1. Introduction

The measurement of temperature is crucial in countless scientific investigations and industrial productions. Among all the methods for determining temperature, considerable interests have focused on luminescence-based measurements because they can offer excellent accuracy, resolution, repeatability, and stability [1,2]. Optical temperature sensors based on the fluorescence intensity ratio (FIR) technique takes advantage of the temperature-dependent fluorescence intensities of two thermally coupled emitting levels of rare earth ions (RE³⁺) and have garnered much attention in recent years due to their potential applications in electrical power stations, oil refineries, coal mines, and biocompatible temperature probe, etc [3–6]. Compared with other temperature measuring methods, luminescent thermometry technique can improve the sensitivity and reduce the dependence of measurement condition, such as fluorescence loss and electromagnetic compatibility problems. More importantly, by using the FIR method, researchers have acquired the temperature of a single living cell [7].

RE³⁺ doped upconversion luminescence (UCL) is of importance in lasers, solar cells, three-dimensional displays, and luminescent probes in microscopy owing to the unique optical properties arising from the intra 4f transitions of RE³⁺. Among all the RE³⁺, Er³⁺ is an ideal candidate for UCL since its rich and ladder-like electronic energy level structure. By comparing the emission intensities from thermally coupled electronic levels of ²H_11/2 and ⁴S_3/2, Er³⁺ doped UCL materials have been widely investigated as optical thermometry over the past few years [8–10]. Moreover, in 2012, Zhang and associates reported that the upconversion emissions from the ⁴G_11/2 → ⁴I_15/2 (384 nm) and ²H_9/2 → ⁴I_15/2 (408 nm) transitions of Er³⁺ could be used for measuring temperature as well [11]. Er³⁺ ions have rich energy level structures in UV region. In our previous studies, UCLs of Er³⁺ in the range from 240 nm to 350 nm were obtained with NIR (980 nm or 1560 nm) laser excitation [12,13]. However, until now, no results about optical thermometry have been reported mainly concerning the UCLs of Er³⁺ in the UV region.

In this letter, we presented an observation of temperature sensor based on the UV UC emissions of Er³⁺. Upon excitation with NIR radiation of 980 nm, UV UC emissions of Er³⁺ in the region of 240 − 350 nm were observed in hexagonal phase sodium lutetium fluoride (NaLuF₄) nanocrystals. The emission intensity ratios of 256 nm to 276 nm of Er³⁺ were investigated systematically by changing the temperature of the sample. The ⁴D_7/2 and ⁴G_9/2 states of Er³⁺ were verified to be thermally coupled levels. Especially, this UV-based sensor of Er³⁺ showed a higher sensitivity than the corresponding green-based nanothermometer from room temperature to 330 K, which demonstrated their great potential for application as high-performance optical thermometry.

2. Experimental

Yb³⁺-Er³⁺ codoped β-NaLuF₄ sample was synthesized via the high-temperature thermal decomposition method according to the procedure described in Ref [14]. X-ray powder diffraction (XRD) analysis revealed that the sample was hexagonal phase, all diffraction peaks can be indexed as the β-NaLuF₄ (JCPDS No. 27−0726). The corresponding morphological analysis with transmission electron microscope (TEM, Hitachi H-600) showed that the nanocrystals were uniform nanospheres with the particle size of about 50 nm. The UCL spectra in the UV region were recorded by a spectrophotometer (Hitachi F-4500) which equipped with a power-controllable 980 nm CW diode laser as the excitation source. The temperature of the sample was controlled by using a set of home-made equipment.

3. Results and discussion

Figure 1(a) shows the UCL spectra of NaLuF₄:Yb³⁺, Er³⁺ nanocrystals at room temperature with the laser excitation of 980 nm. Apart from the well-known green and red upconversion emissions of Er³⁺, the sample emitted UV UC fluorescence in the region of 230 − 350 nm, as shown in Fig. 1(a). The emissions that peaked at 244 nm, 256 nm, 276 nm, 288 nm, 305 nm, 317 nm, and 355 nm were originated from the ²I_11/2 → ⁴I_15/2, ⁴D_7/2 → ⁴I_15/2, ⁴G_9/2 → ⁴I_15/2, ²D_5/2 → ⁴I_15/2, ²K_13/2 → ⁴I_15/2, ²P_3/2 → ⁴I_15/2, and ⁴D_7/2 → ⁴I_11/2 transitions of Er³⁺ ions, respectively [13,15]. In addition, Fig. 1(b) depicts the double logarithmic plots of the emission intensity I_f as a function of pump power I_IR. For an unsaturated UC process, I_f ∝ I_IRⁿ, where n is the number of IR photons absorbed per upconverted photon emitted [16]. Under 980 nm excitation, both the n values were around 5 (4.87 ± 0.09 and 4.69 ± 0.07) for the UV UC emissions of 256 nm and 276 nm, which indicated that the ⁴D_7/2 → ⁴I_15/2 and ⁴G_9/2 → ⁴I_15/2 transitions of Er³⁺ in the Yb³⁺-Er³⁺ codoped NaLuF₄ nanocrystals came from five-photon UC processes.

 

Fig. 1 (a) UC emission spectra of NaLuF₄:Yb³⁺/Er³⁺ nanocrystals in the range of 230 − 350 nm; (b) the log-log plots of emission intensity versus excitation power for ⁴D_7/2 → ⁴I_15/2 and ⁴G_9/2 → ⁴I_15/2 transitions of Er³⁺.

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To further recognize the UC mechanisms in the sample, possible UC processes are schematically given in the energy level diagrams of Yb³⁺ and Er³⁺, as shown in Fig. 2. In Yb³⁺-Er³⁺ codoped systems, both the energy transfer (ET) from Yb³⁺ ions to Er³⁺ and the nonradiative relaxation (NR) from excited Er³⁺ are involved and played important roles in populating the high-energy states of Er³⁺. The possible UC population processes for the UV levels of Er³⁺ can be clearly described as follow: ⁴I_15/2 $\stackrel{\text{ET}}{\to }$ ⁴I_11/2 $\stackrel{\text{ET}}{\to }$ ⁴F_7/2 $\stackrel{\text{NR}}{\to }$ ²H_11/2, ⁴S_3/2 $\stackrel{\text{ET}}{\to }$ ²G_7/2 $\stackrel{\text{NR}}{\to }$ ⁴G_11/2, ²H_9/2 $\stackrel{\text{ET}}{\to }$ ²D_5/2 $\stackrel{\text{NR}}{\to }$ ²K_13/2 $\stackrel{\text{ET}}{\to }$ ²I_11/2 $\stackrel{\text{NR}}{\to }$ ⁴D_7/2, ⁴G_9/2.

 

Fig. 2 Energy level diagrams of Yb³⁺ and Er³⁺ ions, and possible UC processes.

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To verify that the ⁴D_7/2 and ⁴G_9/2 levels were thermally coupled, the 256 nm and 276 nm UCLs of Er³⁺ were studied by changing the sample temperature from 303 K to 523 K. Figure 3 shows the normalized UV UCL spectra of the NaLuF₄:Yb³⁺, Er³⁺ nanocrystals in the wavelength range from 250 nm to 284 nm. They were recorded at 303, 398, and 503 K, respectively, and normalized at 256 nm. There was nearly no overlap of two UV UC emission bands originating from the ⁴D_7/2 → ⁴I_15/2 and ⁴G_9/2 → ⁴I_15/2 transitions of Er³⁺, which was in favor of the measurement accuracy of their emission intensities. In addition, it is obvious that the relative intensity ratio of I₂₅₆/I₂₇₆ increased dramatically with the increase of sample temperature. This is because the electronic states of ⁴D_7/2 and ⁴G_9/2 of Er³⁺ are thermally coupled. Similar to the thermally coupled levels of ²H_11/2 and ⁴S_3/2 levels of Er³⁺, the FIR of the emissions from the ⁴D_7/2 → ⁴I_15/2 and the ⁴G_9/2 → ⁴I_15/2 transitions are in accord with Boltzmann distribution, which could be written as:

 

Fig. 3 Temperature-dependent UV UCLs of Er³⁺ in Yb³⁺/Er³⁺ codoped NaLuF₄ nanocrystals. All spectra have been normalized to the emission intensities from the ⁴D_7/2 → ⁴I_15/2 transition of Er³⁺ ions.

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The detailed changes of UV UCLs (256 nm and 276 nm) of Er³⁺ with temperature change from 303 K to 523 K were investigated, as illustrated in Fig. 4(a), where the fluorescence intensities have been normalized at 256 nm as well. Upon increasing the sample temperature, we found that the relative emission intensity of I₂₇₆ decreased gradually. Moreover, the appropriate energy separation between the ⁴D_7/2 and ⁴G_9/2 allows the upper (⁴D_7/2) level to be populated by thermal excitation from the excited ⁴G_9/2 level. Figure 4(b) shows a monolog plot of the fluorescence intensity ratio of UV emissions of Er³⁺ as a function of inverse absolute temperature in the range of 303 − 523 K. The experimental data could be fitted by a straight line with the slope of about 384.

 

Fig. 4 Temperature sensing based on the UV UCLs of Er³⁺ ions. (a) UV UCLs of Er³⁺ from ⁴D_7/2 and ⁴G_9/2 levels at different temperature; (b) monolog plot of the FIR as a function of inverse absolute temperature; (c) FIR relative to the temperature; (d) sensor sensitivity as a function of the temperature in the range from 303 K to 523 K.

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Furthermore, the relationship between the FIR of UV emissions from the ⁴D_7/2/⁴G_9/2 → ⁴I_15/2 transitions of Er³⁺ and the temperature in the region of 303 − 523 K is shown in Fig. 4(c). The value of FIR increased from 1.17 to 2.06 with increase the temperature from 303 K to 523 K. By fitting the experimental data according to equation as described above, the obtained coefficient C and the energy gap ΔE₁₂ were about 4.448 and 384 cm⁻¹, respectively. To better understand the temperature sensing performance of this UV-based nanothermometer, it is necessary for us to investigate its sensor sensitivity S. The value of S is determined by the rate of FIR varies with temperature and it can be defined as:

It is then very important to compare the sensitivities of this UV-based optical thermometer with that of green-based temperature sensor of Er³⁺. Figure 5(a) displays the temperature-dependent green UCLs of Er³⁺ under the excitation of 980 nm. The corresponding sensitivity curve was depicted in Fig. 5(b) (black solid line). Obviously, the S value for the green-based thermometer keeps increasing in the experimental temperature range (303 – 503 K) and reaches the maximum of 0.0073 K⁻¹ at the temperature of 503 K. For comparison, the sensor sensitivities based on the green UC emissions and UV UC emissions of Er³⁺ are also exhibited, as shown in Fig. 5(b). With increasing the experimental temperature, the changes of sensitivities of these two (UV-based and green-based) sensors presented completely opposite trends. In particular, the sensitivities of UV-based sensor are prior to those of the green-based optical thermometer in low temperature range (from room temperature to 330 K). Therefore, the coupled UV emissions of Er³⁺ is more suitable to be applied in low temperature measurement than its green thermal coupled levels, and this property made β-NaLuF₄:Yb³⁺, Er³⁺ nanoparticles better candidate for high-performance optical thermometry.

 

Fig. 5 (a) Temperature-dependent green UCLs of Er³⁺ in Yb³⁺/Er³⁺ codoped NaLuF₄ nanocrystals. All spectra have been normalized to the emission intensities from the ²H_11/2 → ⁴I_15/2 transition of Er³⁺ ions; (b) Temperature dependent sensor sensitivities deduced from the UV UCLs (⁴D_7/2/⁴G_9/2 → ⁴I_15/2) and green UC emissions (²H_11/2/⁴S_3/2 → ⁴I_15/2) of Er³⁺.

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4. Conclusion

In conclusion, an optical temperature sensor based on the shortest wavelength to date has been developed in Yb³⁺-Er³⁺ codoped β-NaLuF₄ nanocrystals. The neighboring ⁴D_7/2 and ⁴G_9/2 levels were verified to be thermally coupled levels and their population ratios were fitted well by the Boltzmann distributions. By using FIR technology, optical temperature sensing based on the thermal coupled levels of ⁴D_7/2 and ⁴G_9/2 was reported in this work for the first time. The obtained sensor sensitivity of this UV-based sensor is prior to that of the green-based optical thermometer in low temperature range (from room temperature to 330 K). This novel property made Er³⁺-based UV UCL materials promising platforms for high-performance optical thermometer.