APPLIED PHYSICS LETTERS 90, 181117 共2007兲
Optical thermometry through infrared excited green upconversion emissions in Er3+ – Yb3+ codoped Al2O3 B. Donga兲,b兲,c兲 and D. P. Liu School of Science, Dalian Nationalities University, Dalian 116600, People’s Republic of China
X. J. Wang,b兲,d兲 T. Yang, and S. M. Miao School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China
C. R. Lib兲,e兲 College of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, People’s Republic of China
共Received 7 February 2007; accepted 8 April 2007; published online 2 May 2007兲 Fluorescence intensity ratio 共FIR兲 variation of green upconversion emissions at 523 and 545 nm in the Er3+ – Yb3+ codoped Al2O3 has been studied as a function of temperature using a 978 nm semiconductor laser diode as an excitation source. In the temperature range of 295– 973 K, the maximum sensitivity and the temperature revolution derived from the FIR technique are approximately 0.0051 K−1 and 0.3 K, respectively. The Er3+ – Yb3+ codoped Al2O3 material with the highest operating temperature up to 973 K, the higher temperature revolution, and the fluorescence efficiency indicated that it is promising for applications in optical high temperature sensor. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2735955兴 Recently, the optical temperature sensors based on the fluorescence intensity ratio 共FIR兲 technique of green upconversion emissions in the Er3+ doped and Er3+ – Yb3+ codoped materials attracted much attention because it can reduce the dependence of measurement condition and improve the sensitivity by measuring the fluorescence intensity originating from 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+, the ratio which is independent of fluorescence loss and fluctuations in excitation intensity.1–6 Some successful works of the FIR technique using green upconversion emission of Er3+ are mainly focused on the glass2,3 and silica matrices.4,5 The Er3+ doped glass possesses higher fluorescence efficiency and lower excitation power, however, the maximum operating temperature is below 523 K.2,3 Though the Er3+ doped silica increased the operating temperature up to 913 K, a lower fluorescence efficiency partially limited the potential application in FIR technique.5 Thus, looking for a proper matrix material with higher fluorescence efficiency and operating temperature in low excitation powers becomes much interesting. Among the Er3+ doped matrix, Al2O3 can enhance the dispersion of Er3+, fluorescence efficiency, decay time, chemical durability, and mechanical strength.7,8 Furthermore, lower stretching frequency of the Er3+ doped Al2O3 implies that it possesses a higher fluorescence efficiency. The greater absorption cross section of about 11.7⫻ 10−21 cm2 of Yb3+, coupled with an efficient energy transfer 共ET兲 from the large spectral overlap between Yb3+ emission of 2F5/2 → 2F7/2 and Er3+ absorption of 4I15/2 → 4I11/2, can significantly improve the upconversion emission properties.9,10 In this letter, we present the preliminary results of Er3+ – Yb3+ codoped a兲
Also at School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China. b兲 Author to whom correspondence should be addressed c兲 Electronic mail:
[email protected] d兲 Electronic mail:
[email protected] e兲 Electronic mail:
[email protected]
Al2O3 optical high temperature sensor with the highest operating temperature of 973 K using FIR technique. The nonaqueous sol-gel method was used for the preparation of the Er3+ – Yb3+ codoped Al2O3 nanoparticles. Al共OC3H7兲3 was first added into the AcAcH and PriOH solution with a concentration of 2 mol l−1, ranging a molar ratio of 1:2 for Al共OC3H7兲3 and AcAcH. After aging for 1 h, hydrolysis of the modified precursor was performed under PriOH environment with a concentration of Al共OC3H7兲3 of 30 g l−1 to a molar ratio of 0.85:1 between H2O and Al共OC3H7兲3. The resulting solution was agitated for 3 h, and then the pH value was adjusted to about 3.0 with concentrated HNO3, forming a clear, slightly yellow, and very stable Al2O3 sol. Finally, Er3+ and Yb3+ were introduced by addition of Er共NO3兲3 · 5H2O and Yb共NO3兲3 · 5H2O with a molar ratio of 0.01:0.1:1 for Er3+ : Yb3+ : Al3+. The Er3+ – Yb3+ codoped Al2O3 sols were heated to the sintering temperature of 823 K, milled into powders, and pressed into thin pieces with dimensions of 5 ⫻ 5 ⫻ 0.2 mm3, then the samples were heated to 1273 K. The sample was placed in a furnace and its temperature increased from 295 to 973 K 共the highest temperature of the furnace兲 with measurement error of ±1.5 K was monitored by a copper constantan thermocouple set to the back face of sample. The green upconversion emission spectra in the wavelength range of 500– 580 nm were detected from the sample using a 978 nm semiconductor laser diode 共LD兲 as an excitation source with excitation power of 0.35 W, corresponding to a power density of 1.75⫻ 102 W / cm2. The green upconversion emissions from the sample were focused onto a single monochromator and detected with a CR131 photomultiplier tube associated with a lock-in amplifier. The spectral resolution of the experimental setup was 0.1 nm. Figure 1 shows a simplified energy level diagram of the green upconversion emissions from the Er3+ – Yb3+ codoped Al2O3 by a 978 nm LD excitation. The Er3+ on 4I11/2 level
0003-6951/2007/90共18兲/181117/3/$23.00 90, 181117-1 © 2007 American Institute of Physics Downloaded 25 Jun 2007 to 130.153.140.143. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 1. Energy level diagram of green upconversion emissions from the Er3+ – Yb3+ codoped Al2O3 by a 978 nm LD excitation.
was populated through the ground state absorption of 4I15/2 + a photon→ 4I11/2 and ET 1 of 2F5/2共Yb3+兲 + 4I15/2共Er3+兲 → 2F7/2共Yb3+兲 + 4I11/2共Er3+兲. Then the excited state absorption of 4I11/2 + a photon 4F7/2, cross relaxation of 4I11/2 + 4I11/2 → 4I15/2 + 4F7/2, and ET 2 of 2F5/2共Yb3+兲 + 4I11/2共Er3+兲 → 2F7/2共Yb3+兲 + 4F7/2共Er3+兲 populated the Er3+ to 4F7/2 level, and a nonradiative decay from 4F7/2 to 2H11/2 and 4S3/2 levels. Finally, the green upconversion emissions centered at about 523 and 545 nm were produced by the transitions of 2 H11/2 → 4I15/2 and 4S3/2 → 4I15/2 of Er3+, respectively. Figure 2 shows the green upconversion emission spectra in the wavelength range of 500– 580 nm for the Er3+ – Yb3+ codoped Al2O3 at the temperatures of 295 and 645 K. The green upconversion emission spectra exhibited two green emission bands centered at about 523 and 545 nm, which were attributed to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er3+, respectively. With increasing the temperature of Er3+ – Yb3+ codoped Al2O3, the peak positions of green upconversion emissions at 523 and 545 nm have no change, however, the FIR of the two emissions varied. The energy gap of about 770 cm−1 between the 2H11/2 and 4S3/2 levels
FIG. 3. Monolog plot of the FIR of green upconversion emissions at 523 and 545 nm as a function of inverse absolute temperature in the range of 295– 973 K.
could be obtained from the green upconversion emission spectra. This energy separation allows the 2H11/2 level to be populated from 4S3/2 level by thermal excitation and a quasithermal equilibrium occurs between the two levels,4 leading to variation in the transitions of 2H11/2 → 4I15/2 and 4 S3/2 → 4I15/2 of Er3+ at the elevated temperature. With the thermalization of population at the two levels and ignoring the effects of self-absorption of the fluorescence, the FIR of green upconversion emissions at 523 and 545 nm can be written as Eq. 共1兲. R⬅
冋 册
− ⌬E I523 N共 2H11/2兲 gHHH = = exp I545 N共 4S3/2兲 g S S S kT
冋 册
= C exp
− ⌬E , kT
共1兲
where N, g, , and are the number of ions, the degeneracy, the emission cross section, and the angular frequency of fluorescence transitions from the 2H11/2 and 4S3/2 levels to 4 I15/2 level, respectively, ⌬E the energy gap between the 2 H11/2 and 4S3/2 levels, k the Boltzmann constant, T the absolute temperature, and the preexponential constant is given by C = gHHH / gSSS. Figure 3 shows a monolog plot of the FIR of green upconversion emissions at 523 and 545 nm as a function of inverse absolute temperature in the range of 295– 973 K. The experimental data are fitted to straight line with the slope of about 964.1. The FIR of green upconversion emissions at 523 and 545 nm relative to the temperature range of 295– 973 K was shown in Fig. 4. The coefficient C value in Eq. 共1兲 is 9.63 according to fitting curve of the experimental data. The sensor sensitivity can be defined as Eq. 共2兲.1,3
冉 冊
dR ⌬E =R − 2 . dT kT
共2兲
The corresponding resultant curve is shown in Fig. 5. At the temperature of 495 K, the sensitivity of Er3+ – Yb3+ codoped Al2O3 reached its maximal value of about 0.0051 K−1. Several temperature cycles were performed and a good repeatability was obtained. Furthermore, there was no any modification in the sample, and the green upconversion emissions
FIG. 2. Green upconversion emission spectra in the wavelength range of 500– 580 nm at the temperatures of 295 and 645 K for the Er3+ – Yb3+ codoped Al2O3 sample. Downloaded 25 Jun 2007 to 130.153.140.143. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 4. FIR of green upconversion emissions at 523 and 545 nm relative to the temperature range of 295– 973 K.
FIG. 5. Sensor sensitivity dR / dT as a function of the temperature in the temperature range of 295– 973 K.
could be measured easily up to the highest temperature of 973 K. In comparison with similar studies of optical temperature sensors,2–6,11 a favorable result in the Er3+ – Yb3+ codoped Al2O3 was achieved here. An approximate sensitivity was also realized by using Er3+ doped BaTiO3, but in a narrower operating temperature range of 333– 466 K.6 Moreover, a high power excitation source of 2.0⫻ 103 W / cm2 had been also used, which is about one magnitude higher than that of applied in the present study.6 The temperature measurement resolution for the Er3+ – Yb3+ codoped Al2O3 was also relatively high, being about 0.3 K by employing a signal division circuitry with a precision of four digits or more. The resolution value is same to that of Er3+ doped chalcogenide glasses of 0.3 K,11 higher than that of Er3+ doped fluoride glass of 2 K 共Ref. 2兲 and Er3+ doped SiO2 of around 1.3 K.5 It should be noted that the optical temperature sensor based on Er3+ – Yb3+ codoped Al2O3 increased the operating temperature to 973 K, which is the highest operating temperature reported up to date. The results above suggested that Al2O3 is a proper matrix material for use in optical high temperature sensor. In conclusions, the optical thermometry in temperature range of 295– 973 K was presented using the FIR technique of green upconversion emissions in the Er3+ – Yb3+ codoped Al2O3. The sensitivity and revolution for the optical temperature sensor based on the Er3+ – Yb3+ codoped Al2O3 are ap-
proximately 0.0051 K−1 and 0.3 K, respectively, using excitation power of 0.35 W readily available from a 978 nm semiconductor LD. The authors acknowledge the technical assistance of B. S. Cao, H. Wang, Z. H. Zhu, and S. Q. Zhou in this research. This work is supported by the National Natural Science Foundation of China 共Grant No. 60477023兲 and Science and Technology Commission of Liaoning Province 共Grant No. 20062137兲. 1
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