Stark sublevels of Er³⁺–Yb³⁺ codoped Gd₂(WO₄)₃ phosphor for enhancing the sensitivity of a luminescent thermometer

Abstract

We report a strategy for enhancing the sensitivity of an optical thermometer by utilizing the Stark sublevels. Under 980 nm excitation, the upconversion emission originating from the Stark levels of Er³⁺ (⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2/⁴F_9/2(1)) is observed in a monoclinic phase Gd₂(WO₄)₃:Er³⁺/Yb³⁺ phosphor, which is synthesized through the co-precipitation method. The temperature sensing behavior is studied over the range of 296–620 K based on thermal coupling levels ²H_11/2/⁴S_3/2(2), ²H_11/2/⁴S_3/2, ²H_11/2/⁴S_3/2(1), ⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1) using fluorescence intensity ratio (FIR) technology. The sensitivity based on Stark sublevels ²H_11/2/⁴S_3/2(2) or ²H_11/2/⁴S_3/2(1) is almost twice more than that based on the traditional ²H_11/2/⁴S_3/2 levels. The obtained maximum sensitivity of the optical thermometer is 16.5 × 10⁻³ K⁻¹ at 395 K (²H_11/2/⁴S_3/2(2)). These results suggest that the use of Stark levels is a promising approach for enhancing the sensitivity of optical thermometers.

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Introduction

Highly sensitive and highly accurate temperature sensing is a challenging task in both scientific research and industrial production. Recently, rare earth (RE) doped materials have attracted great attention as temperature probes, especially for their potential application in intracellular environments and strong electromagnetic fields (without interferences).^1–8 As we know, the FIR technique is an effective approach to achieve accurate measurements by exploiting the temperature-dependent luminescence intensities derived from two thermal coupling levels, which are unaffected by spectrum losses and fluctuations in excitation intensity.^9–11 Photon upconversion (UC) is basically a nonlinear optical effect, which can convert low energy photons to high energy photons via multiphoton processes.^12,13 As one of the most widely used activators, erbium ions (Er³⁺) have abundant ladder-like electronic energy levels.^14–17 Under near-infrared (NIR) excitation, green UC emissions originating from ²H_11/2 and ⁴S_3/2 to ground state transitions are highly efficient and the energy gap between them matches well with the requirement for the thermal coupling levels (200 cm⁻¹ < ΔE < 2000 cm⁻¹).^18–20

According to previous literature, the Stark sublevels are considered as thermal coupling levels due to the small energy gap.^21,22 Jaque et al. investigated the subtissue thermal sensing behavior based on the Stark sublevels (⁴F₅₍₂₎ and ⁴F₅₍₁₎) of Nd³⁺ ions.²³ Dong et al. reported highly penetrating bio-imaging and temperature sensing using the Stark sublevels (³H₄₍₂₎ and ³H₄₍₁₎) of Tm³⁺ ions.²⁴ Yin et al. achieved high sensitivity sensing in the low temperature range using the Stark sublevels (¹G₄₍₂₎ and ¹G₄₍₂₎) of Tm³⁺ ions.²⁵ Although the Stark energy levels of RE ions, such as Tm³⁺ (¹G₄₍₂₎ and ¹G₄₍₁₎), Ho³⁺ (⁵F₅₍₂₎ and ⁵F₅₍₁₎), Nd³⁺ (⁴F_3/2(2) and ⁴F_3/2(1)), and so on have been widely used in optical thermometers,^26–29 few reports focused on the Stark energy levels of Er³⁺ ions. Generally, the larger the energy gap between the thermal coupling levels is, the higher the sensor sensitivity. Because the sensitivity is restricted to the energy gap of the thermal coupling levels, utilizing thermal coupling levels with a large energy gap may be a valid strategy to improve sensitivity. In addition, due to its remarkable thermal stability, distinct crystal structure and low phonon energy, Gd₂(WO₄)₃ is regarded as a promising host material for temperature sensing.^30–32

In the present work, we prepared the monoclinic phase Gd₂(WO₄)₃:10%Er³⁺/10%Yb³⁺ phosphor via the co-precipitation method. Emission bands arising due to the transitions from Stark sublevels were observed. Using FIR technology, the temperature sensing behavior was studied over the range of 296–620 K based on five pairs of thermal coupling levels.

Experimental

The Gd₂(WO₄)₃:10%Er³⁺/10%Yb³⁺ phosphor was synthesized via a co-precipitation method. In a typical procedure, 10 mL of solution containing Gd(NO₃)₃ (1.8 mmol), Er(NO₃)₃ (0.1 mmol) and Yb(NO₃)₃ (0.1 mmol) was slowly added into 30 mL of Na₂WO₄ (3 mmol) solution, and then the mixed solution was magnetically stirred for 30 min. The product was collected by centrifugation (8000 rpm, 10 min) and washed twice with deionized water. The precipitate was dried at 70 °C for 2 h. Finally, the Gd₂(WO₄)₃:Yb³⁺/Er³⁺ phosphor was obtained after being calcined at 900 °C for 4 h. The pH of the Na₂WO₄ solution was adjusted to be lower than 10 using ammonia. Many repeated experiments indicated that a 10%Er³⁺ and 10%Yb³⁺ codoped Gd₂(WO₄)₃ phosphor is an optimal result for relatively strong red emission, which is more suitable for measurement and application.

Upconversion luminescence spectra were measured using HORIBA Jobin Yvon iHR550 spectrometers which employ a photon-counting detection system using a Hamamatsu R928 photomultiplier tube. Powder X-ray diffraction (XRD) patterns were recorded using an X-ray diffractometer (Panalytical Empyrean).

Results and discussion

The XRD pattern of the Gd₂(WO₄)₃:10%Er³⁺/10%Yb³⁺ phosphor is shown in Fig. 1. All the diffraction peaks of the present material match well with the monoclinic phase Gd₂(WO₄)₃ (JCPDS 23-1076) and there is an absence of impurity peaks, indicating that high purity monoclinic phase Gd₂(WO₄)₃ nanoparticles have been successfully synthesized. Under 980 nm excitation, the UC spectra of Gd₂(WO₄)₃:Er³⁺/Yb³⁺ at various temperatures are shown in Fig. 2. The intensity of each emission band shows an obvious decrease with increasing temperature. One red and two green emission bands of the Er³⁺ ions are observed. Fig. 3 gives the schematic for the UC luminescence mechanism for the Gd₂(WO₄)₃:Er³⁺/Yb³⁺ phosphor under 980 nm excitation. It is interesting to find that both the longer wavelength green emission band and the red emission band are strongly divided into two parts, which are attributed to the ⁴S_3/2 and ⁴F_9/2 levels being split into the ⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1) levels, respectively. For a typical upconversion mechanism of an Er³⁺ ion, the population at the ²H_11/2 and ⁴F_9/2 (⁴F_9/2(2)/⁴F_9/2(1)) levels can be realized through two-photon processes.^33,34 The ²H_11/2 and ⁴S_3/2 (⁴S_3/2(2)/⁴S_3/2(1)) levels are mainly populated via multiphonon relaxation of the ⁴F_7/2 level.³⁵ The luminescence bands centered at 525, 544, 551, 657 and 666 nm are derived from the ²H_11/2 → ⁴I_15/2, ⁴S_3/2(2) → ⁴I_15/2, ⁴S_3/2(1) → ⁴I_15/2, ⁴F_9/2(2) → ⁴I_15/2 and ⁴F_9/2(1) → ⁴I_15/2 transitions, respectively. Owing to the small energy gap, these sublevels can be thermally populated from each other indicating that these two levels are thermally coupled. In addition, it is well known that the ²H_11/2 and ⁴S_3/2 levels are thermal coupling levels. Therefore, any pair of the ²H_11/2, ⁴S_3/2(2) and ⁴S_3/2(1) levels is a thermal coupling level. According to the emission spectra (Fig. 2), the energy gaps of the thermal coupling levels are estimated as about 485, 600, 719, 234 and 205 cm⁻¹ for the ²H_11/2/⁴S_3/2(2), ²H_11/2/⁴S_3/2, ²H_11/2/⁴S_3/2(1), ⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1) levels, respectively.

Fig. 1 XRD pattern of Gd₂(WO₄)₃:10%Er³⁺/10%Yb³⁺ and the standard data for monoclinic phase Gd₂(WO₄)₃ (JCPDS 23-1076).

Fig. 2 Temperature-dependent upconversion emission spectra of the Gd₂(WO₄)₃:Er³⁺/Yb³⁺ phosphor with 980 nm excitation. The inset shows the emission spectra in the wavelength range 600–725 nm.

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

Since the relative population of thermal coupling levels follows the Boltzmann distribution, the FIR of the thermal coupling levels can be represented as:^36–38

where I₁ (lower level) and I₂ (upper level) are the integrated intensities of the two thermal coupling levels. A_i, g_i and ω_i are the radiative transition probability, the degeneracy and the angular frequency of corresponding transitions, respectively. T, k and C (C = g₂A₂ω₂/g₁A₁ω₁) are the absolute temperature, the Boltzmann constant and the proportionality factor, respectively. ΔE is the energy gap of the thermal coupling levels.

According to FIR technique, appropriate thermal coupling levels are a vital and decisive factor in the temperature sensing behavior. Thus, the temperature sensing behavior based on three pairs of thermal coupling levels (²H_11/2/⁴S_3/2(2), ²H_11/2/⁴S_3/2 and ²H_11/2/⁴S_3/2(1)) are investigated. The FIRs as a function of temperature are plotted in Fig. 4 over the temperature range 296–620 K. The corresponding data can be fitted with exponential functions, which can be expressed as: I₅₂₅/I₅₄₄ = 24.2 exp(−790/T), I₅₂₅/(I₅₄₄ + I₅₅₁) = 13.36 exp(−890/T), and I₅₂₅/I₅₅₁ = 28.5 exp(−968/T).

Fig. 4 (a) The FIRs based on thermal coupling levels ²H_11/2/⁴S_3/2(2), ²H_11/2/⁴S_3/2(1) and ²H_11/2/⁴S_3/2 as a function of temperature. (b) The sensitivity based on thermal coupling levels ²H_11/2/⁴S_3/2(2), ²H_11/2/⁴S_3/2(1) and ²H_11/2/⁴S_3/2 as a function of temperature.

For sensing applications, the absolute sensitivity (S) is a very important parameter, which has been calculated using the formula:^39–41

The corresponding sensitivity curves are calculated and plotted in Fig. 4(b). Compared with ordinary thermal coupling levels (²H_11/2/⁴S_3/2), the sensitivities of optical thermometers based on both ²H_11/2/⁴S_3/2(2) and ²H_11/2/⁴S_3/2(1) levels increase dramatically. Obviously, the sensitivity based on the ²H_11/2 and ⁴S_3/2(2) levels is the highest over the temperature range of 296–485 K. Upon further increasing the temperature, the sensitivity based on the ²H_11/2 and ⁴S_3/2(1) levels is dominant over the temperature range of 485–620 K. Based on the three pairs of thermal coupling levels, the sensitivities first increase, and then after a certain temperature they gradually decrease as the temperature increases. The corresponding maximal sensitivity values are 16.5 × 10⁻³ K⁻¹ at 395 K (²H_11/2/⁴S_3/2(2)), 15.9 × 10⁻³ K⁻¹ at 485 K (²H_11/2/⁴S_3/2(1)) and 8.1 × 10⁻³ K⁻¹ at 446 K (²H_11/2/⁴S_3/2). These significantly enhanced sensitivities are attributed to the energy gaps of the thermal coupling levels and the corresponding radiative transition probabilities (A_i). The above demonstrations reveal that the high sensitivity of an optical thermometer based on ²H_11/2/⁴S_3/2(2) (296–485 K) and ²H_11/2/⁴S_3/2(1) (485–620 K) was perfectly realized in the Gd₂(WO₄)₃:Er³⁺/Yb³⁺ phosphor, whose sensitivities are about twice those based on the traditional ²H_11/2 and ⁴S_3/2 levels. Table 1 lists the sensitivities of typical Er³⁺/Yb³⁺ doped optical thermometers.

Table 1 Values of the maximum sensitivity of typical Er³⁺/Yb³⁺ doped matrices, including the temperature of the maximum relative thermal sensitivity (S_max) and the temperature range

Material	Smax (× 10^−3 K^−1) (temperature K)	Temperature range (K)	References
Er, Yb:Ba5Gd8Zn4O21	3.2 (490 K)	260–490 K	42
Er, Yb:NaYF4	3.68 (363 K)	223–403 K	43
Er, Yb:YVO4	11.69 (380 K)	300–485 K	44
Er, Yb:SrWO4	14.98 (403 K)	299–518 K	45
Er, Yb:NaGd(WO4)2	11.49 (453 K)	293–573 K	46
Er, Yb:Gd2O3	8.4 (570 K)	298–723 K	47
Er, Yb:YAG	1.7 (404 K)	298–573 K	48
Er, Yb:NaGdTiO4	4.5 (480 K)	300–510 K	49
Er, Yb:Gd2(WO4)3	16.5 (395 K)	296–620 K	This work

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In addition, the temperature sensing behavior based on the Stark sublevels of ⁴S_3/2 and ⁴F_9/2 (⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1)) is investigated over the range 296–620 K. Fig. 5 show the FIRs (a) and the sensitivities (b) based on the Stark sublevels as a function of temperature. It is worth noting that the sensitivities based on the Stark levels exhibit a similar trend whereby they keep decreasing as the temperature increases over the whole temperature range. The sensitivity based on the ⁴S_3/2(2) and ⁴S_3/2(1) levels is higher than that based on ⁴F_9/2(2) and ⁴F_9/2(1), which is mainly attributed to the difference in the energy gap. The maximum sensitivities are observed to be 1.31 × 10⁻³ K⁻¹ and 1.27 × 10⁻³ K⁻¹ at 296 K based on the Stark sublevels of the Er³⁺ ions, ⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1), respectively.

Fig. 5 (a) The FIRs based on the Stark levels ⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1) as a function of temperature. (b) The sensitivities based on the Stark levels ⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1) as a function of temperature.

Conclusion

In summary, the monoclinic phase 10%Er³⁺/10%Yb³⁺ codoped Gd₂(WO₄)₃ phosphor is synthesized successfully via the co-precipitation method and is characterized using XRD. Under 980 nm excitation, visible luminescence arising from Stark sublevels to ground state transitions (⁴S_3/2(2) → ⁴I_15/2 (544 nm), ⁴S_3/2(1) → ⁴I_15/2 (551 nm), ⁴F_9/2(2) → ⁴I_15/2 (657 nm) and ⁴F_9/2(1) → ⁴I_15/2 (666 nm)) is observed. Using the FIR technique, the temperature sensing behavior is investigated based on thermal coupling levels ²H_11/2/⁴S_3/2(2), ²H_11/2/⁴S_3/2, ²H_11/2/⁴S_3/2(1), ⁴S_3/2(2)/⁴S_3/2(1) and ⁴F_9/2(2)/⁴F_9/2(1). The high sensitivity of the optical thermometer based on the ²H_11/2/⁴S_3/2(2) (296–485 K) and ²H_11/2/⁴S_3/2(1) (485–620 K) levels are perfectly realized, whose sensitivities are about twice more than those based on the traditional ²H_11/2 and ⁴S_3/2 levels. The obtained maximum sensitivity of the optical thermometer is 16.5 × 10⁻³ K⁻¹ at 395 K (²H_11/2/⁴S_3/2(2)). These merits indicate that the Gd₂(WO₄)₃:Er³⁺/Yb³⁺ phosphor is robust in high sensitivity temperature sensing.

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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