynczyk, A. Bednarkiewicz, M. Nyk, W. Strek, and M. Samoc, Nanoscale 4, 6959 (2012). 9. K. Zheng, Z. Liu, C. Lv, and W. Qin, J. Mater. Chem. C 1, 5502 (2013). 10. Z. Boruc, M. Kaczkan, B. Fetlinski, S. Turczynski, and M. Malinowski, Opt. Lett. 37, 5214 (2012). 11. W. Xu, X. Gao, L. Zheng, Z. Zhang, and W. Cao, Sens. Actuators B 173, 250 (2012). 12. L. Xing, Y. Xu, R. Wang, W. Xu, and Z. Zhang, Opt. Lett. 39, 454 (2014). 13. S. A. Wade, S. F. Collins, and G. W. Baxter, J. Appl. Phys. 94, 4743 (2003). 14. X. Xie, N. Gao, R. Deng, Q. Sun, Q. H. Xu, and X. Liu, J. Am. Chem. Soc. 135, 12608 (2013). 15. G. Jiang, J. Pichaandi, N. J. Johnson, R. D. Burke, and F. C. van Veggel, Langmuir 28, 3239 (2012). 16. K. Deng, X. Wei, X. Wang, Y. Chen, and M. Yin, Appl. Phys. B 102, 555 (2011). 17. S. Xu, W. Xu, Y. Wang, S. Zhang, Y. Zhu, L. Tao, L. Xia, P. Zhou, and H. Song, Nanoscale 6, 5859 (2014). 18. F. Shi, J. Wang, X. Zhai, D. Zhao, and W. Qin, Cryst. Eng. Commun. 13, 3782 (2011). 19. S. Zhou, K. Deng, X. Wei, G. Jiang, C. Duan, Y. Chen, and M. Yin, Opt. Commun. 291, 138 (2013). 20. L. Xing, Y. Xu, R. Wang, and W. Xu, Opt. Lett. 38, 2535 (2013).
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Strategy for thermometry via Tm3+-doped NaYF4 core-shell nanoparticles Shaoshuai Zhou,1 Guicheng Jiang,2 Xinyue Li,1 Sha Jiang,3 Xiantao Wei,1 Yonghu Chen,1 Min Yin,1 and Changkui Duan1,* 1 2
Department of Physics, University of Science and Technology of China, Hefei 230026, China
Condensed Matter Science and Technology Institute, Harbin Institute of Technology, Harbin 150001, China 3 School of Science, Chongqing University of Post and Telecommunications, Chongqing 400065, China *Corresponding author: [email protected] Received October 14, 2014; accepted October 23, 2014; posted October 29, 2014 (Doc. ID 224808); published November 24, 2014
Optical thermometers usually make use of the fluorescence intensity ratio of two thermally coupled energy levels, with the relative sensitivity constrained by the limited energy gap. Here we develop a strategy by using the upconversion (UC) emissions originating from two multiplets with opposite temperature dependences to achieve higher relative temperature sensitivity. We show that the intensity ratio of the two UC emissions, 3 F2;3 and 1 G4 , of Tm3 in β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell nanoparticles under 980 nm laser excitation exhibits high relative temperature sensitivity between 350 and 510 K, with a maximum of 1.53% K−1 at 417 K. This demonstrates the validity of the strategy, and that the studied material has the potential for high-performance optical thermometry. © 2014 Optical Society of America OCIS codes: (160.5690) Rare-earth-doped materials; (280.4788) Optical sensing and sensors; (300.6280) Spectroscopy, fluorescence and luminescence. http://dx.doi.org/10.1364/OL.39.006687
Recently, optical thermometers based on lanthanide ionactivated luminescent materials have attracted great attention due to their noninvasive mode and quick response [1–12]. In particular, the fluorescence intensity ratio (FIR) method, exploiting the temperature-dependent luminescence of the thermally coupled energy levels (TCELs), has been regarded as a very promising approach for optical temperature sensing by virtue of its reduced dependence on the measurement conditions, such as fluorescence loss and fluctuations of exciting light [13]. Rare earth ions possessing TCELs have been explored in regard to this FIR method during the past several years. For instance, temperature measurement inside an individual HeLa cell was performed using 2 H11∕2 and 4 S3∕2 states of Er3 in NaYF4 :Yb3 ∕Er3 nanoparticles (NPs) by Vetrone et al. [4]. Recently, Ho3 ion doped material was verified to be another promising candidate with higher relative sensitivity [5,6]. TCELs in other lanthanide ions such as Pr3 [7], Nd3 [1,8], Gd3 [9], Dy3 [10], and Tm3 [11,12] were also investigated for optical thermometers. According to the principle of a TCEL-based temperature sensor, the FIR is governed by the Boltzmann distribution ratio [13]. Hence, the relative sensitivity, which is a key parameter of the temperature sensing capability, is proportional to the energy gap of the corresponding TCELs [5]. However, the gap of the TCELs is upper bounded by thermal coupling conditions at about 2000 cm−1 [13]. A survey of the thermometry using the TCEL-based FIR technique reveals that Ho3 -doped NaLuF4 has the largest relative sensitivity in the operational temperature interval from 390 to 780 K [5]. However, further improvement of the sensing sensitivity is difficult to achieve with this TCEL-based FIR technique due to the restriction mentioned above. Herein, we report a strategy for FIR-based temperature sensing in β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 0146-9592/14/236687-04$15.00/0
core-shell NPs by virtue of the upconversion (UC) luminescence of Tm3 under 980 nm laser excitation. Pr3 is deliberately doped into the inert shell to test the laser heating effect. The thermal behaviors of the emissions originating from 3 F2;3 and 1 G4 states were investigated from 302 to 510 K. A high relative sensitivity was achieved and the maximum value reached 1.53% K−1 at 417 K, which is higher than all the previously reported rare earth ions doped temperature sensors using TCELs-based FIR technique. β-NaYF4 :20%Yb3 , 0.5%Tm3 core NPs and β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell NPs were synthesized in high boiling solvents oleic acid and octadecene according to the previously reported protocol [14,15]. The size and morphology of the NPs were acquired using high-resolution transmission electron microscopy (HRTEM, Model JEOL-2010). X-ray diffraction (XRD) patterns were obtained with an x-ray diffractometer (Rigaku-TTR-III) with Cu Kα radiation (λ 0.15418 nm). The emission spectra were obtained by a Jobin–Yvon HRD-1 double monochromator equipped with a Hamamatsu R928 photomultiplier. The signal was analyzed by an EG&G 7265 DSP Lock-in Amplifier. The temperature of the nanopowder was controlled by an attached copper post, whose temperature was controlled by a temperature controller (OMRON E5CC800) with a type-K thermocouple and a heating tube. The morphologies of the core and core-shell NPs are shown in Figs. 1(a) and 1(b), respectively. The core NPs are of uniform size and shape with an average diameter of 24 nm. After the epitaxial growth of the shell, the morphologies of the particles change from isotropic nanospheres into short nanorods with an average size of 27 nmdiameter × 39 nmlength. Both the core and core-shell NPs are monodispersed and can be easily dispersed in cyclohexane. Figure 1(c) shows the XRD patterns of the core and core-shell NPs, which agree well © 2014 Optical Society of America
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Fig. 1. TEM images of (a) β-NaYF4 :20%Yb3 , 0.5%Tm3 core and (b) β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell NPs. (c) XRD patterns of the core and core-shell NPs as well as the standard data of β-NaYF4 (JCPDS No. 16-0334) as a reference.
with the standard data of β-NaYF4 (JCPDS No. 16-0334), indicating a pure crystalline hexagonal phase. The β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 coreshell structure is designed to enhance the UC luminescence of Tm3 , as well as to cut off the energy transfer from Pr3 to Yb3 [16], as depicted in Fig. 2(a). Figure 2(b) shows the comparison of UC luminescence spectra of the core and core-shell NPs (powder) acquired under the same measurement conditions at room temperature. Characteristic emission peaks of Tm3 located at 361, 450, 473, 646, and 696 nm originate from 1 D2 → 3 H6 , 1 D → 3 F , 1 G → 3 H , 1 G → 3 F , and 3 F 3 2 4 4 6 4 4 2;3 → H6 transitions, respectively [17,18]. And the UC emissions are
Fig. 2. (a) Schematic diagram of the core-shell NPs. (b) UC emission spectra of the core and core-shell NPs (powder) under 980 nm laser excitation with power of 47 mW. The inserts are photographs of the UC luminescence for the core and core-shell NPs dispersed in cyclohexane.
Fig. 3. Schematic illustration of the UC processes for the asprepared core-shell NPs under 980 nm laser excitation. The dashed–dotted, dashed, curved, and full arrows indicate photon excitation, energy transfer, multiphonon relaxation, and emission processes, respectively.
dramatically enhanced for the core-shell NPs. As shown in the insert of Fig. 2(b), the core-shell NPs in cyclohexane exhibit bright blue emission under 980 nm laser excitation. It is concluded that a high UC efficiency is achieved in the core-shell NPs. Detailed UC processes for the luminescence of Tm3 are illustrated in Fig. 3 [18]. As for UC materials doped with Yb3 ions for temperature sensing, the heating effect caused by strong absorption of 980 nm excitation should be given more attention, which may lead to an enhanced temperature of the material’s surface compared with the surroundings [19]. Therefore, it is necessary to check and to reduce or even to eliminate this deviation for accurate temperature sensing. Pr3 -doped materials have been investigated for temperature sensing based on its TCELs 3 P1 and 3 P0 , in which the FIR varies with temperature according to a theoretical equation governed by the Boltzmann-type distribution ratio [7]. There is no evident heating effect when excited directly by the blue laser. In this work, before the investigation of the temperature-dependent UC luminescent properties, Pr3 ions doped in the shell were adopted to detect the heating effect caused by a 980 nm laser. Specifically, the sample was irradiated by a 980 nm laser for UC luminescence and a 447 nm laser to excite Pr3 for temperature sensing at the same time. The spot size on the sample illuminated by the 980 nm laser was estimated to be 2 mm × 3 mm. The power was adjusted through neutral density filters. It is demonstrated that the emission spectrum ranging from 510 to 570 nm of Pr3 under excitation of 980 nm with power of 47 mW or below coincides with that obtained when the 980 nm laser was turned off. So we conclude that the heating effect of 980 nm laser can be ignored by using a laser power not exceeding 47 mW. The UC emission spectra of the core-shell NPs in the range from 620 to 740 nm were measured at a series of temperatures between 302 and 510 K under 980 nm excitation of 47 mW. As shown in the normalized spectra in Fig. 4, the emission peak located at 696 nm increases gradually with the enhancement of temperature. What is more, the intensity increase becomes more and more pronounced in the same temperature interval with increasing temperature.
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Fig. 4. UC spectra in the region from 620 to 740 nm for the core-shell NPs at various temperatures from 302 to 510 K. The intensities are normalized to the peak around 646 nm.
The integral emission intensities of the two emission peaks at 646 and 696 nm, originating from the 1 G4 → 3 F4 and 3 F2;3 → 3 H6 transitions, respectively, exhibit remarkably different temperature-dependent behaviors. As shown in Fig. 5(a), the intensity of 3 F2;3 → 3 H6 transition (696 nm) increases monotonously with temperature in the whole investigated temperature range. The main reason for this is the increasing population of 3F 3 2;3 states from the lower H4 state with temperature due to thermal population, which further promotes the radiative transition from 3 F2;3 states [11,12,20]. However,
Fig. 5. (a) Integral emission intensities of 3 F2;3 → 3 H6 and 1 G4 → 3 F4 transitions varied with temperature under 980 nm excitation. (b) Temperature dependence of the ratio R between the integral intensities of 3 F2;3 → 3 H6 and 1 G4 → 3 F4 transitions for the core-shell NPs.
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the intensity of 646 nm first increases with the increase of temperature until 390 K and then decreases. From the energy level diagram, there is an obvious mismatch in the energy levels between Yb3 and Tm3 ; thus the upconverted population of 1 G4 can be affected by the participation of phonons, which is temperature dependent. As a result, the UC luminescence is enhanced due to the phonons’ assistance as temperature rises in a certain temperature range. On the other hand, as the temperature rises further, the population of 1 G4 may also decrease via phonon-assisted cross relaxation, leading to the decrease of UC luminescence. Furthermore, due to the fact that the 1 G4 state is populated from 3 H4 state through the energy transfer ET 3 shown in Fig. 3, the decreased population in 3 H4 state because of the thermal population of 3 F2;3 will also reduce the population of 1 G4 state. To sum up, the dominance of thermal population or phonon-assisted processes results in the different temperature dependences of the two UC emissions. Figure 5(b) is the temperature dependence of the ratio R between the integral emission intensities of 3 F2;3 → 3 H6 and 1 G4 → 3 F4 transitions. The experimental data are well fitted with the following equation: 3499 0.82: R 11485 × exp − T
(1)
Also notable, the slope of the fitting curve, namely the absolute change of the ratio R with temperature, increases gradually as the temperature rises in the range from 300 to 510 K. This remarkable variation of the ratio R relative to the temperature indicates that this material can be used as an optical temperature sensor under near infrared irradiation. Relative sensitivity S R , the relative change of the ratio R with respect to temperature variation, is a key parameter to characterize sensing ability of sensors. For the current material, the relative sensitivity in the range from 300 to 510 K is depicted in Fig. 6, which shows a relatively high temperature sensitivity in a wide temperature range from 350 to 510 K, with the maximal S R value reaching 1.53% K−1 at 417 K. The S R values of Er3 [4]
Fig. 6. Relative sensitivity based on 3 F2;3 → 3 H6 and 1 G4 → 3 F4 transitions of Tm3 as a function of temperature. The S R values of temperature sensors using TCELs in Er3 and Ho3 are presented for comparison.
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and Ho3 [5] doped temperature sensors using a TCELbased FIR technique were also presented in Fig. 6 for comparison. It is noted that the S R values are given only for temperatures above 390 K for Ho3 , due to poor thermal coupling of the two related levels, 5 F1 ∕5 G6 and 5 F ∕5 K , and extremely poorly resolved luminescence 2;3 8 from the upper energy levels 5 F1 ∕5 G6 at lower temperatures. The temperature sensitivities obtained here are superior to those based on TCELs in a wide temperature range. In conclusion, a strategy for FIR-based optical temperature sensing is put forward here to achieve higher relative sensitivity than those based on TCELs. By using the different thermal behaviors of the two UC emissions originating from 3 F2;3 and 1 G4 states of Tm3 in β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell NPs, high relative sensitivities are achieved in a wide temperature range from 350 to 510 K, with the maximum reaching 1.53% K−1 at 417 K. This is superior to other previously reported rare-earth ions doped temperature sensors using a TCEL-based FIR technique in the same temperature range. We conclude that the β-NaYF4 :20%Yb3 , 0.5%Tm3 ∕NaYF4 :1%Pr3 core-shell NPs are potentially excellent optical thermometers with a high relative sensitivity. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11374291, 11274299, 11204292, 11404321, and 11311120047). References 1. W. Xu, Q. Song, L. Zheng, Z. Zhang, and W. Cao, Opt. Lett. 39, 4635 (2014). 2. S. Zheng, W. Chen, D. Tan, J. Zhou, Q. Guo, W. Jiang, C. Xu, X. Liu, and J. Qiu, Nanoscale 6, 5675 (2014).
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