Enhanced temperature sensing performance of Er³⁺, Yb³⁺: PLZT ceramic based on emissions of Stark sublevels

Abstract

In this study, the temperature sensing behaviors of emissions of Stark sublevels in Er³⁺/Yb³⁺ codoped lead lanthanum zirconate titanate (PLZT) transparent ceramic were examined. Based on the temperature-dependent upconversion fluorescence spectra excited by 980 nm, the emission intensities of the Stark sublevels ²H_{11/2(1, 2)}, ⁴S_{3/2(1, 2),} and ⁴F_{9/2(1, 2, 3)} were obtained upon increasing the temperature from 160 to 320 K. All the transitions present strong temperature dependence under the competitive effects of thermal excitation and nonradiative relaxation. The optical temperature sensing properties of ²H_11/2/⁴S_3/2(1), ²H_11/2/⁴S_3/2(2), ²H_11/2/⁴F_9/2(1), ²H_11/2/⁴F_9/2(2), and ²H_11/2/⁴F_9/2(3) were investigated using the fluorescence intensity ratio (FIR) technique. The largest absolute sensitivity S_a of 142.4 × 10^–4 K⁻¹ was obtained based on the FIRs of ²H_11/2/⁴F_9/2(1) at 320 K, which is approximately 26 times larger than that of traditionally used thermal coupled levels ²H_11/2/⁴S_3/2 in the considered sample. In contrast, the maximum relative sensitivity S_r was 2.21% K⁻¹ at 203 K. A comparison of these results with those of other Er³⁺/Yb³⁺ codoped materials reveals that Er³⁺/Yb³⁺: PLZT ceramics are a promising thermometer material at low temperatures. Applying FIRs based on the photoluminescence of Stark sublevels is a practical approach to achieving greater thermometric efficiency.

1 Introduction

Temperature measurements with high sensitivity and accuracy have become vital in many fields including daily life, industrial production, and scientific research [1,2,3]. In recent years, optical thermometry has attracted considerable interest owing to its noncontact characteristics, better accuracy, higher reliability, and broader applications as compared with traditional contact thermometers [4,5,6,7]. It can be used in harsh environments, such as those with high temperatures and pressure, and corrosive areas [8]. Among several optical thermometry methods, the fluorescence intensity ratio (FIR) based on the thermal coupled level transitions of rare-earth ions is believed to be the most promising and reliable for temperature measurements [9,10,11,12,13].

Upconversion luminescence materials codoped with Er³⁺ and Yb³⁺ ions have been studied [14,15,16]. As one of the most widely used activators, Er³⁺ ions have typical thermal coupled levels (TCLs) of ²H_11/2 and ⁴S_3/2, which relative emission intensity presents strong temperature dependence, and were applied to temperature sensing [17, 18]. By codoping Yb³⁺ ions as sensitizers, Er³⁺/Yb³⁺ doped materials can be effectively pumped by a commercial 980 nm laser diode and thus strengthen the intensity of upconversion luminescence [15]. This type of photoluminescence also has some other attractive advantages. In the previous published literatures [19], the absorption and scattering coefficients of human skin, subcutaneous adipose tissue and mucous in the wavelength range from 400 to 2000 nm have been studied. 980 nm have pretty low absorption and scattering coefficient, relative long tissue penetrate distance and non-biological toxicity, which make it possible to be used in biomedicine field. Some literature have reported the UC thermometer using FIR method for biomedical application [20,21,22]. For those broader application and attracted features, searching different optical temperature sensor materials can provide a more favorable opportunity to select different sensing functions according to the practical requirement.

The luminescence intensity and sensor sensitivity are essential for the practical application. Searching TCLs with larger energy gaps have been proposed for developing efficient methods with greater sensitivity [23]. In our previous work, the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels were shown to be thermal coupled multi-levels (TCMLs). The temperature sensitivity of FIR of ²H_11/2/⁴F_9/2 is approximately 4 times larger than that of ²H_11/2/⁴S_3/2 in Er³⁺/Yb³⁺ codoped PLZT ceramics [24]. To investigate the possibility of further enhancing the temperature sensitivity, we studied the thermal luminescence characteristics of Stark sublevels in the same sample. Stark sublevels can also be considered as TCLs for the small level gap. The emissions of small level gaps were studied as another valid strategy for improving sensitivity [25,26,27]. For instance, Liu reported the maximum absolute sensitivity of ²H_11/2 to ⁴S_3/2(1) levels as about onefold higher than that of traditionally used TCLs ²H_11/2/⁴S_3/2 in Er³⁺/Yb³⁺ codoped Ba₃Y₄O₉ [28]. In addition, the sensitivity based on FIRs of ²H_11/2 to ⁴S_3/2(1) or ²H_11/2 to ⁴S_3/2(2) is almost twice enhanced in Er³⁺/Yb³⁺ codoped Gd₂(WO₄)₃ [14]. As a traditional and widely studied material, PLZT transparent ceramic was selected as the host. The lower phonon energy (750 cm⁻¹) and higher refraction index (2.401) of PLZT can result in higher transition probabilities than other rare-earth-doped host like LiNbO₃ [29], and ceramics usually have better mechanical, thermal, and chemical resistances [29, 30].

In this study, the luminescence properties of the Stark sublevels of the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 levels in Er³⁺/Yb³⁺ codoped PLZT transparent ceramic were investigated based on the upconversion spectra under the 980 nm excitation, and the temperature-dependent emission intensities of the Stark sublevels were studied in the temperature range from 160 to 320 K. The FIRs of the Stark sublevels’ emissions were studied for the enhancement of temperature sensing sensitivity. Moreover, the results of this study are helpful for better understanding the temperature characteristics of the Stark sublevels’ relative intensities.

2 Experimental

The Er³⁺/Yb³⁺ codoped PLZT transparent ceramic was fabricated by the mixed-oxide method at Boston Applied Technologies Inc., USA. Then, a 3 mm × 3 mm × 2 mm sample was cut from the bulk ceramic and polished. This sample consists of 65 mol% lead zirconate plus 35 mol% lead titanate and 8 mol% lanthanum in the form of La₂O₃, i.e., PLZT(8/65/35), to which 0.5 mol% Er³⁺ ions in the form of Er₂O₃ and 2.5 mol% Yb³⁺ ions in the form of Yb₂O₃ had been added. The origins of components were PbO, ZrO₂, La₂O₃, TiO₂, Er₂O₃ and Yb₂O₃, respectively. PbO was purchased from Hammond Lead Products, with purity of 99.98%. All other oxides were purchased from Fisher Scientific have purity 99.85% or higher. A typical sintering process of the ceramic was published elsewhere [31]. By the inductively coupled plasma-atomic emission spectrometry (ICP-AES), the quantities of Er³⁺ and Yb³⁺ ions in the sample were determined to be 1.39 × 10²⁰ ions/cm³ and 6.70 × 10²⁰ ions/cm³, respectively. The upconversion spectrum and temperature fluorescence spectra were measured by a steady-state/lifetime spectrofluorometer (FLS 920, Edinburgh Instruments) excited by a 980 nm laser diode with emission slit size 0.5 nm, power 1000 mw, detection sensitivity (≤ 50 cps), integration time of 0.1 s and step of 0.5 nm, in here, the temperature of sample was real-time control by temperature control components of spectrofluorometer FLS 920 with 0.1 °C accuracy.

3 Results and discussion

The temperature-dependent upconversion spectra of Er³⁺/Yb³⁺: PLZT ceramic in the temperature range from 160 to 320 K were measured under 980 nm excitation (Fig. 1). The ²H_11/2 and ⁴S_3/2 levels are split into two sublevels ²H_11/2(1) and ²H_11/2(2) and ⁴S_3/2(1) and ⁴S_3/2(2), and the ⁴F_9/2 level is split into three sublevels: ⁴F_9/2(1), ⁴F_9/2(2), and ⁴F_9/2(3). Seven luminescence band peaks at 527 nm, 539 nm, 551 nm, 564 nm, 655 nm, 665 nm, and 680 nm were found according to the transitions from the ²H_11/2(1), ²H_11/2(2),⁴S_3/2(1), ⁴S_3/2(2), ⁴F_9/2(1), ⁴F_9/2(2), and ⁴F_9/2(3) levels, respectively, to the ground level ⁴I_15/2.

Fig. 1

Temperature-dependent upconversion spectra of the Er³⁺/Yb³⁺: PLZT ceramic excited by 980 nm in the 160–320 K temperature range

Figure 2 shows the energy level scheme and relevant radiative transitions in this material. PLZT(Pb_1-xLa_x(Zr_yTi_1–y)_1–xO₃) have the perovskite structure with the formation of ABO₃, in which La substitutes for Pb²⁺ in the A-site and the B-site vacancies are created for electric balance. When rare earth ion doped in PLZT, Re³⁺ ion (Er³⁺,Yb³⁺) will occupy the position of La³⁺ for their similar ionic radii and charge characters. Since in our sample, the concentration of Yb³⁺ is around five times larger than Er³⁺ ion and Yb³⁺ has a much larger absorption cross-section around 980 nm than that of Er³⁺ ion, which we have discussed in our previous work for the same sample, the 980 nm pumping is mainly absorbed by the Yb³⁺ ion [31]. Thus, the mechanism for the up-conversion emission can be explained as follows.

Fig. 2

Schematic energy level diagram and transitions mechanisms of Er³⁺/Yb³⁺: PLZT ceramic

After absorbing the energy of incident light, the Yb³⁺ ions can be excited from the ground level ²F_7/2 to upper levels ²F_5/2. Due to the close energy level of ²F_5/2 and ⁴I_11/2, the energy transfer process effectively occur between Yb³⁺ and Er³⁺ ion, that is ²F_5/2(Yb³⁺) + ⁴I_15/2 (Er³⁺) → ²F_7/2(Yb³⁺) + ⁴I_11/2 (Er³⁺). Meantime, the Er³⁺ ion in its ground level can also absorb the pumping energy and excited to⁴I_11/2 level. Then, the ions accumulate in Er³⁺:⁴I_11/2 level absorb other photon energy and excited to upper ⁴F_7/2 level. And Yb³⁺ ion can also absorb a second photon and transfer its energy to Er³⁺ ion by ²F_5/2(Yb³⁺) + ⁴I_11/2(Er³⁺) → ²F_7/2(Yb³⁺) + ⁴F_7/2(Er³⁺),which also contribute to the accumulation of ⁴F_7/2 level. Ions in ⁴F_7/2 level rapidly relax to the sublevels of ²H_11/2 and ⁴S_3/2 level, and then some of them radiatively relax to the ground level and generate green emission. The other ions relax to ⁴F_9/2 level. Besides, the accumulation of ⁴F_9/2 level also comes from the excited level ⁴I_13/2 photon absorption and the energy transfer process: ²F_5/2(Yb³⁺) + ⁴I_13/2(Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_9/2(Er³⁺). Finally, the ions in ⁴F_9/2 sublevels decay to the ground level and generate red emission. The cross relaxation between Er³⁺ ions (⁴I_11/2 + ⁴I_11/2 → ⁴I_15/2 + ⁴F_7/2, ⁴I_11/2 + ⁴I_13/2 → ⁴I_15/2 + ⁴F_9/2) also help for the population accumulation of ⁴F_7/2 and ⁴F_9/2 level, which can’t be neglected in the sample with Er³⁺ concentration larger than 0.5 mol% [32]. But since those cross relaxation often obviously take place in the more heavily doped materials [33], and the Er³⁺ ion concentration is pretty low in our sample, those cross relaxation may be less due to the competitive mechanism.

Figure 1 also provides the temperature-dependent properties of the upconversion emissions from 160 to 320 K. The positions of the emission peaks barely changed with the increase of temperature, while the intensities changed remarkably. To further investigate the influence of temperature on the luminescence intensity of each transition, the relative integrated intensities normalized to the intensities at 160 K were calculated (Fig. 3). Here, the intensities of ²H_{11/2 (1)} and ²H_{11/2 (2)} were summed to be 539 nm to reduce calculation errors due to weaker luminescence. Figure 3 shows that all the transitions show strong temperature dependence. With the increase of temperature, the fluorescence integrated intensity of 539 nm evidently increased, while that of I_s (the sum of intensities of 551 nm and 564 nm, ⁴S_{3/2(1, 2)} → ⁴I_15/2) and I_F (the sum of intensities of 655 nm, 665 nm, and 680 nm, ⁴F_{9/2(1, 2, 3)} → ⁴I_15/2) uniformly decreased from 160 to 320 K, as shown in the inset of Fig. 3. This was because of thermal excitation between the TCLs [24].

Fig. 3

Relative luminescence integrated intensities of the emissions of sublevels at different temperatures ranging from 160 to 320 K

In the green emission of I_s, the intensities of 551 nm and 564 nm have the same decreasing trend with temperature, and the ions in the ⁴S_3/2(1) and ⁴S_3/2(2) levels were thermally excited to the ²H_11/2 level simultaneously because of their similar small energy gap. However, in the red emission of I_F, the intensity of 665 nm abnormally increases at 240 K. The abnormal increase can be attributed to the competitive effects of thermal excitation and nonradiative relaxation. The ⁴F_9/2(2) level is laying in the gap of the ⁴F_9/2(1) and ⁴F_9/2(3) levels. With the increase of temperature, the population of the ⁴F_9/2(2) is benefited by the nonradiative relaxation of the ⁴F_9/2(1) level and thermal excitation of the ⁴F_9/2(3) level as well as expended by the nonradiative relaxation to ⁴F_9/2(3) and thermal excitation to ⁴F_9/2(1) level. The ⁴F_9/2(1) level with higher energy provides a thermal excitation barrier for transitions from the ⁴F_9/2(2) to the ⁴S_3/2 level. Therefore, the intensity of 665 nm increases from 160 to 240 K. With a further increase in temperature, the efficiency of thermal excitation increases, and the intensity of 665 nm decreases. Comparing the variation characteristics of intensities of 655, 665, and 680 nm from 240 to 320 K, thermal excitation was more dominant above 240 K, and their intensities show a similar decreasing trend with increasing temperature.

To elucidate the temperature sensing properties of TCMLs, the ratios between the emissions of the ⁴S_{3/2(1, 2)} and ⁴F_{9/2(1, 2, 3)} levels were examined (Fig. 4). The values of different ratios in Fig. 4 do not change above 240 K, indicating that the ions in the ⁴S_{3/2(1, 2)} and ⁴F_{9/2(1, 2, 3)} levels are thermally excited to the ²H_11/2 level simultaneously. For the intensities of 655 nm or 665 nm, the ratios change dramatically under 240 K, which is because of thermal excitation and nonradiative relaxation for lower temperatures. However, the ratios of 551/564, 551/680, and 564/680 remain for the entire experimental temperature range. The most important deduction is that for the lowest level and intermediate ladder level, the relative populations of ⁴S_{3/2(1, 2)} to ⁴F_{9/2(1, 2, 3)} should not follow the Boltzmann distribution, and the FIRs of the levels cannot be used for temperature sensing in this sample.

Fig. 4

a FIRs of the emissions of the sublevels of the ⁴S_3/2 level or ⁴F_9/2 levels, b FIRs of emissions the of sublevels of the ⁴S_3/2 level and ⁴F_9/2 levels

The experimental data of 539/551, 539/564, 539/655, 539/665, and 539/680 were obtained and depicted in Fig. 5. The FIRs of five pairs were fitted with $A{\text{exp}}\left( { - B/T} \right) + C$ [24]. Here, A is determined by the degeneracy, the emission cross-section, and the angular frequency of two levels, B is determined by $\Delta E/k_{{\text{B}}}$, and C is introduced to simplify the equation [34]. The fitting formula are listed in Table 1. The theoretical curves match the experimental results well. The values of B in Table 1 are similar and between the actual $\Delta E/k_{{\text{B}}}$ of ²H_11/2/⁴S_3/2 and ²H_11/2/⁴F_9/2. The intermediate ladder level ⁴S_3/2 can effectively reduce the transition interval limit of TCLs [24]. Meanwhile, the values of B are different owing to laser power fluctuations, the host matrix’s absorption, and the thermal effect of nonradiative relaxation [35].

Fig. 5

Temperature-dependent luminescent behaviors of 539/551, 539/564, 539/655, 539/665, and 539/680 as a function of temperature

Table 1 FIRs of five pairs fitted with the Boltzmann distribution

The absolute sensitivity S_a and relative sensitivity S_r of FIRs were obtained, as shown in Fig. 6. The S_a of the five pairs increase nonlinearly with increasing temperature, among which the S_a of 539/655 increases for the entire temperature range and are maximized at 320 K as 142. 4 × 10^–4 K⁻¹. Meanwhile, S_r of 539/655 is superior to those of other pairs, which is greater than 1.5% K⁻¹ in the temperature range from 170 to 290 K with maximum value of 2.21% K⁻¹ at 203 K. The maximum temperature sensitivities of five pairs are listed in Table 2. The FIRs of Stark sublevels significantly enhanced the absolute sensitivity, and the maximum S_a is approximately 26 times larger than that of traditionally used ²H_11/2/⁴S_3/2 levels, which is also larger than that of other Er³⁺ doped hosts such as Al₂O₃, NaYF₄, YAG, Gd₂O₃, and CaMoO₄ [36,37,38,39,40,41,42,43]. The enhanced sensitivity is attributed to the energy gaps of corresponding thermal levels and the relative radiative transition probabilities [14]. The absolute sensitivity S_a of the Er³⁺/Yb³⁺: PLZT ceramic was compared with other Er³⁺/Yb³⁺ codoped materials based on the FIRs of Stark sublevels, as listed in Table 3. The maximum S_a of the Er³⁺/Yb³⁺: PLZT ceramics is higher than that of most Er³⁺/Yb³⁺ codoped phosphors. Thus, these Er³⁺/Yb³⁺: PLZT ceramics are promising materials for use as optical thermometers for temperatures higher than 160 K because of their enhanced temperature sensitivity.

Fig. 6

a Absolute temperature sensitivity S_a and b relative temperature sensitivity S_r of the FIRs of five pairs

Table 2 Temperature sensitivities of FIRs of the emissions of Er³⁺/Yb³⁺ codoped PLZT ceramic

Table 3 Maximum absolute temperature sensitivities of some Er³⁺/Yb³⁺ codoped hosts based on FIRs of Stark sublevels

4 Conclusion

The temperature-dependent emissions of the Stark sublevels of Er³⁺/Yb³⁺ codoped PLZT transparent ceramic were studied within 160–320 K based on upconversion fluorescence spectra under 980 nm excitation. Notably, ²H_11/2 and ⁴S_3/2 level are split into two sublevels as ²H_{11/2(1, 2)} and ⁴S_{3/2(1, 2)}, and ⁴F_9/2 level is split into three sublevels as ⁴F_{9/2(1, 2, 3)}. All the level transitions have strong temperature dependence. The temperature characteristics of the emissions of the Stark sublevels were studied, and we found that the relative intensities of the lower level ⁴F_9/2 and intermediate step level ⁴S_3/2 in TCMLs cannot be used for temperature sensing. Hence, the FIRs of 539/551, 539/564, 539/655, 539/665, and 539/680 were studied and theoretically fitted for temperature sensing. The maximum values of absolute sensitivity S_a and relative sensitivity S_r were obtained as 142.4 × 10^–4 K⁻¹ at 320 K and 2.21% K⁻¹ at 203 K, respectively. The S_a was enhanced 26 times, while the S_r is greater than 1.5% K⁻¹ in the temperature range from 170 to 290 K. A comparison with other Er³⁺/Yb³⁺ codoped materials shows that Er³⁺/Yb³⁺: PLZT ceramics are a promising thermometer material for temperatures greater than 160 K. Meanwhile, the application of FIRs based on the emissions of Stark sublevels might enhance temperature sensing performance.