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Abstract
The Er3+ ion’s 4F7/2-4I15/2 transition, which has been less reported in the past, is studied in CaWO4:Yb3+/Er3+ phosphors. It has been confirmed that this transition is from the two-photon upconversion mechanism when the 980 nm laser diode is used as the excitation source. Moreover, the transition has the same lifetime with the neighboring lower 2H11/2/4S3/2 states, suggesting that the 4F7/2/4S3/2 states are thermally linked. It is shown that the 4F7/2/4S3/2-4I15/2 emissions can be used for ratiometric temperature measurement. Their relative thermal sensitivity is up to 2820/T2, one of the largest sensitivities reported so far. It is very close to the theoretical maximum 2877/T2.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to the significance of temperature in lots of fields [1–9], many methods have been developed to measure this parameter [10–15]. Among the numerous temperature sensing devices, thermocouple and thermal resistance have been widely used nowadays for their reliable performance, relatively low manufacturing cost, and so forth. With the development of science and technology, it is found that these conventional temperature sensors are not suitable for being used in many occasions. Compared with these contact-type thermometers, optical sensor has attracted much attention because the response time is short and the working model is non-contact [16–20].
The luminescence intensity ratio (LIR) technology is especially attractive as it owns strong anti-jamming ability [21–25]. The lanthanides are always used for luminescent ratiometric thermometry because they have many pairs of thermally coupled states. So far, it has been confirmed that the Yb3+, Er3+, Ho3+, Tm3+, Nd3+, Eu3+, Pr3+, Sm3+, Gd3+ and Dy3+ ions can be used for temperature measurement based on the LIR technology [10–12]. Among these ions, the Er3+ ion is famous for its 2H11/2/4S3/2-4I15/2 transitions, which are spaced by a moderate gap and thus abide by the Boltzmann distribution. This pair of transitions has been studied in a variety of hosts [26–30]. However, the relative sensitivity (Sr) for them is relatively low and decreases sharply with increasing temperature. How to achieve a higher thermal sensitivity becomes a hot point nowadays. Using a pair of states spaced by a larger gap has been confirmed to be a possible way. Compared with the commonly studied 2H11/2/4S3/2 states, the 4F7/2/4S3/2 ones of the Er3+ ion are spaced by a much larger gap. Therefore, using the 4F7/2/4S3/2-4I15/2 transitions for ratiometric temperature measurement is expected to achieve a larger relative sensitivity. Nonetheless, it is difficult to observe the 4F7/2-4I15/2 transition.
Here it is shown that the calcium tungstate is an ideal host for the Er3+ ion. At relatively high temperatures, the 4F7/2-4I15/2 transition can be easily detected with a relatively good signal-to-noise ratio. The population mechanism and thermal effect of this transition are studied. A ratiometric strategy that depends on the 4F7/2/4S3/2-4I15/2 transitions is proposed. Their relative sensitivity is found to be 2820/T2, one of the largest values reported so far. Moreover, this sensitivity is very close to the theoretical maximum 2877/T2.
2. Experimental
2.1 Synthesis of CaWO4:Yb3+/Er3+ phosphors
CaWO4:Yb3+/Er3+ (10/1 mol %) phosphors were fabricated by using the co-precipitation method. In the first place, CaCl2, Na2WO4, Yb(NO3)3 and Er(NO3)3 solutions were prepared. The stoichiometric CaCl2, Yb(NO3)3 and Er(NO3)3 solutions were fully mixed under continuous magnetic stirring. Next, the proper Na2WO4 solution was dropped into the former mixture slowly. Several hours later, the resulting solution was centrifuged to obtain the white powders. Afterwards, these white powders were dried at 353 K for 24 h and sintered at 1523 K for 6 h to form the final phosphors.
2.2 Characterization
The powder X-ray diffraction (XRD) patterns of the as-prepared phosphors were obtained by using PANalytical X’Pert Powder X-ray diffractometer. The morphology of the as-prepared phosphors was investigated by using the field-emission scanning electron microscopy (FE-SEM; SU70, Hitachi, Japan). The continuous-wave 980 nm laser diode (ITC-4005, Thorlabs) was used as the source to excite the as-prepared phosphors. The emission spectra of the phosphors were collected by the system made up of a monochromator (SBP-300, Zolix Instruments), a photomultiplier (PMTH-S1-CR131, Zolix Instruments) and a DAQ (DSC-102, Zolix Instruments). The phosphors’ temperature was controlled by the home-made heating-cooling platform (accuracy:±0.3 K).
3. Results and discussion
Figure 1(a) shows the XRD patterns of the as-prepared phosphors. As can be observed, the positions of the diffraction peaks agree well with the reference data (PDF card no 41-1431). It suggests that the phosphors were fabricated with success. Moreover, they held the tetragonal phase. Figures 1(b) and 1(c) present the SEM images of the as-prepared phosphors. One can see that the morphology of the particles is irregular. The size of these particles ranges from a few hundred nanometers to a few micrometers. The elemental mappings of the samples, depicted in Figs. 1(d)–1(h), demonstrate the existence of Ca, O, W, Yb and Er elements.
Fig. 1. (a) XRD pattern, (b)/(c) SEM images, (d) Ca, (e) O, (f) W, (g) Yb and (h) Er elemental mappings of the as-prepared phosphors.
Following the NIR excitation, the phosphors emit visible green luminescence, which is presented in Fig. 2(a). The 530 and 551 nm emission bands have the relatively strong intensity. They are known to be from the Er3+ ion’s 2H11/2−4I15/2 and 4S3/2−4I15/2 transitions, respectively [31,32]. In the previous literatures, these two emission bands have been widely investigated. In contrast, the 4F7/2−4I15/2 emission band is less reported because it is weak. As shown in Fig. 2(a), it nearly vanishes at room temperature. However, at 813 K, it gains a giant enhancement and can be easily detected by the commonly used photomultipler tube with a relatively good signal-to-noise ratio.
Fig. 2. (a) Emission spectra of the samples at 333/813 K, respectively, following the 980 nm laser diode excitation; (b) Green emission intensity as a function of pump power.
Figure 2(b) shows the green emission intensity as a function of pump power in the double logarithmic plot. As demonstrated previously, the UC emission intensity I can be written as I = QPn, where Q is a constant, P is the pump power and n is the photon number needed to trigger the transition [33]. The slope of the linear fit is 2.0. It suggests that two laser photons at 980 nm are needed to excite the Er3+ ion to generate the green emission.
Figure 3(a) depicts the possible population processes for the Er3+’s green emission. Upon the laser excitation at 980 nm, the Yb3+ ion absorbs the laser photon and goes to its higher 2F5/2 state. Next, the Er3+ ion is excited to the 4I11/2 state first and then to the higher 4F7/2 excited state. The most populations of the 4F7/2 excited state relax quickly to the lower 2H11/2/4S3/2 states. Finally, the 4F7/2/2H11/2/4S3/2−4I15/2 transitions occur, leading to the 490/530/551 nm emission bands, respectively.
Fig. 3. (a) The possible processes for the 490/530/551 nm emissions; (b) Normalized emission spectra of the samples in the 693-813 K temperature range, following the 980 nm laser diode excitation.
One can see from Fig. 2(a) that the 490 nm emission band is very weak at low temperature. Therefore, the thermal effect of the phosphors is only investigated at relatively high temperatures. Figure 3(b) shows the temperature dependent emission spectra of the phosphors in the 693-813 K range. They were normalized to the 551 nm emission band. As can be observed, the 490 nm emission band increases monotonically with the rise of temperature. Therefore, the LIR between the 490 and 551 nm emission bands can be used to indicate temperature.
The LIR between the 490 and 510 nm emission bands is expressed as [12]
where A is a constant, ΔE is the energy gap between the 4F7/2 and 4S3/2 states, k is the Boltzmann constant and T is the absolute temperature. The inset of Fig. 4(a) shows the LIR between the 490/551 nm emission bands in the 693-813 K temperature range. One can see that these experimental data points increase gradually upon increasing the temperature. In addition, they agree well with Eq. (1). It suggests undoubtedly that the 4F7/2/4S3/2 states are thermally linked with each other.Fig. 4. (a) LIR between the 490/551 nm emission bands and its relative sensitivity as a function of temperature in the 693-813 K range; (b) Temperature uncertainty δT for the 490/551 nm emission bands.
The relative sensitivity, Sr, is always used to evaluate the performance optical sensors. It is defined as [34–37]
With the use of this equation, the Sr for the 490/551 nm emission bands is calculated, which is presented in Fig. 4(a). It decreases gradually upon increasing the temperature. At 693 K, the Sr is 0.58% K-1. At 813 K, the Sr goes down to 0.42% K-1. According to Wade et al., the upper limit gap between two thermally linked states is around 2000cm-1 [10]. In this case, the theoretical maximum relative sensitivity is 2877/T2. It is also shown in Fig. 4(a). Obviously, the 490/551 nm emission bands’ relative sensitivity is extremely close to the theoretical maximum. In fact, the optical temperature sensing based on Yb3+/Er3+ doped CaWO4 phosphors had been reported previously [38,39]. For instance, Wang et al. once studied the influence of doping and excitation powers on optical thermometry on the basis of Yb3+/Er3+ doped CaWO4 phosphors. The 2H11/2/4S3/2-4I15/2 transitions’ maximum relative sensitivity was computed to 949/T2. Our group once investigated the temperature sensing ability of this pair of emission bands in micrometer scaled CaWO4 host and obtained the similar results. Unfortunately, the Er3+ ion’s 4F7/2/4S3/2-4I15/2 transitions have never been exploited to measure temperature in CaWO4 host. As presented in Fig. 4(a), the relative sensitivity of the 4F7/2/4S3/2-4I15/2 transitions is up to 2820/T2, which is nearly three times the relative sensitivity for the 2H11/2/4S3/2-4I15/2 transitions.Apart from the relative sensitivity Sr, the temperature uncertainty (or the so-called temperature resolution) δT is also one of the most significant parameters to evaluate the performance of temperature sensors. It is defined as [40,41]
where δLIR/LIR is the relative uncertainty of LIR. In general, this term is related with the instrument settings in the experiment. It can be computed by consecutively collecting the emission spectra for dozens of times [40,41]. Based on our instrumental setup, the δLIR/LIR is calculated to be about 0.6%. By using Eq. (3), the temperature uncertainty δT for the 490/551 nm emission bands is obtained, as presented in Fig. 4(b). At 693 K, the temperature uncertainty is as low as 1.02 K. With increasing the temperature to 813 K, it decreases to 1.40 K. This should be attributed to the fact that the relative sensitivity goes down with the rise of temperature.To further disclose the properties of the 4F7/2/2H11/2/4S3/2 states, the time resolved spectra for them are studied in the 693-813 K temperature range. Figures 5(a)–5(g) present the time resolved spectra of the 490/530/551 nm emission bands at 693 K, 713 K, 733 K, 753 K, 773 K, 793 K and 813 K, respectively. As can be observed, each time resolved spectrum is in good accordance with the following mono-exponential expression [42]
where I(t) and I0 are the intensities of time resolved spectrum at time t and 0, respectively, τ is the lifetime of an emitting state and B is the luminescence background. It can be seen from Figs. 5(a)–5(g) that at each temperature, the 490/530/551 nm emission bands have the same decay behavior. It suggests that these three emission bands own the same lifetime in the 693-813 K temperature range. This is an obvious indication that the 4F7/2/2H11/2/4S3/2 states are thermally linked with each other. By using Eq. (4), the lifetimes for the 490/530/551 nm emission bands in the 693-813 K temperature range were obtained, as shown in Fig. 5(h).Fig. 5. Time resolved spectra of the prepared phosphors at (a) 693 K, (b) 713 K, (c) 733 K, (d) 753 K, (e) 773 K, (f) 793 K, and (g) 813 K, respectively; (h) Comparison of the lifetimes for the 490/530/551 nm emission bands in the 693-813 K temperature range.
The lifetime of an emitting state, τ, can be written as [43]
where AR and WNR are the radiative and non-radiative transition probabilities of an emitting state, respectively. In general, the radiative transition probability AR is taken as the intrinsic property of an emitting state, which is immune to temperature change. In contrast, the non-radiative transition probability WNR strongly depends on temperature. It increases evidently and monotonically with increasing temperature. This makes the lifetime τ decrease with increasing temperature. The lifetimes of the 490/530/551 nm emission bands as a function of temperature, depicted in Fig. 5(h), agree well with these analyses. At the same time, the quantum efficiency η=AR/(AR+WNR) in response to temperature can be analyzed. It decreases monotonically because AR is a constant and WNR increases monotonically with increasing temperature. In fact, the quantum efficiency η can be further expressed as η=ARτ. Therefore, the quantum efficiency η should share the same change law with the lifetime τ.4. Conclusion
To summary, CaWO4:Yb3+/Er3+ phosphors were prepared. Following the 980 nm laser excitation, the Er3+ ion’s 4F7/2-4I15/2 transition was observed. It was confirmed to be from the two-photon upconversion mechanism. It was demonstrated that the 4F7/2/2H11/2/4S3/2 states were thermally linked with each other. The relative sensitivity for the 4F7/2-4I15/2 and 4S3/2-4I15/2 transitions is up to 2820/T2, one of the maximum sensitivity reported so far for the thermometers that depend on thermally coupled states. In addition, this sensitivity is very close to the theoretical maximum 2877/T2. The work can make us deeply understand the 4F7/2-4I15/2 transition and provides us a strategy for highly sensitive thermal detection at relatively high temperatures.
Funding
National Natural Science Foundation of China (NSFC) (61505045, 81571720).
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