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Er3+ co-doped CaMoO4: a promising green upconversion phosphor for optical temperature sensing
Journal of Alloys and Compounds 639 (2015) 325–329
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Yb3+/Er3+ co-doped CaMoO4: a promising green upconversion phosphor for optical temperature sensing Feng Huang a,b, Yan Gao a, Jiangcong Zhou a, Ju Xu a,b, Yuansheng Wang a,b,⇑ a Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China b Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China
a r t i c l e
i n f o
Article history: Received 5 February 2015 Accepted 19 February 2015 Available online 25 March 2015 Keywords: Temperature sensing Optical spectroscopy Upconversion Rare earth
a b s t r a c t In this work, Er3+/Yb3+:CaMoO4 upconversion phosphor is fabricated, and its temperature dependent luminescent performance is investigated. Using fluorescence intensity ratio of the Er3+ 2H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2 emissions in CaMoO4 as thermometric index, the temperature sensitivity is found to be above 0.0095 K1 in the whole temperature range of 303–873 K, while the maximum value reaches as high as 0.0143 K1 at 573 K, revealing this phosphor to be a very promising optical temperature sensing material. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth doped upconversion materials have been widely investigated because of their broad application prospects in biosensing, display and solar cell [1–5]. Recently, applying these materials as temperature sensing medium has attracted great attention [6–10]. In this application, the fluorescence intensity ratio (FIR) of rare earth (RE) ion doped materials is utilized as the label of temperature. Through comparison of the fluorescence intensities from two thermally coupled levels of RE ions, temperature can be detected at a distance from the object. This noncontact thermal measurement is particularly favorable for the operations in electromagnetically and/or thermally harsh environments, where the contacting thermometry technique is unusable, such as monitoring temperature in power stations, oil refineries, coal mines, and building fire [9]. Er3+ is the most studied RE ion for this specific application, and 3+ Yb is usually employed as the co-doped ion to sensitize Er3+. Typically, the 2H11/2 and 4S3/2 levels of Er3+ are chosen as the two thermally coupled levels, whose emission intensity ratio would vary with the temperature and thus act as an index for probing the environment temperature [11,12]. It is well known that the Er3+ 2H11/2 ? 4I15/2 transition is hypersensitive [13], and the
⇑ Corresponding author at: Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.02.228 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
temperature sensitivity of the material varies remarkably for Er3+ ions in different hosts [8–12,14–19]. Hence, searching a suitable Er3+/Yb3+ host is of great significance in thermometry application. To date, several materials have been investigated as the Er3+/ 3+ Yb hosts for thermometry application. Among them, amorphous glasses are the most widely studied. However, the relatively low luminescence intensity and low thermal sensitivity restrict their practical applications. For the other materials, such as ZnO, Al2O3, Yb2Ti2O7, Gd2O3, BaTiO3 and YNbO4, the reported maximal temperature sensitivities are usually in the range of 0.0012– 0.0092 K1 [14–20]. To further promote the performance of the optical temperature sensor, searching other materials with higher temperature sensitivity is highly desired. Finding a host capable to provide suitable crystal field environment surrounding Er3+ dopant to enhance radiative probability of the 2H11/2 ? 4I15/2 hypersensitive transition is a key for obtaining high temperature sensitivity of Er3+. From this perspective, the scheelite typed ABO4 compounds (A = Ba, Sr, Ca; B = Mo, W) are the promising hosts. In such host, the RE dopant ion always inhabits in the A site surrounded with 8 oxygen forming a dodecahedron. The high coordination number and the low symmetry of this site benefit the radiative probability of Er3+ 2H11/2 ? 4I15/2 transition [13]. Temperature sensing properties of the lanthanide doped CaWO4 and SrWO4 had been reported previously [8,9,21], which exhibits certain advantages in both brightness and temperature sensitivity. Especially, the maximal sensitivity detected in Er3+/Yb3+:SrWO4 is the highest record reported so far [21]. However, many other scheelite typed ABO4 compounds are still
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Fig. 1. (a) XRD pattern of the 0.5 mol% Er3+/8 mol% Yb3+ co-doped CaMoO4 powder; bars on the bottom stand for the standard data of tetragonal CaMoO4 (PDF 29-0351); inset is the partially enlarged XRD pattern showing the shift of peaks. (b) SEM micrograph of this Er3+/Yb3+ co-doped CaMoO4 powder. (c) Room temperature upconversion spectrum of the prepared Er3+/Yb3+:CaMoO4 phosphor excited by 980 nm laser; inset is schematic illustration of the upconversion emission for the Er3+/Yb3+ co-doped materials. (d) Logarithmic dependence of 2H11/2 and 4S3/2 emission intensities on power density of excitation.
Fig. 2. (a) Upconversion spectra of the 0.5 mol% Er3+/8 mol% Yb3+ co-doped CaMoO4 phosphor under 980 nm excitation at various temperatures (from 303 K to 873 K); insets show photographs of the sample at 303 K, 423 K and 873 K, respectively. (b) Schematic illustration of Boltzmann distribution at different temperatures, and its effect on the thermal coupled level pair (2H11/2 and 4S3/2). (c) Curves of R versus T measured for the Er3+/Yb3+:CaMoO4 sample in three heating cycles. (d) Monolog plots of R as a function of inverse absolute temperature, fitted by Eq. (1).
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remaining to be explored as the optical temperature sensing medium. For example, CaMoO4, which is also an ideal upconversion host, has not been investigated for the temperature sensing application. It is known that, in the CaMoO4 matrix, the CaAO bonds are shorter than the SrAO bonds in SrWO4, while the Mo6+ ion exhibit smaller radius and thus stronger polarization than that of W6+ [22], as a consequence, the CaMoO4 host would endow the RE dopants with a more intense crystal field environment, and favor the luminescence and temperature sensitivity of Er3+ ions. Upon such consideration, in this work, the Er3+/Yb3+:CaMoO4 phosphor is synthesized, and its temperature dependent luminescent performance is investigated. Evidently, this phosphor exhibits remarkable temperature sensing properties, and therefore is a very promising optical thermometric material. 2. Experimental 2.1. Chemical reagents The used molybdenum trioxide (MoO3), anhydrous calcium chloride (CaCl2), ytterbium chloride six hydrate (YbCl36H2O), erbium chloride six hydrate (ErCl36H2O), citric acid (C6H8O7) and 35% ammonia (NH3H2O) were all purchased from Sinopharm Chemical Reagent Company. 2.2. Synthesis CaMoO4 powder was synthesized following sol–gel method. Typically, 1 mmol MoO3 was dissolved in a mixture of 5 ml 35% ammonia and 5 ml distilled water at 70 °C, forming solution A. In the other hand, 0.8725 mmol CaCl2, 0.08 mmol
Fig. 3. Calculated (red line) and measured sensitivities (circles) of the 0.5 mol% Er3+/8 mol% Yb3+ co-doped CaMoO4 phosphor at temperatures ranging from 303 K to 873 K. For comparison, the calculated sensitivities in various Er3+ doped or Er3+/ Yb3+ co-doped hosts are also presented. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
YbCl36H2O, 0.005 mmol ErCl36H2O and 2 mmol citric acid were dissolved in 10 ml distilled water and stirred at 70 °C for 30 min, forming solution B. Then, solution A was added into solution B. The mixture was heated at 100 °C for drying into a deep blue gel, which was then calcined at 800 °C for 2 h in ambient atmosphere, yielding white powdered products. 2.3. Characterization X-ray Diffraction (XRD) analyses were carried out with a powder diffractometer (DMAX 2500) using Cu Ka radiation (k = 0.154 nm). The microstructure and composition analyses were conducted on a field emission scanning electron microscope (SEM, JSM-6700F) working at 5 kV. Photoluminescence (PL) spectra were measured using a spectrouorometer (FLS920) equipped with a power-controllable 980 nm diode laser (K98S02F) and a temperature controlling stage (THMS 600).
3. Results and discussion As revealed by X-ray Diffraction (XRD) pattern in Fig. 1a, the 0.5 mol% Er3+/8 mol% Yb3+ co-doped powder product is indexed to be the scheelite structured CaMoO4 (PDF 29-0351). It is found that all the XRD peaks shift slightly toward the higher angle side, which had been previously discussed by Hadermann et al. [23]: when RE ions are incorporated into the lattice of scheelite CaMoO4, they usually substitute Ca2+ ions in the dodecahedral sites, accompanied with the generation of cation vacancies for charge compensation. The substitution of Ca2+ (r = 1.12 Å) by the dopant ions with smaller radii (Er3+, r = 1.00 Å; Yb3+, r = 0.98 Å) results in shrinkage of the CaMoO4 lattice. As revealed by scanning electron microscope (SEM) observation (Fig. 1b), the Er3+/ Yb3+:CaMoO4 particles exhibit irregular shapes with sizes of 0.5– 2.0 lm. As demonstrated by the room temperature spectrum (Fig. 1c), the upconversion luminescence of Er3+/Yb3+:CaMoO4 is dominated
Fig. 4. Dependences of C parameter and integrated green emission intensity on Er3+ concentration in CaMoO4.
Table 1 Optical thermometry parameters in various Er3+ doped or Er3+/Yb3+ co-doped hosts. Temperatures corresponding to the maximum sensitivities as well as the excitation wavelengths are also included. Phosphor
C value
DE (cm1)
Maximum sensitivity (K1)
Tmax (K)
Excitation wavelength (nm)
Ref.
CaMoO4 SrWO4 LiNbO3 La2(WO4)3 CaWO4 YNbO4 Gd2O3 Fluoroindate glass Silicate glass Al2O3 BaTiO3 NaYF4
27.1 21.1 32 18.12 15 13.19 3.19 11.2 7.92 9.63 9.97 8.06
733 601 868 707 629 706 518 770 895 670 833 752
0.0143 0.0149 0.0140 0.0097 0.0092 0.0073 0.0039 0.0055 0.0031 0.0051 0.0045 0.0040
575 403 628 510 455 473 300 550 550 495 600 535
980 980 980 971 980 976 976 1480 970 978 980 980
This work [21] [25] [24] [8] [20] [18] [26] [27] [16] [19] [28]
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Fig. 5. (a) Room temperature upconversion spectra of Er3+ 4I9/2?4I15/2 transition in the Er3+/Yb3+:CaMoO4 samples with 0.25 mol% and 2.0 mol% Er3+ concentrations. (b) Schematic illustration of the cross relaxation between Er3+ ions.
by green emissions originated from the 2H11/2, 4S3/2 ? 4I15/2 transitions of Er3+. Moreover, the logarithmic plots of intensity versus pumping power density for the Er3+ 2H11/2 and 4S3/2 emissions exhibit similar slopes (1.73 and 1.68, respectively), as shown in Fig. 1d, implying that slight disturbance in the excitation power would not induce remarkable change in the intensity ratio of the two emissions. These features favor the application of Er3+/ Yb3+:CaMoO4 in temperature sensing [14,17,24]. The upconversion emission spectra at various temperatures are presented in Fig. 2a. The intensity ratio of Er3+ 2H11/2 and 4S3/2 levels exhibits a remarkable dependence on the temperature, ascribing to the thermal coupling between 2H11/2 and 4S3/2 levels of Er3+, as schematically illustrated in Fig. 2b. According to Boltzmann distribution theory, the emission intensity ratio (R) of the two thermally coupled levels can be expressed by the following equation [8,14]:
RB
IH AH GH rH wH DE DE ¼ C exp ¼ exp kB T kB T IS AS GS rS wS
ð1Þ
where IH and IS are the integrated intensities of the 2H11/2 ? 4I15/2 (510–538 nm) and 4S3/2 ? 4I15/2 (540–565 nm) transitions, respectively; A and G the radiative transition rate and degeneracy for 2 H11/2 and 4S3/2 levels of Er3+, respectively; r the response of the detection system at the emission angular frequency w; DE the energy gap between 2H11/2 and 4S3/2 levels; kB the Boltzmann constant; and T the absolute temperature. The pre-exponential constant C is AHGrHw/ASGrSw. Curves of R versus T in three heating cycles measured for the Er3+/Yb3+:CaMoO4 sample are presented in Fig. 2c. Obviously, the dependence of intensity ratio on temperature shows a good repeatability, which is ascribed to the excellent thermal stability of the microstructure in phosphor evidenced by XRD and SEM analyses (Figs. S1–S2, Supplementary Information). As exhibited in Fig. 2d, the plots of Ln(R) versus 1/T conform to linear relationship, and are well fitted by Eq. (1). Through fitting, the parameters C and DE are evaluated to be 27.1 ± 0.4 and 733 ± 7 cm1, respectively. These two parameters are important factors for the sensitivity (S) of temperature detection, as described by the following equation:
S¼
dR DE DE DE ¼ C exp ¼R dT kB T kB T 2 kB T 2
ð2Þ
According to Eq. (2), the value of temperature sensitivity (S) highly depends on the parameter C, and a larger value of C generally results in a higher sensitivity. For the thermometric hosts reported previously, such as ZnO, Al2O3, Yb2Ti2O7, Gd2O3, and BaTiO3, the values of C are usually in the range of 2.8–11.2. Only
a few materials exhibit high C values (above 15). For example, Yang et al. reported that in La2(WO4)3 host the C value is 18.1 [24], while Rai and Swart et al. found that, in SrWO4 host, the C value is as high as 21.1 [21]. The highest C value reported so far is about 32, appeared in the submicron-sized LiNbO3 particles prepared by Cantelar et al. [25]. However, owing to the high mismatch in ionic radius and valence, LiNbO3 can accommodate merely lowconcentration doping of RE ions, therefore cannot yield bright upconversion emission. Herein, upon 8 mol% Yb/0.5 mol% Er codoping, the CaMoO4 host exhibits a remarkable high valued C parameter (up to 27.1), thus can be regarded as one of the most sensitive thermometric materials, to the best of our knowledge. On the other hand, the DE is also an important parameter. As it can be derived from Eq. (2), with the rising of the temperature, the sensitivity (S) would increase to its maximum at a certain temperature (Tmax), and then decrease dramatically. The value of Tmax follows the following equation:
2 dS d R DE DE ¼0 ¼ 2¼R 2 dT dT kB T 3max kB T max Consequently, Tmax = DE/2kB. Obviously, a high valued DE would induce the shift of Tmax to higher temperature, thus extending the detection range to higher temperature region [9]. For the free Er3+ ions, DE between 2H11/2 and 4S3/2 levels is about 700 cm1, while for Er3+ in various hosts this value varies from 500 cm1 to 900 cm1, depending on the crystal field environment surrounding the dopants [13]. In the CaMoO4 host, DE is evaluated to be 733 cm1, higher than those in SrWO4 (601 cm1) and La2(WO4)3 (707 cm1) [21,24]. Owing to the high valued C and DE parameters, the Er3+/ Yb3+:CaMoO4 powder exhibits excellent performance in temperature sensing. As revealed by the curve of sensitivity versus temperature demonstrated in Fig. 3, the sensitivity of Er3+/ Yb3+:CaMoO4 is above 0.0095 K1 in the whole temperature range of 303–873 K, and the maximum value reaches as high as 0.0143 K1 at 573 K, remarkably superior to most of the other materials reported previously. For comparison, the optical thermometry parameters in various Er3+ doped or Er3+/Yb3+ co-doped materials are listed in Table 1. It is worthy to note that, the maximal temperature sensitivity in the presently studied Er3+/ Yb3+:CaMoO4 is comparable with the highest record reported in Er3+/Yb3+:SrWO4 (0.0149 K1 at 403 K) [21], while the former is more sensitive than the latter when the temperature is higher than 430 K (see Fig. 3) owing to the relatively higher values of C and DE parameters in CaMoO4. Furthermore, another outstanding characteristic observed in the present sample is the bright green upconversion emission
F. Huang et al. / Journal of Alloys and Compounds 639 (2015) 325–329
under laser excitation of low power density (1.2 W/cm2) [8,14], as shown in the inset of Fig. 2a. Even at a high temperature of 873 K, the green luminescence is still clearly visible to the naked eye. Since the pumping laser would more or less disturb the surface temperature of the object, and introduce error in temperature detection, the operation of Er3+/Yb3+:CaMoO4 temperature sensor under low power laser excitation is beneficial for enhancing the detection accuracy. The upconversion properties of Er3+/Yb3+:CaMoO4 with various Er3+ concentrations are also investigated. As demonstrated in Fig. 4, with increase of Er3+ concentration from 0.1 mol% to 0.5 mol%, the upconversion emission intensifies, and the C value maintains around 27; when Er3+ concentration further increases, both the emission intensity and the C value reduce remarkably. The degradation of C at high Er3+ concentration could be attributed to the cross relaxation of 2H11/2 + 4I15/2 ? 4I9/2 + 4I13/2 between Er3+ ions, which reduces the population at 2H11/2 state as schematically illustrated in Fig. 5b. As presented in Fig. 5a, for the sample with high Er3+ concentration (2%), the Er3+ 4I9/2 ? 4I15/2 emission at 800 nm is remarkably enhanced, implying a stronger Er3+–Er3+ cross relaxation than those in the samples with low Er3+ concentration. Considering both the emission intensity and the thermal sensitivity, the optimal Er3+ concentration should be in the range of 0.5–1%. 4. Conclusions In summary, the temperature sensing properties of the Er3+/ 3+ Yb :CaMoO4 phosphor are demonstrated for the first time. This material exhibits bright upconversion emission under low power laser excitation, and the temperature sensitivity is found to be above 0.0095 K1 in the whole temperature range of 303–873 K, with the maximum value reaching as high as 0.0143 K1, comparable with the highest record reported so far. Evidently, Er3+/ Yb3+:CaMoO4 is a very promising material for the application in optical thermometry. Acknowledgements This work was supported by the National Natural Science Foundation of China (11204301, 21271170, 51472242, 51172231 and 11304312), and Natural Science Foundation of Fujian (2014J05071).
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2015.02. 228. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
[24] [25] [26] [27] [28]
F. Wang, X.G. Liu, Chem. Soc. Rev. 38 (2009) 976. X.Y. Huang, S.Y. Han, W. Huang, X.G. Liu, Chem. Soc. Rev. 42 (2013) 173. D.Q. Chen, Y.S. Wang, M.C. Hong, Nano Energy 1 (2012) 73. D.Q. Chen, P. Huang, Dalton Trans. 43 (2014) 11299. M.Y. Ding, D.Q. Chen, T.J. Chen, C.H. Lu, Y.R. Ni, Z.Z. Xu, Mater. Lett. 128 (2014) 101. S.A. Wade, S.F. Collins, G.W. Baxter, J. Appl. Phys. 94 (2003) 4743. V.K. Rai, Appl. Phys. B 88 (2007) 297. W. Xu, X.Y. Gao, L.J. Zheng, P. Wang, Z.G. Zhang, W.W. Cao, Appl. Phys. Exp. 5 (2012) 072201. W. Xu, Z.G. Zhang, W.W. Cao, Opt. Lett. 37 (2012) 4865. W. Xu, H. Zhao, Y.X. Li, L.J. Zheng, Z.G. Zhang, W.W. Cao, Sens. Actuators, B 188 (2013) 1096. B.S. Cao, Y.Y. He, Z.Q. Feng, Y.S. Li, B. Dong, Sens. Actuators, B 159 (2011) 8. Y.F. Bai, Y.X. Wang, K. Yang, X.R. Zhang, Y.L. Song, C.H. Wang, Opt. Commun. 281 (2008) 5448. S. Hüfner, Optical Spectra of Transparent Rare Earth Compounds, Academic Press, New York, 1978. B. Dong, B.S. Cao, Y.Y. He, Z. Liu, Z.P. Li, Z.Q. Feng, Adv. Mater. 24 (2012) 1987. X. Wang, X.G. Kong, Y. Yu, Y.J. Sun, H. Zhang, J. Phys. Chem. C 11 (2007) 15119. B. Dong, D.P. Liu, X.J. Wang, T. Yang, S.M. Miao, C.R. Li, Appl. Phys. Lett. 90 (2007) 181117. B.P. Singh, A.K. Parchur, R.S. Ningthoujam, P.V. Ramakrishna, S. Singh, P. Singh, S.B. Rai, R. Maalej, Phys. Chem. Chem. Phys. 16 (2014) 22665. S.K. Singh, K. Kumar, S.B. Rai, Sens. Actuators, A 149 (2009) 16. M.A.R.C. Alencar, G.S. Maciel, C.B. de Araújo, A. Patra, Appl. Phys. Lett. 84 (2004) 4753. A.K. Singh, S.K. Singh, B.K. Gupta, R. Prakash, S.B. Rai, Dalton Trans. 42 (2013) 1065. A. Pandey, V.K. Rai, V. Kumar, V. Kumar, H.C. Swart, Sens. Actuators, B 209 (2015) 352. J.H. Chung, J.H. Ryu, J.W. Eun, J.H. Lee, S.Y. Lee, T.H. Heo, K.B. Shim, Mater. Chem. Phys. 134 (2012) 695. V.A. Morozov, B.I. Lazoryak, S.Z. Shmurak, A.P. Kiselev, O.I. Lebedev, N. Gauquelin, J. Verbeeck, J. Hadermann, G.V. Tendeloo, Chem. Mater. 26 (2014) 3238. Y.M. Yang, C. Mi, SPIE 9044 (2013) 904408. M. Quintanilla, E. Cantelar, F. Cussó, M. Villegas, A.C. Caballero, Appl. Phys. Exp. 4 (2011) 022601. G.S. Maciel, L. de S. Menezes, A.S.L. Gomes, C.B. de Araújo, Y. Messaddeq, A. Florez, M.A. Aegerter, IEEE Photon. Technol. Lett. 7 (1995) 1474. Q. Zhou, C.R. Li, Z.F. Liu, S.F. Li, C.L. Song, Opt. Mater. 30 (2007) 513. S.H. Zhou, K.M. Deng, X.T. Wei, G.C. Jiang, C.K. Duan, Y.H. Chen, M. Yin, Opt. Commun. 291 (2013) 138.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Yb3+/Er3+ co-doped CaMoO4: a promising green upconversion phosphor for optical temperature sensing Feng Huang a,b, Yan Gao a, Jiangcong Zhou a, Ju Xu a,b, Yuansheng Wang a,b,⇑ a Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China b Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China
a r t i c l e
i n f o
Article history: Received 5 February 2015 Accepted 19 February 2015 Available online 25 March 2015 Keywords: Temperature sensing Optical spectroscopy Upconversion Rare earth
a b s t r a c t In this work, Er3+/Yb3+:CaMoO4 upconversion phosphor is fabricated, and its temperature dependent luminescent performance is investigated. Using fluorescence intensity ratio of the Er3+ 2H11/2 ? 4I15/2 and 4S3/2 ? 4I15/2 emissions in CaMoO4 as thermometric index, the temperature sensitivity is found to be above 0.0095 K1 in the whole temperature range of 303–873 K, while the maximum value reaches as high as 0.0143 K1 at 573 K, revealing this phosphor to be a very promising optical temperature sensing material. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth doped upconversion materials have been widely investigated because of their broad application prospects in biosensing, display and solar cell [1–5]. Recently, applying these materials as temperature sensing medium has attracted great attention [6–10]. In this application, the fluorescence intensity ratio (FIR) of rare earth (RE) ion doped materials is utilized as the label of temperature. Through comparison of the fluorescence intensities from two thermally coupled levels of RE ions, temperature can be detected at a distance from the object. This noncontact thermal measurement is particularly favorable for the operations in electromagnetically and/or thermally harsh environments, where the contacting thermometry technique is unusable, such as monitoring temperature in power stations, oil refineries, coal mines, and building fire [9]. Er3+ is the most studied RE ion for this specific application, and 3+ Yb is usually employed as the co-doped ion to sensitize Er3+. Typically, the 2H11/2 and 4S3/2 levels of Er3+ are chosen as the two thermally coupled levels, whose emission intensity ratio would vary with the temperature and thus act as an index for probing the environment temperature [11,12]. It is well known that the Er3+ 2H11/2 ? 4I15/2 transition is hypersensitive [13], and the
⇑ Corresponding author at: Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.jallcom.2015.02.228 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
temperature sensitivity of the material varies remarkably for Er3+ ions in different hosts [8–12,14–19]. Hence, searching a suitable Er3+/Yb3+ host is of great significance in thermometry application. To date, several materials have been investigated as the Er3+/ 3+ Yb hosts for thermometry application. Among them, amorphous glasses are the most widely studied. However, the relatively low luminescence intensity and low thermal sensitivity restrict their practical applications. For the other materials, such as ZnO, Al2O3, Yb2Ti2O7, Gd2O3, BaTiO3 and YNbO4, the reported maximal temperature sensitivities are usually in the range of 0.0012– 0.0092 K1 [14–20]. To further promote the performance of the optical temperature sensor, searching other materials with higher temperature sensitivity is highly desired. Finding a host capable to provide suitable crystal field environment surrounding Er3+ dopant to enhance radiative probability of the 2H11/2 ? 4I15/2 hypersensitive transition is a key for obtaining high temperature sensitivity of Er3+. From this perspective, the scheelite typed ABO4 compounds (A = Ba, Sr, Ca; B = Mo, W) are the promising hosts. In such host, the RE dopant ion always inhabits in the A site surrounded with 8 oxygen forming a dodecahedron. The high coordination number and the low symmetry of this site benefit the radiative probability of Er3+ 2H11/2 ? 4I15/2 transition [13]. Temperature sensing properties of the lanthanide doped CaWO4 and SrWO4 had been reported previously [8,9,21], which exhibits certain advantages in both brightness and temperature sensitivity. Especially, the maximal sensitivity detected in Er3+/Yb3+:SrWO4 is the highest record reported so far [21]. However, many other scheelite typed ABO4 compounds are still
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Fig. 1. (a) XRD pattern of the 0.5 mol% Er3+/8 mol% Yb3+ co-doped CaMoO4 powder; bars on the bottom stand for the standard data of tetragonal CaMoO4 (PDF 29-0351); inset is the partially enlarged XRD pattern showing the shift of peaks. (b) SEM micrograph of this Er3+/Yb3+ co-doped CaMoO4 powder. (c) Room temperature upconversion spectrum of the prepared Er3+/Yb3+:CaMoO4 phosphor excited by 980 nm laser; inset is schematic illustration of the upconversion emission for the Er3+/Yb3+ co-doped materials. (d) Logarithmic dependence of 2H11/2 and 4S3/2 emission intensities on power density of excitation.
Fig. 2. (a) Upconversion spectra of the 0.5 mol% Er3+/8 mol% Yb3+ co-doped CaMoO4 phosphor under 980 nm excitation at various temperatures (from 303 K to 873 K); insets show photographs of the sample at 303 K, 423 K and 873 K, respectively. (b) Schematic illustration of Boltzmann distribution at different temperatures, and its effect on the thermal coupled level pair (2H11/2 and 4S3/2). (c) Curves of R versus T measured for the Er3+/Yb3+:CaMoO4 sample in three heating cycles. (d) Monolog plots of R as a function of inverse absolute temperature, fitted by Eq. (1).
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remaining to be explored as the optical temperature sensing medium. For example, CaMoO4, which is also an ideal upconversion host, has not been investigated for the temperature sensing application. It is known that, in the CaMoO4 matrix, the CaAO bonds are shorter than the SrAO bonds in SrWO4, while the Mo6+ ion exhibit smaller radius and thus stronger polarization than that of W6+ [22], as a consequence, the CaMoO4 host would endow the RE dopants with a more intense crystal field environment, and favor the luminescence and temperature sensitivity of Er3+ ions. Upon such consideration, in this work, the Er3+/Yb3+:CaMoO4 phosphor is synthesized, and its temperature dependent luminescent performance is investigated. Evidently, this phosphor exhibits remarkable temperature sensing properties, and therefore is a very promising optical thermometric material. 2. Experimental 2.1. Chemical reagents The used molybdenum trioxide (MoO3), anhydrous calcium chloride (CaCl2), ytterbium chloride six hydrate (YbCl36H2O), erbium chloride six hydrate (ErCl36H2O), citric acid (C6H8O7) and 35% ammonia (NH3H2O) were all purchased from Sinopharm Chemical Reagent Company. 2.2. Synthesis CaMoO4 powder was synthesized following sol–gel method. Typically, 1 mmol MoO3 was dissolved in a mixture of 5 ml 35% ammonia and 5 ml distilled water at 70 °C, forming solution A. In the other hand, 0.8725 mmol CaCl2, 0.08 mmol
Fig. 3. Calculated (red line) and measured sensitivities (circles) of the 0.5 mol% Er3+/8 mol% Yb3+ co-doped CaMoO4 phosphor at temperatures ranging from 303 K to 873 K. For comparison, the calculated sensitivities in various Er3+ doped or Er3+/ Yb3+ co-doped hosts are also presented. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
YbCl36H2O, 0.005 mmol ErCl36H2O and 2 mmol citric acid were dissolved in 10 ml distilled water and stirred at 70 °C for 30 min, forming solution B. Then, solution A was added into solution B. The mixture was heated at 100 °C for drying into a deep blue gel, which was then calcined at 800 °C for 2 h in ambient atmosphere, yielding white powdered products. 2.3. Characterization X-ray Diffraction (XRD) analyses were carried out with a powder diffractometer (DMAX 2500) using Cu Ka radiation (k = 0.154 nm). The microstructure and composition analyses were conducted on a field emission scanning electron microscope (SEM, JSM-6700F) working at 5 kV. Photoluminescence (PL) spectra were measured using a spectrouorometer (FLS920) equipped with a power-controllable 980 nm diode laser (K98S02F) and a temperature controlling stage (THMS 600).
3. Results and discussion As revealed by X-ray Diffraction (XRD) pattern in Fig. 1a, the 0.5 mol% Er3+/8 mol% Yb3+ co-doped powder product is indexed to be the scheelite structured CaMoO4 (PDF 29-0351). It is found that all the XRD peaks shift slightly toward the higher angle side, which had been previously discussed by Hadermann et al. [23]: when RE ions are incorporated into the lattice of scheelite CaMoO4, they usually substitute Ca2+ ions in the dodecahedral sites, accompanied with the generation of cation vacancies for charge compensation. The substitution of Ca2+ (r = 1.12 Å) by the dopant ions with smaller radii (Er3+, r = 1.00 Å; Yb3+, r = 0.98 Å) results in shrinkage of the CaMoO4 lattice. As revealed by scanning electron microscope (SEM) observation (Fig. 1b), the Er3+/ Yb3+:CaMoO4 particles exhibit irregular shapes with sizes of 0.5– 2.0 lm. As demonstrated by the room temperature spectrum (Fig. 1c), the upconversion luminescence of Er3+/Yb3+:CaMoO4 is dominated
Fig. 4. Dependences of C parameter and integrated green emission intensity on Er3+ concentration in CaMoO4.
Table 1 Optical thermometry parameters in various Er3+ doped or Er3+/Yb3+ co-doped hosts. Temperatures corresponding to the maximum sensitivities as well as the excitation wavelengths are also included. Phosphor
C value
DE (cm1)
Maximum sensitivity (K1)
Tmax (K)
Excitation wavelength (nm)
Ref.
CaMoO4 SrWO4 LiNbO3 La2(WO4)3 CaWO4 YNbO4 Gd2O3 Fluoroindate glass Silicate glass Al2O3 BaTiO3 NaYF4
27.1 21.1 32 18.12 15 13.19 3.19 11.2 7.92 9.63 9.97 8.06
733 601 868 707 629 706 518 770 895 670 833 752
0.0143 0.0149 0.0140 0.0097 0.0092 0.0073 0.0039 0.0055 0.0031 0.0051 0.0045 0.0040
575 403 628 510 455 473 300 550 550 495 600 535
980 980 980 971 980 976 976 1480 970 978 980 980
This work [21] [25] [24] [8] [20] [18] [26] [27] [16] [19] [28]
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Fig. 5. (a) Room temperature upconversion spectra of Er3+ 4I9/2?4I15/2 transition in the Er3+/Yb3+:CaMoO4 samples with 0.25 mol% and 2.0 mol% Er3+ concentrations. (b) Schematic illustration of the cross relaxation between Er3+ ions.
by green emissions originated from the 2H11/2, 4S3/2 ? 4I15/2 transitions of Er3+. Moreover, the logarithmic plots of intensity versus pumping power density for the Er3+ 2H11/2 and 4S3/2 emissions exhibit similar slopes (1.73 and 1.68, respectively), as shown in Fig. 1d, implying that slight disturbance in the excitation power would not induce remarkable change in the intensity ratio of the two emissions. These features favor the application of Er3+/ Yb3+:CaMoO4 in temperature sensing [14,17,24]. The upconversion emission spectra at various temperatures are presented in Fig. 2a. The intensity ratio of Er3+ 2H11/2 and 4S3/2 levels exhibits a remarkable dependence on the temperature, ascribing to the thermal coupling between 2H11/2 and 4S3/2 levels of Er3+, as schematically illustrated in Fig. 2b. According to Boltzmann distribution theory, the emission intensity ratio (R) of the two thermally coupled levels can be expressed by the following equation [8,14]:
RB
IH AH GH rH wH DE DE ¼ C exp ¼ exp kB T kB T IS AS GS rS wS
ð1Þ
where IH and IS are the integrated intensities of the 2H11/2 ? 4I15/2 (510–538 nm) and 4S3/2 ? 4I15/2 (540–565 nm) transitions, respectively; A and G the radiative transition rate and degeneracy for 2 H11/2 and 4S3/2 levels of Er3+, respectively; r the response of the detection system at the emission angular frequency w; DE the energy gap between 2H11/2 and 4S3/2 levels; kB the Boltzmann constant; and T the absolute temperature. The pre-exponential constant C is AHGrHw/ASGrSw. Curves of R versus T in three heating cycles measured for the Er3+/Yb3+:CaMoO4 sample are presented in Fig. 2c. Obviously, the dependence of intensity ratio on temperature shows a good repeatability, which is ascribed to the excellent thermal stability of the microstructure in phosphor evidenced by XRD and SEM analyses (Figs. S1–S2, Supplementary Information). As exhibited in Fig. 2d, the plots of Ln(R) versus 1/T conform to linear relationship, and are well fitted by Eq. (1). Through fitting, the parameters C and DE are evaluated to be 27.1 ± 0.4 and 733 ± 7 cm1, respectively. These two parameters are important factors for the sensitivity (S) of temperature detection, as described by the following equation:
S¼
dR DE DE DE ¼ C exp ¼R dT kB T kB T 2 kB T 2
ð2Þ
According to Eq. (2), the value of temperature sensitivity (S) highly depends on the parameter C, and a larger value of C generally results in a higher sensitivity. For the thermometric hosts reported previously, such as ZnO, Al2O3, Yb2Ti2O7, Gd2O3, and BaTiO3, the values of C are usually in the range of 2.8–11.2. Only
a few materials exhibit high C values (above 15). For example, Yang et al. reported that in La2(WO4)3 host the C value is 18.1 [24], while Rai and Swart et al. found that, in SrWO4 host, the C value is as high as 21.1 [21]. The highest C value reported so far is about 32, appeared in the submicron-sized LiNbO3 particles prepared by Cantelar et al. [25]. However, owing to the high mismatch in ionic radius and valence, LiNbO3 can accommodate merely lowconcentration doping of RE ions, therefore cannot yield bright upconversion emission. Herein, upon 8 mol% Yb/0.5 mol% Er codoping, the CaMoO4 host exhibits a remarkable high valued C parameter (up to 27.1), thus can be regarded as one of the most sensitive thermometric materials, to the best of our knowledge. On the other hand, the DE is also an important parameter. As it can be derived from Eq. (2), with the rising of the temperature, the sensitivity (S) would increase to its maximum at a certain temperature (Tmax), and then decrease dramatically. The value of Tmax follows the following equation:
2 dS d R DE DE ¼0 ¼ 2¼R 2 dT dT kB T 3max kB T max Consequently, Tmax = DE/2kB. Obviously, a high valued DE would induce the shift of Tmax to higher temperature, thus extending the detection range to higher temperature region [9]. For the free Er3+ ions, DE between 2H11/2 and 4S3/2 levels is about 700 cm1, while for Er3+ in various hosts this value varies from 500 cm1 to 900 cm1, depending on the crystal field environment surrounding the dopants [13]. In the CaMoO4 host, DE is evaluated to be 733 cm1, higher than those in SrWO4 (601 cm1) and La2(WO4)3 (707 cm1) [21,24]. Owing to the high valued C and DE parameters, the Er3+/ Yb3+:CaMoO4 powder exhibits excellent performance in temperature sensing. As revealed by the curve of sensitivity versus temperature demonstrated in Fig. 3, the sensitivity of Er3+/ Yb3+:CaMoO4 is above 0.0095 K1 in the whole temperature range of 303–873 K, and the maximum value reaches as high as 0.0143 K1 at 573 K, remarkably superior to most of the other materials reported previously. For comparison, the optical thermometry parameters in various Er3+ doped or Er3+/Yb3+ co-doped materials are listed in Table 1. It is worthy to note that, the maximal temperature sensitivity in the presently studied Er3+/ Yb3+:CaMoO4 is comparable with the highest record reported in Er3+/Yb3+:SrWO4 (0.0149 K1 at 403 K) [21], while the former is more sensitive than the latter when the temperature is higher than 430 K (see Fig. 3) owing to the relatively higher values of C and DE parameters in CaMoO4. Furthermore, another outstanding characteristic observed in the present sample is the bright green upconversion emission
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under laser excitation of low power density (1.2 W/cm2) [8,14], as shown in the inset of Fig. 2a. Even at a high temperature of 873 K, the green luminescence is still clearly visible to the naked eye. Since the pumping laser would more or less disturb the surface temperature of the object, and introduce error in temperature detection, the operation of Er3+/Yb3+:CaMoO4 temperature sensor under low power laser excitation is beneficial for enhancing the detection accuracy. The upconversion properties of Er3+/Yb3+:CaMoO4 with various Er3+ concentrations are also investigated. As demonstrated in Fig. 4, with increase of Er3+ concentration from 0.1 mol% to 0.5 mol%, the upconversion emission intensifies, and the C value maintains around 27; when Er3+ concentration further increases, both the emission intensity and the C value reduce remarkably. The degradation of C at high Er3+ concentration could be attributed to the cross relaxation of 2H11/2 + 4I15/2 ? 4I9/2 + 4I13/2 between Er3+ ions, which reduces the population at 2H11/2 state as schematically illustrated in Fig. 5b. As presented in Fig. 5a, for the sample with high Er3+ concentration (2%), the Er3+ 4I9/2 ? 4I15/2 emission at 800 nm is remarkably enhanced, implying a stronger Er3+–Er3+ cross relaxation than those in the samples with low Er3+ concentration. Considering both the emission intensity and the thermal sensitivity, the optimal Er3+ concentration should be in the range of 0.5–1%. 4. Conclusions In summary, the temperature sensing properties of the Er3+/ 3+ Yb :CaMoO4 phosphor are demonstrated for the first time. This material exhibits bright upconversion emission under low power laser excitation, and the temperature sensitivity is found to be above 0.0095 K1 in the whole temperature range of 303–873 K, with the maximum value reaching as high as 0.0143 K1, comparable with the highest record reported so far. Evidently, Er3+/ Yb3+:CaMoO4 is a very promising material for the application in optical thermometry. Acknowledgements This work was supported by the National Natural Science Foundation of China (11204301, 21271170, 51472242, 51172231 and 11304312), and Natural Science Foundation of Fujian (2014J05071).
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2015.02. 228. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
[24] [25] [26] [27] [28]
F. Wang, X.G. Liu, Chem. Soc. Rev. 38 (2009) 976. X.Y. Huang, S.Y. Han, W. Huang, X.G. Liu, Chem. Soc. Rev. 42 (2013) 173. D.Q. Chen, Y.S. Wang, M.C. Hong, Nano Energy 1 (2012) 73. D.Q. Chen, P. Huang, Dalton Trans. 43 (2014) 11299. M.Y. Ding, D.Q. Chen, T.J. Chen, C.H. Lu, Y.R. Ni, Z.Z. Xu, Mater. Lett. 128 (2014) 101. S.A. Wade, S.F. Collins, G.W. Baxter, J. Appl. Phys. 94 (2003) 4743. V.K. Rai, Appl. Phys. B 88 (2007) 297. W. Xu, X.Y. Gao, L.J. Zheng, P. Wang, Z.G. Zhang, W.W. Cao, Appl. Phys. Exp. 5 (2012) 072201. W. Xu, Z.G. Zhang, W.W. Cao, Opt. Lett. 37 (2012) 4865. W. Xu, H. Zhao, Y.X. Li, L.J. Zheng, Z.G. Zhang, W.W. Cao, Sens. Actuators, B 188 (2013) 1096. B.S. Cao, Y.Y. He, Z.Q. Feng, Y.S. Li, B. Dong, Sens. Actuators, B 159 (2011) 8. Y.F. Bai, Y.X. Wang, K. Yang, X.R. Zhang, Y.L. Song, C.H. Wang, Opt. Commun. 281 (2008) 5448. S. Hüfner, Optical Spectra of Transparent Rare Earth Compounds, Academic Press, New York, 1978. B. Dong, B.S. Cao, Y.Y. He, Z. Liu, Z.P. Li, Z.Q. Feng, Adv. Mater. 24 (2012) 1987. X. Wang, X.G. Kong, Y. Yu, Y.J. Sun, H. Zhang, J. Phys. Chem. C 11 (2007) 15119. B. Dong, D.P. Liu, X.J. Wang, T. Yang, S.M. Miao, C.R. Li, Appl. Phys. Lett. 90 (2007) 181117. B.P. Singh, A.K. Parchur, R.S. Ningthoujam, P.V. Ramakrishna, S. Singh, P. Singh, S.B. Rai, R. Maalej, Phys. Chem. Chem. Phys. 16 (2014) 22665. S.K. Singh, K. Kumar, S.B. Rai, Sens. Actuators, A 149 (2009) 16. M.A.R.C. Alencar, G.S. Maciel, C.B. de Araújo, A. Patra, Appl. Phys. Lett. 84 (2004) 4753. A.K. Singh, S.K. Singh, B.K. Gupta, R. Prakash, S.B. Rai, Dalton Trans. 42 (2013) 1065. A. Pandey, V.K. Rai, V. Kumar, V. Kumar, H.C. Swart, Sens. Actuators, B 209 (2015) 352. J.H. Chung, J.H. Ryu, J.W. Eun, J.H. Lee, S.Y. Lee, T.H. Heo, K.B. Shim, Mater. Chem. Phys. 134 (2012) 695. V.A. Morozov, B.I. Lazoryak, S.Z. Shmurak, A.P. Kiselev, O.I. Lebedev, N. Gauquelin, J. Verbeeck, J. Hadermann, G.V. Tendeloo, Chem. Mater. 26 (2014) 3238. Y.M. Yang, C. Mi, SPIE 9044 (2013) 904408. M. Quintanilla, E. Cantelar, F. Cussó, M. Villegas, A.C. Caballero, Appl. Phys. Exp. 4 (2011) 022601. G.S. Maciel, L. de S. Menezes, A.S.L. Gomes, C.B. de Araújo, Y. Messaddeq, A. Florez, M.A. Aegerter, IEEE Photon. Technol. Lett. 7 (1995) 1474. Q. Zhou, C.R. Li, Z.F. Liu, S.F. Li, C.L. Song, Opt. Mater. 30 (2007) 513. S.H. Zhou, K.M. Deng, X.T. Wei, G.C. Jiang, C.K. Duan, Y.H. Chen, M. Yin, Opt. Commun. 291 (2013) 138.