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Nd3+ codoped Bi4Ti3O12 microcrystals
Journal of Luminescence 221 (2020) 117095
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Non-contact fluorescence intensity ratio optical thermometer based on Yb3þ/Nd3þ codoped Bi4Ti3O12 microcrystals Hao Chen , Gongxun Bai *, Qinghua Yang , Youjie Hua , Shiqing Xu , Liang Chen ** Institute of Optoelectronic Materials and Devices, China Jiliang University, Hangzhou, 310018, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Upconversion Near-infrared Optical thermometry Lanthanide Yb3þ/Nd3þ ions
Optical thermometer based on the non-contact fluorescence intensity ratio technique of two thermally-couple levels has huge potential applications in many fields, such as the electric power industry and in-situ physio logical measurements. In the paper, the Yb3þ/Nd3þ codoped Bi4Ti3O12 nanocrystal ratio thermometer has pre pared by solid-phase sintering. The two thermally-couple levels are Nd3þ:4F7/2 (emission peak around 753 nm) and Nd3þ:4F5/2 (emission peak at 805 nm), respectively. The optical thermometer has based on the upconversion in near-infrared luminescence under 980 nm excitation with low power of 1.87 W/cm2. Due to the relatively low pump power, the effect of the excitation heat effect on the material is reduced. This resulted in a relatively high relative sensitivity (SR ¼ 1% K 1) in the temperature range of 323–573 K. In addition to the characteristics, the excellent repeatability (σ ¼ 1.5%) of the fluorescence intensity ratio between the two Nd3þ emission peaks makes Yb3þ/Nd3þ codoped Bi4Ti3O12 microcrystals a promising non-contact near-infrared ratio thermometer.
1. Introduction
generating upconversion luminescence, which affects the accuracy of non-contact temperature measurement [1]. Therefore, there is a certain demand for materials with low pump source power, high sensitivity and good repeatability [4]. (see Table 1) Upconversion refers to the process by which a luminescent ion rea ches a higher energy excited state from a ground state through contin uous multiphoton absorption [27,28]. Because of the long life of metastable energy levels, the photon energy can exist in a certain metastable state for a certain period of time, and no relaxation occurs before the next photon is absorbed [24,28]. This is equaling to the transition from the ground state to the higher excited state through the multiphoton process and emits light. The Yb3þ/Nd3þ codoped upcon version materials are candidates for ratiometric thermometers [3]. The two TCLs of Nd3þ (4F7/2/4F5/2 and 4F5/2/4F3/2) have an energy gap of ~1000 cm 1. Due to the upconversion emission band of Nd3þ (750–950 nm) and the excitation source (such as 980 nm diode laser) locate in the first biological window (650–1000 nm), so that it also supports the application of wide spectrum measurement [2,3,7,18]. The host plays an important role in the temperature sensing for lanthanide doped ther mometers. And the Bi4Ti3O12 has the low photon energy and high thermal stability. Thermally enhanced phonon assisted from Yb3þ to Nd3þ plays an
Temperature is an important parameter that characterizes the state of an object [1–7]. Among them, real-time and non-contact temperature monitoring plays an important role in industrial production, examina tion, medical and other applications [7–12]. Nowadays, temperature-dependent luminescence of materials doped with lantha nide ions have attracted great attention [5,13]. Since the lanthanide optical thermometry is based on the fluorescence intensity ratio (FIR) of two thermal coupling levels (TCLs) [2,4,14]. FIR’s non-contact ther mometers offer the advantages of no direct contact, high detection sensitivity, and fast response. For FIR-based thermometers, the two signals from the TCL need to vary significantly with temperature, and only the energy gap (ΔE) between the two TCLs is in the range of 200–2000 cm 1 [1,15]. Too large or too small energy gap can result in weak coupling and a strong overlap of the TCL signal [16]. Many rare earth ions (such as Eu3þ, Tb3þ, Dy3þ, Er3þ, Nd3þ, Tm3þ and Ho3þ) can be used as TCLs for non-contact thermometers [16–27]. L. Marciniak et al. have developed LiLaP4O12:Tm3þ,Yb3þ,Eu3þ nanocrystals for op tical thermometry. It is found that high sensitivity is 0.1% K 1 under pump power 40 W/cm2 [26]. At the same time, there is also a part of the thermal effect caused by the excitation of the excitation source when
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Bai), [email protected] (L. Chen). https://doi.org/10.1016/j.jlumin.2020.117095 Received 29 October 2019; Received in revised form 2 February 2020; Accepted 3 February 2020 Available online 4 February 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.
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Journal of Luminescence 221 (2020) 117095
2. Experimental
Table 1 Temperature sensitivity measured with different doped rare earth ions. Luminescent material
Emission Wavelength (nm)
Excitation Wavelength (nm)
Relative Sensitivity (K 1)
References
NaYF4: Nd–Yb Gd2O3: Er–Yb CaWO4: Yb–Nd La2O3: Er–Yb NaYF4: Er–Yb Y2Ti2O7: Yb–Er YF3: Yb–Er
720–950 510–565 297–420
980 980 889
1033/T2 1100/T2 1459/T2
[32] [15] [29]
510–570 510–580 298–673
980 920 564
814/T2 1028/T2 812/T2
[33] [34] [35]
260–490
688
997/T2
[17]
Sample preparation: The microcrystalline sample having a nominal composition of Bi4-x-yTi3O12:xYb3þ/yNd3þ (x ¼ 0.2, y ¼ 0.05) (BTO: Yb–Nd) is synthesized using a high temperature solid state reaction. High purity of Bi2O3, Nd2O3, TiO2, and Yb2O3 as the starting materials without further purification. A stoichiometric batch of 15 g starting materials has thoroughly mixed in an agate mortar and then sintered in an alumina crucible in a bottom loading furnace at 800 � C for 4 h and 720 � C for 4 h under ambient conditions, respectively with two inter mediate grinding steps. Characterization details: The phase purity of the crystallite mea sures by the X-ray diffraction in steps of 0.01� /s, and the counting time per step is 1 s. The SEM by using the SU8010 FE-SEM HITACHI, Japan. And the energy dispersive spectrometer (EDS) is acquired using EDAX the company of the United States TEAM Apollo XL EDS. The diffuse reflectance spectrum (DRS) of the 400–1200 nm spectral region record with a spectrophotometer in steps of 2 nm with an integration time of 0.5 s and equip with an integrating sphere. The dependent upconversion emission spectrum in the temperature range of 323–573 K measure in a home optical system. The 980 nm continuous wave laser uses as an excitation source. The temperature controller sets the operating tem perature of the laser to 293 K. The power density of the laser sets to 1.87 W/cm2, and a CCD spectrometer uses as a detector to record the upconversion emission spectrum. The power density is obtained by the power of the laser dividing the luminous spot area (~0.16016 cm2) of the sample. Temperature measurement experiment: The prepared Yb–Nd codoped bismuth titanate crystallites were placed inside a heating platform where the temperature control in the range of 293–553 K by a high temperature fluorescence controller (Orient KOJI, TAP-02). The sample was held at the given temperature for 5 min to let it fully equilibrate and then get upconversion measurement at the defined
important role in the significant increase in Nd3þ emissions [6]. In addition, due to the low phonon energy of Bi4Ti3O12, the low multi-phonon non-radiative relaxation of Nd3þ may also be the cause of this phenomenon. The Bi4Ti3O12 crystals with the ferroelectric property have been reported as a good luminescent host for lanthanide ions [3]. In this work, the non-contact thermometer is made of Yb3þ/Nd3þ codoped with Bi4Ti3O12 microcrystals which have synthesized using solid state reaction, with scanning electron microscopy (SEM) and X-ray diffraction (XRD) to verify the structure of Bi4Ti3O12 crystal, using 980 laser to excite Bi4Ti3O12 to study its upconversion luminescence [3,18]. And under the repetitive verification of the experiment, it can be found that the FIR formula has a very low error rate [1,5,22]. The excellent sensitivity of the Yb3þ/Nd3þ codoped Bi4Ti3O12 material is excited by a very low pump source [2]. Excellent repeatability and high sensitivity make this material one of the superior probing materials for non-contact thermometers [22].
Fig. 1. (a) The powder XRD pattern of BTO: Yb–Nd microcrystals and tabulated standard diffraction patterns of trigonal Bi4Ti3O12 (blue). (b)The DRS of the material. The label in figure marks the energy level of Nd3þ and Yb3þ respectively. (c)The SEM image of the prepared microcrystals. (d) The energy dispersive spectrum (EDS) of the prepared microcrystals. 2
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Journal of Luminescence 221 (2020) 117095
Fig. 2. (a) The upconversion emission spectrum of BTO: Yb–Nd in the temperature range 323–503 K. (b) The upconversion emission spectra in the temperature range 503–573 K. (c) The upconversion emission spectra in the temperature range 323–573 K. (d) The integrated emission intensity of the NIR1 (red histogram) and the NIR2 (blue histogram) varies with temperature, the temperature range is 323–573 K.
are the electronic transition of Nd3þ from the ground state 4I9/2 to the excited state 4G5/2 þ 4G7/2, 2H11/2, 4F9/12, 4S3/2 and 4F3/2, respectively, as marked in Fig. 1b. It worth noting that a strong overlap between the absorption bands of Yb3þ: 2F7/2 → 2F5/2 and Nd3þ: 4I9/2 → 4F3/2 at ~830 nm can be looked in Fig. 1b. According to the DRS, it is found that the too large ΔE value of 4F3/2/4F7/2 results the weak thermal coupling between two levels. In addition, the 4F3/2 (~900 nm) emission band strongly overlaps Yb3þ thermal simulation of upconversion emissions, which is often used for laser cooling. The signal interference between Yb3þ and Nd3þ can be made Nd3þ emission band. Therefore, only the 4 F5/2 and 4F7/2 are considered for the proportional thermometers study. And Fig. 1c shows an SEM image of BTO: Yb–Nd microcrystals, these are about 10–40 μm in diameter. The Bi4Ti3O12 powder particles synthe sized by the solid phase method have a random shape, and have a certain degree of agglomeration. Fig. 1d shows the obvious peak of Yb and Nd, which is a good illustration for the co-doping of Bi4Ti3O12. According to the spectral changes of temperature changes, the tem perature characteristics can be inferred. Fig. 2a illustrates the upcon version emission spectra of BTO: Yb–Nd at the 980 nm laser excitation source under the relatively low power density (~1.87 W/cm2) form 323–573 K. The spectral intensity of 805 nm starts to show a downward trend after the temperature (573 K) in Fig. 2b. Fig. 2c can also clearly see the comprehensive intensity change. The emission profile of Nd3þ is located in the first biological window (650–1000 nm), which spans a very broad spectral from 730 to 855 nm. The 713–775 nm (maximum 751 nm) has defined as NIR1 and 785–855 nm (maximum 805 nm) as NIR2, respectively. Fig. 2d shows the area integral intensity of NIR1 and NIR2. They are the electronic transitions of Nd3þ from the ground state 4 I9/2 to the excited states 4F7/2 and 4F5/2, respectively. Fig. 2d summa rizes the dependence of the integrated emission intensity of NIR1 and NIR2 on the FIR of NIR1/NIR2 at a temperature step of 10 K. The NIR1
temperature. The pump laser turns on for a minimum time to avoid any laser-induced heating during the measurements. In detail, we would turn off laser stabilized for 5 min after each measurement, and then take on the next time. 3. Results and discussion Herein, the correctness of the experimental design is proved by various effective test methods. Fig. 1a shows the XRD of BTO: Yb–Nd microcrystals. The crystal structure of Bi4Ti3O12 is a typical Bi-based layered structure of oxygen-containing octahedron, adopts the typical orthorhombic D-type structure (space group: Cmmm (65); PDF#35–0795). Its crystal structure consists of a tantalum oxide layer [Bi2O3] and a perovskite-like layer [Bi4Ti3O12] [28]. In the figure, where θ ¼ 29.98 is due to Bi2O3. The co-doping of a small amount of the Yb3þ/Nd3þ in Bi4Ti3O12 resulted in a diffraction peak of the Bi4Ti3O12 angular movement to a high angle compared to undoped samples, indicating that the Nd3þ and the Yb3þ are in the Bi4Ti3O12 lattice. The ionic radius of the Bi3þ is bigger than that of the Nd3þ and Yb3þ. The Bi3þ ions are displaced by smaller the Nd3þ and Yb3þ ions, and that will cause the systole of the unit cell of Bi4Ti3O12. It is universally acknowledged that the Bragg law of nλ ¼ 2dsin(θ), will result in the diffraction peak to move to a high angle. Also, due to the huge difference of the ion size between Yb3þ and Bi3þ, this leads to the low solubility of Yb3þ in the Bi4Ti3O12 crystal phase. Fig. 1b shows the DRS of BTO: Yb–Nd in the 250–1200 nm spectral range. It easy to found a sharp peak at ~980 nm and a wide shoulder at ~930 nm in the broad absorption bands from 850 to 1200 nm, it is showing the electronic transitions of Yb3þ form the ground state of 2F7/2 to excited state multiple of 2F5/2 [29]. And the other broad absorption bands with the sharps at ~586, 638, 678, 745 and 800 nm are ascribed to the absorption of Nd3þ. These 3
H. Chen et al.
Journal of Luminescence 221 (2020) 117095
Fig. 3. (a) Dependence of Nd3þ: 4F5/2 on power. The solid red line is the linear fit of the experimental data. (b) The schematic diagram of the energy level diagram of Yb3þ and Nd3þ, and the mechanism of upconversion emission and TP under excitation of laser in the 980 nm. PEAT and TP represent phonon-assisted energy transfer and heat groups, respectively.
electrons are pumped from Yb3þ: 2F7/2 to Yb3þ: 2F5/2 under the 980 nm excitation. Nd3þ is excited from the energy transfer of Yb3þ to Nd3þ in the 4I9/2 state to 4F3/2 and 4F5/2. Because the energy gap ΔE between the energy levels of Yb3þ: 2F7/2 and Nd3þ: 2F3/2 is ~1000 cm 1. About the Bi4Ti3O12 has very low phonon energy ~270 cm 1 [31], it can compensate by ~3–4 phonons required for energy transfer from phonon energy from Yb3þ: 2F7/2 to Nd3þ: 2F3/2. The process of the PEAT has been verified in other crystal doped Yb3þ and Nd3þ materials [2,21,27,32]. The approximately linear process of the excitation power density of the Nd3þ emission (Fig. 3a) also indicates the PEAT upconversion mecha nism of Yb3þ/Nd3þ in Bi4Ti3O12. The ΔE values of 4F3/2/4F5/2 and 4 F5/2/4F7/2 are ~1000 cm 1, and the TP between these levels results in a strong emission of Nd3þ: 4F7/2 and Nd3þ: 4F5/2 levels, with maximum values at 765 and 825 nm (Fig. 2a and b). Describe the emission intensity of a given excited state with tem perature by Equation (1):
and the NIR2 exhibit very weak thermal quenching effects at tempera ture. Due to the effective transport phonon (TP) between Nd3þ: 2F5/2 and Nd3þ: 2F3/2 the emission intensity of NIR2 increases by temperature. At 503 K, the initial intensity of NIR2 relative to 323 K is increased by ~4 times. Higher temperatures (>503 K) will result in lower emission in tensity of NIR2. However, the blackbody radiation at high temperatures (>503 K) slightly reduces the certainty of NIR2 by measure. Similarly, due to the effective TP between Nd3þ: 2F7/2 and Nd3þ: 2F5/2, the NIR1 increase significantly with temperature, but is far less than the increase of NIR. We observe the gain factor is 11 at 543 K that relative to the initial emission intensity of 323 K. The highest emission temperature of NIR1 (max ¼ 805 nm) is slightly higher than NIR2 (max ¼ 745 nm). The high thermal quenching temperatures of the BTO: Yb–Nd is 543 K (NIR1) and 473 K (NIR2), respectively, which are the highest readings for Yb3þ/Nd3þ codoped upconversion emissions [30]. The reason why NIR1 is higher than NIR2 is that the difference of TP between Nd3þ: 2F7/2 and Nd3þ: 2F5/2. This unique feature allows accurate temperature measurements over the entire temperature range, with the FIR of NIR1/NIR2 increasing significantly from 0.06 (323 K) to 0.27 (573 K). The FIR of NIR1/NIR2 varies greatly and can be applied to FIR ther mometers. Fig. 3a shows a double logarithmic plot of upconversion emission intensity variation and excitation power density from 0.16 to 1.96 W/cm2 in the range of NIR2. Within the power density range, through a double logarithmic graph obtained by fitting a linear slope value can be obtained in 1.33729 � 0.00479. This indicates a two-photon process in which phonon-assisted Nd3þ upconversion emission occurs at 980 nm excitation [2]. Fig. 3b diagrammatic shows upconversion emission and TP mechanisms of BTO: Yb–Nd. The
E
I ∝ gAhv⋅ekB T
(1)
Where the g is the degeneracy of the state, the A is the spontaneous emission rate, the h is the Planck constant, the v is the frequency, the E is the energy level, the kB is the Boltzmann constant, the T is the absolute temperature [5]. It can use the Boltzmann distribution to describe the FIR of two TCLs by Equation (2):
Fig. 4. (a) The FIR varies according to temperature, (b) The absolute temperature sensitivity (SA) and relative temperature sensitivity (SR) vary according to temperature. 4
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Journal of Luminescence 221 (2020) 117095
Fig. 5. (a) Repeatability of FIR thermometry. (b) Standard deviation and temperature uncertain (σ) based on three the cooling and heating cycles.
I2 I1 � � g2 A2 hv2 ΔE21 ¼ exp g1 A1 hv1 kB T � � ΔE21 ¼ B exp kB T
value (9.9 � 10 4 K 1) is obtained at 573 K, and get the minimum SA (6.6 � 10 4 K 1) at 343 K. The SA value is generated in the maximum illuminating region of NIR1 and NIR2, which also shows that the ma terial can obtain better sensitivity at higher temperatures. The SR value is determined to be 1248.64/T2. In the temperature range studied (323–573 K), the highest SR value is 0.38% K 1 at the highest measured temperature of 573 K, starting from 1.19% K 1 at the initial temperature of 323 K and falling to the highest temperature in the measurement range. The temperature uncertain (△Tmin) in Fig. 5 can be determined based on the σ and SA.
FIR ¼
(2)
wherein, the I1 is the fluorescence intensity TCL of Nd3þ: 2F5/2; the I2 is the fluorescence intensity TCL of the Nd3þ: 2F7/2; the B is the propor tional constant of the temperature-independent, and the ΔE21 is the energy gap between the TCLs. The experimental data of the FIR has fitted by the equation to obtain the energy gap ΔE between the TCLs. In Fig. 4a is the best fit plot of experimental FIR (NIR1/NIR2) data. Where r2 ¼ 0.99685. The fitted energy gap (ΔEf) value of 867.2 cm 1. The obtained experimental en ergy gap (ΔEm) is 893 cm 1 in Fig. 2. From this, an error value (ε ¼ 2.9%) of the energy gap can be got. � � �ΔEm ΔEf � ε¼ � 100% (3) ΔEm
ΔT ¼
4. Conclusions
(4)
It reports Yb3þ/Nd3þ codoped Bi4Ti3O12 crystallites based on NIR ratio thermometers, operating temperatures ranging from 323 to 573 K. 4 S3/2/4F7/2 (NIR1) and 4F5/2 (NIR2) TCL energy gap ΔE for ratios ther mometer. NIR1 and NIR2 TCL’s weak thermal quenching and high peak emission maximum temperatures provide high sensing accuracy and a wide range of temperature sensing. The two thermal coupling levels of Nd3þ (4F7/2/4F5/2 and 4F5/2/4F3/2) have located in the first biological window (650–1000 nm), it can support the application of wide spectrum measurement. From 0.06 (323 K) to 0.27 (573 K), the FIR of NIR1/NIR2 increased significantly. The larger NIR1/NIR2 variation interval FIR enables it to be applied to a ratiometric thermometer. The relative sensitivity value has determined to be 1248/T2. In the temperature range studied (323–573 K), the maximum SR value is 1.2% K 1 at 323 K, and as the temperature increased, the temperature rose from room temperature to the highest temperature of the experimental equipment, and the SR value also relatively increased to 0.4% K 1. The △T (~10 6) obtained from cyclic repeatability experiments of three independent environments from 323 to 573 K also demonstrate the feasibility and superiority of the material as the main material for non-contact tem perature measurement.
According to Equation, it can find that SA is related to the FIR value, there is an error in the comparison between different systems. Recently, the relative temperature sensitivity defined as the ratio of SA to FIR is often used as a different standard parameter for quantitative comparison of system temperature sensitivity [2]. It is the normalized change of FIR with temperature: SR ¼
1 ∂FIR ΔE ⋅ ¼ FIR ∂T 2 kB T 3
(5)
In addition, the following Equation is used to define Tmax as the maximum SA temperature obtained [29]: � � ∂2 FIR ΔE ΔE ¼ FIR ⋅ 2 ⋅ (6) 3 2 ∂T kB T max kB T 3max Tmax ¼
ΔE 2kB
(8)
The σ less than 1.5% indicates better repeatability for the FIR mea surements of three heating and cooling cycles for the BTO: Yb–Nd (temperature range 323–573 K, step size 10 K). At the same time, the small value of △Tmin value also achieves good repeatability. Almost a few measured errors in multiple repetitive experiments, which is one of the root causes of barium titanate as a remote temperature measuring material.
The SA is defined as the absolute FIR change with temperature and can be calculated using:
∂FIR ΔE SA ¼ ¼ FIR⋅ kB T 3 ∂T
σ SR
(7)
Therefore, the ΔE value directly is proportional to the Tmax value. According to the formula, the material will get the Tmax at 624 K. The higher the Tmax value, the higher the sensitivity at high temperatures. Fig. 4b shows the SA and SR values as a function of temperature from 323 to 573 K. In the experimental temperature range, the maximum SA
CRediT authorship contribution statement Hao Chen: Writing - original draft, Writing - review & editing. 5
H. Chen et al.
Journal of Luminescence 221 (2020) 117095
Gongxun Bai: Conceptualization, Supervision, Writing - review & editing. Qinghua Yang: Methodology. Youjie Hua: Methodology. Shiqing Xu: Conceptualization, Resources. Liang Chen: Methodology, Supervision.
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Acknowledgments This work was financed by the National Natural Science Foundation of China (Grant No. 61775203, 61705214), the National Key Research and Development Project (Grant No. 2017YFF0210800), Zhejiang Pro vincial Natural Science Foundation of China (No. LZ20F050001, LD18F050001), Jiangxi Provincial Key Laboratory of Optoelectronics and Communications (Grant No. 201910EC001). References [1] P. Huang, W. Zheng, Z. Gong, W. You, J. Wei, X. Chen, Rare earth ion and transition metal ion–doped inorganic luminescent nanocrystals: from fundamentals to biodetection, Mater. Today Nano. 5 (2019) 100031. [2] G. Gao, D. Busko, S. Kauffmann-Weiss, A. Turshatov, I.A. Howard, B.S. Richards, Wide-range non-contact fluorescence intensity ratio thermometer based on Yb3þ/ Nd3þ co-doped La2O3 microcrystals operating from 290 to 1230 K, J. Mater. Chem. C 6 (2018) 4163–4170. [3] E. Pan, G. Bai, B. Ma, L. Lei, L. Huang, S. Xu, Reversible enhanced upconversion luminescence by thermal and electric fields in lanthanide ions doped ferroelectric nanocomposites, Sci. China Mater. 63 (2020) 110–121. [4] S. Yu, D. Tu, W. Lian, J. Xu, X. Chen, Lanthanide-doped near-infrared II luminescent nanoprobes for bioapplications, Sci. China Mater. 62 (2019) 1071–1086. [5] F. Hu, J. Cao, X. Wei, X. Li, J. Cai, H. Guo, Y. Chen, C.K. Duan, M. Yin, Luminescence properties of Er3þ-doped transparent NaYb2F7 glass-ceramics for optical thermometry and spectral conversion, J. Mater. Chem. C 4 (2016) 9976–9985. [6] D. Jaque, F. Vetrone, Luminescence nanothermometry, Nanoscale 4 (2012) 4301–4326. [7] M. Kamimura, T. Matsumoto, S. Suyari, M. Umezawa, K. Soga, Ratiometric nearinfrared fluorescence nanothermometry in the OTN-NIR (NIR II/III) biological window based on rare-earth doped β-NaYF4 nanoparticles, J. Mater. Chem. B 5 (2017) 1917–1925. [8] S. Sekiyama, M. Umezawa, S. Kuraoka, T. Ube, M. Kamimura, K. Soga, Temperature sensing of deep abdominal region in mice by using over-1000 nm near-infrared luminescence of rare-earth-doped NaYF4 nanothermometer, Sci. Rep. 8 (2018) 16979. [9] H. Dong, L. Sun, C. Yan, Energy transfer in lanthanide upconversion studies for extended optical applications, Chem. Soc. Rev. 44 (2015) 1608–1634. [10] M. Sun, H. Dong, A. Dougherty, Q. Lu, D. Peng, W. Wong, B. Huang, L. Sun, C. Yan, Nanophotonic energy storage in upconversion nanoparticles, Nano energy 56 (2019) 473–481. [11] G. Gao, D. Busko, R. Joseph, A. Turshatov, I.A. Howard, B.S. Richards, High quantum yield single-band green upconversion in La2O3:Yb3þ,Ho3þ microcrystals for anticounterfeiting and plastic recycling, Part. Part. Syst. Char. 36 (2019) 1800462. [12] Z. Wu, G. Bai, Q. Hu, D. Guo, C. Sun, L. Ji, L. Ming, L. Li, P. Li, J. Hao, Effects of dopant concentration on structural and near-infrared luminescence of Nd3þ doped beta-Ga2O3 thin films, Appl. Phys. Lett. 106 (2015) 91–94. [13] Y. Wang, K. Zheng, S. Song, D. Fan, H. Zhang, X. Liu, Remote manipulation of upconversion luminescence, Chem. Soc. Rev. 47 (2018) 6473–6485. [14] X. Liu, Z. Xu, C. Chen, D. Tian, L. Yang, X. Luo, A.A. Al Kheraif, J. Lin, Carbon quantum dot-sensitized and tunable luminescence of Ca19Mg2(PO4)14:Ln3þ(Ln3 þ ¼Eu3þ and/or Tb3þ) nanocrystalline phosphors with abundant colors via a sol–gel process, J. Mater. Chem. C 7 (2019) 2361–2375.
6
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Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Non-contact fluorescence intensity ratio optical thermometer based on Yb3þ/Nd3þ codoped Bi4Ti3O12 microcrystals Hao Chen , Gongxun Bai *, Qinghua Yang , Youjie Hua , Shiqing Xu , Liang Chen ** Institute of Optoelectronic Materials and Devices, China Jiliang University, Hangzhou, 310018, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: Upconversion Near-infrared Optical thermometry Lanthanide Yb3þ/Nd3þ ions
Optical thermometer based on the non-contact fluorescence intensity ratio technique of two thermally-couple levels has huge potential applications in many fields, such as the electric power industry and in-situ physio logical measurements. In the paper, the Yb3þ/Nd3þ codoped Bi4Ti3O12 nanocrystal ratio thermometer has pre pared by solid-phase sintering. The two thermally-couple levels are Nd3þ:4F7/2 (emission peak around 753 nm) and Nd3þ:4F5/2 (emission peak at 805 nm), respectively. The optical thermometer has based on the upconversion in near-infrared luminescence under 980 nm excitation with low power of 1.87 W/cm2. Due to the relatively low pump power, the effect of the excitation heat effect on the material is reduced. This resulted in a relatively high relative sensitivity (SR ¼ 1% K 1) in the temperature range of 323–573 K. In addition to the characteristics, the excellent repeatability (σ ¼ 1.5%) of the fluorescence intensity ratio between the two Nd3þ emission peaks makes Yb3þ/Nd3þ codoped Bi4Ti3O12 microcrystals a promising non-contact near-infrared ratio thermometer.
1. Introduction
generating upconversion luminescence, which affects the accuracy of non-contact temperature measurement [1]. Therefore, there is a certain demand for materials with low pump source power, high sensitivity and good repeatability [4]. (see Table 1) Upconversion refers to the process by which a luminescent ion rea ches a higher energy excited state from a ground state through contin uous multiphoton absorption [27,28]. Because of the long life of metastable energy levels, the photon energy can exist in a certain metastable state for a certain period of time, and no relaxation occurs before the next photon is absorbed [24,28]. This is equaling to the transition from the ground state to the higher excited state through the multiphoton process and emits light. The Yb3þ/Nd3þ codoped upcon version materials are candidates for ratiometric thermometers [3]. The two TCLs of Nd3þ (4F7/2/4F5/2 and 4F5/2/4F3/2) have an energy gap of ~1000 cm 1. Due to the upconversion emission band of Nd3þ (750–950 nm) and the excitation source (such as 980 nm diode laser) locate in the first biological window (650–1000 nm), so that it also supports the application of wide spectrum measurement [2,3,7,18]. The host plays an important role in the temperature sensing for lanthanide doped ther mometers. And the Bi4Ti3O12 has the low photon energy and high thermal stability. Thermally enhanced phonon assisted from Yb3þ to Nd3þ plays an
Temperature is an important parameter that characterizes the state of an object [1–7]. Among them, real-time and non-contact temperature monitoring plays an important role in industrial production, examina tion, medical and other applications [7–12]. Nowadays, temperature-dependent luminescence of materials doped with lantha nide ions have attracted great attention [5,13]. Since the lanthanide optical thermometry is based on the fluorescence intensity ratio (FIR) of two thermal coupling levels (TCLs) [2,4,14]. FIR’s non-contact ther mometers offer the advantages of no direct contact, high detection sensitivity, and fast response. For FIR-based thermometers, the two signals from the TCL need to vary significantly with temperature, and only the energy gap (ΔE) between the two TCLs is in the range of 200–2000 cm 1 [1,15]. Too large or too small energy gap can result in weak coupling and a strong overlap of the TCL signal [16]. Many rare earth ions (such as Eu3þ, Tb3þ, Dy3þ, Er3þ, Nd3þ, Tm3þ and Ho3þ) can be used as TCLs for non-contact thermometers [16–27]. L. Marciniak et al. have developed LiLaP4O12:Tm3þ,Yb3þ,Eu3þ nanocrystals for op tical thermometry. It is found that high sensitivity is 0.1% K 1 under pump power 40 W/cm2 [26]. At the same time, there is also a part of the thermal effect caused by the excitation of the excitation source when
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (G. Bai), [email protected] (L. Chen). https://doi.org/10.1016/j.jlumin.2020.117095 Received 29 October 2019; Received in revised form 2 February 2020; Accepted 3 February 2020 Available online 4 February 2020 0022-2313/© 2020 Elsevier B.V. All rights reserved.
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Journal of Luminescence 221 (2020) 117095
2. Experimental
Table 1 Temperature sensitivity measured with different doped rare earth ions. Luminescent material
Emission Wavelength (nm)
Excitation Wavelength (nm)
Relative Sensitivity (K 1)
References
NaYF4: Nd–Yb Gd2O3: Er–Yb CaWO4: Yb–Nd La2O3: Er–Yb NaYF4: Er–Yb Y2Ti2O7: Yb–Er YF3: Yb–Er
720–950 510–565 297–420
980 980 889
1033/T2 1100/T2 1459/T2
[32] [15] [29]
510–570 510–580 298–673
980 920 564
814/T2 1028/T2 812/T2
[33] [34] [35]
260–490
688
997/T2
[17]
Sample preparation: The microcrystalline sample having a nominal composition of Bi4-x-yTi3O12:xYb3þ/yNd3þ (x ¼ 0.2, y ¼ 0.05) (BTO: Yb–Nd) is synthesized using a high temperature solid state reaction. High purity of Bi2O3, Nd2O3, TiO2, and Yb2O3 as the starting materials without further purification. A stoichiometric batch of 15 g starting materials has thoroughly mixed in an agate mortar and then sintered in an alumina crucible in a bottom loading furnace at 800 � C for 4 h and 720 � C for 4 h under ambient conditions, respectively with two inter mediate grinding steps. Characterization details: The phase purity of the crystallite mea sures by the X-ray diffraction in steps of 0.01� /s, and the counting time per step is 1 s. The SEM by using the SU8010 FE-SEM HITACHI, Japan. And the energy dispersive spectrometer (EDS) is acquired using EDAX the company of the United States TEAM Apollo XL EDS. The diffuse reflectance spectrum (DRS) of the 400–1200 nm spectral region record with a spectrophotometer in steps of 2 nm with an integration time of 0.5 s and equip with an integrating sphere. The dependent upconversion emission spectrum in the temperature range of 323–573 K measure in a home optical system. The 980 nm continuous wave laser uses as an excitation source. The temperature controller sets the operating tem perature of the laser to 293 K. The power density of the laser sets to 1.87 W/cm2, and a CCD spectrometer uses as a detector to record the upconversion emission spectrum. The power density is obtained by the power of the laser dividing the luminous spot area (~0.16016 cm2) of the sample. Temperature measurement experiment: The prepared Yb–Nd codoped bismuth titanate crystallites were placed inside a heating platform where the temperature control in the range of 293–553 K by a high temperature fluorescence controller (Orient KOJI, TAP-02). The sample was held at the given temperature for 5 min to let it fully equilibrate and then get upconversion measurement at the defined
important role in the significant increase in Nd3þ emissions [6]. In addition, due to the low phonon energy of Bi4Ti3O12, the low multi-phonon non-radiative relaxation of Nd3þ may also be the cause of this phenomenon. The Bi4Ti3O12 crystals with the ferroelectric property have been reported as a good luminescent host for lanthanide ions [3]. In this work, the non-contact thermometer is made of Yb3þ/Nd3þ codoped with Bi4Ti3O12 microcrystals which have synthesized using solid state reaction, with scanning electron microscopy (SEM) and X-ray diffraction (XRD) to verify the structure of Bi4Ti3O12 crystal, using 980 laser to excite Bi4Ti3O12 to study its upconversion luminescence [3,18]. And under the repetitive verification of the experiment, it can be found that the FIR formula has a very low error rate [1,5,22]. The excellent sensitivity of the Yb3þ/Nd3þ codoped Bi4Ti3O12 material is excited by a very low pump source [2]. Excellent repeatability and high sensitivity make this material one of the superior probing materials for non-contact thermometers [22].
Fig. 1. (a) The powder XRD pattern of BTO: Yb–Nd microcrystals and tabulated standard diffraction patterns of trigonal Bi4Ti3O12 (blue). (b)The DRS of the material. The label in figure marks the energy level of Nd3þ and Yb3þ respectively. (c)The SEM image of the prepared microcrystals. (d) The energy dispersive spectrum (EDS) of the prepared microcrystals. 2
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Journal of Luminescence 221 (2020) 117095
Fig. 2. (a) The upconversion emission spectrum of BTO: Yb–Nd in the temperature range 323–503 K. (b) The upconversion emission spectra in the temperature range 503–573 K. (c) The upconversion emission spectra in the temperature range 323–573 K. (d) The integrated emission intensity of the NIR1 (red histogram) and the NIR2 (blue histogram) varies with temperature, the temperature range is 323–573 K.
are the electronic transition of Nd3þ from the ground state 4I9/2 to the excited state 4G5/2 þ 4G7/2, 2H11/2, 4F9/12, 4S3/2 and 4F3/2, respectively, as marked in Fig. 1b. It worth noting that a strong overlap between the absorption bands of Yb3þ: 2F7/2 → 2F5/2 and Nd3þ: 4I9/2 → 4F3/2 at ~830 nm can be looked in Fig. 1b. According to the DRS, it is found that the too large ΔE value of 4F3/2/4F7/2 results the weak thermal coupling between two levels. In addition, the 4F3/2 (~900 nm) emission band strongly overlaps Yb3þ thermal simulation of upconversion emissions, which is often used for laser cooling. The signal interference between Yb3þ and Nd3þ can be made Nd3þ emission band. Therefore, only the 4 F5/2 and 4F7/2 are considered for the proportional thermometers study. And Fig. 1c shows an SEM image of BTO: Yb–Nd microcrystals, these are about 10–40 μm in diameter. The Bi4Ti3O12 powder particles synthe sized by the solid phase method have a random shape, and have a certain degree of agglomeration. Fig. 1d shows the obvious peak of Yb and Nd, which is a good illustration for the co-doping of Bi4Ti3O12. According to the spectral changes of temperature changes, the tem perature characteristics can be inferred. Fig. 2a illustrates the upcon version emission spectra of BTO: Yb–Nd at the 980 nm laser excitation source under the relatively low power density (~1.87 W/cm2) form 323–573 K. The spectral intensity of 805 nm starts to show a downward trend after the temperature (573 K) in Fig. 2b. Fig. 2c can also clearly see the comprehensive intensity change. The emission profile of Nd3þ is located in the first biological window (650–1000 nm), which spans a very broad spectral from 730 to 855 nm. The 713–775 nm (maximum 751 nm) has defined as NIR1 and 785–855 nm (maximum 805 nm) as NIR2, respectively. Fig. 2d shows the area integral intensity of NIR1 and NIR2. They are the electronic transitions of Nd3þ from the ground state 4 I9/2 to the excited states 4F7/2 and 4F5/2, respectively. Fig. 2d summa rizes the dependence of the integrated emission intensity of NIR1 and NIR2 on the FIR of NIR1/NIR2 at a temperature step of 10 K. The NIR1
temperature. The pump laser turns on for a minimum time to avoid any laser-induced heating during the measurements. In detail, we would turn off laser stabilized for 5 min after each measurement, and then take on the next time. 3. Results and discussion Herein, the correctness of the experimental design is proved by various effective test methods. Fig. 1a shows the XRD of BTO: Yb–Nd microcrystals. The crystal structure of Bi4Ti3O12 is a typical Bi-based layered structure of oxygen-containing octahedron, adopts the typical orthorhombic D-type structure (space group: Cmmm (65); PDF#35–0795). Its crystal structure consists of a tantalum oxide layer [Bi2O3] and a perovskite-like layer [Bi4Ti3O12] [28]. In the figure, where θ ¼ 29.98 is due to Bi2O3. The co-doping of a small amount of the Yb3þ/Nd3þ in Bi4Ti3O12 resulted in a diffraction peak of the Bi4Ti3O12 angular movement to a high angle compared to undoped samples, indicating that the Nd3þ and the Yb3þ are in the Bi4Ti3O12 lattice. The ionic radius of the Bi3þ is bigger than that of the Nd3þ and Yb3þ. The Bi3þ ions are displaced by smaller the Nd3þ and Yb3þ ions, and that will cause the systole of the unit cell of Bi4Ti3O12. It is universally acknowledged that the Bragg law of nλ ¼ 2dsin(θ), will result in the diffraction peak to move to a high angle. Also, due to the huge difference of the ion size between Yb3þ and Bi3þ, this leads to the low solubility of Yb3þ in the Bi4Ti3O12 crystal phase. Fig. 1b shows the DRS of BTO: Yb–Nd in the 250–1200 nm spectral range. It easy to found a sharp peak at ~980 nm and a wide shoulder at ~930 nm in the broad absorption bands from 850 to 1200 nm, it is showing the electronic transitions of Yb3þ form the ground state of 2F7/2 to excited state multiple of 2F5/2 [29]. And the other broad absorption bands with the sharps at ~586, 638, 678, 745 and 800 nm are ascribed to the absorption of Nd3þ. These 3
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Journal of Luminescence 221 (2020) 117095
Fig. 3. (a) Dependence of Nd3þ: 4F5/2 on power. The solid red line is the linear fit of the experimental data. (b) The schematic diagram of the energy level diagram of Yb3þ and Nd3þ, and the mechanism of upconversion emission and TP under excitation of laser in the 980 nm. PEAT and TP represent phonon-assisted energy transfer and heat groups, respectively.
electrons are pumped from Yb3þ: 2F7/2 to Yb3þ: 2F5/2 under the 980 nm excitation. Nd3þ is excited from the energy transfer of Yb3þ to Nd3þ in the 4I9/2 state to 4F3/2 and 4F5/2. Because the energy gap ΔE between the energy levels of Yb3þ: 2F7/2 and Nd3þ: 2F3/2 is ~1000 cm 1. About the Bi4Ti3O12 has very low phonon energy ~270 cm 1 [31], it can compensate by ~3–4 phonons required for energy transfer from phonon energy from Yb3þ: 2F7/2 to Nd3þ: 2F3/2. The process of the PEAT has been verified in other crystal doped Yb3þ and Nd3þ materials [2,21,27,32]. The approximately linear process of the excitation power density of the Nd3þ emission (Fig. 3a) also indicates the PEAT upconversion mecha nism of Yb3þ/Nd3þ in Bi4Ti3O12. The ΔE values of 4F3/2/4F5/2 and 4 F5/2/4F7/2 are ~1000 cm 1, and the TP between these levels results in a strong emission of Nd3þ: 4F7/2 and Nd3þ: 4F5/2 levels, with maximum values at 765 and 825 nm (Fig. 2a and b). Describe the emission intensity of a given excited state with tem perature by Equation (1):
and the NIR2 exhibit very weak thermal quenching effects at tempera ture. Due to the effective transport phonon (TP) between Nd3þ: 2F5/2 and Nd3þ: 2F3/2 the emission intensity of NIR2 increases by temperature. At 503 K, the initial intensity of NIR2 relative to 323 K is increased by ~4 times. Higher temperatures (>503 K) will result in lower emission in tensity of NIR2. However, the blackbody radiation at high temperatures (>503 K) slightly reduces the certainty of NIR2 by measure. Similarly, due to the effective TP between Nd3þ: 2F7/2 and Nd3þ: 2F5/2, the NIR1 increase significantly with temperature, but is far less than the increase of NIR. We observe the gain factor is 11 at 543 K that relative to the initial emission intensity of 323 K. The highest emission temperature of NIR1 (max ¼ 805 nm) is slightly higher than NIR2 (max ¼ 745 nm). The high thermal quenching temperatures of the BTO: Yb–Nd is 543 K (NIR1) and 473 K (NIR2), respectively, which are the highest readings for Yb3þ/Nd3þ codoped upconversion emissions [30]. The reason why NIR1 is higher than NIR2 is that the difference of TP between Nd3þ: 2F7/2 and Nd3þ: 2F5/2. This unique feature allows accurate temperature measurements over the entire temperature range, with the FIR of NIR1/NIR2 increasing significantly from 0.06 (323 K) to 0.27 (573 K). The FIR of NIR1/NIR2 varies greatly and can be applied to FIR ther mometers. Fig. 3a shows a double logarithmic plot of upconversion emission intensity variation and excitation power density from 0.16 to 1.96 W/cm2 in the range of NIR2. Within the power density range, through a double logarithmic graph obtained by fitting a linear slope value can be obtained in 1.33729 � 0.00479. This indicates a two-photon process in which phonon-assisted Nd3þ upconversion emission occurs at 980 nm excitation [2]. Fig. 3b diagrammatic shows upconversion emission and TP mechanisms of BTO: Yb–Nd. The
E
I ∝ gAhv⋅ekB T
(1)
Where the g is the degeneracy of the state, the A is the spontaneous emission rate, the h is the Planck constant, the v is the frequency, the E is the energy level, the kB is the Boltzmann constant, the T is the absolute temperature [5]. It can use the Boltzmann distribution to describe the FIR of two TCLs by Equation (2):
Fig. 4. (a) The FIR varies according to temperature, (b) The absolute temperature sensitivity (SA) and relative temperature sensitivity (SR) vary according to temperature. 4
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Journal of Luminescence 221 (2020) 117095
Fig. 5. (a) Repeatability of FIR thermometry. (b) Standard deviation and temperature uncertain (σ) based on three the cooling and heating cycles.
I2 I1 � � g2 A2 hv2 ΔE21 ¼ exp g1 A1 hv1 kB T � � ΔE21 ¼ B exp kB T
value (9.9 � 10 4 K 1) is obtained at 573 K, and get the minimum SA (6.6 � 10 4 K 1) at 343 K. The SA value is generated in the maximum illuminating region of NIR1 and NIR2, which also shows that the ma terial can obtain better sensitivity at higher temperatures. The SR value is determined to be 1248.64/T2. In the temperature range studied (323–573 K), the highest SR value is 0.38% K 1 at the highest measured temperature of 573 K, starting from 1.19% K 1 at the initial temperature of 323 K and falling to the highest temperature in the measurement range. The temperature uncertain (△Tmin) in Fig. 5 can be determined based on the σ and SA.
FIR ¼
(2)
wherein, the I1 is the fluorescence intensity TCL of Nd3þ: 2F5/2; the I2 is the fluorescence intensity TCL of the Nd3þ: 2F7/2; the B is the propor tional constant of the temperature-independent, and the ΔE21 is the energy gap between the TCLs. The experimental data of the FIR has fitted by the equation to obtain the energy gap ΔE between the TCLs. In Fig. 4a is the best fit plot of experimental FIR (NIR1/NIR2) data. Where r2 ¼ 0.99685. The fitted energy gap (ΔEf) value of 867.2 cm 1. The obtained experimental en ergy gap (ΔEm) is 893 cm 1 in Fig. 2. From this, an error value (ε ¼ 2.9%) of the energy gap can be got. � � �ΔEm ΔEf � ε¼ � 100% (3) ΔEm
ΔT ¼
4. Conclusions
(4)
It reports Yb3þ/Nd3þ codoped Bi4Ti3O12 crystallites based on NIR ratio thermometers, operating temperatures ranging from 323 to 573 K. 4 S3/2/4F7/2 (NIR1) and 4F5/2 (NIR2) TCL energy gap ΔE for ratios ther mometer. NIR1 and NIR2 TCL’s weak thermal quenching and high peak emission maximum temperatures provide high sensing accuracy and a wide range of temperature sensing. The two thermal coupling levels of Nd3þ (4F7/2/4F5/2 and 4F5/2/4F3/2) have located in the first biological window (650–1000 nm), it can support the application of wide spectrum measurement. From 0.06 (323 K) to 0.27 (573 K), the FIR of NIR1/NIR2 increased significantly. The larger NIR1/NIR2 variation interval FIR enables it to be applied to a ratiometric thermometer. The relative sensitivity value has determined to be 1248/T2. In the temperature range studied (323–573 K), the maximum SR value is 1.2% K 1 at 323 K, and as the temperature increased, the temperature rose from room temperature to the highest temperature of the experimental equipment, and the SR value also relatively increased to 0.4% K 1. The △T (~10 6) obtained from cyclic repeatability experiments of three independent environments from 323 to 573 K also demonstrate the feasibility and superiority of the material as the main material for non-contact tem perature measurement.
According to Equation, it can find that SA is related to the FIR value, there is an error in the comparison between different systems. Recently, the relative temperature sensitivity defined as the ratio of SA to FIR is often used as a different standard parameter for quantitative comparison of system temperature sensitivity [2]. It is the normalized change of FIR with temperature: SR ¼
1 ∂FIR ΔE ⋅ ¼ FIR ∂T 2 kB T 3
(5)
In addition, the following Equation is used to define Tmax as the maximum SA temperature obtained [29]: � � ∂2 FIR ΔE ΔE ¼ FIR ⋅ 2 ⋅ (6) 3 2 ∂T kB T max kB T 3max Tmax ¼
ΔE 2kB
(8)
The σ less than 1.5% indicates better repeatability for the FIR mea surements of three heating and cooling cycles for the BTO: Yb–Nd (temperature range 323–573 K, step size 10 K). At the same time, the small value of △Tmin value also achieves good repeatability. Almost a few measured errors in multiple repetitive experiments, which is one of the root causes of barium titanate as a remote temperature measuring material.
The SA is defined as the absolute FIR change with temperature and can be calculated using:
∂FIR ΔE SA ¼ ¼ FIR⋅ kB T 3 ∂T
σ SR
(7)
Therefore, the ΔE value directly is proportional to the Tmax value. According to the formula, the material will get the Tmax at 624 K. The higher the Tmax value, the higher the sensitivity at high temperatures. Fig. 4b shows the SA and SR values as a function of temperature from 323 to 573 K. In the experimental temperature range, the maximum SA
CRediT authorship contribution statement Hao Chen: Writing - original draft, Writing - review & editing. 5
H. Chen et al.
Journal of Luminescence 221 (2020) 117095
Gongxun Bai: Conceptualization, Supervision, Writing - review & editing. Qinghua Yang: Methodology. Youjie Hua: Methodology. Shiqing Xu: Conceptualization, Resources. Liang Chen: Methodology, Supervision.
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