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Calibration of optical temperature sensing of Ca1-xNaxMoO4:Yb3+,Er3+ with intense green up-conversion luminescence
Accepted Manuscript 3+ 3+ Calibration of optical temperature sensing of Ca1-xNaxMoO4:Yb ,Er with intense green up-conversion luminescence Tao Pang, Weijian Wan, Danlei Qian, Ziling Liu PII:
S0925-8388(18)33204-3
DOI:
10.1016/j.jallcom.2018.08.309
Reference:
JALCOM 47397
To appear in:
Journal of Alloys and Compounds
Received Date: 4 June 2018 Revised Date:
29 August 2018
Accepted Date: 30 August 2018
Please cite this article as: T. Pang, W. Wan, D. Qian, Z. Liu, Calibration of optical temperature sensing 3+ 3+ of Ca1-xNaxMoO4:Yb ,Er with intense green up-conversion luminescence, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.08.309. 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.
ACCEPTED MANUSCRIPT Calibration of optical temperature sensing of Ca1-xNaxMoO4:Yb3+,Er3+ with intense green up-conversion luminescence Tao Pang*, Weijian Wan, Danlei Qian, Ziling Liu College of science, Huzhou University, Zhejiang Huzhou 313000, China Corresponding author. E-mail address: [email protected]
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*
Abstract
High luminescence intensity is crucial for realizing luminescent temperature sensing with a large
signal-to-noise
ratio.
In
this
work,
the
temperature
sensing
behavior
of
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Ca1-xNaxMoO4:Yb3+,Er3+ with intense green up-conversion luminescence was studied. It has been demonstrated that the single-band green emission was dependent on the Yb3+-MoO42- dimer 4
F9/2. In addition, it was
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sensitization as well as the low multi-phonon relaxation rate of 4S3/2
found that the uncorrected thermometer could not accurately measure temperature due to thermal effects. A simple method for calibration of up-conversion temperature sensing was reported. Due to high luminescence intensity and temperature sensitivity, Ca1-xNaxMoO4:Yb3+,Er3+ is a promising candidate for up-conversion temperature sensing.
1. Introduction
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Keywords: Up-conversion luminescence; Temperature sensing; Thermal effects
In recent years, temperature sensing based on fluorescence intensity ratio (FIR) of two up-conversion emission bands has drawn great interest[1-3]. As an important luminescent center,
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Er3+ is suitable for the research of temperature sensing because of the moderate energy gap between 2H11/2 and 4S3/2[4]. In order to realize the up-conversion temperature sensing with a large
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signal-to-noise ratio, Yb3+ must be introduced as a sensitizer to improve the absorption at 980 nm. Nevertheless, Yb3+/Er3+ co-doping generally leads to intense red emission[5-7], which is not conducive to the development of a temperature sensor based on the FIR of two green emissions. According to the population channel of red emission level, choosing fluoride as matrix can effectively suppress the red emission, but there are two inherent defects: (1) low heat temperature[2]; (2) large spectral overlap between the two green emissions[8]. In contrast, oxides with low phonon energy are the more suitable host materials[9-11]. On the other hand, since the up-conversion luminescence is accompanied by a strong thermal effects, this temperature sensing based on up-conversion luminescence inevitably suffers from the
ACCEPTED MANUSCRIPT problem of temperature measurement accuracy[12]. It has been demonstrated that temperature sensing based on Er3+ up-conversion luminescence requires the establishment of a temperature measurement standard at low pumping power[13,14]. However, the thermal effects can not be completely eliminated by reducing the pumping power since the up-conversion luminescence is
the true temperature is worth further exploration.
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quite inefficient[15]. Therefore, how to use this optical temperature sensing technology to reflect
In this paper, we report the Ca1-xNaxMoO4:Yb3+,Er3+ phosphors with single-band green up-conversion emission, and study the related luminescence mechanism. More importantly, we
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examine the influence of thermal effects on temperature sensing behavior and provide a simple method of correction. These studies are helpful for the development and application of the
2. Experimental 2.1 Preparation of phosphors
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temperature sensing based on Er3+ luminescence.
According to the stoichiometric ratio, weigh a certain amount of CaCO3 (AR), Na2CO3 (AR), H24Mo7N6O24 (AR), Yb(NO3)3•6H2O (99.99%) and Er(NO3)3•6H2O (99.99%). After grinding and
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mixing in an agate mortar, the powders were transferred to a corundum crucible and calcined at a temperature of 1173 K for 4 hours to obtain the target product. The chemical composition of all
Table 1
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samples is shown in Table 1.
2.2 Characterization
X-ray diffraction (XRD) analysis was performed at 40 kV and 30 mA using a Smartlab9
X-ray generator with Cu Kα (λ=1.5406Å) radiation. The 2θ scan range was 10-80º with a step size of 0.02º. Up- and down-conversion spectra were recorded by a Hitachi F-4600 fluorescence spectrophotometer, where the excitation light sources were power tunable 980 nm semiconductor laser and 150 W xenon light, respectively. The digital image of up-conversion luminescence was recorded by an iphone 6s. The luminescence decay curves are measured by a FLS980 phosphorescence lifetime spectrometer using a 980 nm pulse laser as excitation source. A temperature controlling system that consisted of a sample cell with heating element, a thermal
ACCEPTED MANUSCRIPT insulation layer, an air pump and a K-type thermocouple temperature controller was used to investigate the temperature dependent luminescence. Its temperature controlling and measuring uncertainty was less than 0.2 °C. Data were collected every 20 K in the temperature range of 300-500 K. In order to ensure the reliability of the collected data, the sample was thermostated for
3. Results and discussion 3.1 Structure analysis
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Fig. 1
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calculating the integral area of the corresponding emission band.
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2 min at each collection temperature. In this paper, all emission intensities were obtained by
As shown in Fig. 1a, the diffraction peaks of all the samples are in agreement with the standard data in JCPDS No. 85-1267. No impurity phases are observed, indicating that the as-prepared products are pure orthorhombic CaMoO4, and Yb3+, Er3+ and Na+ are successfully incorporated into the lattice of CaMoO4. Because of relatively close ionic radius and valence
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between doping ions and Ca2+, Yb3+ (r=0.868 Å with CN=6), Er3+ (r=0.89 Å with CN=6) and Na+ (r=1.02 Å with CN=6) occupy the sites of Ca2+ (r=1.00 Å with CN=6) [6,16,17]. Further, it is seen from Fig. 1b that Ca forms [CaO8] polyhedron with eight neighboring O, and each two neighboring octahedrons have one edge with the nearest Ca2+ spacing of 3.8712 Å, which exceeds
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the critical distance of exchange interaction[18]. Therefore, there is no exchange of electrons
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between the doped lanthanide ions. 3.2 Up-conversion luminescence
Fig. 2
As can be seen from Fig. 2, Na+ doping has a strong influence on up-conversion luminescence. Doping with 2%Na+ makes the intensity of green emission increased by 86%, but higher concentrations of Na+ doping results in the quenching of luminescence. In order to understand the inner mechanism, Fig. 3 shows the up-conversion decay curves of 1#, 2# and 3# samples. Owing to the non-single-exponential feature of these decay curves, the average decay
ACCEPTED MANUSCRIPT lifetime is evaluated based on the following Eq. (1) [19]:
τ =∫
I (t )tdt (1)
∫ I (t )dt
where I(t) is the time-dependent luminescence intensity. It is of interest to note that with
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increasing the Na+ content, the lifetime firstly decreases and then increases. Ca2+ vacancies will be formed after incorporating Yb3+ and Er3+ into CaMoO4 lattice. However, with introduction of Na+, the number of cationic vacancies must be reduced. Consequently, the substitution of Na+ for Ca2+ causes a certain degree of lattice expansion (see Fig.1 and S2). Of course, the lattice expansion is
R ≈ 2(
3V 1/3 ) 4π CN
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also related to the ionic radius. According to Eq. (2), (2)
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(R represents the average distance between rare earth ions, V is the volume of the crystallographic cell, C is the concentration of doping ions, N is the number of lattice sites occupied by rare earth ions in the unit cell) [20], the R value will gradually increase as the Na+ concentration increases. The interaction between rare earth ions has a 1/RS (S is a positive integer taking the values of 6, 8, 10, which correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions,
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respectively) dependence on the distance between two ions[21], thus the energy transfer and cross relaxation are not the main factor affecting the change in luminescence lifetime. In addition, the emission peak positions are completely consistent in all three samples, indicating that the
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influence of Na+ doping on the energy level structure of Er3+ was negligible. Due to the almost constant energy gap, the variation of lifetime is independent of the multi-phonon relaxation. Taken
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together, the change in luminescence lifetime is only related to the radiation transition process[19]. In other words, Na+ doping mainly affects the radiation transition probability. When doped with 2% Na+, the probability of radiation transition is enhanced, which makes the luminescence intensity strengthen. However, in the case of 4% Na+ doping, the local symmetry around Er3+ may be enhanced again owing to decrease of cationic vacancies. As a result, the up-conversion luminescence is quenched.
Fig. 3
ACCEPTED MANUSCRIPT Fig.4
The contribution of faint far-red to the red stimulus is small, so the dominant wavelength (~541 nm) is located in the middle of the two green emissions that is the most sensitive band of
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the human eye (see S1). The greater luminous efficiency of green emission combined with the higher luminescence efficiency of 2% Na+ doping allow the bright green luminescence to be observed under indoor lighting conditions[22]. Generally, Yb3+/Er3+ co-doping produces strong green and red emissions at 980 nm radiation. However, almost single-band green emission is
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observed from Fig.2. To clarify the problem, the down-shifting luminescence is also studied. Based on the excitation spectrum, 378 nm was chosen as the excitation wavelength. It can be seen
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from Fig. 4a that when the 2# sample is excited with 378 nm at 300 K, only strong green emission is produced. And no red emission is observed even if the sample temperature is raised to 500 K. From this, it is inferred that the multi-phonon relaxation rate of 4S3/2 to 4F9/2 is low since it is the only way to produce red down-conversion emission[23]. In order to verify whether cross-relaxation between Er3+ ions and non-radiative decay of 4I11/2 to 4I13/2 occurs, Fig. 4b gives the up-conversion
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spectra of 4#, 5# and 6# samples. Intensity ratio of green and red emissions (IG/IR) decreases with increasing Er3+ concentration, indicating that the cross-relaxation process is gradually strengthened. However, when the concentration of Er3+ reaches 8%, the luminescence is still dominated by green emission, suggesting that the cross-relaxation in the sample with 1% Er3+ is
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inefficient. Moreover, it is deduced that the probability of non-radiative decay of 4I11/2 to 4I13/2 is
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also low, since the energy gap between 4I11/2 and 4I13/2 is 521 cm-1 larger than that of 4S3/2-4F9/2[24]. The study of down-shifting luminescence in Fig. 4c indicates that there is an energy-back-transfer (EBT) process in the samples co-doped with Yb3+ and Er3+, viz. 4S3/2(Er3+) + 2F7/2(Yb3+) → 4
I13/2(Er3+) + 2F5/2(Yb3+) + phonon[21]. This process is helpful for the population of the 4I13/2 level,
so if the two-step energy transfer of Yb3+ to Er3+ dominates the up-conversion luminescence, the IG/IR in Yb3+/Er3+ co-doped samples should be less than that of Er3+ single-doped samples, and the larger the Yb3+ concentration, the smaller the IG/IR[5,21,25,26]. However, the opposite results in Fig. 4d suggest that the two-step energy transfer is not the primary mechanism. According to Dong et al.[11], the possible up-conversion mechanism is as follows: Firstly, the Yb3+-MoO42- dimer promotes Er3+ directly to the 2H11/2/4S3/2 levels, and then the EBT is responsible for the population
ACCEPTED MANUSCRIPT of 4I13/2. Finally, the 4F9/2 is populated by the energy transfer from Yb3+ to Er3+: 4I13/2(Er3+) + 2
F5/2(Yb3+) → 4F9/2(Er3+) + 2F7/2(Yb3+). Because the dimer sensitization is dominated, the intensity
of green emission is much larger than that of red emission.
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3.3 Temperature sensing
Fig.5
As well known, the relative intensity ratio of two green emissions (IH/IS) coming from Er3+
(3)
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IH ∆E = C exp(− ) IS kT
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follows the formula below [27]:
where C is a constant, ∆E represents the energy gap between 2H11/2 and 4S3/2, k is Boltzmann constant, T is absolute temperature of sample. Since IH/IS is only related to the temperature, the dependence of IH/IS on radiation time shown in Fig. 5 reflects the relationship between temperature and radiation time. And the temperature can be calculated by the following Eq. (4)
a
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T=
ln(
IH ) − ln b IS
(4)
Where a and b are –∆E/k and C in Eq. (3), respectively. However, we encounter difficulties in
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determining the ∆E and C. As shown in Fig. 6(a) and 6(b), when we use two different spectral scanning methods, there is a clear difference in ∆E obtained from the fitting of Eq. (3). Therefore,
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there is a large deviation in the calculated temperature (see table 2). Particularly, the value calculated by using the fitting function (TM1) deviates significantly from the actual temperature. For example, when t = 0 min, the calculated temperature is 8.2 K lower than the control temperature of the heating device.
Fig.6
To reveal the underlying causes, S3 and S4 give the functions established with different pumping powers and spectral scanning modes. As can be seen from table 2, the greater the power
ACCEPTED MANUSCRIPT and the longer the radiation time, the worse the accuracy of the thermometer. Also, it is found that the higher the accuracy of the thermometer, the closer ∆E is to the spectral value in S5 (794 cm-1). High pumping power leads to greater thermal effects, so the above phenomena may be related to thermal effects. Thermometers based on down-conversion luminescence have been previously
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considered unaffected by thermal effects [28]. Therefore, S6 gives the fitting function (TM7) based on the down-conversion luminescence at various temperatures. Obviously, the ∆E determined by TM7 is closer to the true value, which fully confirms that the thermal effect strongly influences the measurement accuracy. However, it is noted that the thermometer TM7 still shows the
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inaccurate temperature. This can be attributed to the EBT process and Stokes shift losses. In other words, down-conversion luminescence is also affected by the thermal effects. Just the heat
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generation rate is so fast that we have no way to see the process of temperature rise directly as in intense laser radiation (see S7 and S8). This fact can be further confirmed by the results shown in Fig.5, S9 and table 2. Under 980 nm excitation with a pumping power of 48 mW, no appreciable temperature rise is observed, but the problem of poor temperature measurement accuracy still exist. Recently, we have confirmed that the thermal effect is dependent on the Yb3+ sensitization, and the
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higher the Yb3+concentration, the greater the influence of thermal effect[5]. Therefore, Er3+ single doping should be less affected by thermal effects. From Fig.6, S3, S4 and S10, we believe that Er3+ single doping combined with pulse excitation or low pumping power can eliminate the
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influence of thermal effects. Nevertheless, the low luminescence intensity (see S11) limits its real application. In summary, it has been demonstrated that the uncorrected thermometer, especially
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those that are sensitized with Yb3+ and excited with high power, are difficult to accurately measure temperature due to thermal effects.
Table 2
Fig. 7
According to the discussion above, the real temperature (TT) of the radiation point is the algebraic sum of the temperature (TR) read by the temperature control device and the temperature rise (∆T) caused by the thermal effect, and the corresponding mathematical expression is as
ACCEPTED MANUSCRIPT follows:
TT = TR + ∆T
(5)
Therefore, when calibrating the up-conversion thermometer, ∆T must be considered. And the corrected principle should be that after the ∆T is introduced, the fitted ∆E is consistent with the 4
I15/2
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spectral value. Take TM2 as an example, since the integrate intensity for 2H11/2/4S3/2
transition maintained a good stability at varying temperatures in the 300-500 K range (see Fig. 7), the ∆T of each data collection point should be approximately equal. From Fig. 8 and table 2, when the ∆T is taken as 36.2 K, a corrected thermometer (TM9) is obtained. It must be emphasized that
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some experimental data deviating from the fitted curve may be related to temperature fluctuations (see S12). The similar phenomena have also been observed in the literatures [31-33]. In our
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opinion, they may be related to the fact that thermal equilibrium is a dynamic process. A method recently reported by our group[31] is used to evaluate the reliability of TM9. As shown in Fig. 9, when bringing the IH/IS values at various pumping power into TM9 (where C and ΔE/k take the values 30.18 and 1142.03, respectively), it is found that the fitted ambient temperature is about 303.55 K, which is about 2.65 K larger than the ambient temperature monitored with a
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proportional-integral-differential (PID) controller equipped with a K-type thermocouple. Considering that the temperature fluctuations can exceed 4 K under thermal equilibrium (see table
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2), we think that the corrected thermometer TM9 is reliable.
Fig.8
Fig.9
The rate at which IH/IS varies with a change in temperature is called as absolute temperature
sensitivity, which represents the sensing ability of the thermometer and is defined as[31]
Sensitivity =
d ( I H / I S ) I H ∆E = ( ) dT I S kT 2
(6)
The absolute sensitivity of the TM9 as a function of the temperature is shown in Fig. 10. It can be found that the maximum sensitivity is 0.0142 K-1 in our temperature measurement range.
ACCEPTED MANUSCRIPT Compared to several typical Er3+-based up-conversion thermometers (see table 3), the excellent sensing ability means that the Ca1-xNaxMoO4:Yb3+,Er3+ up-conversion phosphors are potentially applicable as the optical thermometric materials.
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Fig. 10
Table 3
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Conclusion
Ca1-xNaxMoO4:Yb3+,Er3+ up-conversion phosphors with orthorhombic structure are
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prepared by a simple solid-state method. Na+ doping has a strong influence on luminescence of CaMoO4:Yb3+,Er3+. When the concentration of Na+ is 2%, the strongest up-conversion luminescence is obtained. Since the Yb3+-MoO42- dimer sensitization dominates the upconversion luminescence and the multi-phonon relaxation rate of 4S3/2
4
F9/2 is quite low, the phosphor
produces nearly single-band green up-conversion emission under 980 nm excitation. The
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research on temperature sensing indicates that the thermometers based on the relative intensity ratio of two green emissions from Er3+ suffers from severe temperature measurement accuracy problems. To reflect the true temperature, a simple method for calibration of temperature sensing is reported. Compared to several typical Er3+-based up-conversion thermometers, the
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Ca1-xNaxMoO4:Yb3+,Er3+ shows excellent sensing capability, and is therefore potentially
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applicable as the optical thermometric material. Acknowledgements
This word was supported by the National Natural Science foundation of China (Grant No.
11747312) and Huzhou University’s Scientific Research Project (Grant No. 2016XJXM23). References
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of 1#, 2# and 3# samples (a) as well as crystal structure diagram of CaMoO4 (b) Fig. 2 Up-conversion spectra of various samples under 980 nm excitation, inset: digital image of
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up-conversion luminescence for 2# sample at the pumping power of 688 mW (a); the integrated intensity of green emission (b)
Fig. 3 Up-conversion decay curves of 554 nm emission in 1#, 2# and 3# samples
Fig.4 (a) Excitation and down-conversion spectra of 2# sample; (b) up-conversion spectra of 4#,
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5# and 6# samples as well as the dependence of IG/IR on Er3+ concentration; (c) down-conversion spectra of 2#, 4#, 7# and 8# samples as well as the EBT process; (d) up-conversion spectra of 2#,
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4#, 7# and 8# samples as well as the dependence of IG/IR on Yb3+ concentration. It should be pointed out that all spectra shown in (b) and (d) are normalized to the emission peak at 554 nm. Fig.5 Dependence of IH/IS on radiative time for 2# sample under 980 nm excitation with pump power of 48 mW. The sample temperature is kept at 300 K.
Fig.6 Thermometers TM1 (a) and TM2 (b) based on two different spectral scanning modes under
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980 nm excitation with an excitation power of 188 mW. (a) Continuous scanning mode: during the entire data collection, the laser continuously irradiates on the sample; (b) Intermittent scan mode: the acquisition interval of every two data points is 2 min, and the laser radiation time is about 3s. 4
I15/2 transition at various temperatures
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Fig.7 Integrate intensities for 2H11/2/4S3/2
Fig.8 Corrected dependence of IH/IS on temperature
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Fig.9 IH/IS of 2# sample at various pumping powers as well as the temperature of irradiation spot calculated by TM9
Fig. 10 Temperature sensitivity of TM9 as a function of the temperature
ACCEPTED MANUSCRIPT Table 1 Chemical composition of various samples sample
Chemical composition (mol %) Na+
Yb3+
Er3+
Mo6+
1#
91
0
8
1
100
2#
89
2
8
1
100
3#
87
4
8
1
4#
97
2
0
1
5#
94
2
0
4
6#
90
2
0
8
7#
81
2
16
8#
67
2
30
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Ca2+
100 100 100
1
100
1
100
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100
ACCEPTED MANUSCRIPT Table 2 Comparison between different thermometers
T1 * (K)
T2 * (K)
T3 * (K)
T3-T2 (K)
∆E/k
∆E (cm-1)
TM1
291.80
289.57
293.80
4.23
928.17
645.15
TM2
300.17
297.88
302.21
4.33
960.48
667.60
TM3
299.38
297.06
301.45
4.39
940.27
653.56
TM4
294.51
292.23
296.55
4.32
925.62
643.37
TM5
288.93
286.71
290.92
4.21
911.33
633.44
TM6
282.59
280.37
284.58
4.21
872.54
606.48
TM7
299.50
297.48
301.30
TM8
303.15
300.83
305.23
TM9
334.67
332.28
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Thermo-meters
1079.87
750.59
4.4
963.12
669.44
4.52
1142.03
793.80
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336.80
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*T1, T2 and T3 represent the temperatures of point 1, point 2 and point 3 in Figure 5, respectively
ACCEPTED MANUSCRIPT Table 3 Optical paramers of several typical thermometers based on Er3+ luminescence Excitation
Sensitivity
Temperature
wavelength (nm)
(K-1)
range (K)
NaGdTiO4:Yb3+/Er3+
980
0.0083
100-1000
[29]
NaY(MoO4)12:Yb3+/Er3+
980
0.0097
303-523
[30]
SrWO4:Yb3+/Er3+
980
0.011
299-518
[1]
CaF2:Yb3+/Er3+
980
0.014
295-723
[14]
TM9
980
0.0142
336.2-536.2
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ACCEPTED MANUSCRIPT Highlights 3+
3+
1. The ratiometric temperature sensing behavior of Ca1-xNaxMoO4:Yb ,Er
upconversion
phosphor is investigated. 2. It is found that the temperature sensing suffers from the problem of temperature measurement
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3. A simple and reliable method of correction is reported.
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accuracy.
S0925-8388(18)33204-3
DOI:
10.1016/j.jallcom.2018.08.309
Reference:
JALCOM 47397
To appear in:
Journal of Alloys and Compounds
Received Date: 4 June 2018 Revised Date:
29 August 2018
Accepted Date: 30 August 2018
Please cite this article as: T. Pang, W. Wan, D. Qian, Z. Liu, Calibration of optical temperature sensing 3+ 3+ of Ca1-xNaxMoO4:Yb ,Er with intense green up-conversion luminescence, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.08.309. 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.
ACCEPTED MANUSCRIPT Calibration of optical temperature sensing of Ca1-xNaxMoO4:Yb3+,Er3+ with intense green up-conversion luminescence Tao Pang*, Weijian Wan, Danlei Qian, Ziling Liu College of science, Huzhou University, Zhejiang Huzhou 313000, China Corresponding author. E-mail address: [email protected]
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*
Abstract
High luminescence intensity is crucial for realizing luminescent temperature sensing with a large
signal-to-noise
ratio.
In
this
work,
the
temperature
sensing
behavior
of
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Ca1-xNaxMoO4:Yb3+,Er3+ with intense green up-conversion luminescence was studied. It has been demonstrated that the single-band green emission was dependent on the Yb3+-MoO42- dimer 4
F9/2. In addition, it was
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sensitization as well as the low multi-phonon relaxation rate of 4S3/2
found that the uncorrected thermometer could not accurately measure temperature due to thermal effects. A simple method for calibration of up-conversion temperature sensing was reported. Due to high luminescence intensity and temperature sensitivity, Ca1-xNaxMoO4:Yb3+,Er3+ is a promising candidate for up-conversion temperature sensing.
1. Introduction
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Keywords: Up-conversion luminescence; Temperature sensing; Thermal effects
In recent years, temperature sensing based on fluorescence intensity ratio (FIR) of two up-conversion emission bands has drawn great interest[1-3]. As an important luminescent center,
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Er3+ is suitable for the research of temperature sensing because of the moderate energy gap between 2H11/2 and 4S3/2[4]. In order to realize the up-conversion temperature sensing with a large
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signal-to-noise ratio, Yb3+ must be introduced as a sensitizer to improve the absorption at 980 nm. Nevertheless, Yb3+/Er3+ co-doping generally leads to intense red emission[5-7], which is not conducive to the development of a temperature sensor based on the FIR of two green emissions. According to the population channel of red emission level, choosing fluoride as matrix can effectively suppress the red emission, but there are two inherent defects: (1) low heat temperature[2]; (2) large spectral overlap between the two green emissions[8]. In contrast, oxides with low phonon energy are the more suitable host materials[9-11]. On the other hand, since the up-conversion luminescence is accompanied by a strong thermal effects, this temperature sensing based on up-conversion luminescence inevitably suffers from the
ACCEPTED MANUSCRIPT problem of temperature measurement accuracy[12]. It has been demonstrated that temperature sensing based on Er3+ up-conversion luminescence requires the establishment of a temperature measurement standard at low pumping power[13,14]. However, the thermal effects can not be completely eliminated by reducing the pumping power since the up-conversion luminescence is
the true temperature is worth further exploration.
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quite inefficient[15]. Therefore, how to use this optical temperature sensing technology to reflect
In this paper, we report the Ca1-xNaxMoO4:Yb3+,Er3+ phosphors with single-band green up-conversion emission, and study the related luminescence mechanism. More importantly, we
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examine the influence of thermal effects on temperature sensing behavior and provide a simple method of correction. These studies are helpful for the development and application of the
2. Experimental 2.1 Preparation of phosphors
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temperature sensing based on Er3+ luminescence.
According to the stoichiometric ratio, weigh a certain amount of CaCO3 (AR), Na2CO3 (AR), H24Mo7N6O24 (AR), Yb(NO3)3•6H2O (99.99%) and Er(NO3)3•6H2O (99.99%). After grinding and
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mixing in an agate mortar, the powders were transferred to a corundum crucible and calcined at a temperature of 1173 K for 4 hours to obtain the target product. The chemical composition of all
Table 1
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samples is shown in Table 1.
2.2 Characterization
X-ray diffraction (XRD) analysis was performed at 40 kV and 30 mA using a Smartlab9
X-ray generator with Cu Kα (λ=1.5406Å) radiation. The 2θ scan range was 10-80º with a step size of 0.02º. Up- and down-conversion spectra were recorded by a Hitachi F-4600 fluorescence spectrophotometer, where the excitation light sources were power tunable 980 nm semiconductor laser and 150 W xenon light, respectively. The digital image of up-conversion luminescence was recorded by an iphone 6s. The luminescence decay curves are measured by a FLS980 phosphorescence lifetime spectrometer using a 980 nm pulse laser as excitation source. A temperature controlling system that consisted of a sample cell with heating element, a thermal
ACCEPTED MANUSCRIPT insulation layer, an air pump and a K-type thermocouple temperature controller was used to investigate the temperature dependent luminescence. Its temperature controlling and measuring uncertainty was less than 0.2 °C. Data were collected every 20 K in the temperature range of 300-500 K. In order to ensure the reliability of the collected data, the sample was thermostated for
3. Results and discussion 3.1 Structure analysis
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Fig. 1
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calculating the integral area of the corresponding emission band.
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2 min at each collection temperature. In this paper, all emission intensities were obtained by
As shown in Fig. 1a, the diffraction peaks of all the samples are in agreement with the standard data in JCPDS No. 85-1267. No impurity phases are observed, indicating that the as-prepared products are pure orthorhombic CaMoO4, and Yb3+, Er3+ and Na+ are successfully incorporated into the lattice of CaMoO4. Because of relatively close ionic radius and valence
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between doping ions and Ca2+, Yb3+ (r=0.868 Å with CN=6), Er3+ (r=0.89 Å with CN=6) and Na+ (r=1.02 Å with CN=6) occupy the sites of Ca2+ (r=1.00 Å with CN=6) [6,16,17]. Further, it is seen from Fig. 1b that Ca forms [CaO8] polyhedron with eight neighboring O, and each two neighboring octahedrons have one edge with the nearest Ca2+ spacing of 3.8712 Å, which exceeds
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the critical distance of exchange interaction[18]. Therefore, there is no exchange of electrons
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between the doped lanthanide ions. 3.2 Up-conversion luminescence
Fig. 2
As can be seen from Fig. 2, Na+ doping has a strong influence on up-conversion luminescence. Doping with 2%Na+ makes the intensity of green emission increased by 86%, but higher concentrations of Na+ doping results in the quenching of luminescence. In order to understand the inner mechanism, Fig. 3 shows the up-conversion decay curves of 1#, 2# and 3# samples. Owing to the non-single-exponential feature of these decay curves, the average decay
ACCEPTED MANUSCRIPT lifetime is evaluated based on the following Eq. (1) [19]:
τ =∫
I (t )tdt (1)
∫ I (t )dt
where I(t) is the time-dependent luminescence intensity. It is of interest to note that with
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increasing the Na+ content, the lifetime firstly decreases and then increases. Ca2+ vacancies will be formed after incorporating Yb3+ and Er3+ into CaMoO4 lattice. However, with introduction of Na+, the number of cationic vacancies must be reduced. Consequently, the substitution of Na+ for Ca2+ causes a certain degree of lattice expansion (see Fig.1 and S2). Of course, the lattice expansion is
R ≈ 2(
3V 1/3 ) 4π CN
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also related to the ionic radius. According to Eq. (2), (2)
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(R represents the average distance between rare earth ions, V is the volume of the crystallographic cell, C is the concentration of doping ions, N is the number of lattice sites occupied by rare earth ions in the unit cell) [20], the R value will gradually increase as the Na+ concentration increases. The interaction between rare earth ions has a 1/RS (S is a positive integer taking the values of 6, 8, 10, which correspond to dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions,
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respectively) dependence on the distance between two ions[21], thus the energy transfer and cross relaxation are not the main factor affecting the change in luminescence lifetime. In addition, the emission peak positions are completely consistent in all three samples, indicating that the
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influence of Na+ doping on the energy level structure of Er3+ was negligible. Due to the almost constant energy gap, the variation of lifetime is independent of the multi-phonon relaxation. Taken
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together, the change in luminescence lifetime is only related to the radiation transition process[19]. In other words, Na+ doping mainly affects the radiation transition probability. When doped with 2% Na+, the probability of radiation transition is enhanced, which makes the luminescence intensity strengthen. However, in the case of 4% Na+ doping, the local symmetry around Er3+ may be enhanced again owing to decrease of cationic vacancies. As a result, the up-conversion luminescence is quenched.
Fig. 3
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The contribution of faint far-red to the red stimulus is small, so the dominant wavelength (~541 nm) is located in the middle of the two green emissions that is the most sensitive band of
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the human eye (see S1). The greater luminous efficiency of green emission combined with the higher luminescence efficiency of 2% Na+ doping allow the bright green luminescence to be observed under indoor lighting conditions[22]. Generally, Yb3+/Er3+ co-doping produces strong green and red emissions at 980 nm radiation. However, almost single-band green emission is
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observed from Fig.2. To clarify the problem, the down-shifting luminescence is also studied. Based on the excitation spectrum, 378 nm was chosen as the excitation wavelength. It can be seen
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from Fig. 4a that when the 2# sample is excited with 378 nm at 300 K, only strong green emission is produced. And no red emission is observed even if the sample temperature is raised to 500 K. From this, it is inferred that the multi-phonon relaxation rate of 4S3/2 to 4F9/2 is low since it is the only way to produce red down-conversion emission[23]. In order to verify whether cross-relaxation between Er3+ ions and non-radiative decay of 4I11/2 to 4I13/2 occurs, Fig. 4b gives the up-conversion
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spectra of 4#, 5# and 6# samples. Intensity ratio of green and red emissions (IG/IR) decreases with increasing Er3+ concentration, indicating that the cross-relaxation process is gradually strengthened. However, when the concentration of Er3+ reaches 8%, the luminescence is still dominated by green emission, suggesting that the cross-relaxation in the sample with 1% Er3+ is
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inefficient. Moreover, it is deduced that the probability of non-radiative decay of 4I11/2 to 4I13/2 is
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also low, since the energy gap between 4I11/2 and 4I13/2 is 521 cm-1 larger than that of 4S3/2-4F9/2[24]. The study of down-shifting luminescence in Fig. 4c indicates that there is an energy-back-transfer (EBT) process in the samples co-doped with Yb3+ and Er3+, viz. 4S3/2(Er3+) + 2F7/2(Yb3+) → 4
I13/2(Er3+) + 2F5/2(Yb3+) + phonon[21]. This process is helpful for the population of the 4I13/2 level,
so if the two-step energy transfer of Yb3+ to Er3+ dominates the up-conversion luminescence, the IG/IR in Yb3+/Er3+ co-doped samples should be less than that of Er3+ single-doped samples, and the larger the Yb3+ concentration, the smaller the IG/IR[5,21,25,26]. However, the opposite results in Fig. 4d suggest that the two-step energy transfer is not the primary mechanism. According to Dong et al.[11], the possible up-conversion mechanism is as follows: Firstly, the Yb3+-MoO42- dimer promotes Er3+ directly to the 2H11/2/4S3/2 levels, and then the EBT is responsible for the population
ACCEPTED MANUSCRIPT of 4I13/2. Finally, the 4F9/2 is populated by the energy transfer from Yb3+ to Er3+: 4I13/2(Er3+) + 2
F5/2(Yb3+) → 4F9/2(Er3+) + 2F7/2(Yb3+). Because the dimer sensitization is dominated, the intensity
of green emission is much larger than that of red emission.
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3.3 Temperature sensing
Fig.5
As well known, the relative intensity ratio of two green emissions (IH/IS) coming from Er3+
(3)
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IH ∆E = C exp(− ) IS kT
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follows the formula below [27]:
where C is a constant, ∆E represents the energy gap between 2H11/2 and 4S3/2, k is Boltzmann constant, T is absolute temperature of sample. Since IH/IS is only related to the temperature, the dependence of IH/IS on radiation time shown in Fig. 5 reflects the relationship between temperature and radiation time. And the temperature can be calculated by the following Eq. (4)
a
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T=
ln(
IH ) − ln b IS
(4)
Where a and b are –∆E/k and C in Eq. (3), respectively. However, we encounter difficulties in
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determining the ∆E and C. As shown in Fig. 6(a) and 6(b), when we use two different spectral scanning methods, there is a clear difference in ∆E obtained from the fitting of Eq. (3). Therefore,
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there is a large deviation in the calculated temperature (see table 2). Particularly, the value calculated by using the fitting function (TM1) deviates significantly from the actual temperature. For example, when t = 0 min, the calculated temperature is 8.2 K lower than the control temperature of the heating device.
Fig.6
To reveal the underlying causes, S3 and S4 give the functions established with different pumping powers and spectral scanning modes. As can be seen from table 2, the greater the power
ACCEPTED MANUSCRIPT and the longer the radiation time, the worse the accuracy of the thermometer. Also, it is found that the higher the accuracy of the thermometer, the closer ∆E is to the spectral value in S5 (794 cm-1). High pumping power leads to greater thermal effects, so the above phenomena may be related to thermal effects. Thermometers based on down-conversion luminescence have been previously
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considered unaffected by thermal effects [28]. Therefore, S6 gives the fitting function (TM7) based on the down-conversion luminescence at various temperatures. Obviously, the ∆E determined by TM7 is closer to the true value, which fully confirms that the thermal effect strongly influences the measurement accuracy. However, it is noted that the thermometer TM7 still shows the
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inaccurate temperature. This can be attributed to the EBT process and Stokes shift losses. In other words, down-conversion luminescence is also affected by the thermal effects. Just the heat
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generation rate is so fast that we have no way to see the process of temperature rise directly as in intense laser radiation (see S7 and S8). This fact can be further confirmed by the results shown in Fig.5, S9 and table 2. Under 980 nm excitation with a pumping power of 48 mW, no appreciable temperature rise is observed, but the problem of poor temperature measurement accuracy still exist. Recently, we have confirmed that the thermal effect is dependent on the Yb3+ sensitization, and the
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higher the Yb3+concentration, the greater the influence of thermal effect[5]. Therefore, Er3+ single doping should be less affected by thermal effects. From Fig.6, S3, S4 and S10, we believe that Er3+ single doping combined with pulse excitation or low pumping power can eliminate the
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influence of thermal effects. Nevertheless, the low luminescence intensity (see S11) limits its real application. In summary, it has been demonstrated that the uncorrected thermometer, especially
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those that are sensitized with Yb3+ and excited with high power, are difficult to accurately measure temperature due to thermal effects.
Table 2
Fig. 7
According to the discussion above, the real temperature (TT) of the radiation point is the algebraic sum of the temperature (TR) read by the temperature control device and the temperature rise (∆T) caused by the thermal effect, and the corresponding mathematical expression is as
ACCEPTED MANUSCRIPT follows:
TT = TR + ∆T
(5)
Therefore, when calibrating the up-conversion thermometer, ∆T must be considered. And the corrected principle should be that after the ∆T is introduced, the fitted ∆E is consistent with the 4
I15/2
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spectral value. Take TM2 as an example, since the integrate intensity for 2H11/2/4S3/2
transition maintained a good stability at varying temperatures in the 300-500 K range (see Fig. 7), the ∆T of each data collection point should be approximately equal. From Fig. 8 and table 2, when the ∆T is taken as 36.2 K, a corrected thermometer (TM9) is obtained. It must be emphasized that
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some experimental data deviating from the fitted curve may be related to temperature fluctuations (see S12). The similar phenomena have also been observed in the literatures [31-33]. In our
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opinion, they may be related to the fact that thermal equilibrium is a dynamic process. A method recently reported by our group[31] is used to evaluate the reliability of TM9. As shown in Fig. 9, when bringing the IH/IS values at various pumping power into TM9 (where C and ΔE/k take the values 30.18 and 1142.03, respectively), it is found that the fitted ambient temperature is about 303.55 K, which is about 2.65 K larger than the ambient temperature monitored with a
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proportional-integral-differential (PID) controller equipped with a K-type thermocouple. Considering that the temperature fluctuations can exceed 4 K under thermal equilibrium (see table
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2), we think that the corrected thermometer TM9 is reliable.
Fig.8
Fig.9
The rate at which IH/IS varies with a change in temperature is called as absolute temperature
sensitivity, which represents the sensing ability of the thermometer and is defined as[31]
Sensitivity =
d ( I H / I S ) I H ∆E = ( ) dT I S kT 2
(6)
The absolute sensitivity of the TM9 as a function of the temperature is shown in Fig. 10. It can be found that the maximum sensitivity is 0.0142 K-1 in our temperature measurement range.
ACCEPTED MANUSCRIPT Compared to several typical Er3+-based up-conversion thermometers (see table 3), the excellent sensing ability means that the Ca1-xNaxMoO4:Yb3+,Er3+ up-conversion phosphors are potentially applicable as the optical thermometric materials.
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Fig. 10
Table 3
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Conclusion
Ca1-xNaxMoO4:Yb3+,Er3+ up-conversion phosphors with orthorhombic structure are
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prepared by a simple solid-state method. Na+ doping has a strong influence on luminescence of CaMoO4:Yb3+,Er3+. When the concentration of Na+ is 2%, the strongest up-conversion luminescence is obtained. Since the Yb3+-MoO42- dimer sensitization dominates the upconversion luminescence and the multi-phonon relaxation rate of 4S3/2
4
F9/2 is quite low, the phosphor
produces nearly single-band green up-conversion emission under 980 nm excitation. The
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research on temperature sensing indicates that the thermometers based on the relative intensity ratio of two green emissions from Er3+ suffers from severe temperature measurement accuracy problems. To reflect the true temperature, a simple method for calibration of temperature sensing is reported. Compared to several typical Er3+-based up-conversion thermometers, the
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Ca1-xNaxMoO4:Yb3+,Er3+ shows excellent sensing capability, and is therefore potentially
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applicable as the optical thermometric material. Acknowledgements
This word was supported by the National Natural Science foundation of China (Grant No.
11747312) and Huzhou University’s Scientific Research Project (Grant No. 2016XJXM23). References
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[32] L.L. Tong, X.P. Li, J.S. Zhang, S. Xu, J.S. Sun, H. Zheng, Y.Q. Zhang, X.Q. Zhang, R.N. Hua,
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H.P. Xia, B.J. Chen, Opt. Express 25 (2017) 16047-16058.
[33] L.L. Tong, X.P. Li, J.S. Zhang, S.Xu, J.S. Sun, L.H. Cheng, H. Zheng, Y.Q. Zhang, X.Q.
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Zhang, R.N. Hua, H.P. Xia, B.J. Chen, Sensor. Actuat. B 246 (2017) 175-180.
ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of 1#, 2# and 3# samples (a) as well as crystal structure diagram of CaMoO4 (b) Fig. 2 Up-conversion spectra of various samples under 980 nm excitation, inset: digital image of
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up-conversion luminescence for 2# sample at the pumping power of 688 mW (a); the integrated intensity of green emission (b)
Fig. 3 Up-conversion decay curves of 554 nm emission in 1#, 2# and 3# samples
Fig.4 (a) Excitation and down-conversion spectra of 2# sample; (b) up-conversion spectra of 4#,
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5# and 6# samples as well as the dependence of IG/IR on Er3+ concentration; (c) down-conversion spectra of 2#, 4#, 7# and 8# samples as well as the EBT process; (d) up-conversion spectra of 2#,
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4#, 7# and 8# samples as well as the dependence of IG/IR on Yb3+ concentration. It should be pointed out that all spectra shown in (b) and (d) are normalized to the emission peak at 554 nm. Fig.5 Dependence of IH/IS on radiative time for 2# sample under 980 nm excitation with pump power of 48 mW. The sample temperature is kept at 300 K.
Fig.6 Thermometers TM1 (a) and TM2 (b) based on two different spectral scanning modes under
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980 nm excitation with an excitation power of 188 mW. (a) Continuous scanning mode: during the entire data collection, the laser continuously irradiates on the sample; (b) Intermittent scan mode: the acquisition interval of every two data points is 2 min, and the laser radiation time is about 3s. 4
I15/2 transition at various temperatures
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Fig.7 Integrate intensities for 2H11/2/4S3/2
Fig.8 Corrected dependence of IH/IS on temperature
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Fig.9 IH/IS of 2# sample at various pumping powers as well as the temperature of irradiation spot calculated by TM9
Fig. 10 Temperature sensitivity of TM9 as a function of the temperature
ACCEPTED MANUSCRIPT Table 1 Chemical composition of various samples sample
Chemical composition (mol %) Na+
Yb3+
Er3+
Mo6+
1#
91
0
8
1
100
2#
89
2
8
1
100
3#
87
4
8
1
4#
97
2
0
1
5#
94
2
0
4
6#
90
2
0
8
7#
81
2
16
8#
67
2
30
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1
100
1
100
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T1 * (K)
T2 * (K)
T3 * (K)
T3-T2 (K)
∆E/k
∆E (cm-1)
TM1
291.80
289.57
293.80
4.23
928.17
645.15
TM2
300.17
297.88
302.21
4.33
960.48
667.60
TM3
299.38
297.06
301.45
4.39
940.27
653.56
TM4
294.51
292.23
296.55
4.32
925.62
643.37
TM5
288.93
286.71
290.92
4.21
911.33
633.44
TM6
282.59
280.37
284.58
4.21
872.54
606.48
TM7
299.50
297.48
301.30
TM8
303.15
300.83
305.23
TM9
334.67
332.28
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1079.87
750.59
4.4
963.12
669.44
4.52
1142.03
793.80
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336.80
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ACCEPTED MANUSCRIPT Table 3 Optical paramers of several typical thermometers based on Er3+ luminescence Excitation
Sensitivity
Temperature
wavelength (nm)
(K-1)
range (K)
NaGdTiO4:Yb3+/Er3+
980
0.0083
100-1000
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NaY(MoO4)12:Yb3+/Er3+
980
0.0097
303-523
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SrWO4:Yb3+/Er3+
980
0.011
299-518
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CaF2:Yb3+/Er3+
980
0.014
295-723
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980
0.0142
336.2-536.2
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1. The ratiometric temperature sensing behavior of Ca1-xNaxMoO4:Yb ,Er
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phosphor is investigated. 2. It is found that the temperature sensing suffers from the problem of temperature measurement
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3. A simple and reliable method of correction is reported.
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accuracy.