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Abstract
Fluorescence intensity ratio (FIR) temperature sensors provide an effective method to control or study fine variations in physical and biological research because of their high sensitivity, accuracy, and spatial resolution. However, it is difficult to maintain high sensitivity over a wide temperature range using FIR temperature sensors because of the limits of the Boltzmann distribution law. In this study, sensitivity amplification for a wide temperature range in FIR thermometry based on GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles is achieved. The mechanism of the non-monotonic temperature dependence of the relative sensitivity (Sr) is studied. The results demonstrate that the Sr stably keeps ∼2.4% per K over a wide temperature range of 303–753 K, thus providing a basis for the extensive application of FIR temperature sensors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Temperature is one of the most fundamental parameters in the fields of physical, chemical, and biological research [1–4]. As one type of optical temperature sensing method, fluorescence intensity ratio (FIR) thermometry provides a high precision and temperature sensitivity and has, therefore, attracted considerable interest [5–8]. Increasing the sensitivity and accuracy and expanding the temperature range of FIR sensors has become a research challenge in recent years [9–12].
Mostly, FIR thermometry is based on the ratio of fluorescence intensity originating from a pair of thermally coupled energy levels (TCELs) of rare earth (RE) ions. In this situation, the temperature dependence of FIR follows the Boltzmann law, and the FIR is expressed as FIR = Aexp(-ΔE/kBT), where A is a constant, ΔE is the energy gap, kB is the Boltzmann constant, and T is the absolute temperature [13–16]. Sensitivity is a crucial parameter in sensing applications, which determines the rate of change between the FIR and the temperature variation. The relative sensitivity (Sr) is defined as Sr = (1/FIR)(∂FIR/∂T); thus, Sr now becomes Sr = ΔE/kBT2 [17,18]. In this case, the Sr of the FIR decreases significantly as the temperature increases (because of the T−2 term), which limits the temperature measurement performance at high temperatures [19,20]. Recently, a new thermometric strategy was developed to enhance Sr at high temperatures, as reported in our previous work [21]. This approach produced an Sr of 1.4% per K at a high temperature of 633 K, using GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles [21]. However, the sensitivity first increases and then decreases over the measured temperature range. Unstable sensitivity prevents the accuracy of the FIR sensors from being improved over a wide temperature range. Thus, achieving both stable and high sensitivity over a wide temperature range is the key to enhancing the performance of FIR temperature sensors.
In this study, an optimized FIR thermometric strategy, based on the hybrid particles GdVO4:Eu3+ and Al2O3:Cr3+, was designed. This method successfully achieves stable and high sensitivity over a wide temperature range of 303–753 K. The mechanism of the non-monotonic temperature dependence of Sr is examined, and it is demonstrated that both the temperature and excitation wavelength determine the sensitivity. The defined mono-sensitivity (κ) reveals the relationship between the Sr of hybrid particles and each luminous material constituent of the hybrid particles. By controlling the excitation wavelength, Sr remains approximately 2.4% per K over the entire temperature range, which is valuable for highly accurate FIR temperature sensors.
2. Design of the FIR thermometry sensitivity
To maintain the high sensitivity of the FIR temperature measurement over a wide temperature range, firstly, it is necessary to study the influencing factors and changing laws of the Sr. For hybrid particles, if the Sr between the hybrid particles and constituent materials can be established, then a method to increase the sensitivity can be achieved. Thus, a method to achieve a high Sr of FIR thermometry based on hybrid particles is proposed.
In principle, FIR is the ratio of the two luminescence intensities, I1 and I2, and can be expressed as
where I1 and I2 represent the intensities of the luminous materials that constitute the hybrid materials. Furthermore, Sr can be written as3. Materials and experiments
GdVO4:Eu3+ was selected as one of the components of the hybrid particles because of its highly efficient red-light-emitting material characteristics and enhanced fluorescence intensity with increasing temperature [22–24]. Al2O3:Cr3+ was chosen as another component because both Cr3+ and Eu3+ ions strongly absorb UV radiation, which means that GdVO4:Eu3+ and Al2O3:Cr3+ can be excited at the same wavelength. Moreover, the fluorescence peak of Cr3+ is very narrow, and it cannot overlap with the fluorescence bands of Eu3+ ions [25]. As a result, we chose to use the hybrid GdVO4:Eu3+ and Al2O3:Cr3+ particles as the temperature sensitive material for the FIR temperature sensor. GdVO4:2%Eu3+ and Al2O3:2%Cr3+ particles were prepared via the high-temperature solid-state method [21]. Then, GdVO4:Eu3+ and Al2O3:Cr3+ particles were fully ground and mixed in a weight ratio of 1:1. Without a second sintering process, the mixed particles were pressed into a disk that yielded a hybrid sample directly.
The crystal phases of GdVO4:Eu3+ and Al2O3:Cr3+ were characterized using a PANalytical X’Pert Powder X-ray diffractometer (CuKα radiation, λ = 1.5406 Å). The morphology of the hybrid particle sample was observed using a Hitachi S-4300 scanning electron microscope. During the fluorescence spectrum measurement, the fluorescence spectra of both GdVO4:Eu3+ and Al2O3:Cr3+ were obtained under a 405-nm laser (Thorlabs) excitation at a pump power of 7 mW/cm2. Furthermore, the fluorescence spectra were measured using a grating spectrometer (Zolix, Omni-λ300), and a photomultiplier (Zolix, PMTH-S1-CR131) was employed as the detector. To study the influence of excitation wavelengths on sensitivity, excitation spectra were detected under a xenon light source. A grating spectrometer was used to separate the multi-wavelength light from the Xe light source, and a quartz lens was used to focus the monochromatic light to improve the excitation efficiency. The excitation spectra were obtained for a temperature range of 303–753 K by monitoring the 5D0–7F2 emission of Eu3+ and the 2E–4A2 emission of Cr3+. For all spectra measurements, the temperature was controlled by a heating stage with an accuracy of ± 0.2 K.
4. Results and discussions
The X-ray diffraction patterns of GdVO4:Eu3+ and Al2O3:Cr3+ are shown in Figs. 1(a) and 1(b), respectively. The positions of the standard patterns with numbers 86-0996 and 10-0173 from the JCPDS (top panel) are in good agreement with those of GdVO4:Eu3+ and Al2O3:Cr3+ particles (bottom panel), respectively. This demonstrates that the crystal phases of GdVO4:Eu3+ and Al2O3:Cr3+ are tetragonal and hexagonal, respectively. The morphology of the hybrid particles is presented in Fig. 1(c), indicating that the size of these particles is at the submicron scale. These results show that the hybrid GdVO4:Eu3+ and Al2O3:Cr3+ particles were well prepared.
Fig. 1. (a, b) X-ray diffraction patterns of GdVO4:Eu3+ and Al2O3:Cr3+ particles, respectively. Red lines are the standard patterns from the JCPDS, and blue lines are the measured patterns. (c) Scanning electron microscopy image of hybrid GdVO4:Eu3+ and Al2O3:Cr3+ particles.
Because hybrid particles are composed of two kinds of luminescent materials, studying their respective fluorescence characteristics is valuable for revealing the mechanism responsible for the superior characteristics of hybrid GdVO4:Eu3+ and Al2O3:Cr3+ particles. Because the two emitting centers, Eu3+ and Cr3+, of hybrid particles need to be excited using the same light source, the fluorescence properties and excitation spectrum range of GdVO4:Eu3+ and Al2O3:Cr3+ were investigated. The emission and excitation spectra of GdVO4:Eu3+ and Al2O3:Cr3+ were measured at 303 K, as shown in Figs. 2(a) and 2(b), respectively. The emission spectra of both GdVO4:Eu3+ and Al2O3:Cr3+ were measured using a 405-nm excitation. As illustrated in Fig. 2(c), which shows the simplified down-conversion process, when pumped by a 405 nm laser, Eu3+ absorbs near-ultraviolet radiation from the ground state 7F0 to 5D3, and then relaxes to 5D0 via a non-radiative relaxation process. In addition, a 618-nm photoluminescence is caused by the transition of 5D0 to 7F2 in GdVO4:Eu3+, as shown in Fig. 2(a). Moreover, Cr3+ was selected as another hybrid particle luminescence center owing to its high fluorescence efficiency. The 2E–4A2 transition of Cr3+-doped Al2O3 occurs around 693 nm, as depicted in Fig. 2(b). The two fluorescence bands of Eu3+ and Cr3+ are distant enough from each other so that the overlap between the two fluorescence emissions is avoided, which is beneficial for reducing the temperature measurement error.
Fig. 2. Emission and excitation spectra of (a) GdVO4:Eu3+ and (b) Al2O3:Cr3+particles at 303 K, respectively; (c) Simplified energy level diagram of Eu3+ and Cr3+.
In addition, both Eu3+ and Cr3+ can be excited by UV and near UV light, meaning that their excitation spectra overlap. Hence, there are various choices of excitation light wavelengths. Here, excitation spectra are studied in order to select the appropriate excitation lights. The excitation spectra of GdVO4:Eu3+ and Al2O3:Cr3+ were detected by monitoring the 5D0–7F2 and 2E–4A2 transitions, as shown in Fig. 2(a) and (b). Interestingly, a charge transfer band (CTB) of GdVO4:Eu3+ particles exists during the transition of VO43- to Eu3+ at approximately 325 nm [26–28]. The CTB of GdVO4:Eu3+ results in a strong broadband absorption of UV radiation, which is much stronger than the absorption peaks originating from the excited state of the Eu3+ ion at approximately 400 nm. Thus, the GdVO4:Eu3+ sample suggests more efficient luminescence emission under broadband excitation wavelengths of 225–400 nm. In view of the excitation spectrum of Al2O3:Cr3+, Cr3+ ions can be excited by light sources in the 325–475 nm range. Therefore, the hybrid particles can be effectively excited in a wide range of 350–400 nm.
In the excitation wavelength range, if the change in excitation wavelength has no effect on the κ of GdVO4:Eu3+ and Al2O3:Cr3+, then according to Eq. (4), the Sr of the hybrid particles at a specific temperature should be a constant that cannot be artificially controlled. Therefore, the variability of κ as a function of the excitation wavelength becomes the key to obtaining a stable and high sensitivity over a wide temperature range. In order to determine the influence of temperature and excitation wavelength on the sensitivity simultaneously, variable temperature excitation spectra are measured and analyzed. Figure 3(a) shows the temperature dependence of the excitation spectra of GdVO4:Eu3+ obtained by monitoring the fluorescence intensity at 618 nm. The broad excitation band around 330 nm at 473 K is attributed to the CTB of VO43-. The intensity of the CTB is significantly stronger than the intensity of the excitation band of Eu3+ derived from the 7F0–5DJ multiplet transitions at approximately 397 nm. As the temperature increases, the CTB obviously broadens and shifts towards longer wavelengths (lower energies) [22–24], gradually covering the excitation peak of Eu3+ at approximately 397 nm and enhancing the fluorescence intensity excited with a 405-nm laser, as illustrated in Fig. 3(c). The red shift of the CTB that occurs with increasing temperature, accompanied by band broadening, can achieve various trends of fluorescence changes with different excitation wavelengths.
Fig. 3. (a) Excitation spectra of GdVO4: Eu3+ particles at 473, 573 K, and 673 K; (b) Excitation spectra of Al2O3:Cr3+ particles at 313, 453, and 613 K; (c) Temperature dependence from 303 K to 753 K for emission bands of GdVO4: Eu3+ at 618 nm and Al2O3:Cr3+ at 693 nm.
Figure 3(b) shows the temperature dependence of the excitation spectra of Al2O3:Cr3+, which can be excited from 350 nm to 450 nm in the measurement range. As a transition metal ion, the outer electronic configuration of Cr3+ is 3d3, which is easily modulated by the lattice structure. Therefore, the fluorescence emission can be strongly influenced by temperature due to the drastic lattice vibrations caused by changing environmental factors. Thus, the large nonradiative transition results in a rapid decrease in the fluorescence intensity with increasing temperature, as shown in Fig. 3(c). Figure 3(c) shows that the fluorescence intensities of GdVO4:Eu3+ and Al2O3:Cr3+ change in opposite directions as the temperature increases, meaning that their mono-sensitivities are opposite. Therefore, as described in Eq. (4), the Sr of the hybrid particles can be enhanced. Remarkably because the CTB of GdVO4:Eu3+ is introduced, the κ of GdVO4:Eu3+ shows different temperature dependences for different excitation wavelengths. Moreover, the broadening and red shifting of the CTB increase the overlap of the excitation spectra bands between GdVO4:Eu3+ and Al2O3:Cr3+. Therefore, the excitation wavelengths that can be selected become more abundant. As mentioned above, the Sr of the hybrid particles is shown to be related to both the temperature and excitation wavelength. Hence, by modulating the excitation wavelength, a stable and high temperature measurement sensitivity can be achieved using these hybrid particles.
We further systematically studied the temperature and excitation wavelength dependences of κ, providing support for selecting a suitable excitation wavelength to achieve a highly sensitive temperature measurement over a wide temperature range. Excitation spectra were measured at intervals of 5 K in the temperature range of 303–813 K. For each excitation wavelength in the measurements, κ is derived by the derivation of ∂I/∂T, as depicted in Eq. (3). The temperature and excitation wavelength dependences of the mono-sensitivity κ1 GdVO4:Eu3+ and κ2 of Al2O3:Cr3+ particles are shown in Fig. 4(a) and (b), respectively. As shown in Fig. 4(a) and (b), a positive κ1 and negative κ2 can be obtained over the entire temperature range of 303–753 K, which is consistent with the previous theoretical design. The contour line in Fig. 4 indicates that a certain κ can exist in a continuous temperature interval with various excitation wavelengths. The maximum absolute values of κ1 and κ2 can reach 2.3% per K for the best excitation wavelength ranges of 325–450 nm and 350–500 nm, respectively. Generally, the fluorescence intensities of a majority of FIR thermometry decreases with increasing temperature, which results in both values of κ being negative [29–31]. For GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles, FIR no longer follows the Boltzmann distribution, and Sr in the hybrid particles should be recalculated using Eq. (4). Because the hybrid particles exhibit opposite values for κ1 and κ2, Sr can be regarded as Sr=|κ1|+|κ2|. Therefore, Sr is notably enhanced and not limited by high-temperature conditions, which makes it possible to maintain stable sensitivities over a wide temperature range.
Fig. 4. Contour map of mono-sensitivity (a) κ1 of GdVO4:Eu3+ and (b) κ2 of Al2O3:Cr3+ by monitoring at 618 nm and 693 nm, respectively; the horizontal axis represents the excitation wavelength and the vertical axis represents temperature.
Derived from Eq. (4), the temperature and excitation wavelength dependence of the Sr of the hybrid particles are shown in Fig. 5(a). The maximum Sr reaches 2.6% per K, which corresponds to an ultra-sensitive advantage. Moreover, it is obvious that the Sr value is larger than 1.5% per K over the entire temperature range detected within the 375–475 nm excitation wavelengths. Figure 5(b) shows the maximum Sr of the hybrid particles at each temperature by controlling the excitation wavelengths. With the advantage of changing the excitation wavelength, Sr exhibits a non-monotonic temperature dependence, and oscillates around 2.4% per K over the entire temperature range of 303–753 K. Thus, this FIR thermometry based on hybrid particles not only achieves high sensitivity at high temperatures, but also maintains the good stability of Sr. The hybrid GdVO4:Eu3+ and Al2O3:Cr3+ particles provide the FIR thermometry with high and stable sensitivity over a wide temperature range of 303–753 K, and this represents an excellent advantage in the high-temperature sensing range above 500 K that is used in fluorescence measurement techniques.
Fig. 5. (a) 3D surface with profile lines of relative sensitivity Sr of GdVO4:Eu3+ and Al2O3:Cr3+hybrid particles; (b) Maximum relative sensitivity from 303 K to 753 K corresponding to different excited wavelengths.
5. Conclusions
In this study, a highly sensitive and stable FIR thermometry based on GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles was achieved over a wide temperature range. Sr exhibits a non-monotonic temperature dependence and stably maintains approximately 2.4% per K over the range of 303–753 K. Furthermore, the effect of the excitation wavelengths and temperature on Sr was revealed. The CTB of GdVO4:Eu3+ particles broadens and shifts with increasing temperature, which makes the sensitivity affected by the excitation wavelength factor. Thus, the Sr of the hybrid particles can maintain a high value by changing the excitation wavelengths. Our findings indicate the sensitive detection characteristics of the GdVO4:Eu3+ and Al2O3:Cr3+ hybrid particles, which play an important role in improving the accuracy of fluorescence thermometry and provides a novel path for the design and optimization of FIR temperature sensors based on hybrid particle materials.
Funding
National Natural Science Foundation of China (61505045, 81571720).
Disclosures
The authors declare no conflicts of interest.
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