Optical temperature sensing in β-NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ based on thermal, quasi-thermal and non-thermal coupling levels

Abstract

Three methods for optical temperature sensing are investigated in the NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ phosphor, which is prepared by a hydrothermal method. The temperature-dependent luminescence is investigated under 980 nm excitation. Utilizing fluorescence intensity ratio (FIR) technique, the temperature sensing behaviors are studied in the range of 300–600 K based on thermal coupling levels (²H_11/2 and ⁴S_3/2 of Er³⁺), “quasi-thermal coupling levels” (⁴F_7/2 of Er³⁺ and ³F₂ of Tm³⁺) and non-thermal coupling levels (¹D₂ and ¹G₄ of Tm³⁺), respectively. The maximum sensitivity is 604 × 10⁻⁴ K⁻¹ at 300 K, which is based on non-thermal coupling levels of Tm³⁺. Meanwhile, the temperature dependent emission colors are discussed, which are changed from bluish green to light blue with the temperature rising. These significant results, high sensitivity and the temperature dependent multicolor emissions, indicate that the NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ phosphor is robust for optical temperature sensing.

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1. Introduction

Trivalent rare earth (RE³⁺) doped upconversion (UC) materials are extensively investigated because of their attractive applications, such as in display devices, remote temperature sensors, cell imaging, biomedical diagnostics and therapeutics.^1–6 Their unique optical characters arise from their intra-4f transitions which are allowed due to the mixing of configurations having opposite parity.^7–9 Generally, the abundant 4F^N electronic states lead to multicolor emission, which is tunable from ultraviolet (UV) to NIR.^10–14 Under near-infrared (NIR) excitation, RE³⁺ doped host materials can emit visible light through two-photon or multi-photon processes.^15–18 Up to now, NaLuF₄ has been known as one of the most excellent host material due to low phonon energy and highly luminescent efficiency.^19,20

Temperature is a significant parameter for both scientific and industrial fields, thus, high accuracy and high sensitivity temperature measurement is crucial. FIR technique, based on measurement of luminescent intensities from two thermal coupling levels of RE ions, can realize accurate measurement, because it is independent of spectrum losses and fluctuations of exciting intensity.^21–24 The energy gap between these two thermal coupling levels should be in the range of 200–2000 cm⁻¹. Generally, the larger the energy gap is the higher sensor sensitivity. Therefore, further enhancement of the sensing sensitivity is a challenge due to the restriction of the energy gap between the thermal coupling levels. Therefore, improving the sensitivity is urgently needed, and lots of researches are reported, such as the SPR method,²⁵ organic molecules modification,^26,27 core–shell structure^28–30 and the non rare earth ions doped method,^31–34 and so on. These methods are based on thermal coupling levels of RE³⁺. Heretofore, few results on optical thermometry have been reported mainly concerning the non-thermal coupling levels of RE³⁺.

Herein, we report a strategy to enhance the sensitivity of optical thermometer, based on non-thermal coupling levels. The NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ phosphor is prepared by hydrothermal method. According to FIR technique, the temperature sensing behaviors are studied based on three pairs of levels (thermal coupling levels (²H_11/2 and ⁴S_3/2 of Er³⁺), “quasi-thermal coupling levels” (⁴F_7/2 of Er³⁺ and ³F₂ of Tm³⁺) and non-thermal coupling levels (¹D₂ and ¹G₄ of Tm³⁺)), respectively. Meanwhile, multicolor emission in the visible region was discussed in detail at different temperature.

2. Experimental

The OA-capped NaLuF₄:20%Yb³⁺/2%Er³⁺/1%Tm³⁺ phosphor was synthesized by a hydrothermal method. In a typical procedure, 18 mL OA was slowly added into the 12 mL methanol solution containing NaOH (15 mmol). The solution was magnetically stirred for 30 min at room temperature. Then, 6 mL deionized water solution of 1 mmol of RE(NO₃)₃ (Lu³⁺ : Yb³⁺ : Er³⁺ : Tm³⁺ = 79 : 18 : 2 : 1) was added to the above mixture and stirred for another 30 min. A 6 mL solution of NH₄F (5 mmol) was added to the resultant solution to form a colloidal suspension. Then, the above solution was moved into a autoclave and kept at 180 °C for 12 h. The resultant product was collected by centrifugation (5 min, 8000 rpm). Finally, the product is washed with ethanol, deionized water and cyclohexane several times, and dried at 60 °C for 2 h.

Upconversion luminescence spectrum were recorded by HORIBA Jobin Yvon iHR550 Spectrometers which employs a photon-counting detection system using a Hamamatsu R928 photomultiplier tube. Powder X-ray diffraction patterns were measured by Panalytical Empyrean diffractometer.

3. Results and discussions

The crystal structure was characterized using the XRD pattern. It is clear found that the pattern is be consistent with standard values of hexagonal phase NaLuF₄ (JCPDS no. 27-0726), as shown in Fig. 1. The UC spectrum of NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺, under the 980 nm laser excitation, are shown in Fig. 2. The characteristic emissions, originated from the ¹D₂ → ³F₄ (450 nm), ¹G₄ → ³H₆ (475 nm), ³F₂ → ³H₆ (695 nm) of Tm³⁺ and ²H_11/2 → ⁴I_15/2 (525 nm), ⁴S_3/2 → ⁴I_15/2 (545 nm) and ⁴F_9/2 → ⁴I_15/2 (660 nm) of Er³⁺ transitions, are observed respectively. Fig. 3 gives the schematic of UC mechanism for the NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ with the 980 nm laser excitation. The ¹D₂, ¹G₄, ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels are populated via four-, three-, two-, two- and two- photons process, respectively.

Fig. 1 XRD pattern of NaLuF₄:20%Yb³⁺/2%Er³⁺/1%Tm³⁺ and the standard date for hexagonal NaLuF₄ (JCPDS no. 27-0726).

Fig. 2 (a) Upconversion emission spectrum of NaLuF₄:Yb³⁺/Er³⁺ and NaLuF₄:Yb³⁺/Tm³⁺. (b) Temperature-dependent upconversion emission spectrum of NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺.

Fig. 3 Schematic energy level diagram for Yb³⁺, Er³⁺ and Tm³⁺ system and the proposed UC emission mechanism under 980 nm excitation.

It is interesting to found that the intensity of luminescence exhibits a significantly temperature-dependent behavior. The intensity of each emission band shows an obvious decrease with the temperature increasing apart from emission band at 695 nm (Tm:³F₂ → ³H₆ transition). The intensity decreasing is attributed to multiphonon relaxation rising, and different multiphonon relaxation rate is responsible for the variety of relative intensity. It is well known that luminescent intensity largely depend on matrix material.^35,36 Wang et al. reported that thermal excitation (³H₄ → ³F₂ transition) is responsible for the intensity rising (³F₂ → ³H₆ transition) with the temperature increasing in LiNbO₃:Tm³⁺/Yb³⁺ phosphor.³⁷ Meanwhile,Xu et al. presented that the decrease of luminescent intensity (³F₂ → ³H₆ transition) is attributed to the multiphonon relaxation rising in α-NaYb(Mn)F₄:Tm³⁺@NaYF₄ phosphor.³⁸ In addition, the energy transfer (ET) (⁴F_9/2 (Er³) → ³F₂ (Tm³⁺)) is an efficient pathway,^39,40 because the energy gap (∼400 cm⁻¹) is much closer to the maximal phonon energy of the host.^41,42 Thus, these results suggest that the increase of emission intensity at 695 nm may be attributed to influence of matrix material and ET from Er³⁺ to Tm³⁺.

The FIR can be written as^43–45

where I is the integral intensity of emission band, ΔE is the energy gap between two levels of RE ions, C, T and k are a constant, absolute temperature and the Boltzmann constant, respectively. For sensing application, the absolute sensitivity (denote as S) is a very important parameter, which has been calculated using the formula:^46–48

The FIR variation with temperature is plotted in Fig. 4. In the case of Er³⁺, the ²H_11/2 and ⁴S_3/2 levels (thermal coupling levels) can be populated by thermal excitation. Therefore, the FIR arising from the ²H_11/2 and ⁴S_3/2 levels of Er³⁺ to the ground state transitions (denote as R_Er) can be used for the thermometer by using the following formula:

Fig. 4 The FIR based on thermal coupling levels (R_Er), “quasi-thermal coupling levels” (R_Tm–Er) and non-thermal coupling levels (R_Tm) as a function of temperature.

The energy gap between the ¹G₄ and ¹D₂ levels (ΔE_DG ≈ 6476 cm⁻¹) does not meet the requirement of eqn (1) application (200–2000 cm⁻¹). Therefore, the FIR originated from the¹G₄ → ³H₆ and ¹D₂ → ³F₄ of Tm³⁺ transitions (denote as R_Tm) do not allow an estimation of temperature in the same approach.⁴⁹ The experimental data for R_Tm (I₄₇₅/I₄₅₀) as a function of the temperature can be fitted by a polynomial function.

R_Tm = I₄₇₅/I₄₅₀ = C₁ + B₁T + B₂T² = −5.75 + 6.27 ×10⁻²T − 1.72 × 10⁻⁵T² (4)

Due to non-resonant energy matching between the Er³⁺ and Tm³⁺ energy states, the multiphonon assisted energy transfer rate (denote as K_ET) described by Mott–Seitz model:^50,51

Thus, the intensity of emission band at 660 nm (Er³⁺) and 695 nm (Tm³⁺) shows the temperature dependent behavior. In addition, The energy mismatch (ΔE ≈ 400 cm⁻¹) between ⁴F_7/2 state of Er³⁺ and ³F₂ state of Tm³⁺ meet the requirement of thermal coupling levels (200–2000 cm⁻¹). Therefore, the ⁴F_7/2 state of Er³⁺ and the ³F₂ state of Tm³⁺ can regard as a “quasi-thermal coupling levels”, and the FIR based on ³F₂ → ³H₆ (Tm³⁺) and ⁴F_9/2 → ⁴I_15/2 (Er³⁺) transition (denote as R_Tm–Er) shows a significant temperature dependence. The experimental data for R_Tm–Er (I₆₉₅/I₆₆₀) as a function of the temperature can be fitted by a polynomial function (Fig. 4).

R_Tm–Er = I₆₉₅/I₆₆₀ = 0.88 − 6.30 × 10⁻³T + 1.16 × 10⁻⁵T² (6)

The corresponding sensitivity curves are given in Fig. 5. Obviously, the sensitivity (denote as S_Tm) of optical thermometer based on non-thermal coupling levels (¹D₂ and ¹G₄ level of Tm³⁺) is highest in the whole temperature range, and the obtained maximum sensitivity is 604 × 10⁻⁴ K⁻¹ at 300 K. In view of “quasi-thermal coupling levels” (⁴F_7/2 of Er³⁺ and ³F₂ of Tm³⁺), the sensitivity (denote as S_Tm–Er) increases with temperature rising, and obtained maximum is 76 × 10⁻⁴ K⁻¹ at 600 K. In the case of thermal coupling levels (²H_11/2 and ⁴S_3/2 of Er³⁺), the sensitivity (denote as S_Er) firstly increases and then decreases. The maximum sensitivity (S_Er) is 77 × 10⁻⁴ K⁻¹ at 500 K. Table 1 lists the sensitivities of several typical optical thermometer doped with different RE³⁺ ions.

Fig. 5 The sensitivity based on thermal coupling levels (S_Er), “quasi-thermal coupling levels” (S_Tm–Er) and non-thermal coupling levels (S_Tm) as a function of temperature.

Table 1 Values of the maximum sensitivity (S_Max) of different material, including the corresponding transitions and the temperature range

Material	Ions (transitions)	SMax (10^−4 K^−1)	T (K)	References
NaYF4:Yb, Er	Er (^2H11/2, ^4S3/2 → ^4I15/2)	36.8	223–403	52
BaTiO3:Yb, Er, Zn	Er (^2H11/2, ^4S3/2 → ^4I15/2)	47.5	120–505	53
NaLuF4:Yb, Er	Er (^4D7/2, ^4G9/2 → ^4I15/2)	52	303–523	54
NaLuF4:Yb, Tm, Gd	Gd (^6I9/2, ^6I7/2 → ^8S7/2)	29	298–523	55
NaYbF4:Tm, Mn@NaYF4	Tm (^1G4 → ^3H5, ^3H4 → ^3H6)	24	123–423	38
NaGdF4:Yb, Tm@NaGdF4:Tb, Eu	Tb (^5D4 → ^7F5), Eu (^5D0 → ^7F2)	12	125–300	50
NaLuF4:Yb, Tm, Er	Tm (^1D2 → ^3F4, ^1G4 → ^3H6)	604 (STm)	300–600	This work
	Er (^2H11/2, ^4S3/2 → ^4I15/2)	77 (SEr)	300–600	This work
	Tm (^3F2 → ^3H6), Er (^4F9/2 → ^4I15/2)	76 (STm–Er)	300–600	This work

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The relative intensity of the blue colors (400–500 nm) and green colors (500–600 nm) decreases, contrarily, and the red light (600–750 nm) increases with the increase of temperature (Fig. 6). The CIE coordinates for the luminescent spectra of NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ phosphor with different temperature (300, 400, 500 and 600 K) are located in the bluish green and light blue regions, as given in Fig. 7, respectively. Thus, the multicolor emission of temperature dependence can be also applied in temperature sensing.

Fig. 6 Relative intensity of blue, green and red light of NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ changed with temperature.

Fig. 7 Color coordinates of NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ phosphor at different temperature (300 K, 400 K, 500 K and 600 K).

4. Conclusions

In summary, the hexagonal phase NaLuF₄:20%Yb³⁺/2%Er³⁺/1%Tm³⁺ phosphor was synthesized by a hydrothermal method. Utilizing 980 nm laser excitation, the luminescent intensity exhibits a significantly temperature-dependent behavior. Using the FIR technique, three methods are adopted for investigating temperature sensing, which is based on thermal coupling levels (²H_11/2 and ⁴S_3/2 of Er³⁺), “quasi-thermal coupling levels” (⁴F_7/2 of Er³⁺ and ³F₂ of Tm³⁺) and non-thermal coupling levels (¹D₂ and ¹G₄ level of Tm³⁺), respectively. It is found that the sensitivity based on non-thermal coupling levels (¹D₂ and ¹G₄ level of Tm³⁺) is highest in the whole temperature range (300–600 K). The maximum sensitivity is 604 × 10⁻⁴ K⁻¹ at 300 K. In addition, the luminescent color exhibit temperature-dependent behavior, which display from bluish green to light blue. These merits indicate that NaLuF₄:Yb³⁺/Er³⁺/Tm³⁺ phosphor can be applied in optical temperature sensing, due to sensing performance of high sensitivity and temperature dependent multicolor emissions in the visible region.

Acknowledgements

This work has been supported by the grant of National Natural Science Foundation of China under project No. 11374079, 11474078 and 61275117.

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