Investigation of SrB4O7: Sm2+ as a Multimode Temperature Sensor

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Investigation of SrB4O7: Sm2+ as a Multimode Temperature Sensor with High Sensitivity Zhongmin Cao, Xiantao Wei, Lu Zhao, Yonghu Chen, and Min Yin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10917 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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Investigation of SrB4O7:Sm2+ as a Multimode Temperature Sensor with High Sensitivity

Zhongmin Cao, Xiantao Wei, Lu Zhao, Yonghu Chen*, Min Yin∗

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

Abstract Sm2+-doped SrB4O7 was synthesized for high-sensitivity thermometry. A high thermal-sensitive fluorescence intensity ratio and fluorescence lifetime were achieved in a wide temperature range. At 500 K, the relative sensitivity of the temperature sensing was 2.16% K-1 for the fluorescence intensity ratio and 3.36% K-1 for the fluorescence lifetime. Furthermore, the fluorescence color shifted dramatically from deep red at room temperature to green at 700 K. Based on this color change, a visible temperature field was obtained on quartz glass covered with our sample, which made the thermal conduction and distribution visible to the human eye. The temperature of the temperature field was determined using two methods. These outstanding properties, combined with the high sensitivity, multimode for temperature sensing and thermal stability of the sample, make SrB4O7:Sm2+ a promising material for highly sensitive thermometry applications. Key word: thermometry, temperature field, fluorescence intensity ratio, fluorescence color, decay lifetime, high sensitivity

1. Introduction



Corresponding authors: [email protected] (Y. Chen) and [email protected] (M. Yin).

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Rare-earth-ion-doped materials have drawn great attention as non-contact optical temperature sensors that are able to operate in environments where traditional thermometers are not feasible, such as in the tissue of living organisms

1-3

and

submicron temperature sensing. The temperature dependence of the decay lifetime 4-14 and the fluorescence intensity ratio (FIR)

15-27

are the two main mechanisms for

fluorescent thermometry and have the merits of rapid response, high sensitivity and spatial resolution. Sm2+ has the same 4f6 ground state configuration as Eu3+ but with a much lower excited 4f55d1 configuration than that of Eu3+ 6. The electric dipole (ED) transitions between the energy levels of the 4f6 and 4f55d1 configuration are parity allowed and strong, in contrast to the weak ED transitions between the 4f6 levels, which are intrinsically forbidden but allowed by mixing states of opposite parity due to odd-parity crystal-field interactions. While 4f orbitals are shielded by the outer electrons and are insensitive to environmental changes, 5d orbits strongly interact with ligands, leading to large crystal-field splitting and broader emission bands. This has facilitated the application of Sm2+ ions in pressure sensing 6, 30. The relation of temperature to the optical properties of Sm2+ has been researched and discussed

6, 7

but has not been exploited for temperature-sensing applications. SrB4O7 was recently reported as a host lattice in applications such as light conversion stability

28

and pressure sensing

29

. SrB4O7 has excellent thermal and chemical

28

, and the tetrahedral structure of the BO4 can stabilize divalent rare earth

ions, even at high temperature in an oxidizing atmosphere

28

. These characteristics

make SrB4O7 a good host to incorporate Sm2+ for wide-range temperature-sensing applications. In

this

work,

we

conducted

extensive

measurements

of

the

temperature-dependent luminescent properties of as-prepared SrB4O7:Sm2+ samples for thermometry applications. Excellent sensitivity and stability were achieved with our sample over a very broad temperature range. The outstanding performance of

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SrB4O7:5% Sm2+ makes it a promising candidate for highly sensitive optical thermometer applications.

2. Experimental The synthesis was performed using commercially available reagents. All chemicals were analytical grade and were used as received without further purification. The solid-state reaction method

28

was used for the synthesis of

SrB4O7:5% Sm2+. In the process of preparation, SrCO3, H3BO3 and Sm2O3 were used as the starting materials in the proper stoichiometric ratio. After grinding in an agate mortar, the powder was preheated to 750 ℃ for 5 h. The mixture was finely powdered and heated to 850 ℃ under a reducing atmosphere for 10 h to obtain the final product. The crystal structures of the synthesized samples were determined by X-ray diffraction (XRD) (Rigaku-TTR-III) with Cu Kα radiation (λ = 0.15418 nm) in the 2θ range from 10° to 70°. Excitation spectra were obtained with a Hitachi 850 fluorescence spectrophotometer using a 150 W xenon lamp as the excitation light source. The emission spectra were obtained by charge coupled device (CCD) (Andor DU401A-BVF)under excitation with a 355 nm laser. The laser power was 20 mW. The signal was analyzed using an EG&G 7265 DSP lock-in amplifier. The decay curves of the sample were measured at each temperature using a Tektronix TDS2024 digital storage oscilloscope.

3. Results and discussion 3.1 Structure The crystal structure and phase purity of the studied samples were verified by XRD. Fig. 1 shows the XRD patterns of the SrB4O7:5% Sm2+ sample, as well as the standard bulk sample (JCPDS No. 15-0801). Comparison of the XRD patterns shows

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that all the positions of the diffraction peaks are identical, indicating that the final products of the synthesis are well crystallized.

3.2 Photoluminescence spectra Fig. 2 (a) shows the excitation and emission spectra of SrB4O7:5% Sm2+. Both spectra were recorded at room temperature. The collection time of the emission spectra was less than 1 second. The emission spectrum under 355 nm excitation is presented on the right and consists of three sets of peaks located from 680 nm to 750 nm, corresponding to the 5D0→7FJ Transitions originating from the

(J = 0-2) 5

transitions, as identified in the figure.

D0 level were dominant because at room

temperature, the electrons could relax non-radiatively from the upper excited energy levels to the 5D0 levels, while the thermal population of the upper excited energy levels remained negligible. The excitation spectrum of the sample monitored at 684 nm emission is shown in Fig. 2 (a) (left). There are strong and broad transitions to 4f55d1 energy levels. Thus, the sample can be effectively excited using a 355 nm laser. In order to examine thermal stability of Sm2+ in SrB4O7, the sample was heat-treated at different temperature from 600 ℃ to 800 ℃ in air. The emission spectra of the sample after the heat treatment at different temperatures are recorded and shown in Fig. 2 (b). The results clearly show that the Sm2+ remains unoxidized even when temperature increases to 800 ℃. Thus, the SrB4O7: Sm2+ is very stable at very high temperatures. This result broadened the application of the SrB4O7: Sm2+.

3.3 Temperature-dependent spectra The temperature-dependent emission spectra under 355 nm excitation are shown in Fig. 3 with increasing temperature from room temperature to 723 K. The heating

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device controlled the temperature with precision of approximately 0.5 K and reached equilibrium within 5 min at the target temperature. Each spectrum in the figure is normalized to the intensity of the 684 nm peak for ease of comparison. The weak peaks near 627 nm, 636 nm and 643 nm are attributed to 5D1→7FJ (J = 0, 1, 2) transitions, whose intensities increase with temperature. When the temperature exceeded 373 K, a broad band located at approximately 585 nm appeared, which is attributed to the 5d→4f (4f55d1→7F0) transition, with intensity increasing with temperature as a result of increased thermal population. The schematic coordinate diagram of the Sm2+ energy level structure is plotted in Fig. 4. Sm2+ ions are first excited with a 355 nm laser into the high-lying 4f55d1 energy levels, relax non-radiatively to lower 4f55d1 and 5DJ (J = 2, 1, 0) levels, and finally produce the observed visible emission. The 585 nm emission band originates from the 4f55d1 energy levels just above 5DJ (J = 2, 1, 0), so the intensity increases with temperature as a result of the increased thermal population. The FIR-temperature relation of two thermally coupled energy levels (TCELs) is given by23: FIR = I1 I 2 = B exp ( −∆E kBT ),

(2)

where B is a constant, ∆E is the effective energy difference, kB is the Boltzmann constant, I1 and I2 refer to the intensities of the bands used for FIR. Equation (2) shows that once the temperature around the emitters is given, the excitation power or efficiency of the detection does not affect the FIR value. The energy levels that are too far apart in energy cannot be thermally coupled by emission and absorption phonons. There is a rule of thumb that the upper limit for this is approximately 2000 cm-1 in rare earth ions

23

. Here, the gap between the lowest

4f55d level and 5D0 of 4f6 is greater than 2000 cm-1, but thermal coupling is achieved due to the stronger electron-phonon coupling involving 4f(n-1)5d levels.

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The temperature dependence of FIR585nm/684nm (integrated from 540 nm to 585 nm and 680 nm to 690 nm) is plotted in Fig. 5 (a). The inset shows the logarithmic coordinates of Fig. 5 (a). The result is well-fitted by the following expression: FIR = A exp ( −5419 T ) ,

(3)

in which the effective energy difference is 3763 cm-1. The FIR585nm/684nm changes very slowly at the beginning. The FIR value barely changes below 373 K, which indicates that the energy levels of the 4f55d1 configuration and 4f6 are not yet completely thermally coupled. This could be inferred from the inset of Fig. 5, in which the FIR values below 373 K are not fitted exactly. When the temperature exceeds 373 K, the FIR values are fit perfectly. Utilizing FIR580nm/684nm of SrB4O7:5% Sm2+ for temperature sensing has many advantages. First, the sensitivity of the sample is very large. Sensitivity is defined as the rate of change of the intensity ratio per degree of temperature. The absolute and relative sensitivities can be obtained from the definitions and Eq. (3) as: Sab = dFIR/dT= FIR⋅∆E/kT2,

(4)

Sre = Sab/FIR = ∆E/kT2.

(5)

where ∆E is the effective energy gap deduced from the FIR-temperature relation. The obtained absolute and relative temperature sensitivities are plotted in Fig. 5 (b). According to Eq. (4), the FIR value affects the absolute sensitivity. Because different FIR values can be obtained from materials using different procedures, it is difficult to reflect how sensitive the materials really are to temperature changes via absolute sensitivity. The relative sensitivity, however, shows the changing ratio per degree. Thus, it is more objective to use the relative sensitivity to measure the thermal sensitivity of the material. Conventionally, we use the relative sensitivity at 500 K to show the sample performance in temperature-sensing applications. The relative sensitivity of the as-prepared samples at 500 K is 2.16%.

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To compare the performance of our sample with other thermographic materials, we collected recent FIR results in Table 1, which shows that our SrB4O7:5% Sm2+ sample has much larger relative sensitivity than the other materials. Furthermore, the luminescent intensity of most materials is dramatically quenched at high temperature, while that of our sample remains very high at approximately 600 K (Fig. 6 (b)). We also note that the FIR value of our sample varies near unit, which is favorable compared to those with FIR orders of magnitude away from unit for high accuracy measurement.

3.4 Temperature-dependent luminescence lifetime To further examine the thermal coupling between different luminescent energy levels, we explored the luminescent decay times by monitoring emission at 585 nm (4f55d1), 636 nm (5D1) and 684 nm (5D0) for a series of temperatures between 300 K and 673 K. The results are plotted in Fig. 6 (a). These three decay times are the same within systematic error at a given temperature, indicating that these three luminescent levels are in thermal equilibrium. When the temperature increases, the decay time decreases slowly before 400 K but almost exponentially after 500 K. The relative decay time temperature sensitivity can be defined as: Stau = abs (dtau/dT)/tau),

(6)

where tau is the luminescent decay time. The relative sensitivity of the FIR method usually decreases as temperature increases. However, the sensitivity of the SrB4O7:5% Sm2+ sample was 3.36%·K-1 at 550 K and was almost constant after 490 K, which is very high compared to those reported for FIR applying the decay lifetime method

21-27

. Comparing with recent studies

7-14

, the prepared sample has higher sensitivity

and a wider temperature sensing range. This constant high sensitivity makes it very effective and accurate for temperature sensing.

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In addition to the high sensitivity, SrB4O7:5% Sm2+ has advantages in temperature sensing in lifetime monitoring. First, because all the emission peaks’ lifetimes show the same behavior, all the emission bands ranging from 470 nm to 750 nm can be used for temperature sensing. The vast spectral detection range makes the signal collection more convenient and less dependent on experimental conditions. Second, most optical thermometer materials are severely quenched at high temperatures. Thus, it is more difficult to obtain high-quality signals at high temperature. In our case, although the decay time decreases by approximately 800-fold between 293 K and 673 K, the 470 nm-705 nm integrated luminescence intensity decreases, as shown in Fig. 6 (b), by only approximately 15-fold. The increase in intensity in Fig. 6 (b) is caused by the better response of the spectrometer to the wavelength range of 450-600 nm than 700 nm. As the temperature increases, the 5d-4f emission at approximately 585 nm grows stronger; thus, the overall intensity increases. However, after 500 K, thermal quenching becomes increasingly dominant, and the overall intensity decreases. As a result, the signal of our sample remains good at very high temperature. The uncertainties of measurement of the FIR method and the decay lifetime method were determined by experiment. According to the Eq. (4), given the sensitivity and the uncertainty (dFIR or dtau) at a certain temperature, the precision of the temperature (∆T) could be calculated. dFIR and dtau were obtained through repeated measurements. The error range of temperature derived from an obtained FIR or decay lifetime value is within 1.0 K, which is approximately the heater's temperature precision. This result indicates that our sample has excellent repeatability and is thus suitable for temperature sensing.

3.5 Temperature field As the FIR of the broadband over the sharp peaks varies dramatically with temperature, a significant change in luminous chromaticity from deep red to green was observable by the naked eye when the temperature increased from room

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temperature to 723 K. This can be exploited for temperature distribution detection, which is important in many industrial applications. To examine its potential application, we built a temperature-gradient device that could provide a constant high temperature (723 K) at one end and a low temperature (300 K) at the other. The high-temperature end of the device consisted of a heating rod and a thermocouple thermometer. The low temperature end was a water-cooling device. The sample was coated on a quartz glass with uniform thickness, and the glass was placed on the temperature-gradient device. The quartz glass was fixed with the head on high temperature end and the foot on low temperature end. A 10 W ultraviolet lamp was used to excite the sample. By observing the luminous chromaticity distribution (Fig. 7(a)), the real-time temperature distribution was obtained. The decay lifetime (Fig. 7(b)) and emission spectra (Fig. 7(c)) of a few locations were recorded to measure the temperature. The chromaticity distribution graph can be used to detect real-time temperature distribution at a glance. The temperatures of the labeled locations were obtained using the two methods. The FIR-temperature relation and lifetime-temperature relation were used to calibrate the temperature of the labeled location. The result shows that above 450 K, the average difference in temperature of the two modes was within 1 K. The deviation of two method in low-temperature area is caused by two factors. First, the FIR values in the low-temperature area do not fit well with the fitting formula. The calibration of the temperature with the fitting formula will introduce systematic error for sensing mode of FIR. Second, the decay lifetime changes very slowly when the temperature is below 450 K; therefore its sensitivity decreases dramatically at low-temperature area. So the precision of the measurement for sensing mode of lifetime decreases dramatically at low temperature accordingly. The temperature-sensing test shows that this multimode thermometer material can be used for practical applications at high temperatures with a temperature precision of 1 K.

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4. Conclusion In

summary,

Sm2+-doped

SrB4O7 was

synthesized

to

investigate

its

temperature-dependent luminescent properties and the possibility for practical thermo-sensing applications. The sample possesses many advantages as an optical thermometer, such as excellent luminescence intensity, extremely high sensitivity and wide spectral range for detection. The relative sensitivity of our sample reaches 3.36%·K-1 at 500 K for the sensing mode of decay lifetime, which surpasses most reported materials. A visible temperature field was also obtained using our sample as a chromatic real-time temperature probe, which makes the temperature distribution observable to the naked eye. Sm2+ remains unoxidized at 800 ℃, making the material applicable to high-temperature environments.

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Table 1 Comparison of recent reported temperature sensing materials based on FIR technique, and the relative sensitivities at 500 K are listed. ∆E/kB (K)

SR (% K-1)

Ref.

P1, 3P0

657.7

0.263

[21]

4

F7/2, 4F3/2

2805

1.12

[27]

NaLuF4:Gd3+

6

P5/2, 6P7/2

667

0.267

[22]

BaYF5:Dy3+

4

I15/2, 4F9/2

1373

0.549

[24]

PbTiO3:Yb3+,Ho3+

5

F4, 5F5

4163

0.77

[26]

NaBiTiO3:Er3+

2

H11/2, 4S3/2

947

0.53

[25]

SrB4O7:Sm2+

4f55d1, 5D0

5419

2.16

This work

Materials

TCELs*

NaYF4:Pr3+

3

NaYF4:Nd3+

*Thermally Coupled Energy Levels

Table 1

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Acknowledgements This work was financially supported by the National Key Basic Research Program of China (2013CB921800), The National Key Research and Development Program of China (2016YFB0701001), the National Natural Science Foundation of China (11274299, 11374291, 11574298 and 11404321).

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of

Ho3+

and

Yb3+

Codoped

0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3

Multifunctional Ceramics. Ceram. Int. 2016, 42, 11309-11313. (27) Tian, X.; Wei, X.; Chen, Y.; Duan, C.; Yin, M. Temperature Sensor Based on Ladder-level Assisted Thermal Coupling and Thermal-enhanced Luminescence in NaYF4: Nd3+. Opt. Express 2015, 22, 30333-30345. (28) Sun, J.; Zhu, J.; Liu, X.; Du, H. Luminescence Properties of SrB4O7: Sm2+ for Light Conversion Agent. J. Rare Earths 2012, 30, 1084-1087. (29) Jing, Q.; Wu, Q.; Liu, L.; Xu, J.; Bi, Y.; Liu, Y.; Chen, H.; Liu, S.; Zhang, Y.; Xiong, L.; Li, Y.; Liu, J. An Experimental Study on SrB4O7:Sm2+ as a Pressure Sensor. J. Appl. Phys. 2013, 113 023507.

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Fig. 1 XRD pattern of the SrB4O7:5% Sm2+ powder sample and the standard data of bulk SrB4O7 (JCPDS No. 15-0801). 177x133mm (300 x 300 DPI)

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Fig. 2 (a) Excitation spectrum of the as-prepared sample monitoring 684 nm emission peak of the sample (left) and emission spectrum of the as-prepared SrB4O7:5% Sm2+ sample under excitation of 355 nm laser (right). (b) Emission spectra of the sample after the heat treatment at different temperatures. 99x152mm (300 x 300 DPI)

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Fig. 3 Temperature dependent emission spectra of the SrB4O7:5% Sm2+ sample with temperature elevating from room temperature to 723 K. 85x54mm (300 x 300 DPI)

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Fig. 4 Schematic energy level diagram of the Sm2+ ions in SrB4O7. The excitation wavelength is 355 nm (purple arrow). 57x75mm (300 x 300 DPI)

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Fig. 5 (a) The relation between temperature and fluorescence intensity ratio of 585 nm band and 684 nm band of the SrB4O7:5% Sm2+ sample. The inset shows the logarithmic coordinate figure of Fig. 5 (a). (b) Relative sensitivity and absolute sensitivity of the FIR method. 78x107mm (300 x 300 DPI)

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Fig. 6 (a) Temperature dependent fluorescence lifetime of 585 nm (5f55d1→7F0), 636 nm (5D1→7F0) and 684 nm (5D0→7F0) band of the SrB4O7:5% Sm2+ sample. (b) The relation between temperature and Integrated intensity of the overall spectra. 99x151mm (300 x 300 DPI)

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Fig. 7 (a) Luminescence color distribution on one piece of quartz glass. The SrB4O7:5% Sm2+ sample was coated on the glass with homogeneous thickness. (b) The decay lifetime of different temperatures corresponding to the labeled locations. (c) The emission spectra of different temperatures corresponding to the labeled locations. 99x163mm (300 x 300 DPI)

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