Spectral-Kinetic Characterization of YF3: Eu3+ and YF3: (Eu3+, Nd3+) Nanoparticles for Optical Temperature Sensing

1. Introduction

Optical temperature sensing methods based on the use of inorganic phosphors have been intensively developed during the last decade [1]. In these methods, temperature reading is performed via analysis of the luminescence signal, which should be temperature-dependent. In turn, the temperature dependence of the luminescence signal should be known [2,3]. Among a huge variety of inorganic phosphors including oxides and quantum dots, rare-earth-doped fluoride nanoparticles pay a special role due to high chemical and mechanical stability, bright narrow luminescence peaks [4], and, in some cases low cytotoxicity [5,6]. Yttrium fluoride is considered very promising host-matrix due to low phonon energy (around 500 cm^-1), which leads to the decrease in the non-radiative transition probability. The water-based synthesis procedures are usually cheap, easy, and environmentally friendly. In addition, in this host a high down-conversion quantum yield for rare-earth (RE) ion pair was achieved [7]. The YF₃ matrix provides substitution of Y³⁺ ions by RE³⁺ ones without valence change or charge compensation. Finally, in our previous work [8], we developed a hypothesis, that thermal expansion of YF₃ also contributes in the temperature sensitivity of the RE spectral-kinetic characteristics. Thus, it is interesting to study another ion pair in this promising matrix.

In its turn, the choice of doping ion(s) is also a challenging task. Indeed, it depends on the application [4]. For medical applications including hyperthermia, the phosphors should operate in the so-called biological window, where the biological tissues are partially transparent [8,9]. In the case of in vitro studies, the excitation in this spectral range is also desirable because the operation in the biological window provides the lack of autofluorescence from cells. For industrial applications, for example, for temperature mapping of microcircuits such strict restrictions are not so significant [10]. However, very important characteristics of the optical temperature sensors are absolute (S_a) and relative (S_r) temperature sensitivities. These characteristics express the rate of change of the luminescence parameters with the temperature [1]. The higher rate provides higher sensitivity which leads to the more easiness and accuracy of temperature measurements. In the case of single-doped phosphors, the temperature sensitivity of spectral characteristics is commonly based on the presence of two thermally coupled electron levels sharing their electron populations according to the Boltzmann law [2,3]. The main disadvantage of these systems is relatively low temperature sensitivity depending on the energy gap between these two levels. Moreover, it is difficult to manipulate the energy gap due to the fact, that 4f electron shell is shielded by the 5s shell. To increase temperature sensitivity, double-doped phosphors can be utilized [2]. Indeed, there are more temperature-dependent processes, that can synergize increasing the temperature-dependence of spectral-kinetic characteristics. One of the most common and interesting way to increase the sensitivity is to analyze two emissions of heteronymous ions. However, these emissions should stem from two interacting energy levels. For example, in Tb³⁺, Eu³⁺:YF₃ phosphors there are two pairs of interacting energy levels: ⁵D₃ (Tb³⁺) - ⁵L₆ (Eu³⁺) and ⁵D₄ (Tb³⁺) - ⁵D₁ (Eu³⁺) under Tb³⁺ excitation (377 nm corresponding to the ⁷F₆ - ⁵D₃ absorption band of Tb³⁺). In this case, the temperature-dependent parameter is the luminescence intensity ratio between Tb³⁺: ⁵D₄→⁷F₅ transition (I₅₄₂) and the Eu³⁺: ⁵D₀→⁷F₄ one (I₆₉₀) [11]. The S_a was equal to 0.0013 in the 300 - 550 K temperature range. The efficiency of interaction between two levels rises with the temperature increase due to phonon-assisted nature of this interaction. This fact explains the temperature sensitivity of the above-mentioned system. In its turn, down-conversion optical temperature sensors based on Nd³⁺/Yb³⁺ [12,13], Pr³⁺/Yb³⁺, Er³⁺/Yb³⁺ [14] ion pair (where the first ion serves as a donor of energy) are also based on the same mechanism. In our previous work, we suggested, that for Nd³⁺/Yb³⁺:YF₃ nanoparticles the thermal expansion and, as a consequence, the decrease of distance between interacting ions also contribute in the temperature sensitivity of the spectral-kinetic characteristics [8,13]. After literature analysis, we concluded, that the Eu³⁺/Nd³⁺ system in capable of demonstrating notable temperature sensitivity under Eu³⁺ excitation [15]. Here, the interacting energy levels are ⁵D₃ (Eu³⁺) - ²P_1/2 (Nd³⁺) and ⁵D₀ (Eu³⁺) - ⁴G_5/2 (Nd³⁺) under Eu³⁺ excitation (λ_exc = 394 nm corresponding to the ⁷F₀ - ⁵L₆ absorption band of Eu³⁺). However, this system is significantly less studied compared to other ion pairs based on Eu³⁺ and Nd³⁺. The objective of this work was to make conclusion about the possible application of Eu³⁺:YF₃ and Eu³⁺, Nd³⁺:YF₃ nanoparticles in optical temperature sensing analyzing such characteristics as S_a and S_r. The tasks were:

• synthesis and physicochemical characterization of the samples (size, morphology, and the phase composition)
• spectral-kinetic characterization to choose optimal Eu³⁺ and Nd³⁺ concentrations
• spectral-kinetic characterization in order to understand the influence of the annealing procedure on spectral-kinetic characteristics
• The calculation of S_a and S_r.

2. Materials and Methods

The nanoparticles were synthesized via the co-precipitation method in distilled water with subsequent hydrothermal treatment [4,16]. The detailed synthesis procedure is described in our previous work devoted to rare-earth-doped YF₃ nanoparticles [8]. According to the work [17], the annealing of the obtained YF₃ nanoparticles at 400 °C in air does not lead to the formation of impurity phases hence we choice the annealing at 400 °C in air for 3 hours. The excitation of the nanoparticles was performed via LOTIS TII tunable laser LT-2211A (λ_ex (Eu³⁺) = 394 nm, (pulse duration and repetition were 10 ns and 10 Hz, respectively) (Belarus, Minsk). The spectra were recorded via StellarNet (CCD) spectrometer (Tampa, FL, USA). The kinetic characterizetion was carried out via monochromator connected with photomultiplier tube FEU-62 and digital oscilloscope Rhode&Schwartz with 1 GHz bandwidth (Munich, Germany). The phase composition of the samples was studied by means of X-ray diffraction (XRD) via Bruker D8 diffractometer with Cu K_α-radiation (Billerica, MA, USA). The XRD simulation was carried out using VESTA software [18]. The experiments were performed in the 10 - 320 K temperature range via so-called “cold finger” method. The temperature control was carried out via thermostatic cooler “CRYO industries” having LakeShore Model 325 (Westerville, OH, USA) temperature controller. The IR reflection measurements were carried out using a BrukerVertex80v Fourier spectrometerat near-normal (Θ≈15°) and oblique (Θ≈75°) light incidence on the sample at room temperature. The morphology of the samples was studied using Hitachi HT7700 Exalens transmission electron microscope (TEM)) with 100 kV accelerating voltage (TEM mode) (Tokyo, Japan). The average diameter of the nanoparticles was calculated from the 2D TEM images. Statistics were based on the analysis of 100 nanoparticles. To get the average diameter (D) of the nanoparticles, the area (in squire nanometers) of each nanoparticle from TEM image was equated to the area of a circle (π⋅D²/4), where π = 3.14, and D is the diameter. The obtained histogram was approximated via Lognornal function where ±1 standard deviation was determined.

There are three molar concentration values of Eu³⁺: 2.5, 5.0, and 7.5 %. The choice of these concentrations was based on the decision of obtaining the brightest Eu³⁺ luminescence. Indeed, at 1.0 mol.% the luminescence signal demonstrated low brightness, which is expected to be even lower after Nd³⁺ addition. In turn, the samples contained the above-mentioned concentrations demonstrated an opposite tendency. For the higher Eu³⁺ concentrations, the concentration quenching leads to the decrease of the Eu³⁺ luminescence.

We synthesized 14 samples. The list of them is presented in Table S1 of supplementary file. Briefly, there were three samples, (Eu³⁺: 2.5, 5.0, and 7.5 %):YF₃. Then, each sample was divided into two equal parts. One part was annealed and the second was not. Based on the obtained spectral-kinetic data, we selected four samples (Eu³⁺: 2.5 and 5.0%):YF₃ (annealed and not annealed). For these samples, we took several combinations of Eu³⁺/Nd³⁺ and selected the most convenient for further study.

3. Results and Discussion

3.1. Physicochemical Characterization of the Nanoparticles

The phase composition of the YF₃ doped particles was confirmed via XRD. In particular, the normalized XRD patterns of YF₃: Eu³⁺ (2.5 mol.%) nanoparticles before and after annealing (400 °C, 4 hours) and the YF₃ simulation are presented in Figure 1.

The XRD patterns of both samples located on the one plot are presented in Figure S1 of supplementary file. The X-ray diffraction patterns are consistent with the simulation and the literature data and correspond to the orthorhombic structure of YF₃ [17,19] and to the number 074-0911 of the Inorganic Crystal Diffractions Database (ICDD) of orthorombic YF₃ (Pnma space group). The well-defined YF₃ peaks, the absence of impurity, and amorphous phases are clearly seen. It can be seen from Figure 1, that the sample after annealing has narrower diffraction peaks. The XRD peak narrowing can be related to many factors including the change in the size and the removal of the defects. To investigate the contribution of the size to XRD peak narrowing, we performed the TEM imaging of the samples. Transmission electron microscopy (TEM) images of the YF₃: Eu³⁺ (2.5 mol.%) nanoparticles before (a) and after (b) annealing in air (400 °C, 4 hours) are presented in Figure 2a,b, respectively.

It can be seen, that the annealing procedure does not affect the morphology of the nanoparticles. Specifically, both types of nanoparticles has primary oval shape. The size distribution histograms of YF₃: Eu³⁺ (1 mol.%) nanoparticles before and after annealing are represented in Figure 3a,b, respectively.

The size distribution histogram is not perfectly fitted by any peak function, probably, due to the non-spherical shape of the particles. However, the LogNormal approximation gives an estimation of the average size. The LogNormal fitting determined 139 ± 2 and 132 ± 3 nm average diameters before and after annealing, respectively. Anyway, the size of the particle is almost not changed. In addition, the diameter is larger than 15 nm, hence, the influence of the surface can be neglected [20]. Indeed, according to this work, the main unique difference between nanosized crystals and bulk ones is that the number of ions located on the surface of the nanoparticles and the number of ions located in the nanoparticle volume are comparable. The rare-earth ions located on the nanoparticle’s surface have different ligand surroundings compared to rare-earth ions inside the volume. The different surroundings lead to the different spectral-kinetic properties. However, according to the cited work, for rare-earth trifluorides, for nanoparticles larger than 15 nm, the surface ions do not make a serious contribution in the spectral-kinetic properties in opposite to volume ions and nanoparticles are more similar to bulk crystals in terms of spectral-kinetic properties. Since the size of the nanoparticles is almost not changed after the annealing, it can be suggested, that the XRD peak narrowing can be related to the removal of the defects (for instance water molecules captured during the synthesis procedure [16,21]) after annealing. To verify this assumption, infrared (IR) spectroscopy was performed (Figure 4).

The spectrum of the not annealed sample demonstrates a wide band in the 2800 - 3750 cm^-1 range. This peak corresponds to the stretching frequencies of the O–H groups of water molecules. The wide peak located between 1417 and 1800 cm^-1 is also explained by fluctuations in the bonds of organic groups arising from the fluorinating agent. It can be concluded, that the annealing procedure (400 °C, 4 hours in air) is effective for the removal of the molecules cantoning OH groups. Finally, it can be suggested, that the XRD peak narrowing can be related to the presence of such defects as captured water molecules. Indeed, the presence of additional impurities in the nanoparticle’s volume leads to the formation of microstrain (the fluctuations of the distances between the interatomic lattice spacing). Finally, it can be concluded, that the Eu³⁺:YF₃ nanoparticles have a desirable orthorhombic phase composition. The annealing procedure (400 °C, 4 hours) almost does not affect the diameter of the nanoparticles (139 ± 2 and 132 ± 3 nm before and after annealing, respectively) leading to the water removal from the nanoparticles.

3.2. Temperature-Dependent Spectral-Kinetic Characterization of Single-Doped YF₃: Eu³⁺

The energy level diagram of Eu³⁺/Nd³⁺ system is represented in Figure 5 (the Eu³⁺ - Nd³⁺ energy transfer will be discussed in the corresponding section). The optical excitation of Eu³⁺ is carried out at 394 nm (⁷F₀ - ⁵L₆ absorption band).

Further YF₃: Eu³⁺ (2.5; 5.0 and 7.5 mol.%) samples were synthesized and the spectral and kinetic characteristics for YF₃: Eu³⁺ (2.5 mol.%) annealed and after synthesis sample is shown in the Figure 6a,b, respectively.

It can be seen, that the shape of the spectra is independent of the annealing procedure. In turn, the luminescence decay curves have a one-exponential character. The luminescence decay rate decreases after annealing. We also associate this phenomenon with the partial elimination of such defects as water molecules, which were mentioned above. Thus, Eu³⁺ in the annealed samples has fewer channels of depopulating of excited states. The temperature evolution of the YF₃: Eu³⁺ (2.5 mol.%) luminescence spectra is presented in Figure 7.

It can be seen, that the spectrum shape in the 570 - 750 nm range is independent of temperature. This phenomenon can be explained by the lack of thermally coupled electron levels in the Eu³⁺ energy level structure. However, in the ~ 400 - 570 nm range, there is a broadband luminescence whose intensity rises with the increase of temperature. This luminescence was observed earlier in YF₃ matrix [12,17]. The broadband luminescence is associated with “rare-earth-fluoride vacancy” complex formation. This luminescence affects the intensity of Eu³⁺ peak at ~ 560 nm. For this reason, for kinetic characterization, we studied the luminescence peaks from 570 - 750 nm range. Figure 5 shows the luminescence decay time as function of temperature in the 80 - 320 K temperature range. The corresponding luminescence decay time curves are presented in Figure S2 of the supplementary file.

Figure 8 expresses the main tendency that the luminescence decay time linearly decreases with the increase in temperature. Usually, such a tendency is related to the increase in the probability of multiphonon relaxation with the increase of temperature. The same linear temperature dependence of luminescence decay time values for Pr³⁺ was observed in [22], which was also explained by multiphonon relaxation. However, the values of the slope for YF₃: Eu³⁺ nanoparticles are slightly higher compared to the above-mentioned Pr³⁺-based phosphors. As it was discussed above, the annealed nanoparticles demonstrate higher values of decay time compared to the nanoparticles without annealing. The luminescence decay time values decrease with the increase of Eu³⁺ concentration, which can be explained by the concentration quenching. The slope values are presented in Table 1.

After annealing, the slope values decrease with the increase of Eu³⁺ concentration. Probably, the Eu³⁺ content influences on the amount of luminescence quenchers, hence, the contribution of temperature-dependent multiphonon relaxation in the temperature sensitivity of decay time decreases. For not annealed YF₃: Eu³⁺ (5.0 and 7.5%) samples, the slope values are notably higher, which can be related to the increased amount of quenchers. Here, the contribution of temperature-dependent multiphonon relaxation on these quenchers in the temperature sensitivity of decay time is higher. Nevertheless, the difference in the slope values requires additional investigation.

3.3. Temperature-Dependent Spectral Characterization of Double-Doped YF₃:(Eu³⁺, Nd³⁺)

For the purposes of temperature sensing, the high temperature dependence of luminescent parameters is desirable. In this case, the YF₃: Eu³⁺ (2.5 and 5.0 %): annealed nanoparticles the slope of the τ_decay (T) function is the most pronounced (Table 1). We chose them for further doping with Nd³⁺ ions. Indeed, the addition of Nd³⁺ can lead to more pronounced temperature dependence of the YF₃: Eu³⁺ spectral-kinetic characteristics. Specifically, it is suggested, that Nd³⁺ provides additional temperature-dependent channel of depopulation of ⁵D₀ level of Eu³⁺. Hence, some luminescence parameters of double-doped YF₃: (Eu³⁺, Nd³⁺) are expected to be more temperature-dependent. Since the electron level structure of both Eu³⁺ and Nd³⁺ ions is difficult, the Eu³⁺ → Nd³⁺ energy transfer process seems to be complex. However, according to the literature data, the energy transfer involves at least ⁵D₃ (Eu³⁺) → ²P_1/2 (Nd³⁺) and ⁵D₀ (Eu³⁺) → ⁴G_5/2 (Nd³⁺) energy transfer processes. We synthesized a set of samples which were also divided into two groups: before and after annealing. We did not observe the reliable signal of Nd³⁺ luminescence for all the not annealed samples. Probably, it is related to the fact, that some Nd³⁺ excited states are close to vibrational states of OH groups. Among the different combinations of Nd³⁺ and Eu³⁺ in YF₃: Eu³⁺, Nd³⁺ samples, it was difficult to obtain several samples with intense luminescence signals of both Nd³⁺ and Eu³⁺ except for YF₃: Eu³⁺ (2.5%), Nd³⁺ (4.0 %) sample. The spectra of YF₃: Eu³⁺, Nd³⁺ samples having different combinations of the doping ions are presented in Figure S3. Specifically, the room temperature spectra of the YF₃: (Eu³⁺ (2.5%), Nd³⁺ (4.0 %)) before and after annealing are presented in Figure S3a of the supplementary file). It can be seen, that the Nd³⁺ luminescence is significantly less intense compared to Eu³⁺ one for the not annealed samples. After the annealing, the intensity of Nd³⁺ emission is higher. In turn, the annealed YF₃: (Eu³⁺ (2.5%), Nd³⁺ (2.0 %)) sample demonstrated low intense Nd³⁺ luminescence under Eu³⁺ excitation. To increase the Nd³⁺ luminescence we enlarged the Nd³⁺ concentration up to 4.0%. The room temperature spectra of the annealed YF₃: (Eu³⁺ (2.5%), Nd³⁺ (4.0 %)), and luminescence decay curves of ⁵D₀ - ⁷F₁ transition of YF₃: (Eu³⁺ (2.5%), Nd³⁺ (0 and 4.0 %)) are presented in Figure 9a,b, respectively.

It can be seen, that the YF₃: (Eu³⁺ (2.5%), Nd³⁺ (4.0 %)) sample has relatively comparable emissions of both Eu³⁺ and Nd³⁺ ions. This sample was chosen for further temperature-dependent spectral-kinetic characterization. The rate of decay of the luminescence intensity significantly decreases with the addition of Nd³⁺ ion (4.0%) compared to the single-doped YF₃: Eu³⁺ (2.5%) sample. This observation allows suggesting, that there is an energy transfer from ⁵D₀ level (Eu³⁺) to ⁴G_5/2 (Nd³⁺). In order to provide higher temperature sensitivity of LIR (luminescence intensity ratio) function, we should take luminescence peaks having opposite dependence on temperature. For example, there is ⁵D₀ (Eu³⁺) → ⁴G_5/2 (Nd³⁺) energy transfer which is phonon-assisted. Hence, the population of ⁴G_5/2 of Nd³⁺ becomes more effective with the temperature increase via depopulation of ⁵D₀ of Eu³⁺. It can be concluded, that the Eu³⁺ (⁵D₀ - ⁷F₁) intensity decreases with the temperature increase. In turn, the Nd³⁺ (⁴F_3/2 - ⁴I_9/2) demonstrated an opposite tendency. It should also be noted, that the decay curve of YF₃: (Eu³⁺ (2.5%), Nd³⁺ (4.0 %)) sample is not single-exponential. It can be related to the fact, that the Eu³⁺ ions are surrounded by different numbers of Nd³⁺ ions. Hence, the rate of depopulation of Eu³⁺ surrounded with different numbers of Nd³⁺ ions is different and the luminescence decay curve becomes nonexponential. The integrated luminescence intensity ratio function (LIR) function can be determined as:

$$LIR=\frac{I_{Eu}}{I_{Nd}}$$

(1)

Additionally, the choice of LIR is illustrated in Figure S4 of the supplementary file. In particular, the integrated intensities for Eu³⁺ and Nd³⁺ ions were taken in the ~ 570 - 605 and 845 - 925 nm ranges, respectively. The spectra detected in 100 - 300 K range and LIR function are represented in Figure 10a,b, respectively.

It can be seen, that the LIR is a decay function which is due to the above-mentioned opposite temperature dependence of both Eu³⁺ (⁵D₀ - ⁷F₁) and Nd³⁺ (⁴F_3/2 - ⁴I_9/2) emissions. Since the Eu³⁺ - Nd³⁺ energy transfer is not resonant, it involves the crystal lattice phonons.

The absolute (S_a) and relative (S_r) temperature sensitivities can be extracted from LIR function using the equations:

$$S_a=\left|\frac{d\left(LIR\right)}{dT}\right|$$

(2)

$$S_r=\frac{1}{LIR}\left|\frac{d\left(LIR\right)}{dT}\right|\ast 100\%$$

(3)

The S_a and S_r functions are presented in Figure 11.

It can be seen, that the highest sensitivity values are in the 80 - 200 K range. The obtained S_a and S_r values are quite competitive. Specifically, the list of world analogs is presented in Table 2.

3.4. Temperature-Dependent Kinetic Characterization of Double-Doped YF₃: Eu³⁺, Nd³⁺

As it was mentioned above, the decay time of the ⁵D₀ - ⁷F₁ (Eu³⁺) emission of annealed single-doped YF₃: Eu³⁺ nanoparticles demonstrated the highest temperature sensitivity in the 80 - 320 K temperature range (Figure 8). It was suggested, that the addition of Nd³⁺ can increase the temperature sensitivity of the decay time of the ⁵D₀ - ⁷F₁ (Eu³⁺) emission via providing an additional temperature-dependent channel depopulating the ⁵D₀ excited state of Eu³⁺. Indeed, the Nd³⁺ significantly shortens the rate of luminescence decay (Figure 9) indicating the energy transfer from Eu³⁺ to Nd³⁺. The ⁵D₀ - ⁷F₁ (Eu³⁺) luminescence decay curves of YF₃: Eu³⁺ (2.5%), Nd³⁺ (4.0 %) sample are presented in Figure 12a.

It can be seen, that the curves are non-exponential in the whole temperature range. To compare the obtained decay time values of double-doped YF₃: (Eu³⁺, Nd³⁺) nanoparticles with the single-doped YF₃: Eu³⁺ ones, we took τ_decay* as time when the normalized luminescence intensity decreases from 1 to 0.1 a.u. The τ_decay* decreases with the temperature increase. This tendency is comparable to the tendency, which is observed for the LIR function (Figure 10) of the same sample. It can be explained by the two factors: the some nonradiative transitions which provided the decreasing character of decay time dependence for single-doped YF₃:Eu³⁺ samples as well as by the additional channel of Eu³⁺ depopulation by Nd³⁺ ions (phonon-assisted energy transfer). In this case, the probability of phonon appearance and as a consequence, the efficiency of the Eu³⁺ decay (without Nd³⁺) and Eu³⁺ - Nd³⁺ energy transfer, increases with the increase of temperature. However, the rate of both LIR and τ_decay* slightly decreases at elevated temperatures. It can be suggested, that there is the activation of back energy transfer from Nd³⁺ to Eu³⁺, which is observed for some donor/acceptor ion pairs at the elevated temperatures [12,28]. The calculated S_a and S_r values are presented in Figure 13.

As it was mentioned above, the main idea of Nd³⁺ co-doping was to increase the temperature sensitivity of the ⁵D₀ - ⁷F₁ (Eu³⁺) luminescence decay time of the single-doped YF₃: Eu³⁺ (2.5%) nanoparticles. For the single-doped YF₃: Eu³⁺ (2.5%) nanoparticles, the decay time linearly decreases with the temperature increase. The slope is equal to 11.0 μs/K (note, that for the lineal dependence y=kx+b, the S_a = |dy/dx| is equal to the slope value (k)). Indeed, we notably increased the S_a from in the 80 - 260 K temperature range.

Table 3. The comparison of the performances of rare-earth-doped inorganic temperature sensors. Luminescence decay time is taken as temperature-dependent parameter.

Sample	Transition, wavelength, and excitation conditions	Max Sa [μs/K]	Max Sr [%/K] in the	Ref.
annealed YF3: Eu3+,Nd3+	Emission of Eu3+ (5D0 - 7F1, ~ 590 nm), λex = 394 nm (7F0 - 5L6 absorption band)	10 - 18 in the 80 - 200 K	0.2 - 0.3, in the 80 - 200 K	This work
β-NaGdF4: Nd3+,Yb3+	Yb3+ (2F5/2 - 2F7/2, ~ 980 nm),λex = 808 nm (4I9/2 - 4F5/2 abs. of Nd3+).	Linear increase from 1.0 (300 K) to 2.8 (at 350 K)	Increases from 0.7 (300 K) to 1.6 (at 350 K)	[29]
Nd0.5RE0.4Yb0.1PO4 (RE = Y, Lu, La, Gd)	Yb3+ (2F5/2 - 2F7/2, ~ 980 nm), λex = 940 nm, 2F7/2 - 2F5/2 absorption band of Yb3+.	0.4 - 1.6 at 300 K	0.5 - 1.2 at 300 K	[30]
LiYXYb1-XF4: Tm3+	λex = 688 nm, 3H6 - 3F2,3 (Tm3+) absorption band of	1.2	0.36	[31]
β-PbF2: Tm3+, Yb3+	Tm3+ (1G4 - 3H6, 478 nm), (2F7/2 - 2F5/2 abs. of Yb3+)	–	0.20 (at 300 K)	[32]
Gd2O2S: Eu3+	Eu3+, 5D0 level, λex = 375 nm (the transition is not specified)	–	Linear decreas: 4.5 (at 280 K) to 3.0 (at 335 K)	[33]
LaGdO3: Er3+/Yb3+	Er3+ (4S3/2 - 4I15/2, 530 nm), (4F9/2 - 4I15/2, 670 nm) (2F7/2 - 2F5/2 abs. of Yb3+)	–	1.79 (4S3/2) and 0.94 (4F9/2) in the 290 - 350 K range.	[34]
TiO2: Sm3+	Sm3+ (4G5/2 - 6H7/2, 612 nm) 438 nm (matrix excitation)		10 %/ºC at 70 ºC	[35]
NaPr(PO3)4	Pr3+ (emission from 3P0, the wavelength is not specified), λex = 488 nm (3H4 - 3P0 absorption band of Pr3+.		Linearl increas: 44·10-4 (at 300 K) to 60·10-4 (at 365 K)	[22]
LaF3: Pr3+	Pr3+ (3P0 - 3H4, 486 nm) λex = 444 nm (3H4 - 3P2 abs. of Pr3+)	0.7·10-3 in the 80 - 320 K.	–	[36]
LaPO4: Nd3+,Er3+	Nd3+ (4F5/2 - 4F11/2 λem = 1055 nm), λex = 808 nm abs. 4I9/2 - 4F5/2)	max value 0.003 at 600 K	max value ~ 2.5 at 600 K	[37]
MOF: Eu3+	Host excitation under 368 nm, λem = 525 nm	linear decrease: ~ 550 us (at 270 K) to ~460 us (at 360 K). The estimated Sa is equal to 1.0 us/K	–	[38]
GAG: Mn3+, Mn4+	λex = 266 nm, λem = 610 nm (5T2 - 5E″ of Mn3+)		2.08 at 249 K	[39]
Pr3+:YAG	Pr3+ (1D2 - 3H4, 617 nm), λex = 488 nm (3H4 - 3P0 absorption band of Pr3+.	linear decrease: ~ 190 us (at 10 K) to ~ 110 us (at 1000 K). The estimated Sa is equal to 0.080 us/K	–	[40]
CaF2: Ho3+	Ho3+ (5F5 - 5I8, λem = 650 nm), λex = 488 nm (5F3 - 5I8 absorption band of Ho3+.	linear decrease: ~ 100 us (at 100 K) to ~ 40 us (at 450 K). The estimated Sa is equal to 0.17 us/K	–	[40]
LiPr(PO3)4	Pr3+ (emission from 3P0, the wavelength is not specified) λex = 488 nm (3H4 - 3P0 absorption band of Pr3+.	0.0044 K-1 in the 300 - 365 K range	The Sa increases almost linearly from 0.44 %/K (at 300 K) to 0.65 %/K (at 365 K)	[22]

Table 3. The comparison of the performances of rare-earth-doped inorganic temperature sensors. Luminescence decay time is taken as temperature-dependent parameter.

Sample	Transition, wavelength, and excitation conditions	Max Sa [μs/K]	Max Sr [%/K] in the	Ref.
annealed YF3: Eu3+,Nd3+	Emission of Eu3+ (5D0 - 7F1, ~ 590 nm), λex = 394 nm (7F0 - 5L6 absorption band)	10 - 18 in the 80 - 200 K	0.2 - 0.3, in the 80 - 200 K	This work
β-NaGdF4: Nd3+,Yb3+	Yb3+ (2F5/2 - 2F7/2, ~ 980 nm),λex = 808 nm (4I9/2 - 4F5/2 abs. of Nd3+).	Linear increase from 1.0 (300 K) to 2.8 (at 350 K)	Increases from 0.7 (300 K) to 1.6 (at 350 K)	[29]
Nd0.5RE0.4Yb0.1PO4 (RE = Y, Lu, La, Gd)	Yb3+ (2F5/2 - 2F7/2, ~ 980 nm), λex = 940 nm, 2F7/2 - 2F5/2 absorption band of Yb3+.	0.4 - 1.6 at 300 K	0.5 - 1.2 at 300 K	[30]
LiYXYb1-XF4: Tm3+	λex = 688 nm, 3H6 - 3F2,3 (Tm3+) absorption band of	1.2	0.36	[31]
β-PbF2: Tm3+, Yb3+	Tm3+ (1G4 - 3H6, 478 nm), (2F7/2 - 2F5/2 abs. of Yb3+)	–	0.20 (at 300 K)	[32]
Gd2O2S: Eu3+	Eu3+, 5D0 level, λex = 375 nm (the transition is not specified)	–	Linear decreas: 4.5 (at 280 K) to 3.0 (at 335 K)	[33]
LaGdO3: Er3+/Yb3+	Er3+ (4S3/2 - 4I15/2, 530 nm), (4F9/2 - 4I15/2, 670 nm) (2F7/2 - 2F5/2 abs. of Yb3+)	–	1.79 (4S3/2) and 0.94 (4F9/2) in the 290 - 350 K range.	[34]
TiO2: Sm3+	Sm3+ (4G5/2 - 6H7/2, 612 nm) 438 nm (matrix excitation)		10 %/ºC at 70 ºC	[35]
NaPr(PO3)4	Pr3+ (emission from 3P0, the wavelength is not specified), λex = 488 nm (3H4 - 3P0 absorption band of Pr3+.		Linearl increas: 44·10-4 (at 300 K) to 60·10-4 (at 365 K)	[22]
LaF3: Pr3+	Pr3+ (3P0 - 3H4, 486 nm) λex = 444 nm (3H4 - 3P2 abs. of Pr3+)	0.7·10-3 in the 80 - 320 K.	–	[36]
LaPO4: Nd3+,Er3+	Nd3+ (4F5/2 - 4F11/2 λem = 1055 nm), λex = 808 nm abs. 4I9/2 - 4F5/2)	max value 0.003 at 600 K	max value ~ 2.5 at 600 K	[37]
MOF: Eu3+	Host excitation under 368 nm, λem = 525 nm	linear decrease: ~ 550 us (at 270 K) to ~460 us (at 360 K). The estimated Sa is equal to 1.0 us/K	–	[38]
GAG: Mn3+, Mn4+	λex = 266 nm, λem = 610 nm (5T2 - 5E″ of Mn3+)		2.08 at 249 K	[39]
Pr3+:YAG	Pr3+ (1D2 - 3H4, 617 nm), λex = 488 nm (3H4 - 3P0 absorption band of Pr3+.	linear decrease: ~ 190 us (at 10 K) to ~ 110 us (at 1000 K). The estimated Sa is equal to 0.080 us/K	–	[40]
CaF2: Ho3+	Ho3+ (5F5 - 5I8, λem = 650 nm), λex = 488 nm (5F3 - 5I8 absorption band of Ho3+.	linear decrease: ~ 100 us (at 100 K) to ~ 40 us (at 450 K). The estimated Sa is equal to 0.17 us/K	–	[40]
LiPr(PO3)4	Pr3+ (emission from 3P0, the wavelength is not specified) λex = 488 nm (3H4 - 3P0 absorption band of Pr3+.	0.0044 K-1 in the 300 - 365 K range	The Sa increases almost linearly from 0.44 %/K (at 300 K) to 0.65 %/K (at 365 K)	[22]

It can be concluded, that the studied YF₃: (Eu³⁺,Nd³⁺) sample demonstrates the highest S_a values and competitive S_r ones, especially in the 80 - 200 K range. Many of the above-mentioned phosphors do not demonstrate the co competitive S_r values in this temperature range of their optical characteristics were not studied. Hence, the Optical temperature sensors, operating in this range are highly demanded in cryogenic and spices industries.

4. Conclusions

The YF₃: (Eu³⁺, Nd³⁺) nanoparticles were synthesized via the co-precipitation method in distilled water with subsequent hydrothermal treatment. Then, the powders were divided into two groups: not annealed and annealed at 400 °C in air for 4 hours. The phase composition of the YF₃ doped particles was confirmed via XRD. In particular, XRD patterns correspond to the orthorhombic structure of YF₃ host-matrix without impurity and amorphous phases. After the annealing procedure, the samples have narrower diffraction peaks. According to the TEM imaging, the annealing procedure insignificantly affects the morphology of the nanoparticles. The average diameter was determined as 139 ± 2 and 132 ± 3 nm before and after annealing, respectively. The IR spectroscopy showed the presence of water in the not annealed nanoparticles. In turn, after annealing, the presence of the water was not observed. It was suggested, that the narrowing of the XRD peaks is related to the removal of water and to the improvement of nanoparticle crystallinity. The annealing procedure does not affect the shape of the luminescence spectrum of YF₃: Eu³⁺ (2.5, 5.0, and 7.5 mol.%) nanoparticles. In addition, the spectrum shape of these samples is independent of temperature in the 80 - 320 K range. However, after annealing the luminescence decay time (τ_decay) increases. The τ_decay linearly descends with the increase of temperature. The slope values of the annealed YF₃: Eu³⁺ (2.5 and 5.0 mol. %) nanoparticles were the highest (110·10^-4 and 67·10^-4, μs/K in the whole 80 - 320 K range, respectively) thus, these samples were chosen for further doping with Nd³⁺. Moreover, the obtained slope value 110·10^-4 μs/K (S_a) is very competitive surpassing many counterparts. We synthesized a set of YF₃: (Eu³⁺ (2.5 and 5.0 mol. %), Nd³⁺ (2.0, 4.0 mol.%)) annealed and not annealed samples. In the case of not annealed samples, the Nd³⁺ emission intensity was negligible compared to Eu³⁺ one for all the samples. It was explained by the fact, that water molecules quench Nd³⁺ emission because the Nd³⁺ excited states are resonant to some vibrational states of OH groups. In turn, the annealed samples shoved more intense Nd³⁺ emission under Eu³⁺ excitation. In particular, YF₃: (Eu³⁺ (2.5%), Nd³⁺ (4.0 %)) sample demonstrated the highest Nd³⁺ intensity and it was chosen for further LIR (I_Eu/I_Nd) characterization. The maximum S_a and S_r values based on the LIR function were 0.67 K^-1 (at 80 K) and 0.86 %·K^-1 (at 154 K), respectively. As it was mentioned above, the single-doped YF₃: Eu³⁺ (2.5. %) nanoparticles showed the linearly decreasing τ_decay (T) function (⁵D₀ - ⁷F₁ emission) with the slope value 110·10^-4 μs/K. The main idea of Nd³⁺ co-doping was to increase this slope value by increasing the rate of τ_decay (T) descent via the addition of one more temperature-dependent channel of ⁵D₀ excited state depopulation. Indeed, we managed to increase the slope up to 180·10^-4 μs/K at 80 K and to obtain very competitive S_r = 0.3 %/K at 80 K. This result is one of the highest compared to the world analogs.

Finally, it can be concluded that relativity new Eu³⁺/Nd³⁺ donor/acceptor ion pair showed very competitive performances via both LIR and luminescence decay time dependencies on temperature in the visible spectral range. It paves the way toward submicron temperature mapping and time-resolved temperature sensing in broad temperature range including physiological one. The notable temperature sensitivities at liquid nitrogen temperatures make the studied phosphors promising for space industry.