- Email: [email protected]
Yb3+ phosphor
Optical Materials 86 (2018) 278–285
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optical thermometry based on anomalous temperature-dependent 1.53 μm infrared luminescence of Er3+ in BaMoO4: Er3+/Yb3+ phosphor
T
Ruoshan Lei∗, Xin Liu, Feifei Huang, Degang Deng, Shilong Zhao, Hui Xu, Shiqing Xu∗∗ College of Materials Science and Engineering, China Jiliang University, Hangzhou, 310018, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Near-infrared luminescence Negative thermal quenching Optical temperature sensing Rare earth
The temperature dependences of 1.53 μm near infrared and green upconversion (UC) emissions of Er3+ ion have been investigated in Er3+/Yb3+: BaMoO4 powders upon 980 nm excitation. Interestingly, an anomalous enhancement of 1.53 μm luminescence intensity with increasing temperature is observed, which is interpreted based on the phonon-assisted nonradiative process from 4I11/2 to 4I13/2 level. Moreover, the emission intensities from various Stark transitions 4I13/2(Yi)→4I15/2(Zj) show different changing rates with temperature, which is in favor of optical thermometry application based on fluorescence intensity ratio (FIR) technique. The results show that the utilization of FIR-1 (I1504/I1531) from two Stark transitions with the same power-dependence is beneficial to obtain consistent results at different excitation power densities, and avoid measurement error resulted from the power density variation. Besides, when FIR-Green (I533/I557) is used as the thermometric index, a high absolute sensitivity of 115 × 10−4 at 430 K can be obtained. Therefore, Er3+/Yb3+: BaMoO4 is promising for optical thermometry.
1. Introduction Nowadays, great affords have been made to perform the fast and accurate temperature measurement of the objects at different scales. Among the various types of thermometry methods, the fluorescence intensity ratio (FIR) technique has attracted growing interest owing to its particular advantages of non-contact, rapid response, strong antijamming capability, as well as high spatial resolution, which is favorable for temperature detection of the objects in harsh conditions or at submicron scales, such as high-voltage power stations, oil refineries, and engine turbo blade for spacecraft, etc [1–8]. In general, the noncontact FIR technique is implemented by calculating the intensity ratio of two emission bands with diverse temperature-dependences from rare earth (RE) ions [1–8]. For example, several kinds of thermally coupled levels (TCLs) are widely studied for FIR-based thermometry, such as the 2 H11/2/4S3/2 levels of Er3+ [9,10], 3F2,3/3H4 levels of Tm3+ [11], 5 D0/5D1 levels of Eu3+ [12] and so on. Furthermore, a few other kinds of strategy have been recently proposed to design optical temperature sensing materials, such as the combination of RE3+ and transition metal ions (Eu3+/Mn2+ [8,13], Tb3+/Mn4+ [14], etc.), the usage of two nonthermally coupled levels (Er3+: 4F9/2/Tm3+: 3F2 [15], Tm3+:3F2,3/ Tm3+: 1G4(b) [4], etc.), and the RE/defects dual-emitting systems (Yb3+:Na0.5Bi0.5TiO3 [16], Dy3+: Gd2Ti2O7 [17], etc.). However, in ∗
most cases, the emission intensities of RE ions decrease or even disappear with an elevation in temperature owing to the thermal quenching effect, leading to the reduction of signal to noise ratio and difficulty in temperature measurement [18,19]. Hence, it becomes an urgent task to solve the above problem. On the other hand, in the previous studies of FIR-based thermometry, it was normally to fit one calibration curve describing the relation between FIR and temperature at a low excitation power for temperature determination, which was widely believed to be insensitive to power fluctuations. However, it is not correct if the emission bands selected to construct thermometer have different pump power dependencies, which is the case in most of photoluminescence experiments [2]. Recently, Xu et al. have reported that the 2H11/2 and 4 S3/2 levels in Er3+/Yb3+: CaWO4 have different power dependencies, which results in inconsistent calibration curves at two different pump powers (40 and 100 mW) based on FIR of 2H11/2 to 4S3/2. Consequently, when the FIR data recorded at 100 mW are substituted into the calibration curve fitted at 40 mW, a measurement error of up to 20 K is caused at 478 K [20]. It indicates that FIR from two emission bands with different power dependencies could only be applied at a strictly fixed power, which is hard to realize in practice. For instance, the alteration of distance between the laser and object would cause the change of excitation powers. Therefore, it is very necessary to realize
Corresponding author. Corresponding author. E-mail addresses: [email protected] (R. Lei), [email protected] (S. Xu).
∗∗
https://doi.org/10.1016/j.optmat.2018.10.024 Received 1 August 2018; Received in revised form 5 September 2018; Accepted 13 October 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 86 (2018) 278–285
R. Lei et al.
higher angel side, revealing Er3+ and Yb3+ ions are diffused into the host lattice. This is because the substitution of Ba2+ ions by the smaller Er3+ and Yb3+ ions can cause the shrinkage of host lattice and the reduction of crystal lattice constants. Besides, as shown in Fig. 1 (b), the powders exhibit an irregular morphology with their sizes in the micrometer range, which is common for the phosphors prepared by high temperature solid state reaction method. Fig. 2(a) shows the 980 nm laser excited temperature-dependent 1.53 μm luminescence spectra of the sample, which were recorded at the excitation power density of 16.7 mW/mm2. The typical peaks located at 1504, 1521, 1531 and 1543 nm are from the Stark transitions 4 I13/2 (Yi)→4I15/2 (Zj) of Er3+ ions [26]. As far as we know, there has been no systematical investigation on the crystal field splitting of 4I13/2 and 4I15/2 levels of Er3+ ion in BaMoO4 matrix. Hence, we have referred to the analysis of Stark levels in Er3+:Y2O3 [27,28]. Accordingly, the main emission peak positioned at 1531 nm originates from Y1→Z1 transition, where Y1 and Z1 correspond to the lowest energies in 4I13/2 and 4I15/2 levels, respectively. The Y1→Z1 transition exhibits the largest emission cross section, which explains the reason why it belongs to the peak with the strongest intensity [28]. What's more, Fig. 2(a) reveals that there is an unexpected overall luminescence enhancement of 1.53 μm near-infrared emission in Er3+/Yb3+:BaMoO4 powders with rising temperature. The integrated intensity in the range of 1460–1600 nm at 553 K is doubled, compared with that at room temperature. Hence, a high signal-to-noise ratio at elevated temperatures can be achieved in the near-infrared luminescence of BaMoO4:Er3+/ Yb3+ phosphor, which is beneficial for the practical applications. Fig. 2(b) shows the UC emission spectra in the range of 500–700 nm upon 980 nm excitation, which were recorded at 293 and 553 K, respectively. The sample has the strong green emission bands peaked at 533 nm and 557 nm originating from 2H11/2+4S3/2 → 4I15/2 and the rather weak red band peaked at 659 nm originating from 4F9/2 → 4I15/2 transition of Er3+ ions. With the increment of temperature from 293 K to 553 K, the green band peaked at 533 nm enhances slightly; while the green band centered at 557 nm and the red band centered at 659 nm weaken considerably. As 2H11/2 and 4S3/2 are thermally coupled levels, the population redistribution from 4S3/2 level to the upper-lying 2H11/2 level occurs due to the thermal excitation effect at higher temperatures. Hence, the intensity of 533 nm emission is increased, while that of 557 nm emission is decreased with increasing temperature. Meanwhile, the multi-phonon relaxation process becomes active with a rise in temperature, which promotes the non-radiative relaxation process and causes the reduction of all UC luminescence intensities [18,19,29]. Obviously, the marked quenching of 557 nm emission could be harmful for the application in practice. To better understand the anomalous near-infrared luminescence enhancement with temperature, the population and transition processes of Er3+ ions in BaMoO4:Er3+/Yb3+ system under a 980 nm excitation should be taken into consideration. As shown in Fig. 2(c), Er3+ ion at
optical temperature sensing with power-independent output of indicating signals. Among the various trivalent RE ions, Er3+ ion is the most popular one for temperature sensors, which has the TCL of 2H11/2 and 4S3/2 with an appropriate energy gap of ∼800 cm−1, and relatively high green emission efficiencies to the ground state [21,22]. Meanwhile, in the near-infrared range, the 4I13/2 → 4I15/2 emission transition of Er3+ ion at ∼1.5 μm is a standard telecommunication wavelength, which has been extensively studied previously for various optical applications [23–25]. However, the investigation of using 1.5 μm near-infrared luminescence of Er3+ ion for optical temperature sensing was quite scarce, not to mention the effect of excitation power density. Herein, we have studied the temperature dependences of 1.53 μm and green UC emissions of Er3+ ion in BaMoO4: Er3+/Yb3+ powders. The mechanism of the inverse thermal quenching effect observed in 1.53 μm luminescence has been discussed. Three types of FIRs (I1504/ I1531, I1521/I1531 and I533/I557) have been selected for temperature sensing. The dependence of the temperature sensing behaviors on the choice of FIR type has been explored. The influence of excitation power density on the calibration curves and measurement results has been analyzed in detail. 2. Experimental BaMoO4 co-doped with 1mol%Er3+ and 3mol%Yb3+ was synthesized by high temperature solid state reaction method. BaCO3 (A.R.), MoO3 (A.R.), Er2O3 (99.99%) and Yb2O3 (99.99%) were used as the starting materials. In a typical synthesis, the weighed reactants were fully milled in anhydrous alcohol, which was conducted in a QM-1SP4 planetary ball mill with agate containers and balls at a rotational speed of 100 rpm for 4 h. Then, the uniform mixture was calcined at 800 °C for 2 h in air, and was ground after cooling down to room temperature. The crystal structure of the final product was measured by a Bruker D2 PHASER Diffractometer with Cu Kα radiation (λ = 0.154 nm) in the 2θ range from 15° to 80°. A HITACHI TM 3000 field-emission scanning electron microscope (FESEM) was used to characterize the sample morphology. The luminescence spectra were checked by a FLUOROLOG3/Jobin Yvon spectrofluorometer with an external 980 nm continuous wave laser diode (spot size: 2 × 4 mm) as the excitation source. The temperature of the sample was adjusted by using a TAP-02 temperature controlling system, which was also precisely monitored by a copper-constantan thermocouple. 3. Results and discussion Fig. 1(a) shows that the diffraction peak positions of the product are consistent with those of tetragonal scheelite-type phase of BaMoO4 (PDF card No.29–0193) and no extra impurity phases can be detected. It can be further noticed that all the peaks move slightly towards the
Fig. 1. XRD pattern (a) and FESEM image (b) of BaMoO4: 1mol%Er3+, 3mol%Yb3+ powders. 279
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 2. (a) Temperature-dependent near-infrared luminescence spectra upon a 980 nm excitation. (b) The UC luminescence spectra in the range of 500–700 nm measured at 293 and 553 K, respectively. (c) A simplified energy levels diagram of Er3+ ions in BaMoO4 along with the main UC and near-infrared luminescence processes.
4 I15/2 state can be excited to 4I11/2 state via ground state absorption (GSA) process and/or energy transfer (ET) process from Yb3+ to Er3+ upon 980 nm excitation. Some Er3+ ions at 4I11/2 state can be further promoted to 4F7/2 state by ET2 process, and then relax to the 2H11/2, 4 S3/2 and 4F9/2 states, resulting in radiative transitions of 2H11/2 → 4I15/ 4 4 4 4 2 (533 nm), S3/2 → I15/2 (557 nm) and F9/2 → I15/2 (659 nm), re3+ 4 spectively. Meanwhile, other Er ions at I11/2 state can nonradiatively (multiphonon) relax to 4I13/2 state, and radiate to 4I15/2 state for the emission around 1.5 μm. Since the multiphonon-assisted nonradiative transition process enhances with temperature [29,30], the population of 4I13/2 from 4I11/2 state is expected to increase at higher temperatures, leading to the inverse thermal quenching behavior in 1.5 μm emission of Er3+ ions. Fig. 3(a) displays the double logarithmic plots of the intensities from the Stark transitions 4I13/2 (Yi)→4I15/2 (Zj) versus excitation power densities. The slopes for 1504, 1521, 1531 and 1543 nm are 0.83, 0.78, 0.83 and 0.81, respectively. The identical slopes for the emissions at 1504 and 1531 nm suggest that their dependence on excitation power density is similar. Hence, the FIR of 1504 nm to 1531 nm may be insusceptible to excitation power density. Oppositely, the difference of power dependences between the 1521 and 1531 nm emissions is the largest, indicating that the FIR of 1521 nm to 1531 nm could be
sensitive to the pump powers. Further, the linear dependency of the integrated emission intensity with the excitation power density confirms that one-photon excitation mechanism is responsible for 1.5 μm near-infrared luminescence (the inset of Fig. 3(a)). On the other hand, Fig. 3(b) shows that the slopes for the 533, 557 and 659 nm emissions are 1.59, 1.68 and 1.25, respectively. It reveals that two photons are required for the population of 2H11/2, 4S3/2 and 4F9/2 levels, as discussed above. And the different slopes for 2H11/2 → 4I15/2 and 4S3/2 → 4 I15/2 transitions indicate that the FIR of 533 nm to 557 nm may be also dependent on pump power. Fig. 4(a) and (b) illustrate the temperature-dependent 1.5 μm nearinfrared luminescence and green UC emission spectra in the range of 293–553 K, which are normalized to 1531 nm and 557 nm emission intensities, respectively. As shown in Fig. 4(a), almost all peaks enhance their intensities gradually relative to the central peak (1531 nm) with the increment of temperature. Hence, the increasing rate of 1531 nm emission with temperature is smaller than those of other peaks, which is mainly resulted from the different populations in the various Stark levels. Since the Stark levels 4I13/2(Y1) and 4I15/2(Z1) have the largest populations, it is expected that their populations exhibit smaller alterations with temperature than the other less populated upper Stark levels. Herein, the different increasing rates of the peaks with
Fig. 3. The log-log plots of (a) near infrared and (b) UC emission intensities versus pump power density. Inset of Fig. 3(a) shows the integrated intensity of 1.5 μm luminescence as a function of pump power density. 280
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 4. Normalized temperature-dependent (a) 1.53 μm and (b) green UC emission spectra upon 980 nm excitation with the power density of 16.7 mW/mm2. Plots of (c) FIR-1, (d) FIR-2 and (e) FIR-Green as a function of temperature under two different power densities of 16.7 and 33.4 mW/mm2.
excitation power density has also been observed (Fig. 4(e)). The best fits are FIR = 19.2exp(-906/T) for 16.7 mW/mm2 and FIR = 18.5exp (-870/T) for 33.4 mW/mm2, respectively. It indicates that both FIR-2 and FIR-Green could only be applied at a fixed excitation power; otherwise unavoidable measurement errors can be caused. On the contrary, since the dependences of 1504 and 1531 nm emissions on pump power densities are identical, the calibration curve for FIR-1 keeps the same at the two power densities. Consequently, the power density variation from 16.7 to 33.4 mW/mm2 would not lead to the inconsistent measurement results. The above results suggest that the unavoidable measurement error could be caused by using the power-dependent FIRs, when the corresponding calibration curve and FIR data are not recorded at the same excitation power. However, this error has often been disregarded in the previous studies. For the sake of verification, the influence of excitation power and FIR data on the measurement result is studied. Thus, Eq. (1) has been converted as:
temperature have drawn our special interest of studying their temperature sensing behaviors based on FIR technique. Similarly, Fig. 4(b) reveals that the emission intensity of 533 nm enhances with respect to that around 557 nm, which can lead to the change of intensity ratio between 533 and 557 nm emissions with an elevation in temperature. Accordingly, the typical FIR-1 (I1504/I1531), FIR-2 (I1521/I1531) and FIR-Green (I533/I557) are chosen to apply for temperature sensing, since they have different power dependences as discussed above. Fig. 4(c), (d) and (e) show the variations of FIR-1, FIR-2 and FIR-Green values as a function of temperature, respectively, which are measured at two pump power densities of 16.7 and 33.4 mW/mm2. The relationship between FIR and temperature (T) can be expressed as:
FIR = A exp(−B / T )
(1)
For FIR-1 and FIR-2, A and B are constants determined by fitting the experimental data. T is absolute temperature. Diversely, for FIR-Green, B denotes ΔE / k based on the Boltzmann distribution law, in which ΔE is the energy gap between 2H11/2 and 4S3/2 levels, and k is the Boltzmann constant (0.695) [31,32]. As shown in Fig. 4(d), the data of FIR-2 measured at 16.7 and 33.4 mW/mm2 cannot be fitted with the same temperature calibration curve. The best fits by using Eq. (1) are FIR = 0.73exp(-104/T) and FIR = 0.74exp(-110/T) for the excitation power densities of 16.7 and 33.4 mW/mm2, respectively. Similarly, when the temperature sensing is based on FIR-Green, the variation of calibration curve with the
T=
B ln A − ln FIR
(2)
where all the terms denote their meanings described above and are obtained respectively based on the fitting results. As shown in Fig. 5(b), when the FIR-2 recorded at 33.4 mW/mm2 are substituted into the calibration curve measured at 16.7 mW/mm2 (FIR = 0.73exp(-104/T), hence T = 104/(ln0.73-lnFIR)), there is a difference between the 281
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 5. Experimental temperature reading from thermocouple versus calculated temperature based on (a) FIR-1; (b) FIR-2 and (c) FIR-Green. The temperature is calculated by the substitution of the corresponding FIRs measured at 33.4 mW/mm2 into the calibration curve fitted at 16.7 mW/mm2. The dashed line corresponding to y = x is drawn as a guide for better observation.
the increment of pump power from 16.7 to 33.4 mW/mm2 resulted in a temperature rise of the sample, the calibration curves of FIR-1 would also show an obvious difference, which disagrees with the result shown in Fig. 4(c). Hence, the different calibration curves of FIR-2 (I1521/I1531) obtained at 16.7 and 33.4 mW/mm2 are not caused by optical heating effect, but the different power-dependencies of 1521 and 1531 nm emissions. As well known, the absolute sensing sensitivity (Sa) and relative sensing sensitivity (Sr) are important parameters for temperature sensors. Sa describes the change rate in FIR with temperature, which can be written as [2,3,31,32]:
calculated temperature and the experimental one reading from thermocouple. For instance, the temperature is calculated to be 544 K, when the experimental temperature is only 533 K, indicating an error of ∼11 K. The similar measurement error caused by pump power variation has also been observed, when FIR-Green is applied as the thermometric index (Fig. 5(c)). The maximal temperature difference between the calculated value and experimental one is ∼22 K at 533 K. Conversely, Fig. 5(a) reveals that the calculated temperature matches well with the experimental value, when FIR-1 data are used for temperature estimation. The temperature deviation is found to be around ± 1.5 K. Obviously, FIR-1 is more competent for the practical optical thermometry than FIR-2 and FIR-Green due to its power-independent signal output. Hence, it is better to choose two emissions with the same power dependence for temperature sensing, which can solve the problem from the pump power variation. Moreover, the influence of laser induced heating effect should be taken into consideration. Fig. 6 presents the normalized 1.5 μm luminescence spectra for the two excitation power densities of 16.7 and 33.4 mW/mm2. No alterations can be observed, revealing that the laser induced heating effect is quite small. This is because the excitation power densities altered within a small scale, and the power of 33.4 mW/mm2 was still at a relatively low level. Besides, provided that
Sa =
dFIR B = FIR 2 dT T
(3)
For the comparison between the different kinds of optical temperature sensors, Sa is often normalized to the measured FIR value, giving rise to Sr [2,3,31,32]:
Sr =
1 dFIR B = 2 FIR dT T
(4)
Figs. 7 and 8 illustrate the corresponding Sa and Sr curves as a function of temperature, respectively. It can be seen that the Sa and Sr values based on FIR-1 and FIR-2 decrease gradually with temperature. Besides, the pump power density has no influence on the sensing sensitivity based on FIR-1. The maximal Sa and Sr values are ∼5.32 × 10−4 K−1 and ∼0.095%K−1 at 293 K, respectively (Figs. 7(a) and 8(a)). Conversely, when optical thermometer is based on FIR-2, the Sa and Sr values at 33.4 mW/mm2 are slightly larger than those at 16.7 mW/mm2 (Figs. 7(b) and 8(b)). The maximal Sa and Sr values are ∼6.5 × 10−4 K−1 and ∼0.129%K−1 respectively at 293 K. In the nearinfrared range, Nd3+ is the most studied RE ion for temperature sensors, which is based on FIR of 4F3/2(R1) →4I9/2 to 4F3/2(R2) →4I9/2 transitions. The maximal Sr values of these Nd3+ based optical thermometers are normally around 0.1%K−1 (for instance, Nd3+:NaYF4 (0.075%K−1) [33], Nd3+: YAG (0.15%K−1) [34], Nd3+: LaF3 (0.1% K−1) [35], etc.), which are similar with those values based on FIR-1 and FIR-2. However, these relative sensitivities are lower than some other kinds of luminescent thermometers operating in the near-infrared range, such as LiLa0.4Nd0.1Yb0.5P4O12 (0.4%K−1) [36],
Fig. 6. Near-infrared luminescence spectra for the two excitation power densities of 16.7 and 33.4 mW/mm2, which are normalized at 1531 nm. 282
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 7. Absolute sensing sensitivity values of (a) FIR-1, (b) FIR-2 and (c) FIR-Green in Er3+, Yb3+: BaMoO4 powders at two different pump power densities.
CaF2:Tm3+,Yb3+ (2%K−1) [37], Yb3+, Er3+: NaYF4@Yb3+, Nd3+: NaYF4 (2%K−1) [38], etc. Besides, Figs. 7(c) and 8(c) illustrate that the Sa and Sr values based on FIR-Green are much higher than those based on FIR-1 and FIR-2. The maximal Sa is ∼115 × 10−4 K−1 at 430 K, while the maximal Sr is ∼1.07%K−1 at 293 K. In fact, the maximal Sa value is larger than many other kinds of Er3+/Yb3+ or Er3+ doped phosphors based on green emissions, such as 65 × 10−4 K−1 for Er3+/ Yb3+/Li3+:NaZnPO4 [39], 72 × 10−4 K−1 for Er3+/Yb3+:YNbO4 [40], 40.7 × 10−4 K−1 for Er3+/Yb3+: Gd2TiO5 [41], 57 × 10−4 K−1 for
Er3+/Yb3+:NaBiF4 [21] and so on. Additionally, the temperature uncertainty of measurement (△T) has also been employed to evaluate the feasibility of materials for optical temperature sensors, which is defined as [2,42,43]:
ΔT =
1 δFIR Sr FIR
(5)
where δFIR is the uncertainty in the determination of FIR (determined by the experimental detection setup), Sr is the relative sensing
Fig. 8. Relative sensing sensitivity values of (a) FIR-1, (b) FIR-2 and (c) FIR-Green at two different pump power densities. 283
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 9. Switching of (a) FIR-1, (b) FIR-2 and (c) FIR-Green induced by alternating temperature between 293 and 553 K. The excitation power density was fixed at 16.7 mW/mm2.
of FIR-1 and FIR-Green may be more appropriate for optical thermometry, allowing for self-referenced temperature determination. We believe the above findings provide useful information for the pursuit of high-performance optical thermometers.
sensitivity. Obviously, larger sensing sensitivity leads to smaller temperature uncertainty at a certain pump power density. Therefore, among the three types of FIR data, FIR-Green is expected to possess the smallest temperature uncertainty. However, it should be pointed out that though FIR-Green has larger sensing sensitivity and smaller temperature uncertainty than FIR-1, the usage of FIR-1 may get more precise temperature detection due to its independence on the excitation power density. Finally, the measurement stability is required for temperature sensors. In this regard, Fig. 9 illustrates the dependences of FIR-1, FIR-2 and FIR-Green on the absolute temperature by performing recycling tests. It can be seen that the three types of FIR data exhibit good repeatability and reversibility after several cycling experiments, which indicates the high stability for continuous use. Overall, the present product possesses some superior properties for temperature determination. The application of FIR-1 can provide a precise temperature reading due to its power-independent output, while the high sensing sensitivity can be achieved based on FIR-Green. Hence, multi-FIR data from the different spectral range could be more accurate than the application of only one type of FIR, which allows for self-referenced temperature determination. So, Er3+/Yb3+: BaMoO4 is a promising phosphor for accurate temperature sensing measurement.
Acknowledgments This work is supported by Natural Science Foundation of Zhejiang Province (LY18E020008 and LR15F050003) and National Science Foundation of China (No. 61605192 and 51602301). References [1] M. Xu, X.M. Zou, Q.Q. Su, W. Yuan, C. Cao, Q.H. Wang, X.J. Zhu, W. Feng, F.Y. Li, Ratiometric nanothermometer in vivo based on triplet sensitized upconversion, Nat. Commun. 9 (2018) 2698-1-7. [2] M. Quintanilla, L.M. Liz-Marzán, Guiding rules for selecting a nanothermometer, Nano Today 19 (2018) 126–145. [3] M.D. Dramićanin, Sensing temperature via downshifting emissions of lanthanidedoped metal oxides and salts. A review, Meth. Appl. Fluoresc. 4 (2016) 042001. [4] G.R. Chen, R.S. Lei, H.P. Wang, F.F. Huang, S.L. Zhao, S.Q. Xu, Temperature-dependent emission color and temperature sensing behavior in Tm3+/Yb3+:Y2O3 nanoparticles, Opt. Mater. 77 (2018) 233–239. [5] L.D. Carlos, F. Palacio, Thermometry at the Nanoscale: Techniques and Selected Applications, first ed., Royal Society of Chemistry, London, 2016. [6] Y.Q. Zhang, S. Xu, X.P. Li, J.S. Zhang, J.S. Sun, H.P. Xia, R.N. Hua, B.J. Chen, Temperature sensing, excitation power dependent fluorescence branching ratios, and photothermal conversion in NaYF4:Er3+/Yb3+ @NaYF4:Tm3+/Yb3+ core-shell particles, Opt. Mater. Express 8 (2018) 368–384. [7] A. Pandey, V.K. Rai, Rare earth doped materials for temperature sensors, in: Y. Dwivedi, S.B. Rai, J.P. Singh (Eds.), Spectroscopic Techniques for Security Forensic and Environmental Applications, Nova Science Publishers, New York, 2011, pp. 279–292. [8] H. Xia, L. Lei, W.Q. Hong, S.Q. Xu, A novel Ce3+/Mn2+/Eu3+ tri-doped GdF3 nanocrystals for optical temperature sensor and anti-counterfeiting, J. Alloy. Comp. 757 (2018) 239–245. [9] W. Xu, Y. Cui, Y.W. Hu, L.J. Zheng, Z.G. Zhang, W.W. Cao, Optical temperature sensing in Er3+-Yb3+ codoped CaWO4 and the laser induced heating effect on the luminescence intensity saturation, J. Alloy. Comp. 726 (2017) 547–555. [10] Z.H. Feng, L. Lin, Z.Z. Wang, Z.Q. Zheng, Low temperature sensing behavior of upconversion luminescence in Er3+/Yb3+ co-doped PLZT transparent ceramic, Opt. Commun. 399 (2017) 40–44. [11] P. Du, L.H. Luo, J. Su, Controlled synthesis and upconversion luminescence of Tm3+-doped NaYbF4 nanoparticles for non-invasion optical thermometry, J. Alloy. Comp. 739 (2018) 926–933.
4. Conclusions The Er3+/Yb3+ co-doped BaMoO4 phosphor has been prepared by high temperature solid state reaction method. An inverse thermal quenching phenomenon is observed in 1.53 μm near-infrared luminescence upon a 980 nm excitation, which guarantees a high signal-tonoise ratio at high temperatures. Three types of FIRs (I1504/I1531, I1521/ I1531 and I533/I557) are selected to investigate the temperature sensing behaviors in detail. The FIR-1 (I1504/I1531) is found to be insusceptible to pump power densities, giving rise to consistent calibration curves and measurement results at two different excitation power densities. Otherwise, though excellent temperature sensing sensitivity with the maximal Sa of ∼115 × 10−4 K−1 can be achieved by using FIR-Green (I533/I557), its power dependence may lead to inconsistent results and measurement errors at different pump powers. Hence, the combination 284
Optical Materials 86 (2018) 278–285
R. Lei et al. [12] S. Senapati, K.K. Nanda, Red emitting Eu:ZnO nanorods for highly sensitive fluorescence intensity ratio based optical thermometry, J. Mater. Chem. C 5 (2017) 1074–1082. [13] F. Huang, D.Q. Chen, Synthesis of Mn2+:Zn2SiO4-Eu3+:Gd2O3 nanocomposites for highly sensitive optical thermometry through the synergistic luminescence from lanthanide-transition metal ions, J. Mater. Chem. C 5 (2017) 5176–5182. [14] D.Q. Chen, S. Liu, Y. Zhou, Z.Y. Wan, P. Huang, Z.G. Ji, Dual-activator luminescence of RE/TM: Y3Al5O12 (RE=Eu3+, Tb3+, Dy3+; TM=Mn4+, Cr3+) phosphors for selfreferencing optical thermometry, J. Mater. Chem. C 4 (2016) 9044–9051. [15] M.Y. Ding, D.Q. Chen, C.H. Lub, J.H. Xi, Z.G. Ji, Z.Z. Xu, Lanthanide-doped LuF3 mesocrystals for optical thermometry, Mater. Lett. 189 (2017) 5–8. [16] Y.P. Huang, L.H. Luo, Up-conversion photoluminescence based on the intrinsic defects in Na0.5Bi0.5TiO3: Yb3+ ceramics, J. Alloy. Comp. 706 (2017) 312–317. [17] S. Ćulubrk, V. Lojpur, S.P. Ahrenkiel, J.M. Nedeljković, M.D. Dramićanin, Noncontact thermometry with Dy3+ doped Gd2Ti2O7 nano-powders, J. Lumin. 170 (2016) 395–400. [18] J.W. Zhao, H.T. Li, Q.H. Zeng, K. Song, X.F. Wang, X.G. Kong, Temperature-dependent upconversion luminescence of NaYF4:Yb3+, Er3+ nanoparticles, Chem. Lett. 42 (2013) 310–312. [19] J.R. Hao, L. Cao, Y. Wei, C.J. Yan, G.G. Li, Improving photoluminescence and thermal stability of CaSi2O2N2:Eu2+ phosphors by codoping lanthanide ions (Ln=Sc, La, Gd, Yb, Lu), Mater. Lett. 211 (2018) 122–125. [20] L.P. Li, L.J. Zheng, W. Xu, Z. Liang, Y. Zhou, Z.G. Zhang, W.W. Cao, Optical thermometry based on the red upconversion fluorescence of Er3+ in CaWO4:Yb3+/Er3+ polycrystalline powder, Opt. Lett. 41 (2016) 1458–1461. [21] P. Du, L.H. Luo, X.Y. Huang, J.S. Yu, Ultrafast synthesis of bifunctional Er3+/Yb3+ codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater, J. Colloid Interface Sci. 514 (2018) 172–181. [22] X. Ming, Q.Y. Meng, S.C. Lü, W.J. Sun, The hydrothermal synthesis and morphology-dependent optical temperature sensing properties of Er3+ doped NaGd (WO4)2 phosphor, J. Lumin. 192 (2017) 196–202. [23] P. Elahi, H. Kalaycıoğlu, H. Li, Ö. Akçaalan, F.Ö. Ilday, 175 fs-long pulses from a high-power single-mode Er-doped fiber laser at 1550 nm, Opt. Commun. 403 (2017) 381–384. [24] Q.W. Wang, J. Li, J.Y. Lin, H.X. Jiang, Enhancement of 1.5 μm emission under 980 nm resonant excitation in Er and Yb codoped GaN epilayers, Appl. Phys. Lett. 109 (2016) 152103. [25] P.S. Yu, L.B. Su, W. Guo, J. Xu, Photoluminescence and energy transfer progress in Er-doped Bi2O3-GeO2 glasses, J. Lumin. 187 (2017) 121–125. [26] E.E. Brown, U. Hömmerich, A. Bluiett, C. Kucera, J. Ballato, S. Trivedi, Near-infrared and upconversion luminescence in Er:Y2O3 ceramics under 1.5 μm excitation, J. Am. Ceram. Soc. 97 (2014) 2105–2110. [27] X.X. Zhang, K.F. Li, K.W. Cheah, X. Zhou, P.A. Tanner, 1.54 and 1.75 μm infrared luminescence of Y2O3:Er3+, Chem. Phys. Lett. 400 (2004) 331–335. [28] J.B. Gruber, K.L. Nash, D.K. Sardar, U.V. Valiev, N. Ter-Gabrielyan, L.D. Merkle, Modeling optical transitions of Er3+ (4f11) in C2 and C3i sites in polycrystalline Y2O3, J. Appl. Phys. 104 (2008) 023101. [29] N. Yuan, D.Y. Liu, X.C. Yu, H.X. Sun, C.G. Ming, W.H. Wong, F. Song, D.Y. Yu,
[30] [31]
[32] [33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
285
E.Y.B. Pun, D.L. Zhang, A biological nano-thermometer based on ratiometric luminescent Er3+/Yb3+-codoped NaGd(WO4)2 nanocrystals, Mater. Lett. 218 (2018) 337–340. D. Yu, J. Ballato, R.E. Riman, Temperature-dependence of multiphonon relaxation of rare-earth Ions in solid-state hosts, J. Phys. Chem. C 120 (2016) 9958–9964. A.Q. Zhang, Z. Sun, G.F. Liu, Z.L. Fu, Z.D. Hao, J.H. Zhang, Y.L. Wei, Ln3+ (Er3+, Tm3+ and Ho3+)-doped NaYb(MoO4)2 upconversion phosphors as wide range temperature sensors with high sensitivity, J. Alloy. Comp. 728 (2017) 476–483. P. Singh, P.K. Shahi, A. Rai, A. Bahadur, S.B. Rai, Effect of Li+ ion sensitization and optical temperature sensing in Gd2O3: Ho3+/Yb3+, Opt. Mater. 58 (2016) 432–438. D. Wawrzynczyk, A. Bednarkiewicz, M. Nyk, W. Strek, M. Samoc, Neodymium(III) doped fluoride nanoparticles as non-contact optical temperature sensors, Nanoscale 4 (2012) 6959–6961. A. Benayas, B. del Rosal, A. Pérez-Delgado, K. Santacruz-Gómez, D. Jaque, G.A. Hirata, F. Vetrone, Nd:YAG near-Infrared luminescent nanothermometers, Adv. Opt. Mater. 3 (2015) 687–694. U. Rocha, C.J. Silva, W.F. Silva, I. Guedes, A. Benayas, L.M. Maestro, M.A. Elias, E. Bovero, F.C.J.M. Veggel, J.A.G. Solé, D. Jaque, Subtissue thermal sensing based on neodymium-doped LaF3 nanoparticles, ACS Nano 7 (2013) 1188–1199. L. Marciniak, A. Bednarkiewicz, M. Stefanski, R. Tomala, D. Hreniak, W. Strek, Near infrared absorbing near infrared emitting highly-sensitive luminescent nanothermometer based on Nd3+ to Yb3+ energy transfer, Phys. Chem. Chem. Phys. 17 (2015) 24315–24321. N.N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, J.E. Ramírez-Hernández, L.M. Maestro, M.C.I. Cruz, F. Sanz-Rodríguez, A. Juarranz, F. Chen, F. Vetrone, J.A. Capobianco, J.G. Solé, M. Bettinelli, D. Jaque, A. Speghini, NIR-to-NIR twophoton excited CaF2:Tm3+,Yb3+ nanoparticles: multifunctional nanoprobes for highly penetrating fluorescence bio-Imaging, ACS Nano 5 (2011) 8665–8671. L. Marciniak, K. Prorok, L. Frances-Soriano, J. Perez-Prieto, A. Bednarkiewicz, A broadening temperature sensitivity range with a core–shell YbEr@YbNd double ratiometric optical nanothermometer, Nanoscale 8 (2016) 5037–5042. L. Mukhopadhyay, V.K. Rai, R. Bokolia, K. Sreenivas, 980 nm excited Er3+/Yb3+/ Li+/Ba2+: NaZnPO4 upconverting phosphors in optical thermometry, J. Lumin. 187 (2017) 368–377. Y.Y. Tian, Y. Tian, P. Huang, L. Wang, Q.F. Shi, C. Cui, Effect of Yb3+ concentration on upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ codoped YNbO4 nanoparticles prepared via molten salt route, Chem. Eng. J. 297 (2016) 26–34. J.S. Liao, Q. Wang, L.Y. Kong, Z.Q. Ming, Y.L. Wang, Y.Q. Li, L.X. Che, Effect of Yb3+ concentration on tunable upconversion luminescence and optically temperature sensing behavior in Gd2TiO5:Yb3+/Er3+ phosphors, Opt. Mater. 75 (2018) 841–849. S. Balabhadra, M.L. Debasu, C.D.S. Brites, L.A.O. Nunes, O.L. Malta, J. Roch, M. Bettinellie, L.D. Carlos, Boosting the sensitivity of Nd3+-based luminescent nanothermometers, Nanoscale 7 (2015) 17261–17267. S. Balabhadra, M.L. Debasu, C.D.S. Brites, R.A.S. Ferreira, L.D. Carlos, Upconverting nanoparticles working as primary thermometers in different media, J. Phys. Chem. C 121 (2017) 13962–13968.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Optical thermometry based on anomalous temperature-dependent 1.53 μm infrared luminescence of Er3+ in BaMoO4: Er3+/Yb3+ phosphor
T
Ruoshan Lei∗, Xin Liu, Feifei Huang, Degang Deng, Shilong Zhao, Hui Xu, Shiqing Xu∗∗ College of Materials Science and Engineering, China Jiliang University, Hangzhou, 310018, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Near-infrared luminescence Negative thermal quenching Optical temperature sensing Rare earth
The temperature dependences of 1.53 μm near infrared and green upconversion (UC) emissions of Er3+ ion have been investigated in Er3+/Yb3+: BaMoO4 powders upon 980 nm excitation. Interestingly, an anomalous enhancement of 1.53 μm luminescence intensity with increasing temperature is observed, which is interpreted based on the phonon-assisted nonradiative process from 4I11/2 to 4I13/2 level. Moreover, the emission intensities from various Stark transitions 4I13/2(Yi)→4I15/2(Zj) show different changing rates with temperature, which is in favor of optical thermometry application based on fluorescence intensity ratio (FIR) technique. The results show that the utilization of FIR-1 (I1504/I1531) from two Stark transitions with the same power-dependence is beneficial to obtain consistent results at different excitation power densities, and avoid measurement error resulted from the power density variation. Besides, when FIR-Green (I533/I557) is used as the thermometric index, a high absolute sensitivity of 115 × 10−4 at 430 K can be obtained. Therefore, Er3+/Yb3+: BaMoO4 is promising for optical thermometry.
1. Introduction Nowadays, great affords have been made to perform the fast and accurate temperature measurement of the objects at different scales. Among the various types of thermometry methods, the fluorescence intensity ratio (FIR) technique has attracted growing interest owing to its particular advantages of non-contact, rapid response, strong antijamming capability, as well as high spatial resolution, which is favorable for temperature detection of the objects in harsh conditions or at submicron scales, such as high-voltage power stations, oil refineries, and engine turbo blade for spacecraft, etc [1–8]. In general, the noncontact FIR technique is implemented by calculating the intensity ratio of two emission bands with diverse temperature-dependences from rare earth (RE) ions [1–8]. For example, several kinds of thermally coupled levels (TCLs) are widely studied for FIR-based thermometry, such as the 2 H11/2/4S3/2 levels of Er3+ [9,10], 3F2,3/3H4 levels of Tm3+ [11], 5 D0/5D1 levels of Eu3+ [12] and so on. Furthermore, a few other kinds of strategy have been recently proposed to design optical temperature sensing materials, such as the combination of RE3+ and transition metal ions (Eu3+/Mn2+ [8,13], Tb3+/Mn4+ [14], etc.), the usage of two nonthermally coupled levels (Er3+: 4F9/2/Tm3+: 3F2 [15], Tm3+:3F2,3/ Tm3+: 1G4(b) [4], etc.), and the RE/defects dual-emitting systems (Yb3+:Na0.5Bi0.5TiO3 [16], Dy3+: Gd2Ti2O7 [17], etc.). However, in ∗
most cases, the emission intensities of RE ions decrease or even disappear with an elevation in temperature owing to the thermal quenching effect, leading to the reduction of signal to noise ratio and difficulty in temperature measurement [18,19]. Hence, it becomes an urgent task to solve the above problem. On the other hand, in the previous studies of FIR-based thermometry, it was normally to fit one calibration curve describing the relation between FIR and temperature at a low excitation power for temperature determination, which was widely believed to be insensitive to power fluctuations. However, it is not correct if the emission bands selected to construct thermometer have different pump power dependencies, which is the case in most of photoluminescence experiments [2]. Recently, Xu et al. have reported that the 2H11/2 and 4 S3/2 levels in Er3+/Yb3+: CaWO4 have different power dependencies, which results in inconsistent calibration curves at two different pump powers (40 and 100 mW) based on FIR of 2H11/2 to 4S3/2. Consequently, when the FIR data recorded at 100 mW are substituted into the calibration curve fitted at 40 mW, a measurement error of up to 20 K is caused at 478 K [20]. It indicates that FIR from two emission bands with different power dependencies could only be applied at a strictly fixed power, which is hard to realize in practice. For instance, the alteration of distance between the laser and object would cause the change of excitation powers. Therefore, it is very necessary to realize
Corresponding author. Corresponding author. E-mail addresses: [email protected] (R. Lei), [email protected] (S. Xu).
∗∗
https://doi.org/10.1016/j.optmat.2018.10.024 Received 1 August 2018; Received in revised form 5 September 2018; Accepted 13 October 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
Optical Materials 86 (2018) 278–285
R. Lei et al.
higher angel side, revealing Er3+ and Yb3+ ions are diffused into the host lattice. This is because the substitution of Ba2+ ions by the smaller Er3+ and Yb3+ ions can cause the shrinkage of host lattice and the reduction of crystal lattice constants. Besides, as shown in Fig. 1 (b), the powders exhibit an irregular morphology with their sizes in the micrometer range, which is common for the phosphors prepared by high temperature solid state reaction method. Fig. 2(a) shows the 980 nm laser excited temperature-dependent 1.53 μm luminescence spectra of the sample, which were recorded at the excitation power density of 16.7 mW/mm2. The typical peaks located at 1504, 1521, 1531 and 1543 nm are from the Stark transitions 4 I13/2 (Yi)→4I15/2 (Zj) of Er3+ ions [26]. As far as we know, there has been no systematical investigation on the crystal field splitting of 4I13/2 and 4I15/2 levels of Er3+ ion in BaMoO4 matrix. Hence, we have referred to the analysis of Stark levels in Er3+:Y2O3 [27,28]. Accordingly, the main emission peak positioned at 1531 nm originates from Y1→Z1 transition, where Y1 and Z1 correspond to the lowest energies in 4I13/2 and 4I15/2 levels, respectively. The Y1→Z1 transition exhibits the largest emission cross section, which explains the reason why it belongs to the peak with the strongest intensity [28]. What's more, Fig. 2(a) reveals that there is an unexpected overall luminescence enhancement of 1.53 μm near-infrared emission in Er3+/Yb3+:BaMoO4 powders with rising temperature. The integrated intensity in the range of 1460–1600 nm at 553 K is doubled, compared with that at room temperature. Hence, a high signal-to-noise ratio at elevated temperatures can be achieved in the near-infrared luminescence of BaMoO4:Er3+/ Yb3+ phosphor, which is beneficial for the practical applications. Fig. 2(b) shows the UC emission spectra in the range of 500–700 nm upon 980 nm excitation, which were recorded at 293 and 553 K, respectively. The sample has the strong green emission bands peaked at 533 nm and 557 nm originating from 2H11/2+4S3/2 → 4I15/2 and the rather weak red band peaked at 659 nm originating from 4F9/2 → 4I15/2 transition of Er3+ ions. With the increment of temperature from 293 K to 553 K, the green band peaked at 533 nm enhances slightly; while the green band centered at 557 nm and the red band centered at 659 nm weaken considerably. As 2H11/2 and 4S3/2 are thermally coupled levels, the population redistribution from 4S3/2 level to the upper-lying 2H11/2 level occurs due to the thermal excitation effect at higher temperatures. Hence, the intensity of 533 nm emission is increased, while that of 557 nm emission is decreased with increasing temperature. Meanwhile, the multi-phonon relaxation process becomes active with a rise in temperature, which promotes the non-radiative relaxation process and causes the reduction of all UC luminescence intensities [18,19,29]. Obviously, the marked quenching of 557 nm emission could be harmful for the application in practice. To better understand the anomalous near-infrared luminescence enhancement with temperature, the population and transition processes of Er3+ ions in BaMoO4:Er3+/Yb3+ system under a 980 nm excitation should be taken into consideration. As shown in Fig. 2(c), Er3+ ion at
optical temperature sensing with power-independent output of indicating signals. Among the various trivalent RE ions, Er3+ ion is the most popular one for temperature sensors, which has the TCL of 2H11/2 and 4S3/2 with an appropriate energy gap of ∼800 cm−1, and relatively high green emission efficiencies to the ground state [21,22]. Meanwhile, in the near-infrared range, the 4I13/2 → 4I15/2 emission transition of Er3+ ion at ∼1.5 μm is a standard telecommunication wavelength, which has been extensively studied previously for various optical applications [23–25]. However, the investigation of using 1.5 μm near-infrared luminescence of Er3+ ion for optical temperature sensing was quite scarce, not to mention the effect of excitation power density. Herein, we have studied the temperature dependences of 1.53 μm and green UC emissions of Er3+ ion in BaMoO4: Er3+/Yb3+ powders. The mechanism of the inverse thermal quenching effect observed in 1.53 μm luminescence has been discussed. Three types of FIRs (I1504/ I1531, I1521/I1531 and I533/I557) have been selected for temperature sensing. The dependence of the temperature sensing behaviors on the choice of FIR type has been explored. The influence of excitation power density on the calibration curves and measurement results has been analyzed in detail. 2. Experimental BaMoO4 co-doped with 1mol%Er3+ and 3mol%Yb3+ was synthesized by high temperature solid state reaction method. BaCO3 (A.R.), MoO3 (A.R.), Er2O3 (99.99%) and Yb2O3 (99.99%) were used as the starting materials. In a typical synthesis, the weighed reactants were fully milled in anhydrous alcohol, which was conducted in a QM-1SP4 planetary ball mill with agate containers and balls at a rotational speed of 100 rpm for 4 h. Then, the uniform mixture was calcined at 800 °C for 2 h in air, and was ground after cooling down to room temperature. The crystal structure of the final product was measured by a Bruker D2 PHASER Diffractometer with Cu Kα radiation (λ = 0.154 nm) in the 2θ range from 15° to 80°. A HITACHI TM 3000 field-emission scanning electron microscope (FESEM) was used to characterize the sample morphology. The luminescence spectra were checked by a FLUOROLOG3/Jobin Yvon spectrofluorometer with an external 980 nm continuous wave laser diode (spot size: 2 × 4 mm) as the excitation source. The temperature of the sample was adjusted by using a TAP-02 temperature controlling system, which was also precisely monitored by a copper-constantan thermocouple. 3. Results and discussion Fig. 1(a) shows that the diffraction peak positions of the product are consistent with those of tetragonal scheelite-type phase of BaMoO4 (PDF card No.29–0193) and no extra impurity phases can be detected. It can be further noticed that all the peaks move slightly towards the
Fig. 1. XRD pattern (a) and FESEM image (b) of BaMoO4: 1mol%Er3+, 3mol%Yb3+ powders. 279
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 2. (a) Temperature-dependent near-infrared luminescence spectra upon a 980 nm excitation. (b) The UC luminescence spectra in the range of 500–700 nm measured at 293 and 553 K, respectively. (c) A simplified energy levels diagram of Er3+ ions in BaMoO4 along with the main UC and near-infrared luminescence processes.
4 I15/2 state can be excited to 4I11/2 state via ground state absorption (GSA) process and/or energy transfer (ET) process from Yb3+ to Er3+ upon 980 nm excitation. Some Er3+ ions at 4I11/2 state can be further promoted to 4F7/2 state by ET2 process, and then relax to the 2H11/2, 4 S3/2 and 4F9/2 states, resulting in radiative transitions of 2H11/2 → 4I15/ 4 4 4 4 2 (533 nm), S3/2 → I15/2 (557 nm) and F9/2 → I15/2 (659 nm), re3+ 4 spectively. Meanwhile, other Er ions at I11/2 state can nonradiatively (multiphonon) relax to 4I13/2 state, and radiate to 4I15/2 state for the emission around 1.5 μm. Since the multiphonon-assisted nonradiative transition process enhances with temperature [29,30], the population of 4I13/2 from 4I11/2 state is expected to increase at higher temperatures, leading to the inverse thermal quenching behavior in 1.5 μm emission of Er3+ ions. Fig. 3(a) displays the double logarithmic plots of the intensities from the Stark transitions 4I13/2 (Yi)→4I15/2 (Zj) versus excitation power densities. The slopes for 1504, 1521, 1531 and 1543 nm are 0.83, 0.78, 0.83 and 0.81, respectively. The identical slopes for the emissions at 1504 and 1531 nm suggest that their dependence on excitation power density is similar. Hence, the FIR of 1504 nm to 1531 nm may be insusceptible to excitation power density. Oppositely, the difference of power dependences between the 1521 and 1531 nm emissions is the largest, indicating that the FIR of 1521 nm to 1531 nm could be
sensitive to the pump powers. Further, the linear dependency of the integrated emission intensity with the excitation power density confirms that one-photon excitation mechanism is responsible for 1.5 μm near-infrared luminescence (the inset of Fig. 3(a)). On the other hand, Fig. 3(b) shows that the slopes for the 533, 557 and 659 nm emissions are 1.59, 1.68 and 1.25, respectively. It reveals that two photons are required for the population of 2H11/2, 4S3/2 and 4F9/2 levels, as discussed above. And the different slopes for 2H11/2 → 4I15/2 and 4S3/2 → 4 I15/2 transitions indicate that the FIR of 533 nm to 557 nm may be also dependent on pump power. Fig. 4(a) and (b) illustrate the temperature-dependent 1.5 μm nearinfrared luminescence and green UC emission spectra in the range of 293–553 K, which are normalized to 1531 nm and 557 nm emission intensities, respectively. As shown in Fig. 4(a), almost all peaks enhance their intensities gradually relative to the central peak (1531 nm) with the increment of temperature. Hence, the increasing rate of 1531 nm emission with temperature is smaller than those of other peaks, which is mainly resulted from the different populations in the various Stark levels. Since the Stark levels 4I13/2(Y1) and 4I15/2(Z1) have the largest populations, it is expected that their populations exhibit smaller alterations with temperature than the other less populated upper Stark levels. Herein, the different increasing rates of the peaks with
Fig. 3. The log-log plots of (a) near infrared and (b) UC emission intensities versus pump power density. Inset of Fig. 3(a) shows the integrated intensity of 1.5 μm luminescence as a function of pump power density. 280
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 4. Normalized temperature-dependent (a) 1.53 μm and (b) green UC emission spectra upon 980 nm excitation with the power density of 16.7 mW/mm2. Plots of (c) FIR-1, (d) FIR-2 and (e) FIR-Green as a function of temperature under two different power densities of 16.7 and 33.4 mW/mm2.
excitation power density has also been observed (Fig. 4(e)). The best fits are FIR = 19.2exp(-906/T) for 16.7 mW/mm2 and FIR = 18.5exp (-870/T) for 33.4 mW/mm2, respectively. It indicates that both FIR-2 and FIR-Green could only be applied at a fixed excitation power; otherwise unavoidable measurement errors can be caused. On the contrary, since the dependences of 1504 and 1531 nm emissions on pump power densities are identical, the calibration curve for FIR-1 keeps the same at the two power densities. Consequently, the power density variation from 16.7 to 33.4 mW/mm2 would not lead to the inconsistent measurement results. The above results suggest that the unavoidable measurement error could be caused by using the power-dependent FIRs, when the corresponding calibration curve and FIR data are not recorded at the same excitation power. However, this error has often been disregarded in the previous studies. For the sake of verification, the influence of excitation power and FIR data on the measurement result is studied. Thus, Eq. (1) has been converted as:
temperature have drawn our special interest of studying their temperature sensing behaviors based on FIR technique. Similarly, Fig. 4(b) reveals that the emission intensity of 533 nm enhances with respect to that around 557 nm, which can lead to the change of intensity ratio between 533 and 557 nm emissions with an elevation in temperature. Accordingly, the typical FIR-1 (I1504/I1531), FIR-2 (I1521/I1531) and FIR-Green (I533/I557) are chosen to apply for temperature sensing, since they have different power dependences as discussed above. Fig. 4(c), (d) and (e) show the variations of FIR-1, FIR-2 and FIR-Green values as a function of temperature, respectively, which are measured at two pump power densities of 16.7 and 33.4 mW/mm2. The relationship between FIR and temperature (T) can be expressed as:
FIR = A exp(−B / T )
(1)
For FIR-1 and FIR-2, A and B are constants determined by fitting the experimental data. T is absolute temperature. Diversely, for FIR-Green, B denotes ΔE / k based on the Boltzmann distribution law, in which ΔE is the energy gap between 2H11/2 and 4S3/2 levels, and k is the Boltzmann constant (0.695) [31,32]. As shown in Fig. 4(d), the data of FIR-2 measured at 16.7 and 33.4 mW/mm2 cannot be fitted with the same temperature calibration curve. The best fits by using Eq. (1) are FIR = 0.73exp(-104/T) and FIR = 0.74exp(-110/T) for the excitation power densities of 16.7 and 33.4 mW/mm2, respectively. Similarly, when the temperature sensing is based on FIR-Green, the variation of calibration curve with the
T=
B ln A − ln FIR
(2)
where all the terms denote their meanings described above and are obtained respectively based on the fitting results. As shown in Fig. 5(b), when the FIR-2 recorded at 33.4 mW/mm2 are substituted into the calibration curve measured at 16.7 mW/mm2 (FIR = 0.73exp(-104/T), hence T = 104/(ln0.73-lnFIR)), there is a difference between the 281
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 5. Experimental temperature reading from thermocouple versus calculated temperature based on (a) FIR-1; (b) FIR-2 and (c) FIR-Green. The temperature is calculated by the substitution of the corresponding FIRs measured at 33.4 mW/mm2 into the calibration curve fitted at 16.7 mW/mm2. The dashed line corresponding to y = x is drawn as a guide for better observation.
the increment of pump power from 16.7 to 33.4 mW/mm2 resulted in a temperature rise of the sample, the calibration curves of FIR-1 would also show an obvious difference, which disagrees with the result shown in Fig. 4(c). Hence, the different calibration curves of FIR-2 (I1521/I1531) obtained at 16.7 and 33.4 mW/mm2 are not caused by optical heating effect, but the different power-dependencies of 1521 and 1531 nm emissions. As well known, the absolute sensing sensitivity (Sa) and relative sensing sensitivity (Sr) are important parameters for temperature sensors. Sa describes the change rate in FIR with temperature, which can be written as [2,3,31,32]:
calculated temperature and the experimental one reading from thermocouple. For instance, the temperature is calculated to be 544 K, when the experimental temperature is only 533 K, indicating an error of ∼11 K. The similar measurement error caused by pump power variation has also been observed, when FIR-Green is applied as the thermometric index (Fig. 5(c)). The maximal temperature difference between the calculated value and experimental one is ∼22 K at 533 K. Conversely, Fig. 5(a) reveals that the calculated temperature matches well with the experimental value, when FIR-1 data are used for temperature estimation. The temperature deviation is found to be around ± 1.5 K. Obviously, FIR-1 is more competent for the practical optical thermometry than FIR-2 and FIR-Green due to its power-independent signal output. Hence, it is better to choose two emissions with the same power dependence for temperature sensing, which can solve the problem from the pump power variation. Moreover, the influence of laser induced heating effect should be taken into consideration. Fig. 6 presents the normalized 1.5 μm luminescence spectra for the two excitation power densities of 16.7 and 33.4 mW/mm2. No alterations can be observed, revealing that the laser induced heating effect is quite small. This is because the excitation power densities altered within a small scale, and the power of 33.4 mW/mm2 was still at a relatively low level. Besides, provided that
Sa =
dFIR B = FIR 2 dT T
(3)
For the comparison between the different kinds of optical temperature sensors, Sa is often normalized to the measured FIR value, giving rise to Sr [2,3,31,32]:
Sr =
1 dFIR B = 2 FIR dT T
(4)
Figs. 7 and 8 illustrate the corresponding Sa and Sr curves as a function of temperature, respectively. It can be seen that the Sa and Sr values based on FIR-1 and FIR-2 decrease gradually with temperature. Besides, the pump power density has no influence on the sensing sensitivity based on FIR-1. The maximal Sa and Sr values are ∼5.32 × 10−4 K−1 and ∼0.095%K−1 at 293 K, respectively (Figs. 7(a) and 8(a)). Conversely, when optical thermometer is based on FIR-2, the Sa and Sr values at 33.4 mW/mm2 are slightly larger than those at 16.7 mW/mm2 (Figs. 7(b) and 8(b)). The maximal Sa and Sr values are ∼6.5 × 10−4 K−1 and ∼0.129%K−1 respectively at 293 K. In the nearinfrared range, Nd3+ is the most studied RE ion for temperature sensors, which is based on FIR of 4F3/2(R1) →4I9/2 to 4F3/2(R2) →4I9/2 transitions. The maximal Sr values of these Nd3+ based optical thermometers are normally around 0.1%K−1 (for instance, Nd3+:NaYF4 (0.075%K−1) [33], Nd3+: YAG (0.15%K−1) [34], Nd3+: LaF3 (0.1% K−1) [35], etc.), which are similar with those values based on FIR-1 and FIR-2. However, these relative sensitivities are lower than some other kinds of luminescent thermometers operating in the near-infrared range, such as LiLa0.4Nd0.1Yb0.5P4O12 (0.4%K−1) [36],
Fig. 6. Near-infrared luminescence spectra for the two excitation power densities of 16.7 and 33.4 mW/mm2, which are normalized at 1531 nm. 282
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 7. Absolute sensing sensitivity values of (a) FIR-1, (b) FIR-2 and (c) FIR-Green in Er3+, Yb3+: BaMoO4 powders at two different pump power densities.
CaF2:Tm3+,Yb3+ (2%K−1) [37], Yb3+, Er3+: NaYF4@Yb3+, Nd3+: NaYF4 (2%K−1) [38], etc. Besides, Figs. 7(c) and 8(c) illustrate that the Sa and Sr values based on FIR-Green are much higher than those based on FIR-1 and FIR-2. The maximal Sa is ∼115 × 10−4 K−1 at 430 K, while the maximal Sr is ∼1.07%K−1 at 293 K. In fact, the maximal Sa value is larger than many other kinds of Er3+/Yb3+ or Er3+ doped phosphors based on green emissions, such as 65 × 10−4 K−1 for Er3+/ Yb3+/Li3+:NaZnPO4 [39], 72 × 10−4 K−1 for Er3+/Yb3+:YNbO4 [40], 40.7 × 10−4 K−1 for Er3+/Yb3+: Gd2TiO5 [41], 57 × 10−4 K−1 for
Er3+/Yb3+:NaBiF4 [21] and so on. Additionally, the temperature uncertainty of measurement (△T) has also been employed to evaluate the feasibility of materials for optical temperature sensors, which is defined as [2,42,43]:
ΔT =
1 δFIR Sr FIR
(5)
where δFIR is the uncertainty in the determination of FIR (determined by the experimental detection setup), Sr is the relative sensing
Fig. 8. Relative sensing sensitivity values of (a) FIR-1, (b) FIR-2 and (c) FIR-Green at two different pump power densities. 283
Optical Materials 86 (2018) 278–285
R. Lei et al.
Fig. 9. Switching of (a) FIR-1, (b) FIR-2 and (c) FIR-Green induced by alternating temperature between 293 and 553 K. The excitation power density was fixed at 16.7 mW/mm2.
of FIR-1 and FIR-Green may be more appropriate for optical thermometry, allowing for self-referenced temperature determination. We believe the above findings provide useful information for the pursuit of high-performance optical thermometers.
sensitivity. Obviously, larger sensing sensitivity leads to smaller temperature uncertainty at a certain pump power density. Therefore, among the three types of FIR data, FIR-Green is expected to possess the smallest temperature uncertainty. However, it should be pointed out that though FIR-Green has larger sensing sensitivity and smaller temperature uncertainty than FIR-1, the usage of FIR-1 may get more precise temperature detection due to its independence on the excitation power density. Finally, the measurement stability is required for temperature sensors. In this regard, Fig. 9 illustrates the dependences of FIR-1, FIR-2 and FIR-Green on the absolute temperature by performing recycling tests. It can be seen that the three types of FIR data exhibit good repeatability and reversibility after several cycling experiments, which indicates the high stability for continuous use. Overall, the present product possesses some superior properties for temperature determination. The application of FIR-1 can provide a precise temperature reading due to its power-independent output, while the high sensing sensitivity can be achieved based on FIR-Green. Hence, multi-FIR data from the different spectral range could be more accurate than the application of only one type of FIR, which allows for self-referenced temperature determination. So, Er3+/Yb3+: BaMoO4 is a promising phosphor for accurate temperature sensing measurement.
Acknowledgments This work is supported by Natural Science Foundation of Zhejiang Province (LY18E020008 and LR15F050003) and National Science Foundation of China (No. 61605192 and 51602301). References [1] M. Xu, X.M. Zou, Q.Q. Su, W. Yuan, C. Cao, Q.H. Wang, X.J. Zhu, W. Feng, F.Y. Li, Ratiometric nanothermometer in vivo based on triplet sensitized upconversion, Nat. Commun. 9 (2018) 2698-1-7. [2] M. Quintanilla, L.M. Liz-Marzán, Guiding rules for selecting a nanothermometer, Nano Today 19 (2018) 126–145. [3] M.D. Dramićanin, Sensing temperature via downshifting emissions of lanthanidedoped metal oxides and salts. A review, Meth. Appl. Fluoresc. 4 (2016) 042001. [4] G.R. Chen, R.S. Lei, H.P. Wang, F.F. Huang, S.L. Zhao, S.Q. Xu, Temperature-dependent emission color and temperature sensing behavior in Tm3+/Yb3+:Y2O3 nanoparticles, Opt. Mater. 77 (2018) 233–239. [5] L.D. Carlos, F. Palacio, Thermometry at the Nanoscale: Techniques and Selected Applications, first ed., Royal Society of Chemistry, London, 2016. [6] Y.Q. Zhang, S. Xu, X.P. Li, J.S. Zhang, J.S. Sun, H.P. Xia, R.N. Hua, B.J. Chen, Temperature sensing, excitation power dependent fluorescence branching ratios, and photothermal conversion in NaYF4:Er3+/Yb3+ @NaYF4:Tm3+/Yb3+ core-shell particles, Opt. Mater. Express 8 (2018) 368–384. [7] A. Pandey, V.K. Rai, Rare earth doped materials for temperature sensors, in: Y. Dwivedi, S.B. Rai, J.P. Singh (Eds.), Spectroscopic Techniques for Security Forensic and Environmental Applications, Nova Science Publishers, New York, 2011, pp. 279–292. [8] H. Xia, L. Lei, W.Q. Hong, S.Q. Xu, A novel Ce3+/Mn2+/Eu3+ tri-doped GdF3 nanocrystals for optical temperature sensor and anti-counterfeiting, J. Alloy. Comp. 757 (2018) 239–245. [9] W. Xu, Y. Cui, Y.W. Hu, L.J. Zheng, Z.G. Zhang, W.W. Cao, Optical temperature sensing in Er3+-Yb3+ codoped CaWO4 and the laser induced heating effect on the luminescence intensity saturation, J. Alloy. Comp. 726 (2017) 547–555. [10] Z.H. Feng, L. Lin, Z.Z. Wang, Z.Q. Zheng, Low temperature sensing behavior of upconversion luminescence in Er3+/Yb3+ co-doped PLZT transparent ceramic, Opt. Commun. 399 (2017) 40–44. [11] P. Du, L.H. Luo, J. Su, Controlled synthesis and upconversion luminescence of Tm3+-doped NaYbF4 nanoparticles for non-invasion optical thermometry, J. Alloy. Comp. 739 (2018) 926–933.
4. Conclusions The Er3+/Yb3+ co-doped BaMoO4 phosphor has been prepared by high temperature solid state reaction method. An inverse thermal quenching phenomenon is observed in 1.53 μm near-infrared luminescence upon a 980 nm excitation, which guarantees a high signal-tonoise ratio at high temperatures. Three types of FIRs (I1504/I1531, I1521/ I1531 and I533/I557) are selected to investigate the temperature sensing behaviors in detail. The FIR-1 (I1504/I1531) is found to be insusceptible to pump power densities, giving rise to consistent calibration curves and measurement results at two different excitation power densities. Otherwise, though excellent temperature sensing sensitivity with the maximal Sa of ∼115 × 10−4 K−1 can be achieved by using FIR-Green (I533/I557), its power dependence may lead to inconsistent results and measurement errors at different pump powers. Hence, the combination 284
Optical Materials 86 (2018) 278–285
R. Lei et al. [12] S. Senapati, K.K. Nanda, Red emitting Eu:ZnO nanorods for highly sensitive fluorescence intensity ratio based optical thermometry, J. Mater. Chem. C 5 (2017) 1074–1082. [13] F. Huang, D.Q. Chen, Synthesis of Mn2+:Zn2SiO4-Eu3+:Gd2O3 nanocomposites for highly sensitive optical thermometry through the synergistic luminescence from lanthanide-transition metal ions, J. Mater. Chem. C 5 (2017) 5176–5182. [14] D.Q. Chen, S. Liu, Y. Zhou, Z.Y. Wan, P. Huang, Z.G. Ji, Dual-activator luminescence of RE/TM: Y3Al5O12 (RE=Eu3+, Tb3+, Dy3+; TM=Mn4+, Cr3+) phosphors for selfreferencing optical thermometry, J. Mater. Chem. C 4 (2016) 9044–9051. [15] M.Y. Ding, D.Q. Chen, C.H. Lub, J.H. Xi, Z.G. Ji, Z.Z. Xu, Lanthanide-doped LuF3 mesocrystals for optical thermometry, Mater. Lett. 189 (2017) 5–8. [16] Y.P. Huang, L.H. Luo, Up-conversion photoluminescence based on the intrinsic defects in Na0.5Bi0.5TiO3: Yb3+ ceramics, J. Alloy. Comp. 706 (2017) 312–317. [17] S. Ćulubrk, V. Lojpur, S.P. Ahrenkiel, J.M. Nedeljković, M.D. Dramićanin, Noncontact thermometry with Dy3+ doped Gd2Ti2O7 nano-powders, J. Lumin. 170 (2016) 395–400. [18] J.W. Zhao, H.T. Li, Q.H. Zeng, K. Song, X.F. Wang, X.G. Kong, Temperature-dependent upconversion luminescence of NaYF4:Yb3+, Er3+ nanoparticles, Chem. Lett. 42 (2013) 310–312. [19] J.R. Hao, L. Cao, Y. Wei, C.J. Yan, G.G. Li, Improving photoluminescence and thermal stability of CaSi2O2N2:Eu2+ phosphors by codoping lanthanide ions (Ln=Sc, La, Gd, Yb, Lu), Mater. Lett. 211 (2018) 122–125. [20] L.P. Li, L.J. Zheng, W. Xu, Z. Liang, Y. Zhou, Z.G. Zhang, W.W. Cao, Optical thermometry based on the red upconversion fluorescence of Er3+ in CaWO4:Yb3+/Er3+ polycrystalline powder, Opt. Lett. 41 (2016) 1458–1461. [21] P. Du, L.H. Luo, X.Y. Huang, J.S. Yu, Ultrafast synthesis of bifunctional Er3+/Yb3+ codoped NaBiF4 upconverting nanoparticles for nanothermometer and optical heater, J. Colloid Interface Sci. 514 (2018) 172–181. [22] X. Ming, Q.Y. Meng, S.C. Lü, W.J. Sun, The hydrothermal synthesis and morphology-dependent optical temperature sensing properties of Er3+ doped NaGd (WO4)2 phosphor, J. Lumin. 192 (2017) 196–202. [23] P. Elahi, H. Kalaycıoğlu, H. Li, Ö. Akçaalan, F.Ö. Ilday, 175 fs-long pulses from a high-power single-mode Er-doped fiber laser at 1550 nm, Opt. Commun. 403 (2017) 381–384. [24] Q.W. Wang, J. Li, J.Y. Lin, H.X. Jiang, Enhancement of 1.5 μm emission under 980 nm resonant excitation in Er and Yb codoped GaN epilayers, Appl. Phys. Lett. 109 (2016) 152103. [25] P.S. Yu, L.B. Su, W. Guo, J. Xu, Photoluminescence and energy transfer progress in Er-doped Bi2O3-GeO2 glasses, J. Lumin. 187 (2017) 121–125. [26] E.E. Brown, U. Hömmerich, A. Bluiett, C. Kucera, J. Ballato, S. Trivedi, Near-infrared and upconversion luminescence in Er:Y2O3 ceramics under 1.5 μm excitation, J. Am. Ceram. Soc. 97 (2014) 2105–2110. [27] X.X. Zhang, K.F. Li, K.W. Cheah, X. Zhou, P.A. Tanner, 1.54 and 1.75 μm infrared luminescence of Y2O3:Er3+, Chem. Phys. Lett. 400 (2004) 331–335. [28] J.B. Gruber, K.L. Nash, D.K. Sardar, U.V. Valiev, N. Ter-Gabrielyan, L.D. Merkle, Modeling optical transitions of Er3+ (4f11) in C2 and C3i sites in polycrystalline Y2O3, J. Appl. Phys. 104 (2008) 023101. [29] N. Yuan, D.Y. Liu, X.C. Yu, H.X. Sun, C.G. Ming, W.H. Wong, F. Song, D.Y. Yu,
[30] [31]
[32] [33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
285
E.Y.B. Pun, D.L. Zhang, A biological nano-thermometer based on ratiometric luminescent Er3+/Yb3+-codoped NaGd(WO4)2 nanocrystals, Mater. Lett. 218 (2018) 337–340. D. Yu, J. Ballato, R.E. Riman, Temperature-dependence of multiphonon relaxation of rare-earth Ions in solid-state hosts, J. Phys. Chem. C 120 (2016) 9958–9964. A.Q. Zhang, Z. Sun, G.F. Liu, Z.L. Fu, Z.D. Hao, J.H. Zhang, Y.L. Wei, Ln3+ (Er3+, Tm3+ and Ho3+)-doped NaYb(MoO4)2 upconversion phosphors as wide range temperature sensors with high sensitivity, J. Alloy. Comp. 728 (2017) 476–483. P. Singh, P.K. Shahi, A. Rai, A. Bahadur, S.B. Rai, Effect of Li+ ion sensitization and optical temperature sensing in Gd2O3: Ho3+/Yb3+, Opt. Mater. 58 (2016) 432–438. D. Wawrzynczyk, A. Bednarkiewicz, M. Nyk, W. Strek, M. Samoc, Neodymium(III) doped fluoride nanoparticles as non-contact optical temperature sensors, Nanoscale 4 (2012) 6959–6961. A. Benayas, B. del Rosal, A. Pérez-Delgado, K. Santacruz-Gómez, D. Jaque, G.A. Hirata, F. Vetrone, Nd:YAG near-Infrared luminescent nanothermometers, Adv. Opt. Mater. 3 (2015) 687–694. U. Rocha, C.J. Silva, W.F. Silva, I. Guedes, A. Benayas, L.M. Maestro, M.A. Elias, E. Bovero, F.C.J.M. Veggel, J.A.G. Solé, D. Jaque, Subtissue thermal sensing based on neodymium-doped LaF3 nanoparticles, ACS Nano 7 (2013) 1188–1199. L. Marciniak, A. Bednarkiewicz, M. Stefanski, R. Tomala, D. Hreniak, W. Strek, Near infrared absorbing near infrared emitting highly-sensitive luminescent nanothermometer based on Nd3+ to Yb3+ energy transfer, Phys. Chem. Chem. Phys. 17 (2015) 24315–24321. N.N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, J.E. Ramírez-Hernández, L.M. Maestro, M.C.I. Cruz, F. Sanz-Rodríguez, A. Juarranz, F. Chen, F. Vetrone, J.A. Capobianco, J.G. Solé, M. Bettinelli, D. Jaque, A. Speghini, NIR-to-NIR twophoton excited CaF2:Tm3+,Yb3+ nanoparticles: multifunctional nanoprobes for highly penetrating fluorescence bio-Imaging, ACS Nano 5 (2011) 8665–8671. L. Marciniak, K. Prorok, L. Frances-Soriano, J. Perez-Prieto, A. Bednarkiewicz, A broadening temperature sensitivity range with a core–shell YbEr@YbNd double ratiometric optical nanothermometer, Nanoscale 8 (2016) 5037–5042. L. Mukhopadhyay, V.K. Rai, R. Bokolia, K. Sreenivas, 980 nm excited Er3+/Yb3+/ Li+/Ba2+: NaZnPO4 upconverting phosphors in optical thermometry, J. Lumin. 187 (2017) 368–377. Y.Y. Tian, Y. Tian, P. Huang, L. Wang, Q.F. Shi, C. Cui, Effect of Yb3+ concentration on upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ codoped YNbO4 nanoparticles prepared via molten salt route, Chem. Eng. J. 297 (2016) 26–34. J.S. Liao, Q. Wang, L.Y. Kong, Z.Q. Ming, Y.L. Wang, Y.Q. Li, L.X. Che, Effect of Yb3+ concentration on tunable upconversion luminescence and optically temperature sensing behavior in Gd2TiO5:Yb3+/Er3+ phosphors, Opt. Mater. 75 (2018) 841–849. S. Balabhadra, M.L. Debasu, C.D.S. Brites, L.A.O. Nunes, O.L. Malta, J. Roch, M. Bettinellie, L.D. Carlos, Boosting the sensitivity of Nd3+-based luminescent nanothermometers, Nanoscale 7 (2015) 17261–17267. S. Balabhadra, M.L. Debasu, C.D.S. Brites, R.A.S. Ferreira, L.D. Carlos, Upconverting nanoparticles working as primary thermometers in different media, J. Phys. Chem. C 121 (2017) 13962–13968.