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Abstract
Luminescence intensity ratio (LIR) thermometry is of great interest, because of its wide applications of noninvasive temperature sensing. Here, a LIR thermometry based on combined ground and excited states absorptions is developed using CaWO4:Tb3+. The ratio of single luminescence (5D4-7F5) intensities under 379 and 413 nm excitations with opposite temperature dependences, attributed to the thermal coupling of ground state 7F6 and excited state 7F5, is used to measure temperature. This LIR method achieves a high relative sensitivity of 2.8% K-1, and can avoid complex spectral splitting by collecting all down-shifting luminescence bands, being a promising accurate luminescence thermometry.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Temperature is a fundamental thermodynamic parameter, the measurement techniques of which account for 80% of the sensor market throughout the world [1–3]. Recently, optical thermometers have been developed and widely applied in the field of surface functionalization, intracellular mapping, microelectromechanical systems, aircraft, and semiconductors, because of the non-intrusive feature [4–11]. Luminescent thermography, especially luminescence intensity ratio (LIR) thermometry based on rare earth (RE) ions doped photoluminescence materials, receives great attentions and shows enormous advantages on high spatial resolution on micro- and nanoscale, high sensitivity and precision [7,12–16].
LIR thermometry mainly includes two bands and single band ratiometric luminescent methods. Two bands ratiometric luminescent thermometry measures the temperature through the ratio of two luminescent signals at a single excitation wavelength. The past 20 years have witnessed a remarkable development in this LIR technique, exhibiting high relative sensitivity (>1% K-1) and spatial resolution (<10 µm) in short acquisition times (<1 ms) [17–19]. In recent years, single band ratiometric luminescent thermometry is a developing LIR method. This method uses the ratio of single luminescent signals at two excitation wavelengths as the optical parameter for temperature sensing. It has the advantages of good flexibility, reliability, and temperature operating range, and can reduce the equipment cost of the LIR temperature measurement system in technology and biomedical science [2,20]. Current single-band-based LIR thermometers mainly depend on Eu3+-doped phosphors. They monitored the 5D0-7F4 (or 5D0-7F1) emission of Eu3+ upon the 7F0, 7F2-5D0 (or 7F0-5D2, 7F2-5D0) absorption processes to obtain the LIR parameters, achieving the LIR thermometry with relative sensitivities (${S_\textrm{r}}$) of ∼2% K-1 [2,20]. However, studies on single-band-based LIR thermometry are still rarely reported, which need to be enriched. Moreover, it remains some problems using Eu3+-based single band LIR thermometry. Firstly, the energy gap ($\Delta E$) between thermal coupling levels 7F0 and 7F2 of Eu3+ is only 875 cm-1, limiting the enhancement of ${S_\textrm{r}}$ according to the Boltzmann law. Secondly, the complex energy level structures of Eu3+ will lead to the spectral overlap originating from adjacent transitions, which brings the requirement for splitting the luminescent bands and induces measurement errors. Therefore, in order to improve the application potential of the LIR technique, further study of single-band-based LIR thermometry is necessary.
Here, we develop the single-band-based LIR thermometry based on CaWO4:Tb3+ by utilizing the ground and excited state absorption processes (GSA and ESA, respectively). The strongest down-shifting luminescence originating from 5D4-7F5 transition of the Tb3+ ion is chosen as the detected single luminescence signal, showing notably opposite temperature dependences upon 379 and 413 nm excitations (corresponding to GSA and ESA). The ratio of this single luminescence intensities is used as the LIR parameter to achieve temperature sensing. The LIR satisfies the Boltzmann type distribution in the measurement range from 333 to 733 K, demonstrating the thermal coupling effect of the ground state 7F6 and the excited state 7F5 of Tb3+. Moreover, the selected $\Delta E$ between the 7F6 and 7F5 states is as large as 2124 cm-1, thus a high ${S_r}$ is obtained as 2.8%K-1 at 333 K. In addition, all down-shifting luminescence bands (ADLBs) ratiometric thermometry is proposed, because of the same temperature-dependent characteristics of the emissions originating from 5D4 energy levels. The ADLBs method collects all the down-shifting luminescence intensities as the optical parameter, simplifying spectral splitting. This work provides a Tb3+-based single-channel luminescence detection LIR thermometry with high sensitivity, showing a good engineering prospect of improving the temperature sensing accuracy, promoting the system flexibility and simplifying the detection setup.
2. Materials and experiments
Tb3+ ion, the 5D4-7F5 emission of which can be induced through both the GSA and ESA processes (as depicted by the simplified energy levels in the inset of Fig. 1(a)), is the suitable photoluminescence center to realize the single-band-based LIR thermometry under two-wavelength excitations. Moreover, the energy level difference between the ground state 7F6 and the first excited state 7F5 of Tb3+ is larger than 2000cm-1, suggesting its good potential of high sensitivity. Hence, we choose Tb3+ ion to achieve a high sensitivity LIR thermometry in this work. To manifest the characteristics of Tb3+-doped materials, CaWO4:5%Tb3+ was prepared via a solid-state method [18], and the emission and excitation spectra were measured [21]. The emission spectra were recorded with a grating spectrometer (Zolix, Omni-λ300) and photomultiplier (Zolix, PMTH-S1-CR131). The excitation spectra were measured from 333 to 733 K by monitoring the 5D4-7F5 emission at 545 nm of Tb3+. In the measurement of excitation spectra, the light from a Xenon broadband light source was split using a grating spectrometer (Zolix, Omni-λ300) as monochromatic excitation light. In addition, the temperature of the CaWO4: Tb3+ sample was controlled by a heating device with an accuracy of ±0.3 K.
Fig. 1. (a) Emission spectrum of CaWO4: Tb3+ at 333 K under 379 nm excitation. The inset is simplified energy levels of Tb3+. (b) Excitation spectra of CaWO4: Tb3+ at 333 K, 453 K, 533 K, and 613 K; in all spectra the 545 nm emission is monitored and the intensity at 413 nm is 100 times magnified.
3. Design of the LIR thermometry
The emission spectrum from 510 nm to 700 nm of the CaWO4: Tb3+ sample is shown in Fig. 1(a). The luminescence bands at 545 nm, 585 nm and 620 nm are derived from the 5D4-7FJ (J=5,4,3) transitions, respectively [22,23]. Compared with other down-shifting luminescence bands, the intensity of the 545 nm luminescence band originating from 5D4 to 7F5 is extremely high, consuming most of the emission energy. The temperature-dependent excitation spectra of the CaWO4: Tb3+ are shown in Fig. 1(b) by monitoring the strongest emission at 545 nm. As shown in the inset of Fig. 1(a), the absorption lines below 400 nm in Fig. 1(b) are attributed to the Tb3+ ion’s absorption from the ground state 7F6 to different higher excited states 5DJ (J=2,3) (that is, GSA processes), while the absorption line at 413 nm originate from 7F5-5D3 absorption (that is, ESA process). Figure 1(b) indicates that the luminescence intensity of Tb3+ at 545 nm decrease in the GSA process but increase in the ESA process with increasing temperature [24,25]. Thus, the opposite temperature dependences of the detected luminescence band utilizing GSA and ESA processes is realized. The ratio $\textrm{LI}{\textrm{R}_{\textrm{ESA}/\textrm{GSA}}}$ of the 545 nm luminescence band intensities yielded by ESA and GSA processes, can change as temperature rises, which can be used to detect the temperature.
Based on the temperature-dependence of the $\textrm{LI}{\textrm{R}_{\textrm{ESA}/\textrm{GSA}}}$, a LIR thermometry system is designed. The schematic of this system is shown in Fig. 2. The system introduces the way of alternate excitations and single-channel luminescence detection. The switch between the two excitation lights can be realized by an optical switch, which has a fast response speed and can be controlled by a program. ${I_{\textrm{exc}1}}$ and ${I_{\textrm{exc}2}}$, in Fig. 2, are the intensities of excitation sources, corresponding to GSA and ESA processes of Tb3+ ions, respectively. In the testing proceeds, as the two light beams ${I_{\textrm{exc}1}}$ and ${I_{\textrm{exc}2}}$ alternately excite the CaWO4: Tb3+ sample, the excitation-induced down-shifting luminescence intensity is switched between ${I_{\textrm{ESA}}}$ and ${I_{\textrm{GSA}}}$. As temperature goes up or down, the detected luminescence intensity signals ${I_{\textrm{ESA}}}$ and ${I_{\textrm{GSA}}}$ have different change trends as shown in the case (a-c). Further, the temperature can be characterized by the relationship between $\textrm{LI}{\textrm{R}_{\textrm{ESA}/\textrm{GSA}}}$ (i. e., the ratio ${I_{\textrm{ESA}}}/{I_{\textrm{GSA}}}$) and temperature.
Fig. 2. Schematic of LIR thermometry system. ${I_{\textrm{exc}1}}$ and ${I_{\textrm{exc}2}}$ represent the intensity of excitation beams, respectively. With the time (t) changes, ${I_{\textrm{exc}1}}$ and ${I_{\textrm{exc}2}}$ alternately excite the CaWO4:Tb3+ sample. ${I_{\textrm{ESA}}}$ and ${I_{\textrm{GSA}}}$ are the down-shifting luminescence intensity induced by ${I_{\textrm{exc}1}}$ and ${I_{\textrm{exc}2}}$, respectively. Using the filter, only luminescence intensity is detected by the detector.
4. Results and discussions
In order to verify the feasibility of the LIR approach, the temperature dependence of the $\textrm{LI}{\textrm{R}_{\textrm{ESA}/\textrm{GSA}}}$ is measured and the physical mechanism is studied. We choose the strongest absorption bands at 379 nm (GSA) and 413 nm (ESA) as the excitation wavelengths, and the luminescence band at 545 nm is the monitored emission. These absorption lines are attributed to 7F6-5D3 and 7F5-5D3 transitions, respectively. Figure 3(a) shows the luminescence band at 545 nm exhibits opposite temperature dependences under wavelengths 379 and 413 nm. In the following, we use ${I_{379}}\; \textrm{and}\; {I_{413}}$ to represent the luminescence intensity upon379 and 413 nm excitations. The temperature-dependence of the $\textrm{LI}{\textrm{R}_{413/379}}$ (${I_{379}}/{I_{413}}{\; }$) from 333 to 733 K, corresponding to $\textrm{LI}{\textrm{R}_{\textrm{ESA}/\textrm{GSA}}}$, is shown in Fig. 3(b). According to the opposite temperature dependences measured by two-wavelength excitations, it is obvious that the $\textrm{LI}{\textrm{R}_{413/379}}$ gradually increases with increasing temperature. Moreover, the fitted line shows that the LIR satisfies the Boltzmann distribution law, expressed as [26]
where A is a constant, $\Delta E$ is the thermal energy gap, ${k_{\textrm{B}\; }}$is the Boltzmann constant, and T is the absolute temperature. The $\Delta E$ between the 7F6 and 7F5 states obtained by the fitting result is approximately 2124 cm-1. This function for the LIR vs. temperature, given in Fig. 3(b), demonstrates that the thermal coupling relationship exists between the 7F6 and 7F5 states. This thermal coupling is the major reason for the opposite temperature dependence of the 5D4-7F5 emission under the 379 and 413 nm excitations. Stronger non-radiation relaxation phenomenon reduces the 545 nm luminescence intensity upon the 379 nm excitation with the rising temperature. Meanwhile, as the temperature increases, the increasing population of the first excited state 7F5, due to the thermal depopulation of 7F6 state, enhances the 545 nm luminescence intensity at the 413 nm excitation. According to this intrinsic thermal coupling feature of Tb3+, the achieved $\textrm{LI}{\textrm{R}_{\textrm{ESA}/\textrm{GSA}}}$ versus temperature can be the effective criterion for temperature sensing.Fig. 3. (a) Integrated luminescence intensities of 545 nm emission in CaWO4: Tb3+ at 379 nm (GSA) and 413 nm (ESA) excitations from 333 to 733 K. Error bars represent the standard deviation from 30 measurements. (b) Luminescence intensity ratio (LIR) of the modulable luminescence intensities at 545 nm upon 379 nm and 413 nm excitations from 333 to 733 K. $\Delta \textrm{LIR}/\textrm{LIR}$ values are on the order of ${10^{ - 2}}$ derived from the standard deviation of LIR.
Moreover, relative sensitivity (${S_\textrm{r}}$) is one of the important parameters for evaluating temperature measurement methods. The ${S_\textrm{r}}$ represents the rate at which the LIR changes with temperature variation, given by
The above designed LIR thermometry uses two-wavelength excitations and detects the single band luminescence signal, and measures temperature in terms of $\textrm{LI}{\textrm{R}_{413/379}}$ versus temperature. The advantages of this approach are discussed as follows. First, as mentioned above, a couple of thermal levels 7F6 and 7F5 used here has a large $\Delta E$, indicating the good potential of high sensitivity. Moreover, the single band luminescence detection shows the good applicability. This way can effectively avoid the perturbation signal caused by adjacent transitions and other environmental factors. All the down-shifting luminescence bands in the wavelength range 510-700 nm originate from the 5D4 level of Tb3+. We studied the temperature dependences of the luminescence bands at 545 nm, 585 nm and 620 nm of Tb3+ as shown in Fig. 5. The intensities of all luminescence upon 379 nm excitation exhibit the same decreasing trends with increasing temperature, while all luminescence upon 413 nm excitation exhibit the same increasing trends, as shown in Fig. 5(c) and Fig. 5(d), respectively. It is obvious that with rising temperature, different luminescence bands have the same temperature response under the specific wavelength excitation. Therefore, other luminescent signals originating from the same 5D4 level do not disturb the temperature dependence of the target signal in single-channel luminescence detection. In this case, any down-shifting luminescence band can be chosen as the detected signal, which increases the diversity of detection choices in order to avoid the wavelength range that background noises cover. Additionally, this feature can optimize the single-luminescence LIR thermometry by detecting the total intensity of all down-shifting luminescence bands (ADLBs). The total intensity of ADLBs can be collected to replace the single 5D4-7F5 luminescence band as the detected signal. By monitoring ADLBs intensity, this method does not have the spectral resolution demand induced by the complex energy level structures of RE ions [28–33], further reducing the measurement errors caused by adjacent transitions. ADLBs intensity is easy to detect by only using a filter to block excitation lights, which can largely simplify the temperature measurement approach and increase flexibility, suggesting its engineering application prospect.
Fig. 5. (a, b) Emission spectra of CaWO4: Tb3+ at 493 K, 573 K, and 653 K upon 379 nm and 413 nm excitations, respectively. (c, d) The temperature dependences of normalized integrated luminescence intensities at 545 nm, 585 nm, and 620 nm upon 379 nm and 413 nm excitations, respectively.
5. Conclusions
In conclusion, a LIR thermometry based on combined ground and excited states absorption processes of CaWO4:Tb3+is developed in this work. This temperature measurement method uses two-wavelength excitations at 379 nm (GSA) and 413 nm (ESA), and monitors the 5D4-7F5 transition emission of Tb3+ ion. The relationship of $\textrm{LI}{\textrm{R}_{\textrm{ESA}/\textrm{GSA}}}$ vs. temperature meets the Boltzmann type function, demonstrating the thermal coupling of 7F6 and 7F5 states. This mechanism is the nature of the opposite temperature dependences of GSA and ESA processes. As temperature increases, the stronger non-radiation relaxation reduces the luminescence intensity upon 379 nm excitation, while the increased population of the 7F5 excited state enhances the luminescence intensity upon 413 nm excitation. This feature of CaWO4: Tb3+ allows us to measure temperature by using the temperature-dependent $\textrm{LI}{\textrm{R}_{\textrm{ESA}/\textrm{GSA}}}$. Moreover, a high ${S_\textrm{r}}$ of 2.8%K-1 at 333 K is achieved, attributed to the large $\Delta E$ between the 7F6 and 7F5 energy levels. In addition, based on the same temperature dependences of different down-shifting luminescence bands, and the ADLBs-based single-channel luminescence detection LIR thermometry is proposed. The designed LIR thermometry can effectively improve reliability of LIR thermometry and realize the characteristics of high sensitivity and good measurement flexibility, being a promising technique for accurate temperature sensing.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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