Investigation on the upconversion luminescence and ratiometric thermal sensing of SrWO₄:Yb³⁺/RE³⁺ (RE = Ho/Er) phosphors

Abstract

SrWO₄ phosphors doped with Ho³⁺(Er³⁺)/Yb³⁺ are successfully prepared by a high temperature solid-state reaction method. The upconversion (UC) luminescence properties of all the samples have been investigated under 980 nm excitation. Strong green emissions are obtained in the SrWO₄:Yb³⁺/Ho³⁺ and SrWO₄:Yb³⁺/Er³⁺ samples with the naked eyes. In a temperature range going from 303 K to 573 K, the UC emission spectra of the phosphors have been measured. Then the temperature sensing properties also have been discussed via fluorescence intensity ratio (FIR) technology. For the SrWO₄:Yb³⁺/Ho³⁺ phosphor, the FIR technologies based on thermal coupling levels (TCLs)(⁵F₄,⁵F₅) and non-thermal coupling levels (non-TCLs)(⁵S₂, ⁵F₄/⁵F₅) are used for investigating the sensitivity. The results show that the maximum absolute sensitivity reaches 0.0158 K⁻¹ with non-TCLs. As for Yb³⁺/Er³⁺ codoped SrWO₄ phosphor, the maximum absolute sensitivity reaches 0.013 K⁻¹ with TCLs (²H_11/2,⁴S_5/2) at a temperature of 513 K. These significant results demonstrate that the SrWO₄:Ho³⁺(Er³⁺)/Yb³⁺ phosphors are robust for optical temperature sensors.

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1. Introduction

As the basic parameter of thermodynamics, temperature measurement plays a vital role in scientific research, industrial production and medicine.¹ In recent years, much attention has been paid to non-contact temperature sensors based on rare earth (RE) ion activated luminescent materials.² Some UC luminescent materials with doping of RE³⁺ ions have been studied for temperature sensors.^3–7 The FIR technology based on the measurement of the temperature-dependent FIR from two excited state energy levels of activators represents an accurate temperature measuring method, because the FIR from the thermally coupled levels (TCLs) of RE ions is independent of the excitation intensity fluctuations, external disturbance and spectrum losses. Thus, reasonable measurement accuracy and sensitivity could be obtained with this method. The variation of FIR is generally caused by a thermally induced population re-distribution among the TCLs. And some RE ions have already been explored for designing FIR thermometry, such as Er³⁺, Tm³⁺, Ho³⁺ et al.^8–13 The sensitivity is an important role of a temperature sensing device. The absolute sensitivity is defined as the rate of change of FIR with change in temperature relative to FIR. Basically, a larger energy gap between the TCLs benefits the enhancement of sensitivity. However, the largest energy mismatches between those TCLs cannot exceed 2000 cm⁻¹, so it is difficult for the further improvement of sensitivity. There is an urgent need to design a new temperature measurement method to obtain high detecting sensitivity. Recently, a new kind of strategy has been proposed to solve the above drawback, which is based on FIR between the non-TCLs of the activators. The FIR derived from non-TCLs is also related to temperature. Compared with the FIR technique based on the TCLs, the FIR technique based on the non-TCLs is no longer limited by the energy gap. The Er³⁺ and Ho³⁺ ions also have non-TCLs pairs in addition to TCLs. So further investigation on temperature sensing properties of Er³⁺ and Ho³⁺ ions based on the FIR technology (TCLs and non-TCLs) is necessary.

Besides activators, temperature sensing properties of UC luminescent materials are also decided by host material. Among various hosts, tungstates have obtained much attention because of their brilliant physical and chemical stability, low phonon threshold energy, and high density.¹⁴ The Ln³⁺ ions also have high doped concentration in tungstates host. So the activators can emit intense and stable fluoresce even at high temperature.¹⁵ SrWO₄ belongs to a body-centered tetragonal system with scheelite crystal structure where WO₄²⁻ molecular ions are loosely bound to Sr²⁺ cations. It has been reported that SrWO₄ is an ideal host for optical temperature sensing materials. Recently, the temperature sensing properties of Ln³⁺ doped tungstates also have been studied. Pandey et al. report that the Er³⁺–Yb³⁺ co-doped SrWO₄ phosphor is a good optical temperature sensing material, and its maximum absolute sensitivity reaches 0.01498 K⁻¹.¹⁶ The high sensitivity of Tm³⁺/Yb³⁺ co-doped SrWO₄ phosphor for optical thermometry is synthesized by Song et al., and when the temperature is 323 K, the absolute sensitivity reaches a maximum of 0.00617 K⁻¹.¹⁷ For Sm³⁺ doped SrWO₄ phosphor and Nd³⁺/Yb³⁺ co-doped SrWO₄ phosphor, they also have good temperature sensing performance.^18,19 Note that, the SrWO₄ material is more suitable to be a UC fluorescence host for designing optical temperature sensor. However, as far as we know, the SrWO₄:Yb³⁺/Ho³⁺ phosphor for optical thermometry has not been investigated. There are even fewer reports on its optical thermometry using FIR technique based on the non-TCLs.

In this paper, the SrWO₄:Yb³⁺/RE³⁺(RE = Er, Ho) phosphors are synthesized by a high temperature solid state reaction. The UC luminescence performances and temperature sensing properties are investigated under 980 nm excitation. The experimental results illustrate that the temperature sensitivity of our phosphors are better than the most reported Ho³⁺(Er³⁺) doped materials. The SrWO₄ doping Ho³⁺(Er³⁺) ions materials are optical temperature sensing material with good application value.

2. Experimental

2.1 Synthesis of the phosphor

The samples are designed according to the molar composition of Sr_1–x−0.01Yb_xHo_0.01WO₄ (x = 0, 1%, 2%, 3%, 4%, 6%, 10%) and Sr_1−y−0.01Yb_yEr_0.01WO₄ (y = 0, 1%, 2%, 3%, 4%), and synthesized by solid-state reaction method. High-purity SrCO₃ (99%), WO₃ (99.8%), Yb₂O₃ (99.99%), Ho₂O₃ (99.99%) and Er₂O₃ (99.99%) are purchased from Aladdin Chemical Reagent Co. Ltd (China). The raw materials are weighed by molar ratio and mixed together. The mixtures thoroughly are ground in a mortar of agate for 60 minutes. The ground mixture is transferred to a crucible ceramics and then heated it in a muffle furnace. First, the temperature rises to 900 °C by 10 °C min⁻¹. The samples are kept at 900 °C for 4 h. Next, temperature is increased by 5 °C min⁻¹ to 1100 °C and kept for 6 h. Finally the SrWO₄:Ho³⁺/Yb³⁺ and SrWO₄:Er³⁺/Yb³⁺ phosphors are obtained.

2.2 Measurement and characterization

The structural formation of the phosphors has been measured using an X-ray diffraction (Rigaku D/Max-2500) in the range of 15° to 80° (2θ), and its radiation source is Cu Kα ray of λ = 0.15406 nm. For morphology and size of the phosphors, the field emission scanning electron microscope (FE-SEM, JEOL JEM-6700F) is used to perform. The UC emission spectra are recorded using the Zolix Omni-λ500 spectrometer under a 980 nm laser (MDL-III-980-2W, China) excitation. The samples are heated using an Orient KOJI TAP-02 high temperature thermometer, among a temperature range of 303 to 573 K, with a temperature control accuracy of 0.1 °C.

3. Result and discussion

3.1 Structural characterization

Fig. 1(a) shows the XRD patterns of the SrWO₄:Ho³⁺/Yb³⁺ phosphors. It can be seen that the diffraction peaks of the samples are very consistent with the standard card of SrWO₄ (JCPDS no. 08-0490). The sharp and strong diffraction peaks show that the synthesized samples are crystallized well. It also means that Sr²⁺ in the SrWO₄ lattice is substituted by Ho³⁺ and Yb³⁺ ions. The XRD patterns of the SrWO₄:Er³⁺/Yb³⁺ samples are shown in Fig. 1(b). The main diffraction peaks of samples are also consistent with the standard card of SrWO₄ (JCPDS no. 08-0490). But an extra weak peak at 29.3° appears in SrWO₄:1%Er³⁺, x%Yb³⁺(x = 2, 3, 4) samples, which may be induced by the minor unreacted component of Yb₂O₃. Once the concentration of the Yb³⁺ ions is over 2%, a part of the Yb³⁺ ions will not be added in the lattice. In addition, the peak at 30°moves to a high angle direction with the concentration of Yb³⁺ ions increasing (see the illustration in Fig. 1(b)), which is attributed to the shrinkage of the crystal lattice caused by the doped Er³⁺ and Yb³⁺ ions substituting the Sr²⁺ sites. The ionic radius of Er³⁺, Yb³⁺ and Sr²⁺(Er³⁺: 0.89 Å, Yb³⁺: 0.86 Å and Sr²⁺: 1.12 Å)are different, so the crystal lattice will produce deformation. The SEM images of SrWO₄:Ho³⁺(Er³⁺)/Yb³⁺ phosphor are shown in Fig. 2. It indicates that the size of the prepared phosphor is almost uniform.

Fig. 1 XRD patterns of SrWO₄: (a) 1%Ho³⁺/x%Yb³⁺(x = 0, 1, 2, 3, 4, 6, 10) phosphors. (b) 1%Er³⁺/x%Yb³⁺(x = 0, 1, 2, 3, 4) phosphors.

Fig. 2 SEM images of SrWO₄: (a) 1%Ho³⁺/2%Yb³⁺ phosphor. (b) 1%Er³⁺/1%Yb³⁺ phosphor.

3.2 Upconversion emission properties

Fig. 3(a) shows the UC emission spectrum of SrWO₄:1%Ho³⁺/2%Yb³⁺ phosphor from 500 to 800 nm under 980 nm excitation. Three characteristic emission bands are presented that the strong green emission (∼540 nm) belongs to the radiative transition from the ⁵S₂/⁵F₄ to ⁵I₈, the red emission (∼663 nm) belongs to the radiative transition from ⁵F₅ to ⁵I₈ and the weak near infrared emission (∼756 nm) corresponds to ⁵F₄ → ⁵I₇ energy level transition of Ho³⁺ ions. Fig. 3(b) shows the UC emission spectra of SrWO₄:1%Ho³⁺/x%Yb³⁺ (x = 0, 1, 2, 3, 4, 6, 10). It is seen that, the intensities of all the UC emission peaks reach maximum values at x = 2. But they decrease if the Yb³⁺ concentration is further increased up to 2%. The Yb³⁺ ion is a high-efficiency sensitizer for Ho³⁺ ion because it has lager absorption cross-sectional area at the near infrared. With increasing the Yb³⁺ ions concentration to 2%, the energy transfer process from Yb³⁺ to Ho³⁺ can be enhanced, resulting in higher UC emission intensity. However, with the further enhancement of the Yb³⁺ concentrations, the distance between the doping ions decreases with the increase of Yb³⁺ concentration. It induces more intense interaction among adjacent Yb³⁺ ions, which cause concentration quenching.²⁰

Fig. 3 (a). The emission spectrum of SrWO₄:1%Ho³⁺/2%Yb³⁺ phosphor from 500 to 800 nm under exciting at 980 nm. (b). The UC emission spectra of SrWO₄:1%Ho³⁺/x%Yb³⁺ (x = 0, 1, 2, 3, 4, 6, 10). (c). Log–log diagram of excitation power density and UC emission intensity. (d). The energy level diagrams of Ho³⁺/Yb³⁺ ions and possible UC emission processes.

In order to explicate the possible UC emission process of the SrWO₄:Ho³⁺/Yb³⁺ sample, the dependence relation of the UC emission intensity via pump power is measured. The relationship between UC emission intensity and pumping power can be described by the following eqn (1):²¹

I ∝ Pⁿ (1)

where I is the UC emission intensity, P is the pumping power and n is the photon numbers which correspond to populate the upper emitting levels. Fig. 3(c) shows the log–log plot of intensity versus pump power and the slopes of the fitted lines of SrWO₄:1%Ho³⁺/2%Yb³⁺ sample. It can be seen that n values are 1.21, 1.24 and 2.27 for green (∼540 nm), red (∼663 nm), and near-infrared (∼756 nm) emissions. It indicates that these UC emission processes are related to the two-photon processes. The energy level diagrams of Ho³⁺/Yb³⁺ ions and possible UC emission processes are shown in Fig. 3(d).

Under 980 nm excitation, the Yb³⁺ ion are excited from the ground state ²F_7/2 to the excited state ²F_5/2 by ground state absorption (GSA) process. Ho³⁺ ions are excited to the excited state levels mainly through the energy transfer (ET) process from Yb³⁺ to Ho³⁺. Through ET1 (²F_5/2 (Yb³⁺) + ⁵I₈ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵I₆ (Ho³⁺)), the Ho³⁺ ions are excited from the ⁵I₈ to the ⁵I₆ state.²² Some Ho³⁺ ions at ⁵I₆ state can nonradiatively (NR) relax to ⁵I₇ state, and then populating the ⁵F₅ state by excited state absorption (ESA) from ⁵I₇ to ⁵F₅. Ho³⁺ ions at ⁵I₆ state can continue to be excited into the ⁵S₂, ⁵F₄ coupling state by ET2 (²F_5/2 (Yb³⁺) + ⁵I₆ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵S₂, ⁵F₄ (Ho³⁺)). Subsequently, the Ho³⁺ ions at the ⁵S₂, ⁵F₄ coupling state will relax to the ⁵F₅ state by NR relaxation, and then a part of the Ho³⁺ ions at the ⁵F₅ state are transferred to the ⁵I₈ state, concurrently red light of 663 nm is emitted. The ⁵S₂, ⁵F₄ coupling state relax to the ⁵I₈ and ⁵I₇ states, resulting in high intensity green light (∼540 nm) and low intensity red light (∼756 nm), respectively.

The UC emission spectrum of SrWO₄:1%Er³⁺/1%Yb³⁺ phosphor is shown in Fig. 4(a). Two UC emission bands are exhibited, the strong green and the weak red emission, which can be attributed to the radiative transitions from ²H_11/2/⁴S_3/2 to ⁴I_15/2 and the ⁴I_9/2 → ⁴I_15/2 of the Er³⁺ ions, respectively. Fig. 4(b) shows the UC emission spectra of SrWO₄:1%Er³⁺/x%Yb³⁺ (x = 0, 1, 2, 3, 4). It can be found that the intensities of two emission bands first increase and then decrease with increasing Yb³⁺ concentrations, and reach a maximum at x = 1. Once the concentration of Yb³⁺ is over 1%, the UC emission intensities rapidly decline. This is mainly attributed to that the energy transfer efficiency from Yb³⁺ to Er³⁺ ion decrease because the limit of the Yb³⁺ that can be stabilized into the matrix. As demonstrated by the extra peaks in the XRD patterns, when the concentration of the Yb³⁺ ions is over 2%, a part of the Yb³⁺ ions will not be added in the lattice.

Fig. 4 (a). The UC emission spectrum of SrWO₄:1%Er³⁺/1%Yb³⁺ phosphor. (b). UC emission spectra of SrWO₄:1%Er³⁺/x%Yb³⁺ (x = 0, 1, 2, 3, 4). (c). Log–log relationship between pump power and UC emission intensity of SrWO₄:1%Er³⁺/1%Yb³⁺ phosphor. (d). The energy level diagrams of Er³⁺/Yb³⁺ ions and possible UC emission processes.

The dependence relationship of the UC emission intensities of SrWO₄:Er³⁺/Yb³⁺ on pump power are shown in Fig. 4(c). For the emission ∼528 nm, 550 nm and 654 nm, the slopes of the fitting experiment data are 1.70, 1.16 and 1.13, which is near 2. It indicates that all emission processes are related to the two-photon processes. Fig. 4(d) shows the energy level diagrams and possible UC emission processes of Er³⁺ and Yb³⁺ ion. Under 980 nm laser excitation, Yb³⁺ ions are excited from the ground state ²F_7/2 to the excited state ²F_5/2 by GSA process. The Er³⁺ ions are excited to the ⁴I_11/2 state from the ground state through the ET1 (²F_5/2 (Yb³⁺) + ⁴I_15/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺)). After that, the Er³⁺ ions at ⁴F_7/2 state can be populated from the ⁴I_11/2 state by the ET3 (²F_5/2 (Yb³⁺) + ⁴I_11/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_7/2 (Er³⁺)). The ²H_11/2 and ⁴S_3/2 states are populated from the ⁴F_7/2 state owing to the NR relaxation, and then the Er³⁺ ions at ²H_11/2 and ⁴S_3/2 states relax to the ⁴I_15/2 state, leading to the green UC emissions (528 nm and 550 nm). The Er³⁺ ions at ⁴I_11/2 state can relax to the ⁴I_13/2 state through the multiphonon relaxation (MPR) process, and then the Er³⁺ ions at ⁴I_13/2 state are excited to the ⁴F_9/2 by ET2 (²F_5/2 (Yb³⁺) + ⁴I_13/2 (Er³⁺) → ²F_7/2 (Yb³⁺) + ⁴F_9/2 (Er³⁺)). Meanwhile, the ⁴F_9/2 state can be also populated from the ⁴S_3/2 state through the NR relaxation. The Er³⁺ at the ⁴F_9/2 state will transit to the ⁴I_15/2 state and thus emitted the red light (∼654 nm).

3.3 Optical temperature-sensing properties

For investigating the optical temperature sensing characteristics of SrWO₄:Ho³⁺/Yb³⁺ phosphor, the emission spectra of the SrWO₄:Ho³⁺/Yb³⁺ sample are measured at different temperature. As shown in Fig. 5(a), it can be seen that the UC emission intensities of all emission band decrease with the temperature increasing. The possible reason is thermal quenching effect.²³ But the decreasing rate of the different emission band is different. Fig. 5(b) shows the UC emission intensity variation of 540 nm (⁵S₂, ⁵F₄ → ⁵I₈) and 756 nm (⁵S₂, ⁵F₄ → ⁵I₇) with temperature increasing. It is clearly observed that the UC emission intensity at 756 nm drops much more slowly than that at 540 nm with the temperature increasing. By considering the thermalization between ⁵S₂ and ⁵F₄ levels, the FIR of the UC emission from (⁵F₄, ⁵S₂) → ⁵I₈ and (⁵F₄, ⁵S₂) → ⁵I₇ transitions can be analyzed by using a four-level system, which was introduced by González-Pérez et al.²⁴ as shown in Fig. 5(c). The ⁵S₂, ⁵F₄, ⁵I₇ and ⁵I₈ levels form a four-level system. The emission intensity is proportional to the population of each energy level and the population of the excited state relates to the temperature. So the FIR can be expressed by the following eqn (2).²⁵where C₁, C₂, C₃ and C₄ are constants about spontaneous emission rate, energy level degeneracy and emission energy. ΔE is the energy gap of the ⁵S₂ and ⁵F₄, k is the Boltzmann constant, and T is the absolute temperature. It can be seen in Fig. 5(d), the FIR of the I_{756 nm}/I_{540 nm} regularly changes with the temperature increasing. It shows a nonlinear variation of FIR value at the temperature range 303 to 573 K. The eqn (2) is used to well fit the data, the C₁, C₂, C₃, C₄ and ΔE are 0.15, −0.3, 2.98, −6.1 and 429.15 cm⁻¹, respectively.

Fig. 5 (a). Temperature-dependent on UC emission spectra of SrWO₄:1%Ho³⁺/2%Yb³⁺ phosphor at 303–573 K. (b). The emission intensity of SrWO₄:1%Ho³⁺/2%Yb³⁺ phosphor center at 540 nm and 756 nm change in the range 303–573 K. (c) Four-level simplified diagram of transitions from adjacent excited levels to two lower levels. (d) The FIR (I_{756 nm}/I_{540 nm}), (e) the absolute sensitivity based on the I_{756 nm}/I_{540 nm} (f) the relative sensitivity based on the I_{756 nm}/I_{540 nm}.

In the experiment, absolute sensitivity (S_a) is a non-negligible evaluation index of temperature, which indicates the absolute change of FIR in unit temperature, as shown in eqn (3).

Relative sensitivity (S_r) is also an important parameter that can be described as:

S_a and S_r of the sample are shown in Fig. 5(e) and (f), which are fitted by eqn (3) and (4). The S_a increases with the temperature enhancement, reaching a maximum of 0.0017 K⁻¹ at 573 K. While the maximum value of S_r is 1.27% K⁻¹ at 573 K.

Fig. 6(a) shows that the emission intensities of the green and red band vary with temperature increasing. It can see that the intensity of red emission decreases more slowly than that of green emission. The energy levels ⁵S₂,⁵F₄ and ⁵F₅ energy levels are far apart, and the particle population at the two energy levels do not follow the Boltzmann-type distribution. The ⁵S₂, ⁵F₄ and ⁵F₅ belong to non-TCLs. So the traditional FIR analysis method based on TCLs is not appropriate for the non-TCLs system. In the previous report, the relation of the non-TCLs FIR and temperature can be fitted by eqn (5).²⁶

Fig. 6 (a) The emission intensity of SrWO₄:1%Ho³⁺/2%Yb³⁺ phosphor center at red (663 nm) and green (540 nm) changes with temperature. (b) The FIR (I_{540 nm}/I_{663 nm}), (c) the absolute sensitivity and (d) the relative sensitivity of SrWO₄:Ho³⁺/Yb³⁺ phosphor as a function of temperature.

Fig. 6(b) shows that the original data for the I_{540 nm}/I_{663 nm} as a function of the temperature fitting with the eqn (5). Consequently the FIR based on ⁵S₂, ⁵F₄/⁵F₅ (Ho³⁺) shows the significant temperature dependence and well fits the eqn (5). The curve of S_a and S_r by calculation and fitting are shown in Fig. 6(c) and (d). The S_a goes down with the increasing of temperature. The maximum S_a is 0.0158 K⁻¹ at 303 K. While the S_r reaches a maximum of 0.41% K⁻¹ at 393 K. The above analysis of the sensitivity of SrWO₄:Ho³⁺/Yb³⁺ based on the TCLs and the non-TCLs shows that the sample has high sensitivity and it has certain application value in optical temperature measurement.

Besides, the Er³⁺–Yb³⁺ co-doped SrWO₄ phosphor has also been studied for its sensing properties. As shown in Fig. 7(a), the UC emission spectra of green light in the different temperature are recorded. The UC luminescence intensities of 528 nm and 550 nm (generated by ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2) vary differently with the change of temperature. At 313 K, the intensity of the two emission bands is almost equal, with the temperature increasing, the luminescence intensity of the 528 nm emission band is gradually increased within 313–433 K, and that of the 550 nm emission band gradually decreases. Then, in the range of 433–573 K, as the temperature increasing, the intensities of two bands also decrease because of the non-radiative relaxation enhancement. But the luminescence intensity of the 550 nm emission band decreases faster than that of 528 nm emission band. When the temperature increases, the electrons at ⁴S_3/2 state are excited. The transition from ⁴S_3/2 to ²H_11/2 occurs. It indicates that the ²H_11/2 and ⁴S_3/2 states of Er³⁺ can be regarded as TCLs, and the electrons population at ²H_11/2 and the ⁴S_3/2 states follows the Boltzmann distribution. The FIR of TCLs can be expressed by the eqn (6):²⁷

Fig. 7 (a) Temperature-dependent green UC emission spectra of 1%Er³⁺/1%Yb³⁺ co-doped SrWO₄ phosphor at 313–573 K. (b) dependence of FIR for I_{528 nm}/I_{550 nm} on absolute temperature. (c and d) The variation of absolute sensitivity and the relative sensitivity for SrWO₄:1%Er³⁺/1% Yb³⁺ phosphor with temperature between 313 K and 573 K.

The changing of FIR of I_{528 nm}/I_{550 nm} with temperature is displayed in Fig. 7(b), which is fitted by eqn (6). According to the fitting curve of the experimental data, the calculated C value is 24.29 and ΔE/k is 1012.9, so the ΔE between ²H_11/2 and ⁴S_3/2 is 703.7 cm⁻¹, which is very close to the experiment value of 758 cm⁻¹, the latter is obtained from the UC emission spectrum. Fig. 7(c) shows the S_a of SrWO₄:1%Er³⁺/1%Yb³⁺ phosphor. The S_a of the phosphor increases at first and then decreases, reaching a maximum of 0.013 K⁻¹ at 513 K. In Fig. 7(d) the S_r also is calculated, and the S_r reaches a maximum of 1.03% K⁻¹ at 313 K.

In addition, thermal resolution (ΔT) is also a key parameter, which can be defined as:

where ΔFIR is the uncertainty of FIR, S_r is the relative sensitivity, and S_a is the absolute sensitivity. The ΔFIR can be calculated by the following equation.where and the corresponding uncertainty ΔFIR are the average and the standard deviation values from 20 measurements at each temperature, FIR_i is the i-th FIR measured at the same temperature and n is the number of measurements.

For SrWO₄:Ho³⁺/Yb³⁺ phosphor, the thermal resolution of temperature measurement based on the non-TCLs (⁵S₂, ⁵F₄/⁵F₅) FIR has been investigated. While the thermal resolution of temperature measurement based on the TCLs (²H_11/2/⁴S_3/2) also has been discussed for SrWO₄:Er³⁺/Yb³⁺ phosphor. The values of ΔFIR and ΔT are presented among the range of 303–573 K in Fig. 8(a) and (b), respectively. It can be seen that the ΔFIR of SrWO₄:Ho³⁺/Yb³⁺ is lower than that of SrWO₄:Er³⁺/Yb³⁺. At 303 K, the ΔT of SrWO₄:Ho³⁺/Yb³⁺ and SrWO₄:Er³⁺/Yb³⁺ have the minimum values of 0.72 K and 1.14 K, respectively. The values of ΔT become increasingly with the temperature enhancement. Therefore, the samples have better accuracy at lower temperature.

Fig. 8 (a) ΔFIR and (b) ΔT of SrWO₄:Ho³⁺/Yb³⁺ based on the non-TCLs (⁵S₂, ⁵F₄/⁵F₅) and SrWO₄:Er³⁺/Yb³⁺ based on the TCLs (²H_11/2/⁴S_3/2) pair at 303–573 K.

In order to correctly evaluate experimental results, the other materials doped with different RE ions are compared in Table 1. It can clearly conclude that the sensitivity of our sample is higher than that of the most of samples in the table.

Table 1 Comparison of the maximum sensitivity of Ho³⁺(Er³⁺)/Yb³⁺ doped in different material

Materials	Energy level	Sr (K^−1)	Sa (K^−1)	Reference
Bi2SiO5:Yb^3+,Tm^3+	^3F3,/2/^1G4	1.95% (300 K)	0.0168 (300 K)	7
NaGdF4:Er^3+/Yb^3+	^2H11/2/^4S3/2	1.29% (303 K)	0.0382 (363 K)	28
LaF3:Er^3+/Yb^3+	^2H11/2/^4S3/2	844/T^−2 (150 K)	0.0025 (400 K)	29
Y2O3:Ho^3+/Yb^3+/Zn^3+	^5F3/^3K8	1067.76/T^−2 (299 K)	0.003 (673 K)	30
NaLa(MnO4)2:Sm^3+/Tb^3+	^4G5/2/^5D4	1.93 (498 K)	0.119 (265 K)	31
BaY2Si3O10:Ho^3+/Yb^3+	^5F5/^5S2,^5F4	0.49% (298 K)	0.023 (298–448 K)	32
BaY2Si3O10:Er^3+/Yb^3+	^4F9/2/^2H11/2	0.78% (298 K)	0.091 (298 K)	32
LaAlO3:Er^3+/Yb^3+	^2H11/2/^4S3/2	575.27/T^−2 (281 K)	0.0032 (281 K)	33
NaY(WO4)2:Er^3+/Yb^3+	^2H11/2/^4S3/2	1043.12/T^−2 (133 K)	0.0112 (515 K)	34
SrWO4:Er^3+/Yb^3+	^2H11/2/^4S3/2	1.03% (313 K)	0.013 (513 K)	This work
SrWO4:Ho^3+/Yb^3+	^5S2/^5F4	1.27% (573 K)	0.0017 (573 K)	This work
SrWO4:Ho^3+/Yb^3+	^5S2, ^5F4/^5F5	0.41% (393 K)	0.0158 (303 K)	This work

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4. Conclusion

In conclusion, the SrWO₄:Ho³⁺(Er³⁺)/Yb³⁺ phosphors are successfully prepared by solid state reaction method. The UC luminescence properties of SrWO₄:Ho³⁺(Er³⁺)/Yb³⁺ samples are investigated under 980 nm excitation. Furthermore, the temperature sensing properties of the Ho³⁺:⁵S₂/⁵F₄ (TCLs), Er³⁺:²H_11/2/⁴S_3/2 (TCLs) and the Ho³⁺:⁵S₂, ⁵F₄/⁵F₅ (non-TCLs) are investigated in the temperature range of 303–573 K. Specifically, the absolute sensitivity value reaches 0.0158 K⁻¹ at 303 K for Ho³⁺:⁵S₂, ⁵F₄/⁵F₅ (non-TCLs) and the relative sensitivity value reaches 0.41% at 393 K, which is superior to most optical temperature sensing materials doped with RE³⁺ ions. It means that the SrWO₄:Ho³⁺/Yb³⁺ can achieve higher absolute sensitivity by FIR of non-TCLs, as well as it has potential application in optical temperature sensors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Natural Science Foundation of China (Grant No: 61705077); Science Foundation of Jilin Province Education Department (JJKH20190853KJ); Project of Jilin Provincial Science and Technology Department (No: 20190303064SF, 20200403072SF); Project of Jilin Province Development and Reform Commission (2019C048-4, 2020C021-5).

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