A new mechanism for temperature sensing based on

Accepted Manuscript Title: A new mechanism for temperature sensing based on the thermal population of 7 F2 state in Eu3+ Author: Shaoshuai Zhou Xinyue Li Xiantao Wei Changkui Duan Min Yin PII: DOI: Reference:

S0925-4005(16)30377-X http://dx.doi.org/doi:10.1016/j.snb.2016.03.082 SNB 19887

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

7-1-2016 18-3-2016 18-3-2016

Please cite this article as: Shaoshuai Zhou, Xinyue Li, Xiantao Wei, Changkui Duan, Min Yin, A new mechanism for temperature sensing based on the thermal population of 7F2 state in Eu3+, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.03.082 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A new mechanism for temperature sensing based on the thermal population of 7F2 state in Eu3+ Shaoshuai Zhou,a Xinyue Li,b Xiantao Wei,b Changkui Duan,b, Min Yinb, a

Department of Physics, Qufu Normal University, Qufu, Shandong 273165, China

b

Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China



Corresponding author. Tel: +86 (551) 63606287. E-mail address: [email protected] (C. Duan), [email protected] (M. Yin). 1

The highlights of this work: 1. A new optical temperature sensing mechanism is proposed based on the temperature-dependent population of the thermally-excited state 7F2 of Eu3+ ion in Y2O3: 5% Eu3+. 2. The thermal coupling property of the ground state 7F0 and its adjacent thermally-excited state 7 F2 of Eu3+ ion was firstly discussed. 3. A high relative sensitivity is achieved to be 3.93% K-1 at 150 K.

2

Abstract The thermal coupling property of the ground state 7F0 and its adjacent thermally-excited state 7

F2 of Eu3+ ion was firstly discussed and a new temperature sensing mechanism was proposed

based on the thermal population of 7F2 state in Y2O3: 5% Eu3+ powder sample. Under a constant 611.2 nm excitation, the fluorescence originating from 5D0 state increases monotonously as the temperature rises in the region from 150 K to 300 K. The reason leading to this temperature-dependent fluorescence behavior was analyzed in detail. It is demonstrated that this present work provides a new mechanism for non-contact optical thermometry.

Keywords: Temperature sensing; Thermal population; Combustion method

1. Introduction Optical temperature sensing based on rare earth (RE) doped luminescent materials has been extensively investigated in recent years. Such non-contact operating mode overcomes many disadvantages or helplessness for traditional contact thermometers. For instance, they can be used in many complex situations where the contact thermometers are not feasible, such as the corrosive environment, intracellular circumstances, and fast-moving objects temperature detection. Up to now, many optical temperature sensing techniques have been developed aimed at achieving high detection sensitivity, such as the temperature-dependent emission intensity [1, 2], fluorescence intensity ratio (FIR) [3-21], and lifetime [22-24]. In particular, the FIR technique based on temperature-dependent FIR between thermally coupled energy levels (TCELs) gained most attentions due to its intrinsic immunity to some external disturbances during the detection process, such as the fluorescence loss, the amount of emitters and the fluctuations of excitation [20]. RE ions possessing TCELs were widely investigated using this approach, such as Nd3+ [6-8], Gd3+ [9], Dy3+ [10], Ho3+ [11], Er3+ [12-16], Tm3+ [17, 18]. However, 980 nm laser excitation was usually required for most of these reported optical thermometers, which may cause heating effect due to the strong absorption of the sensitizer and the low upconversion efficiency, influencing the measurement accuracy especially for low temperature detection [16]. 3

To the best of our knowledge, all of the TCELs-based temperature sensors focus on optically-excited states, the temperature-dependent population of thermally-excited state which is adjacent to the ground state has never been explored for such a purpose. In this work, a new temperature sensing mechanism was proposed based on the thermal population of 7F2 state of Eu3+ ion in Y2O3: 5% Eu3+ powder sample, which is an excellent red phosphor for its efficient luminescence and good thermal stability. We show that the temperature dependence of the Eu3+ population in 7F2 state can be captured through the intensity variation of the luminescence originating from

5

D0 state under a constant 611.2 nm excitation. The emission spectra

corresponding to the transition of 5D0 → 7F4 were recorded at a series of temperatures from 150 K to 300 K under the excitation of 611.2 nm. And the reason of this temperature-dependent variation was analyzed. 2. Experimental Y2O3: 5% Eu3+ powder sample was synthesized via combustion method [25]. Firstly, RE(NO3)3 standard solutions (Y/Eu = 95:5, 6 mmol in total), prepared by dissolving the respective RE2O3 (99.99%) in hot dilute nitric acid, were added into 24 mL aqueous solution containing 0.564 g glycine. Then, the pH value of the mixture was adjusted to 7 using dilute ammonia. After stirring for 20 min, the solution was put into a muffle furnace preheated to 600 °C. Several minutes later, the combustion process finished and fluffy powder was obtained. Finally, the fluffy powder was collected and sintered at 1000 °C for 2 h. Powder X-ray diffraction (XRD) pattern of the Y2O3: 5% Eu3+ powder sample was performed using an X-ray diffractometer (Model Rigaku-TTR-III) with Cu Kα radiation (λ = 0.15418 nm). The morphology was obtained by a Field Emission Scanning Electron Microscope (FESEM, Model JSM-6700F, JEOL, Japan). The emission and excitation spectra were measured by a double monochromator (Model Jobin-Yvon HRD-1) equipped with a Hamamatsu R928 photomultiplier. The excitation source was a tunable laser system (Model Opolette 355 LD OPO system) with laser line width of 4-7 cm-1 and pulse duration of 7 ns. The decay curves were recorded using a Tektronix TDS2024 digital storage oscilloscope. For the measurements at low temperature, the powder sample was pressed into a round tablet with thickness of 0.8 mm and diameter of 8.0 mm. 4

And then the tablet was glued to a copper pedestal fixed in a closed-cycle cryostat with cryogenic glue. Temperature of the sample was controlled by the attached copper pedestal, whose temperature was controlled over the range of 150 K to 300 K by a WC50 helium compressor and a Lake Shore Model 321 temperature controller. 3. Results and discussion The as-prepared powder sample was composed of irregular shaped nanoparticles aggregated seriously, which can be seen from the SEM image shown in Fig. 1(a). The average particle size is about 100 nm. The crystal structure was identified by the XRD pattern in Fig. 1(b). All the diffraction peaks of the sample match well with the standard Y2O3 (JCPDS No. 86-1326), indicating that a pure Y2O3 structure has been successfully obtained. Under a certain excitation, the Eu3+ ions thermally-activated at the lowest stark level of 7F2 multiplet can be excited to 5D0 state due to the resonance absorption, followed by the radiative transitions from 5D0 to 7FJ state. Theoretically, when the temperature varies, the population of Eu3+ ions situated at the lowest stark level of 7F2 multiplet will change correspondingly to meet the Boltzmann distribution among this level and its adjacent levels. As a result, the population of Eu3+ ions in

5

D0 state will vary accordingly under such a constant excitation, leading to a

temperature-dependent emission of 5D0 → 7FJ transition. Therefore, this emission can be used to show the temperature variation. By virtue of this inspiration, we investigated the temperature-dependent emission of 5D0 → 7

F4 transition in the as-prepared Y2O3: 5% Eu3+ powder sample under excitation of 611.2 nm,

which is exactly suitable for the resonance absorption from the lowest stark level of 7F2 multiplet to 5D0 state. The emission spectra from 680 nm to 720 nm were measured at various temperatures from 150 K to 300 K corresponding to the transition of 5D0 → 7F4 [26]. It is clearly found that the luminescent intensity increases monotonously with the rise of temperature from Fig. 2. In order to depict this temperature-dependent variation quantitatively, the integral intensities from 700 nm to 715 nm at various temperatures are calculated, which are shown in Fig. 3. The experimental data can be well fitted with the following equation:

 884  .  T 

I04  157  exp  5

(1)

Where I04 represents the integral emission intensity of 5D0 → 7F4 transition ranging from 700 nm to 715 nm. The insert in Fig. 3 shows the relative sensitivity SR, which is defined as the relative change of the integral intensity I04 with respect to temperature variation. We found that the SR can reach high value at low temperature and 3.93% K-1 is obtained at 150 K. The reason for this temperature-dependent variation is quantitatively discussed in the following. As is known to all, the energy gap of 7F0 and 7F1 and that of 7F1 and 7F2 for Eu3+ ion are just several hundreds of wave numbers, so a thermal equilibrium can be easily realized between the ground state 7F0 and all of the stark energy levels of 7F1 and 7F2 multiplets. When the temperature rises, the populations of high stark energy levels increase. Therefore, the increase of the luminescent intensity I04 with temperature is attributed to the enhanced resonance absorption, resulting from the temperature-dependent thermal population of the lowest stark energy level of 7

F2 multiplet. Fig. 4 is the emission spectra under excitation of 465.2 nm, from which the values of

the energy gaps between 7F0 and these stark levels of 7F1 and 7F2 can be obtained. It is noted that Eu3+ can replace Y sites of C2 (75%) and S6 (25%) point group symmetries, with the forced electric dipole transition of Eu3+ in the latter site parity forbidden. Both 611.2 nm and 465.2 nm predominantly excite Eu3+ in C2 site. Fig. 4 gives the Lorentz fitting of the five 5D0 → 7F2 emission peaks under 465.2 nm excitation. According to the fitting results, the energy gaps between the ground state 7F0 and the five stark levels of 7F2 are obtained to be 872 cm-1, 920 cm-1, 960 cm-1, 1282 cm-1, 1380 cm-1, which agree well with the values tabulated in Ref. [27] for Eu3+ in C2 site. This shows that the emission of Eu3+ in S6 site is negligible as expected. With these energy gaps, the energy diagram of Eu3+ including 7F0 and the stark levels of 7F1 and 7F2 is depicted in Fig. 5 and these energy levels are marked from L0 to L8 as shown. The radiative transitions corresponding to the emission peaks in Fig. 4 are also given in Fig. 5. The populations of Eu3+ ions in these nine energy levels (L0-L8) follow a Boltzmann-type distribution [20]:

 E  Ni  exp  ij  ,i  j. Nj  kT 

(2)

Where Ni (Nj) is the number of ions in level Li (Lj), ΔEij is the energy gap between level Li and level Lj, k is the Boltzmann constant, and T is the absolute temperature. Considering the temperature range involved in this work, we reasonably ignored the 6

populations of 7F3 multiplet and higher energy states because of their rare population at such temperature resulted from the large energy gap separated with the ground state 7F0. Based on this premise, the total number of Eu3+ ions N in these nine energy levels (L0-L8) can be regarded as a constant value. 8

N

i

i 0

N.

(3)

The number of Eu3+ ions in the lowest stark energy level of 7F2 state (L4) N4 can be obtained according to Eq. (2) and Eq. (3). Since the emission intensity I04 is proportional to the population of 5D0 state which is in proportion to N4 under a constant laser excitation of 611.2 nm, the emission intensity I04 is given by

I 04  A 04 04 N4 8 3  E   E   A 04 04 N exp  4 j   1  exp  j 4  kT j 5    kT   j 0 8 3  E   E   B exp  4 j   1  exp  j 4  . j 5  kT   kT   j 0

(4)

Where A, σ04, ω04 are the absorption cross section of the transition from L4 to 5D0 state, the emission cross section and the angular frequency of the fluorescence transition from 5D0 to 7F4 state, ΔE4j (ΔEj4) is the energy gap between the stark energy level L4 and another level Lj, the value of which is obtained from the emission peak positions in Fig. 4, and B is a constant. However, the proportion of 611.2 nm in the whole excitation region contributing to the transition from L4 to 5D0 state, which is relevant to temperature due to the temperature-dependent homogeneous broadening of the excitation spectra, is not taken into account in Eq. (4). As shown in Fig. 6, the excitation spectral line width broadens and the intensity increases when the temperature rises. And at the same time, the proportion of 611.2 nm decreases. Thus, the experimental data should be revised by introducing a weighting factor to coincide with the theoretical Eq. (4). In order to describe this proportion, we define a weighting factor fT as Eq. (5).

fT 

I (611.2)



613

609

I   d 

.

(5)

Where Δλ is the laser spectral line width for 611.2 nm, I(λ) is the peak intensity at wavelength of λ in the excitation spectra. These fT values are listed in Fig. 6, which were obtained from the 7

excitation spectra measured at a series of temperatures. Obviously, the fT value decreases as the temperature rises in the whole investigated temperature region. To eliminate the deviation caused by the temperature-related homogeneous broadening, the experimental data were revised by multiplying by f150/fT. As shown in Fig. 3, the revised experimental data with considering the weighting factor fT were well fitted with Eq. (4), verifying the rationality of our analysis. Moreover, the decay curves for the emission from 5D0 state were measured by monitoring the luminescence at 708 nm under excitation of 611.2 nm pulsed laser. Fig. 7 shows the normalized decay profiles at a series of temperatures, which can be well fitted with a single-exponential function. The lifetime is obtained to be 1.3 ms in the whole investigated temperature range, indicating that there is no thermal quenching contributing to this temperature-dependent emission variation. 4. Conclusion A new mechanism for optical temperature sensing is put forward here based on the temperature-dependent population of thermally-excited state 7F2 of Eu3+ ion in Y2O3: 5% Eu3+ powder sample. Under a constant 611.2 nm excitation, the luminescence corresponding to 5D0 → 7

F4 transition increases monotonously as the temperature rises in the region from 150 K to 300 K.

A high relative sensitivity is achieved to be 3.93% K-1 at 150 K. It is demonstrated that the thermal population of 7F2 state of Eu3+ results in the variation of the population of 5D0 state under the constant excitation of 611.2 nm, further leading to such temperature-dependent fluorescence behavior. Acknowledgments This work was financially supported by the National Key Basic Research Program of China (Grant No. 2013CB921800) and the National Natural Science Foundation of China (Grant Nos. 11374291, 11274299 and 11574298).

8

Biographies

Shaoshuai Zhou is a lecturer in Department of Physics, Qufu Normal University. He received his Ph.D. in University of Science and Technology of China in 2015. His research is focused on the rare earth ions doped luminescent materials for temperature sensing.

Xinyue Li is currently pursuing her Ph.D. in University of Science and Technology of China. Her research is focused on the luminescent materials and their luminescent properties for the application on temperature sensor.

Xiantao Wei is a senior lab master in University of Science and Technology of China. He received his Ph.D. in USTC in 2010. His research is mainly on the mechanism of luminescence dynamics.

Changkui Duan is a professor in Department of Physics, University of Science and Technology of China. He received his Ph.D. in USTC in 1998. His research is mainly on theoretical research of solid light-emitting materials and solid physics-based quantum computing system.

Min Yin is a professor in Department of Physics, University of Science and Technology of China. He received his Ph.D. in USTC in 1995. His research is focused on luminescent materials doped with rare earth ions.

9

References [1] E. H. Song, S. Ding, M. Wu, S. Ye, F. Xiao, G. P. Dong, Q. Y. Zhang, Temperature-tunable upconversion luminescence of perovskite nanocrystals KZnF3: Yb3+, Mn2+, J. Mater. Chem. C 1 (2013) 4209.

[2] L. Shi, C. Li, Q. Su, Temperature-dependent photoluminescence properties of Sr2CeO4: Eu3+ and its modification for use in fluorescence thermometry, Opt. Lett. 36 (2011) 582-584. [3] S. N. Zhao, L. J. Li, X. Z. Song, M. Zhu, Z. M. Hao, X. Meng, L. L. Wu, J. Feng, S. Y. Song, C. Wang, H. J. Zhang, Lanthanide Ion Codoped Emitters for Tailoring Emission Trajectory and Temperature Sensing, Adv. Funct. Mater. 25 (2015) 1463-1469. [4] Y. Zhou, F. Qin, Y. Zheng, Z. Zhang, W. Cao, Fluorescence intensity ratio method for temperature sensing, Opt. Lett. 40 (2015) 4544-4547. [5] X. Wang, Q. Liu, Y. Bu, C. S. Liu, T. Liu, X. Yan, Optical temperature sensing of rare-earth ion doped phosphors, RSC Adv. 5 (2015) 86219-86236. [6] C. Pérez-Rodríguez, L. L. Martín, S. F. León-Luis, I. R. Martín, K. K. Kumar, C. K. Jayasankar, Relevance of radiative transfer processes on Nd3+ doped phosphate glasses for temperature sensing by means of the fluorescence intensity ratio technique, Sens. Actuators, B 195 (2014) 324-331. [7] X. Tian, X. Wei, Y. Chen, C. Duan, M. Yin, Temperature sensor based on ladder-level assisted thermal coupling and thermal-enhanced luminescence in NaYF4: Nd3+, Opt. Express 22 (2014) 30333-30345. [8] W. Xu, Q. Song, L. Zheng, Z. Zhang, W. Cao, Optical temperature sensing based on the near-infrared emissions from Nd3+/Yb3+ codoped CaWO4, Opt. Lett. 39 (2014) 4635-4638. [9] K. Zheng, Z. Liu, C. Lv, W. Qin, Temperature sensor based on the UV upconversion luminescence of Gd3+ in Yb3+–Tm3+–Gd3+ codoped NaLuF4 microcrystals, J. Mater. Chem. C 1 (2013) 5502. [10] Z. Boruc, M. Kaczkan, B. Fetlinski, S. Turczynski, M. Malinowski, Blue emissions in Dy3+ doped Y4Al2O9 crystals for temperature sensing, Opt. Lett. 37 (2012) 5214-5216. [11] S. Zhou, S. Jiang, X. Wei, Y. Chen, C. Duan, M. Yin, Optical thermometry based on upconversion luminescence in Yb3+/Ho3+ co-doped NaLuF4, J. Alloy. Compd. 588 (2014) 654-657. [12] F. Vetrone, R. Naccache, A. Zamarrόn, A. J. de la Fuente, F. Sanz-Rodríguez, L. M. Maestro, E. M. Rodriguez, D. Jaque, J. G. Solé, J. A. Capobianco, Temperature Sensing Using Fluorescent Nanothermometers, ACS Nano 4 (2010) 3254-3258. [13] P. Du, L. Luo, W. Li, Q. Yue, H. Chen, Optical temperature sensor based on upconversion emission in Er-doped ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ceramic, Appl. Phys. Lett. 104 (2014) 152902. [14] B. P. Singh, A. K. Parchur, R. S. Ningthoujam, P. V. Ramakrishna, S. Singh, P. Singh, S. B. Rai, R. Maalej, Enhanced up-conversion and temperature-sensing behaviour of Er3+ and Yb3+ co-doped Y2Ti2O7 by incorporation of Li+ ions, Phys. Chem. Chem. Phys. 16 (2014) 22665-22676. [15] J. Cao, X. Li, Z. Wang, Y. Wei, L. Chen, H. Guo, Optical thermometry based on up-conversion luminescence behavior of self-crystallized K3YF6: Er3+ glass ceramics, Sens. Actuators, B 224 (2016) 507-513. 10

[16] S. Zhou, K. Deng, X. Wei, G. Jiang, C. Duan, Y. Chen, M. Yin, Upconversion luminescence of NaYF4: Yb3+, Er3+ for temperature sensing, Opt. Commun. 291 (2013) 138-142. [17] L. Xing, W. Yang, D. Ma, R. Wang, Effect of crystallinity on the optical thermometry sensitivity of Tm3+/Yb3+ codoped LiNbO3 crystal, Sens. Actuators, B 221 (2015) 458-462. [18] L. Xing, Y. Xu, R. Wang, W. Xu, Z. Zhang, Highly sensitive optical thermometry based on upconversion emissions in Tm3+/Yb3+ codoped LiNbO3 single crystal, Opt. Lett. 39 (2014) 454-457. [19] S. Zhou, G. Jiang, X. Li, S. Jiang, X. Wei, Y. Chen, M. Yin, C. Duan, Strategy for thermometry via Tm3+-doped NaYF4 core-shell nanoparticles, Opt. Lett. 39 (2014) 6687-6690. [20] S. A. Wade, S. F. Collins, G. W. Baxter, Fluorescence intensity ratio technique for optical fiber point temperature sensing, J. Appl. Phys. 94 (2003) 4743. [21] P. Du, L. Luo, J. S. Yu, Low-temperature thermometry based on upconversion emission of Ho/Yb-codoped Ba0.77Ca0.23TiO3 ceramics, J. Alloys Comp. 632 (2015) 73-77. [22] N. Rakov, S. A. Vieira, Q. P. S. Silva, G. S. Maciel, Facile fabrication of Eu3+-doped lanthanum oxyfluoride powders by combustion processes and temperature analysis of its fluorescence for thermal sensor application, Sens. Actuators, B 209 (2015) 407-412. [23] D. Chen, Z. Wan, Y. Zhou, Dual-phase nano-glass-ceramics for optical thermometry, Sens. Actuators, B 226 (2016) 14-23. [24] Ol. A. Savchuk, P. Haro-González, J. J. Carvajal, D. Jaque, J. Massons, M. Aguilό, F. Díaz, Er:Yb:NaY2F5O up-converting nanoparticles for sub-tissue fluorescence lifetime thermal sensing, Nanoscale, 6 (2014) 9727-9733. [25] L. Wu, J. Wen, Y. Qin, M. Yin, S. Xia, A. Ablekem, Experimental and theoretical analysis on the charge transfer band of Y2O3:Eu nanocrystals, J. Rare Earth. 29 (2011) 1009-1012. [26] G. Gao, L. Wondraczek, Spectral asymmetry and deep red photoluminescence in Eu3+-activated Na3YSi3O9 glass ceramics, Opt. Mater. Express 4 (2014) 476-485. [27] R. P. Leavitt, J. B. Gruber, N. C. Chang, C. A. Morrison, Optical spectra, energy levels, and crystal-field analysis of tripositive rare-earth ions in Y2O3. II. Non-Kramers ions in C2 sites, J. Chem. Phys. 76 (1982) 4775-4788.

11

Figure Captions: Fig. 1. (a) SEM image and (b) XRD pattern of the as-prepared Y2O3: 5% Eu3+ powder sample. Fig. 2. Emission spectra corresponding to the 5D0 → 7F4 transition of Eu3+ at a series of temperatures under 611.2 nm excitation. The inset is the schematic diagram for the excitation and emission. L-7F2 denotes the lowest stark level of 7F2 multiplet. Fig. 3. The temperature-dependent integral fluorescence intensity and revised integral fluorescence intensity from 700 nm to 715 nm. The inset is the relative sensitivity curve based on the experimental data.

Fig. 4. Emission spectra under excitation of 465.2 nm at room temperature and the Lorentz fitting for the 5D0 → 7F2 transition. The inset is the amplification from 605 nm to 640 nm. Fig. 5. The schematic stark energy level diagram for 7F0,1,2 states of Eu3+ ion. Fig. 6. Excitation spectra at a series of temperatures monitored at 708 nm and the fT values at different temperatures. Fig. 7. Decay curves for the 5D0 → 7F4 transition at 708 nm at a series of temperatures.

12

Figure 1

Figure 2

13

Figure 3

Figure 4

14

Figure 5

Figure 6

15

Figure 7

16