Optical temperature sensing of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ phosphor

Abstract

The hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ phosphor was prepared by the high temperature solid-state reaction. Novel six-photon ultraviolet up-conversion emissions in the range of 260–320 nm are observed under 1.54 μm excitation. Using the ratio of 523 nm and 542 nm fluorescence emissions, low-temperature optical temperature sensing in the resulting phosphor is studied. The temperature sensitivity of the resulting phosphor in the region of 5–300 K is obtained by measuring the temperature dependent fluorescence intensity ratio of the 523 nm and 542 nm emissions originated from two thermally coupled ²H_11/2 and ⁴S_3/2 emitting levels. The temperature sensitivity with a maximum sensitivity of 0.0022 K⁻¹ at 338 K is obtained experimentally. The results reveal that the resulting phosphor may be a promising material for the sensing system at low temperature.

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1. Introduction

Among the metal ions, rare earth ions have been reported as excellent luminescence centers in powders and glass-ceramics, due to their abundant 4f energy level structures.^1–4 Notably, every rare earth ion has a couple of adjacent thermally coupled levels with a very small energy gap of about 1000 cm⁻¹, such as Er³⁺: ²H_11/2 and ⁴S_3/2, Tm³⁺: ³F_2,3 and ³H₄, and so on. According to the Boltzmann distribution, the populations of these adjacent energy levels change the temperature environment around the rare earth ions.⁵ The luminescence intensity ratio originated from the two adjacent energy levels also changes with temperature. This temperature dependent fluorescence intensity ratio was reported as the precise evaluation scale of optical temperature sensing.^6,7 Recently, based on up-conversion luminescence emissions, optical temperature sensing in the medium temperature range from 300 K to 800 K were explored in Er³⁺ doped fluorotellurite glass under 800 nm excitation, Er³⁺/Yb³⁺ co-doped Y₂SiO₅ powders under 975 nm excitation, Tm³⁺/Yb³⁺ codoped oxyfluoride glass ceramic under 980 nm excitation, and Er³⁺/Yb³⁺ co-doped chalcogenide glass under 1.06 μm excitation.^8–10 The optical thermometry was achieved by measuring fluorescence intensity ratio of two thermally coupled levels ²H_11/2 and ⁴S_3/2 of Er³⁺ ion with slow increase of ambient temperature under a near-infrared diode laser excitation.

However, presently, the optical thermometry has been reported rarely in the low temperature range from 5 K to 300 K. Optical temperature sensors based on infrared up-conversion were mainly studied in the medium temperature range from 300 K to 800 K in oxides and glass ceramics with large phonon energy. In general, these materials have weak fluorescence emissions, due to thermal relaxation induced by large phonon energy.¹¹ It is necessary to increase excitation power of optical pumping source to measure weak fluorescence emission signals with help of highly sensitive spectrophotometers. Instead of oxides and glass ceramics, fluoride crystals were reported as most efficient up-conversion materials, due to their low phonon energy and the high solubility between cations and doping ions.^12–14 Thus, it remains a large space worthy of exploration in optical temperature sensors based on infrared up-conversion of fluoride crystals. Moreover, the thermal self-interference induced by the near-infrared diode laser excitation with the same power density will become smaller and smaller with increasing the wavelength of near-infrared diode laser from 800 nm to 1.06 μm.¹⁵ Thus, optical temperature sensors used long-wavelength near-infrared diode laser as excitation source will be a trend to obtain exact optical thermometry. In this work, we synthesize a novel hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ phosphor as the transducer element and study optical temperature sensing in low temperature range from 5 K to 300 K under 1.54 μm excitation. The transducer converts up-conversion excited green fluorescence emission of Er³⁺ ions into direct temperature information. The selection of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ phosphor as the doping host is motivated by its high chemical and thermal stability as well as low phonon energy, which favors suppression of nonradiative losses and thereby improves the luminescence of the optically active Er³⁺ ions.¹⁶

2. Experimental

The hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ phosphor was synthesized by a high temperature solid-state reaction. The starting materials are reagent grade 99.95% pure NaF and CaF₂, 99.99% pure YF₃ and 99.99% pure ErF₃. The samples were synthesized according to the molar composition of Na_0.82Ca_0.08Er_0.16Y_0.853F₄. Accurately weighed 5 g batches of raw materials were thoroughly mixed and moved into corundum crucibles. The batches with an addition of small amount of a fluoridating agent NH₄HF₂ were heated at 973 K for 6 h in an electric furnace.

Structures of the samples were investigated by X-ray diffraction (XRD) using a X'TRA (Switzerland ARL) equipment provided with Cu tube with Kα radiation at 1.54056 Å in the range of 10° ≤ 2θ ≤ 85°. Luminescence spectra were obtained by an Omni-λ3007 spectrophotometer with a photomultiplier tube equipped with a 1.54 μm laser as the excitation sources. Different temperatures were obtained using a cooling power closed cyclecryo cooler (DE-202).

3. Results and discussion

Fig. 1 shows representative X-ray diffraction pattern and crystal structure of the Na_0.82Ca_0.08Er_0.16Y_0.853F₄ at the room temperature. X-ray diffraction in Fig. 1(a) shows that the sample has the pure hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ structure, and all diffraction peaks can be indexed by the standard powder diffraction file card no.77-0936. The crystal structure of Na_0.82Ca_0.08Er_0.16Y_0.853F₄ is homeotypic to hexagonal β-NaYF₄ (ICSD 51920).¹⁷ Y is substituted by Ca and Er, and its crystal structure is shown in Fig. 1(b). The space group is P63/m and, for the Na_0.82Ca_0.08Er_0.16Y_0.853F₄, the unit cell dimensions are a = 5.986 Å and c = 3.547 Å. Each unit cell contains 1 formula unit. In the unit cell of Na_0.82Ca_0.08Er_0.16Y_0.853F₄, there are 4e and 2d two Na1 and Na2 cation sites, 2d Y, Er and Ca same cation site, and 6 h F anion site, respectively.

Fig. 1 (a) Power XRD pattern and (b) schematic 3D view of the crystal structure of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄.

Fig. 2(a) shows Raman scattering spectrum of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ at room temperature. The excitation light source of the Raman scattering spectrum is a 514 nm semiconductor laser. The result in the inset shows four strong peaks corresponding to phonon modes with energy 560 cm⁻¹, 602 cm⁻¹, 620 cm⁻¹ and 640 cm⁻¹, suggesting the hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ is an excellent up-conversion material with low phonon energy. The diffuse reflection spectrum of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ phosphor is shown in Fig. 2(b). According to the Kubelka–Munk equation in the ref. 18, the band gap of the semiconducting materials can be approximately calculated through measuring its diffuse reflection spectrum. The value of the band gap of the hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ crystal can be calculated approximately as 2.05 eV, shown in the inset of Fig. 2(b). The absorption peaks centered at 523 nm, 651 nm, 800 nm and 978 nm correspond to ⁴I_15/2 → ²H_11/2, ⁴I_15/2 → ⁴F_9/2, ⁴I_15/2 → ⁴I_9/2 and ⁴I_15/2 → ⁴I_11/2 transitions of Er³⁺ ions. A strong and broad absorption band centered at 1.54 μm in the near infrared region from 1.45 μm to 1.58 μm correspond to the ⁴I_15/2 → ⁴I_13/2 multiplet transitions of Er³⁺ ions.

Fig. 2 (a) Raman scattering spectrum and (b) diffuse reflection spectrum of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ at room temperature.

Under 1.54 μm excitation, with a pump-power density of 60 mW mm⁻², the hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ crystal emits ultraviolet and visible up-conversion fluorescence, as shown in Fig. 3. Five ultraviolet emission peaks centred at 270 nm, 298 nm, 328 nm, 358 nm, and 378 nm originate from the ⁴G_9/2 → ⁴I_15/2, ²K_13/2 → ⁴I_15/2, ²P_3/2 → ⁴I_15/2, ²G_9/2 → ⁴I_15/2, ²G_11/2 → ⁴I_15/2 transitions of Er³⁺ ion. Four visible and infrared emission peaks centred at 407 nm, 523 nm, 541 nm, 658 nm, and 805 nm originate from the ²H_9/2 → ⁴I_15/2, ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, ⁴F_9/2 → ⁴I_15/2, and ⁴I_9/2 → ⁴I_15/2 transitions of Er³⁺ ion. As reported, ultraviolet emissions of Er³⁺ were obtained in Yb³⁺–Er³⁺ codoped β-NaYF₄ by the energy transfer (ET) from Er³⁺ to Yb³⁺ and subsequently the back ET from Yb³⁺ to Er³⁺.¹⁹ However, ultraviolet emissions of Er³⁺ in the hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ crystal are observed without the assistance of Yb³⁺ ions. Thus, it is necessary to explore the ultraviolet and visible up-conversion processes. In order to understand the up-conversion mechanism, we investigate the pumping power dependence of up-conversion fluorescence intensity. For an unsaturated up-conversion process, the emission intensity I_UC is proportional to the n^th power of the infrared excitation intensity I_IR:

I_UC ∝ Iⁿ_IR, (1)

where n is the number of infrared photons absorbed per upconverted photon emitted.²⁰ Fig. 4 shows the double logarithmic plots of excitation power dependence of upconversion intensity under 1.54 μm excitation. If as a function of I_IR under 1.54 μm excitation, in which the n values are obtained from the slope of the linear fit. The values of slope n are 5.81, 5.71, 4.91, 5.01, 3.67, 2.86, 2.61, and 1.85 for the 270 nm, 298 nm, 328 nm, 378 nm, 406 nm, 542 nm, 658 nm, and 805 nm emissions, respectively, as shown in Fig. 4. These n values reveal that the above transitions may originate from six-, six-, five-, five-, four-, three-, three-, and two-photon up-conversion processes, respectively. The up-conversion emissions peaked at 270 nm and 298 nm come from the ⁴G_9/2 → ⁴I_15/2 and ²K_13/2 → ⁴I_15/2 transitions, which demonstrates that populating the ⁴G_9/2 and ²K_13/2 levels needs six 1.54 μm photons. The up-conversion emissions peaked at 328 nm and 378 nm originate from the transitions ²P_3/2 → ⁴I_15/2 and ²G_11/2 → ⁴I_15/2, which demonstrates that populating the ²P_3/2 and ²G_11/2 levels needs five1.54 μm photons. The up-conversion emissions peaked at 406 nm originates from the transition ²H_11/2 → ⁴I_15/2, indicating that populating the ²H_11/2 level needs four 1.54 μm photons. The up-conversion emissions peaked at 542 nm and 658 nm come from the transitions ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2, indicating that populating the ⁴S_3/2 and ⁴F_9/2 levels needs three 1.54 μm photons. The up-conversion emissions peaked at 805 nm comes from the transition ⁴I_9/2 → ⁴I_15/2, indicating that populating the ⁴I_9/2 level needs two 1.54 μm photons. The number of photons decrease sharply when the excitation power exceeds some digital, owing to the competition between the linear decay and the upconversion processes for the depletion of the intermediate excited states.²⁰

Fig. 3 Upconversion emission spectrum of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ at room temperature under 1.54 μm excitation.

Fig. 4 Log intensity vs. log excitation power density of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄.

To further recognize the up-conversion mechanism in the sample, possible up-conversion processes are schematically given in the energy level diagram of Er³⁺, as shown in Fig. 5. The energy transfer between Er³⁺ ions is thought as the main upconversion mechanism, when the Er³⁺ concentration of optical active center is high.¹ The Er³⁺–Er³⁺ energy transfer under 1.54 μm excitation is achieved by the following thirteen transfer processes:

(1) ⁴I_15/2 (Er³⁺) + 1.54 μm photon → ⁴I_13/2 (Er³⁺), (2)

(2) ⁴I_13/2 (Er³⁺) + ⁴I_15/2 (Er³⁺) → ⁴I_15/2 (Er³⁺) + ⁴I_13/2 (Er³⁺), (3)

(3) ⁴I_15/2 (Er³⁺) + 1.54 μm photon → ⁴I_13/2 (Er³⁺), (4)

(4) ⁴I_13/2 (Er³⁺) + ⁴I_13/2 (Er³⁺) → ⁴I_15/2 (Er³⁺) + ⁴I_9/2 (Er³⁺), (5)

(5) ⁴I_15/2 (Er³⁺) + 1.54 μm photon → ⁴I_13/2 (Er³⁺), (6)

(6) ⁴I_13/2 (Er³⁺) + ⁴I_9/2 (Er³⁺) → ⁴I_15/2 (Er³⁺) + ²H_11/2 (Er³⁺), (7)

(7) ²H_11/2 (Er³⁺) → ⁴S_3/2 (Er³⁺) + phonons (8)

(8) ⁴I_15/2 (Er³⁺) + 1.54 μm photon → ⁴I_13/2 (Er³⁺), (9)

(9) ⁴I_13/2 (Er³⁺) + ⁴S_3/2 (Er³⁺) → ⁴I_15/2 (Er³⁺) + ²H_9/2 (Er³⁺), (10)

(10) ⁴I_15/2 (Er³⁺) + 1.54 μm photon → ⁴I_13/2 (Er³⁺), (11)

(11) ⁴I_13/2 (Er³⁺) + ²H_9/2 (Er³⁺) → ⁴I_15/2 (Er³⁺) + ²P_3/2 (Er³⁺), (12)

(12) ⁴I_15/2 (Er³⁺) + 1.54 μm photon → ⁴I_13/2 (Er³⁺), (13)

(13) ⁴I_13/2 (Er³⁺) + ²P_3/2 (Er³⁺) → ⁴I_15/2 (Er³⁺) + ⁴D_7/2 (Er³⁺). (14)

Fig. 5 Six-step upconversion process between two erbium Er³⁺ ions under 1.54 μm excitation.

As a result, the ⁴G_9/2, ²K_13/2, ²P_3/2, ²G_9/2, ²G_11/2, ²H_9/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, and ⁴I_9/2 levels are populated, and eight emission peaks at 270 nm, 298 nm, 328 nm, 378 nm, 406 nm, 542 nm, 658 nm, and 805 nm emissions are originated from the ⁴G_9/2 → ⁴I_15/2 (270 nm), ²K_13/2 → ⁴I_15/2 (298 nm), ²P_3/2 → ⁴I_15/2 (328 nm), ²G_11/2 → ⁴I_15/2 (378 nm), ²H_11/2 → ⁴I_15/2 (406 nm), ⁴S_3/2 → ⁴I_15/2 (542 nm), ⁴F_9/2 → ⁴I_15/2 (658 nm), and ⁴I_9/2 → ⁴I_15/2 (805 nm) transitions of Er³⁺ ions, respectively.

As reported in literature, ²H_11/2 and ⁴S_3/2 levels of Er³⁺ ion were proved as thermally coupled levels to achieve optical thermometry by measuring fluorescence intensity ratio of two thermally coupled with slow increase of ambient temperature under a near-infrared diode laser excitation.^8,10 To verify that the ²H_11/2 and ⁴S_3/2 levels are thermally coupled in the hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄, the 523 nm and 542 nm upconversion emissions were studied as a function of temperature at 150 K and 300 K. Fig. 6 shows the corresponding emission spectra for the sample at 150 K and 300 K under 1.54 μm excitation. To clearly observe the relative change in the spectra, the emission intensity has been normalized. It can be seen from Fig. 6 that the position of the emission bands hardly changes with the increase of temperature, and an evident enhancement has been achieved for the 523 nm emission at 300 K compared with that at 150 K. In fact, the intensity ratio between 523 nm and 542 nm luminescence obviously varies. The change in the ratio is attributed to the corresponding energy gap between the ²H_11/2 and ⁴S_3/2 levels, which allows the ions on ⁴S_3/2 state to be thermally populated onto the ²H_11/2 state with the increase of temperature.

Fig. 6 The normalized upconversion emission spectra at 150 K and 300 K under 1.54 μm excitation of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄.

In order to verify that the ²H_11/2 and ⁴S_3/2 levels are suitable to be used as optical temperature sensing, the upconversion emission spectra of hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ are studied as a function of temperature ranging from 5 K to 300 K under 1.54 μm excitation. According to the theory in ref. 6 and 7, the FIR (fluorescence intensity ratio) from the thermally coupled levels of active ions is modified as

FIR = I_U/I_L = A exp(−ΔE/k_BT), (15)

where I_U and I_L are intensities of emissions from the upper and the lower thermally coupled levels, A is a constant, ΔE is the energy difference between thermally coupled levels, k_B is the Boltzmann constant, and T is the absolute temperature. Fig. 7(a) presents the FIR between 523 nm and 542 nm emissions as a function of temperature in the range of 5–300 K. The experimental data are fitted by eqn (15). It can be observed that the fittings agree well with the experimental data. The A is fitted to be 1.91, and ΔE is fitted to be 675.3 cm⁻¹, close to the value calculated from the emission spectra (670.27 cm⁻¹). These results confirmed that the ²H_11/2 and ⁴S_3/2 states are thermally coupled levels. Considering practical applications, it is necessary to investigate the variation of sensitivity with temperature. For this purpose, the relative sensitivity S has been defined as

S = FIR × ΔE/k_BT². (16)

Fig. 7 (a) FIR between the 523 nm and 542 nm upconversion emissions as a function of temperature in the range of 5–300 K under 1.54 μm excitation. (b) Sensor sensitivity as a function of the temperature for excitation at 1.54 μm.

Fig. 7(b) presents the S as a function of temperature. It can be seen that the sensitivity keeps increasing in our experimental temperature range and reaches the maximum value 0.00221 K⁻¹ at 338 K. It means that the hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ is sensitive in room temperature area.

As for above route to achieve temperature induced population re-distributions,²¹ the temperature dependent population re-distribution ability (PCA) can be defined as

where I_U and I_L are fluorescence intensity generated by the radiative transitions from the upper and lower thermally coupled levels to ground level. According to eqn (15) and (17), the PCA can be expressed aswhere A is a fitting constant in Fig. 7(a), ΔE is the fitting energy difference between thermally coupled levels, k_B is the Boltzmann constant, and T is the absolute temperature. Fig. 8 shows the temperature dependent PCA of ²H_11/2 and ⁴S_3/2 thermally coupled levels. It can be seen that the value of the PCA is strong dependent on absolute temperature. When the temperature is lower than 100 K, the PCA is very low. On the contrary, the PCA increases sharply, when the temperature is higher than 100 K.

Fig. 8 Temperature dependent PCA of ²H_11/2 and ⁴S_3/2 thermally coupled levels.

4. Conclusion

Novel hexagonal Na_0.82Ca_0.08Er_0.16Y_0.853F₄ phosphor was prepared by high temperature solid-state reaction. UV ultraviolet upconversion emissions centered at 270 nm, 298 nm, 328 nm, 358 nm, and 378 nm are observed under 1.54 μm excitation. Low-temperature optical temperature sensing is studied through measuring temperature dependent fluorescence intensity ratio of 523 nm and 542 nm emissions originated from two thermally coupled ²H_11/2 and ⁴S_3/2 levels. The temperature sensitivity with a maximum sensitivity of 0.0022 K⁻¹ was experimentally obtained at 338 K. The resulting phosphor may be promising materials for the sensing system in low temperature.

Acknowledgements

This work was supported by National Natural Science Foundation of China (NSFC51032002, 11374162, 11274173), the key Project of the National High Technology Research and Development Program (“863” Program) of China (no. 2011AA050526), the Science and Technology Support Plan of Jiangsu Province (BE2011191), Natural Science Youth Foundation of Jiangsu Province (BK20130865), and the Scientific Research Foundation of Nanjing University of Posts and Telecommunications (Grant no. NY213022, and NY213113).

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