Optical temperature sensing of NaYbF4: Tm3+ @ SiO2 core-shell micro-particles induced by infrared excitation

Xiangfu Wang; Jin Zheng; Yan Xuan; Xiaohong Yan

doi:10.1364/OE.21.021596

1. Introduction

In recent years, research on rare-earth ions doped luminescence heterojunction materials has developed tremendously because of their wide applications in nanoscale biological fluorescence label, solar cells, laser materials, tricolor display technology and so on [1–6]. Inorganic fluorides have the advantages of intrinsic low phonon energy and high chemical stability, and thereby are often employed as core-shell host materials to obtain the desirable upconversion and downconversion luminescence emissions. Chen et al. reported a method to achieve efficient dual-mode luminescence of Eu³⁺ in β-NaGdF₄: Yb,Tm/NaGdF₄: Eu core/shell nanocrystals [7]. Capobianco and associates reported a method to enhance the Er³⁺ upconversion luminescence in active-core/active-shell NaGdF₄:Er³⁺,Yb³⁺/NaGdF₄: Yb³⁺ nano-particles [8]. Liu et al. reported a strategy to eliminate deleterious cross-relaxation from energy transfer between activator centre ions by gadolinium sublattice-mediated energy migration in NaGdF₄:Yb,Tm/ NaGdF₄:X nano-particles [9]. Prasad and associates reported the application of α-NaYbF₄:Tm³⁺/CaF₂ core/shell nanoparticles with efficient near-infrared to near-infrared upconversion in high-contrast deep tissue bioimaging [10]. Despite excellent luminescence emissions, the optical-thermal character in the core/shell fluoride nanomaterials has been reported rarely, and it remains a large area worthy of exploration in photoluminescence process.

Among fluoride materials, lanthanide ions doped fluoride nano-crystals are suitable to be as optical temperature sensing materials, due to their low phonon energy, high solubility, and high upconversion efficiency. Some trivalent rare-earth ions have a couple of adjacent thermally coupled levels with very small energy gap about 1000 cm⁻¹, such as Er³⁺: ²H_11/2 and ⁴S_3/2, Tm³⁺: ³F_2,3 and ³H₄, Dy³⁺: ⁴I_15/2 and ⁴F_9/2, Eu³⁺: ⁵D₁ and ⁵D₀, Nd³⁺: ⁴F_5/2 and ⁴F_3/2, and Pr³⁺: ³P₁ and ³P₀ and so on [11–19]. These adjacent energy levels can be thermally populated and depopulated through modifying the host temperature environment around the rare earth ions. The luminescence intensity ratio originated from the two adjacent energy levels will change with external temperature. This temperature dependent fluorescence intensity ratio was reported as the precise probe in optical temperature fiber sensing [19,20]. Presently, optical temperature sensors based on intensity ratios were mainly studied in the medium temperature range from 280 K to 350 K in organic dyes, thermoresponsive polymers, mesoporous silica NPs, QDs, and Ln³⁺-based up-converting NPs and β-diketonate complexes [21–26]. The optical thermometry has been reported rarely in the low temperature range from 5 K to 300 K.

However, lanthanide ions doped fluoride nano-crystals are easy oxidized at high temperature. To overcome this, SiO₂ coated fluoride nano-crystals may be a good core-shell heterojunction materials for optical temperature sensor. The SiO₂ shell protects the NaYbF₄:Tm³⁺ luminescent centre in the core from non-radiative decay caused by surface defects [8], and also from oxidization at high temperature. Among the fluoride crystals, the NaYbF₄ crystals has low phonon energy and high chemical stability, and offers sufficient Yb³⁺ ions with long excited level life as energy-transfer donor ion to sensitize an energy acceptor Tm³⁺ ion under 980~1000 nm excitation [27]. It was reported that the hexagonal Tm³⁺-doped NaYbF₄ crystals emitted strong blue luminescence and weak red luminescence [28], and the cubic Tm³⁺-doped NaYbF₄ crystals emitted strong red luminescence and weak blue luminescence under 980 nm excitation [10]. Optical-thermal character in blue and red wavelength regions will be studied simultaneously in the hexagonal and cubic mixed Tm³⁺-doped NaYbF₄. Thus, in this work, we synthesize the hexagonal and cubic mixed NaYbF₄:Tm³⁺/SiO₂ core-shell micro-particles, and study corresponding optical-thermal property under 980 nm excitation.

2. Experimental

All the chemicals used as received in the synthesis process were not further purified. Deionized water was used throughout. The raw materials are ethanol (AR), NaF (99.95%), 99.99% Yb₂O₃ and Tm₂O₃, hydrochloric acid (AR), tetraethoxysilane (AR).

NaYbF₄:Tm³⁺ micro-particles were first prepared by a solvothermal method. Then tetraethoxysilane (TEOS) hydrolyzed on the surface of NaYbF₄:Tm³⁺ to form the silica shell. At last, NaYbF₄:Tm³⁺/SiO₂ core-shell micro-particles were obtained through annealing. In a typical synthesis of cubic and hexagonal phases mixed NaYbF₄:Tm³⁺ micro-particles, rare earth oxides Ln₂O₃ (Ln = Yb and Tm) were dissolved in hydrochloric acid (1mol Ln₂O₃: 3.5mol Hcl) to prepare 0.2 mol/L LnCl₃ solution. An aqueous solution of lanthanide salt YbCl₃ and TmCl₃ was mixed with 27 mL deionized water and 0.2234g EDTA-2Na under thorough stirring. Then, 8 mL NaF (0.2016 g) solution was dropwise added to the mixture. After vigorous stirring at room temperature for about 30 min, the colloidal solution were transferred into a 60 mL Teflon-lined autoclave, sealed and heated at 193.5 °C for 24 h. The final products were collected, washed several times with ethanol, and purified by centrifugation.

NaYbF₄:Tm³⁺/SiO₂ core-shell micro-particles were prepared by a modified stöber method. 30 mg NaYbF₄:Tm³⁺ micro-particles were dispersed in 80 mL 2-propanol by sonication for 30 min. Then 8.94 mL 28% ammonia, 7.5 mL deionized water, and 0.1 mL TEOS were added into the mixture. The mixture was then placed into an ultrasonic bath and kept for 2 h. The product was collected, washed and dried, and sinteringed sample at 500 °C for 2h.

Structures of the samples were investigated by X-ray diffraction (XRD) using a X'TRA (Switzerland ARL) equipment provided with Cu tube with K_α radiation at 1.54056 Å in the range of 10°≤2θ≤80°. The size and shape of the samples were observed by a transmission electron microscope (JEOL JEM-2100) equipped with an energy dispersive x-ray spectroscope (EDS). Luminescence spectra were obtained by the Edinburgh Instruments FLS920 Spectrophotometer with a photomultiplier tube equipped with one adjustable laser diode as the excitation sources. Different temperatures were obtained using a cooling power closed cyclecryo cooler (DE-202).

3. Results and discussion

Morphology and phase structure of NaYbF₄:1.6%Tm³⁺ nano-spheres and NaYbF₄: 1.6%Tm³⁺ active-core/SiO₂ shell micro-particles are analyzed by transmission electron microscope, selected area electron diffraction, and XRD measurements. Transmission electron microscope and selected area electron diffraction observations in Fig. 1(a,b) confirm the well-dispersed polycrystal structure of NaYbF₄:1.6% Tm³⁺ nano-spheres. The mean size of the NaYbF₄:1.6%Tm³⁺ nano-spheres is evaluated as 140 nm. A transmission electron microscope image of silica-coated NaYbF₄:1.6%Tm³⁺ nano-spheres is displayed in Fig. 1(c). It can be seen clearly that the NaYbF₄:1.6%Tm³⁺ active- core/SiO₂ shell micro-particles have a core-shell structure, and the thickness of the silica shell is about 100 nm. The selected area electron diffraction in Fig. 1(d) shows the single-crystalline phase structure of NaYbF₄: 1.6%Tm³⁺ nano-spheres. The phase conversion from polycrystal to single-crystalline phase structure originated from the heat treatment at 500 °C for 2h. The XRD patterns in Fig. 2 show that diffraction peaks of NaYbF₄:1.6%Tm³⁺ nano-spheres and NaYbF₄:1.6%Tm³⁺ active- core/SiO₂ shell micro-particles are assigned to the hybrid cubic and hexagonal phase NaYbF₄, and the cubic α-NaYbF₄ was main constituent.

Fig. 1 Transmission electron microscope micrographs and selected area electron diffraction patterns of (a,b) NaYbF₄: 1.6%Tm³⁺ nano-spheres, (c,d) NaYbF₄: 1.6% Tm³⁺ active-core/SiO₂ shell micro-particles.

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Fig. 2 Power XRD patterns of (a) NaYbF₄:1.6%Tm³⁺ nano-spheres, (b) NaYbF₄: 1.6%Tm³⁺ active-core/SiO₂ shell micro-particles, (c) the standard data for α-NaYbF₄ (JCPDS 77-2043), (d) the standard data for β-NaYbF₄ (JCPDS 27–1427).

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Figure 3(a) shows photoluminescence spectra of NaYbF₄:Tm³⁺ nano-spheres under 980 nm excitation at room temperature. It can be seen that the photoluminescence spectra of Tm³⁺ ions consist of five emission peaks at 365 nm, 450 nm, 478 nm, 617 nm, 697 nm, and 798 nm, which are assigned to the ¹D₂→³H₆ (365 nm), ¹D₂→³F₄ (450 nm), ¹G₄→³H₆ (478 nm), ¹G₄→³F₄ (617 nm), ³F_2,3→³H₆ (798 nm), and ³H₄→³H₆ transitions of Tm³⁺ ions, respectively. The intensity of 450 nm and 478 nm blue emissions is found to increase and then decrease with increase of the Tm³⁺ concentration, a maximum existing at about 1.6mol%. The intensity of 697 nm and 798 nm emissions increase with increase of the Tm³⁺ concentration, which is orignated from the concentration quenching caused by the cross relaxation energy transfer ¹D₂ + ³H₆ → ³F₂ + ³H₅ between Tm³⁺ ions [29,30]. Thus, NaYbF₄:1.6%Tm³⁺ nano-spheres is chosen to be as active-core for NaYbF₄:1.6%Tm³⁺/SiO₂ core-shell micro-particles. Figure 3(b) shows photoluminescence spectra of NaYbF₄:1.6%Tm³⁺/ SiO₂ nano-spheres and core-shell micro-particles under 980 nm excitation at room temperature. The shifting of the peaks is not observed between nano-spheres and core-shell micro-particles. The intensity of 365 nm, 450 nm and 478 nm emission peaks of the NaYbF₄:1.6% Tm³⁺/SiO₂ core-shell micro-particles is a little weaker than that of the NaYbF₄:1.6%Tm³⁺ nano-spheres. The intensity of 697 nm and 798 nm emission peaks of the NaYbF₄:1.6% Tm³⁺/SiO₂ core-shell micro-particles are much weaker than that of the NaYbF₄:1.6%Tm³⁺ nano-spheres. The decrease of infrared emission intensity is originated from a fact that SiO₂ shell changed the surface property of the NaYbF₄:1.6%Tm³⁺ nano-spheres and inhibited the quenching from surface defect [8].

Fig. 3 (a) Photoluminescence spectra of NaYbF₄:xTm³⁺ (x = 0.4%, 0.8%,1.6%, 2.4%, and 3.2%) nano-spheres under 980 nm excitation, (b) Photoluminescence spectra of NaYbF₄:1.6%Tm³⁺ nano-spheres and NaYbF₄:1.6%Tm³⁺/SiO₂ core-shell micro-particles under 980 nm excitation at room temperature.

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The NaYbF₄:1.6%Tm³⁺/SiO₂ core-shell micro-particles are suitable to be used as optical temperature sensor using Tm³⁺ upconversion emissions; because the SiO₂ shell protects the NaYbF₄:1.6%Tm³⁺ core from oxidation. To understand the properties related to optical temperature sensors, the temperature dependent Tm³⁺ anti-Stokes emissions from the NaYbF₄:Tm³⁺/SiO₂ core-shell micro-particles were studied in the temperature range of 100~700 K, and the pumping power of 980 nm diode laser is set at 137 mW/mm². The emission spectra at different temperatures are shown in Fig. 4. It is interesting to observe that the intensities for these emissions are greatly decreased as temperature rises without changing the peak positions of the emissions. The intensities for 450 nm and 798 nm emissions decreased sharply and that of 478 nm and 697 nm emissions decreased slowly. In other words, the intensity decrease with temperature for different peaks in the emission spectrum is different. This change is originated from local ligand fields around Tm³⁺ sites, which can be expressed by the effective bandwidth from emission spectra [31]. From the emission spectra of different temperatures, the effective bandwidth Δλ_eff of the emission band can be obtained:

{Δ λ}_{e f f} = \frac{\int I (λ) d λ}{I_{\max}},

where I(λ) is the emission intensity at wavelength λ and I_max is the emission intensity at peak emission wavelength. The effective bandwidth Δλ_eff usually changes with local ligand fields around Tm³⁺ sites. Figure 5 shows temperature dependent effective bandwidth Δλ_eff of anti-Stokes fluorescence emissions of NaYbF₄:Tm³⁺/ SiO₂ core-shell micro-particles. With the temperature increase from 100 k to 700K, the values Δλ_eff for the 697 nm (³F_2,3→³H₆) emission inhomogeneously increases from 16.76 nm to 22.94 nm, and the values Δλ_eff for the 798 nm emission homogeneously increases from 19.79 nm to 25.35 nm. The values Δλ_eff for the 450 nm (¹D₂→³F₄) emission firstly increases and then decreases with the temperature increase, a maximum 11.85 nm existing at about 600 K. Similarly, the values Δλ_eff for the 478 nm (¹G₄→³H₆) emission firstly increases and then decreases with the temperature increase, a maximum 13.82 nm existing at about 300 K. The inhomogeneous broadening of the emission spectra shows that the NaYbF₄:1.6%Tm³⁺/SiO₂ core-shell micro-particles would be suitable to be as optical temperature sensor through modifying Tm³⁺ upconversion emissions from thermally coupled levels.

Fig. 4 Temperature dependent Tm³⁺ anti-stokes fluorescence emissions from the NaYbF₄: Tm³⁺/sio₂ core-shell micro-particles.

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Fig. 5 Temperature dependent effective bandwidth Δλ_eff of anti-Stokes fluorescence emissions of NaYbF₄: Tm³⁺ active-core/SiO₂ shell micro-particles.

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In order to verify that the ¹D₂ and ¹G₄, ³F₂ and ³H₄ levels are thermally coupled, the upconversion emission spectra of NaYbF₄:1.6%Tm³⁺/SiO₂ core-shell micro-particles are studied as a function of temperature ranging from 100 K to 700 K under 980 nm excitation. According to the theory in Ref [19,20], the ratio of the luminescence R from each thermally coupled levels of active ions is modified as

R = \frac{I_{U}}{I_{L}} = A e^{\frac{— Δ E}{K_{B} T}} + B

where I_U and I_L are intensities of emissions from the upper and the lower thermally coupled levels, A and B are constants, A is a constant that depends on the experimental system and intrinsic spectroscopic parameters [20]. ΔE is the energy difference between thermally coupled levels, k_B is the Boltzmann constant, and T is the absolute temperature. Figure 6(a) shows the R between 697 nm and 798 nm upconversion emissions of Tm³⁺ ions as a function of temperature in the range of 100~700 K under 980nm excitation. The R is obtained throughthe intensity ratio between the corresponding 697 nm and 798 nm emission spectra. The R between 697 nm and 798 nm emissions increases sharply with temperature increase. The experimental data are fitted by Eq. (2). It can be observed that the fittings agree well with the experimental data. ΔE is fitted to be 1794 cm⁻¹, close to the experimental value 1815.9 cm⁻¹ calculated from the emission spectra. These results confirm that the ³F₂ and ³H₄ are thermally coupled levels in the temperature range from 400K to 700 K [19]. Figure 6(b) shows the R between 450 nm and 478 nm upconversion emissions of Tm³⁺ ions as a function of temperature in the range of 100~700 K under 980 nm excitation. The R is obtained through the ratio between the integrated areas under the corresponding 450 nm and 478 nm emission spectra. Differently, the R between 450 nm and 478 nm firstly decreases sharply and then increases slowly with temperature increase. This trend suggests that it exists nonradiative relaxation between ¹D₂ and ¹G₄ in the temperature range of 100~400 K and temperature dependent population re-distribution in the temperature range of 400~700 K. The experimental data in the temperature range of 400~700 K is fitted by Eq. (2). Only at higher temperatures the behavior of the intensity ratio R shows that the population of the states is following the laws of thermalization. It can be observed that the fittings only agree with the experimental data in high temperature range. However, the ΔE is fitted to be 2268.5cm⁻¹, deviating to the experimental value 6476.76 cm⁻¹ calculated from the emission spectra. These results confirm that the ¹D₂ and ¹G₄ are not suitable thermally coupled levels for optical temperature sensors.

Fig. 6 (a) R between the 697 nm and 798 nm, (b) R between the 450 nm and 478 nm, upconversion emissions as a function of temperature in the range of 100~700 K under 980 nm excitation.

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The mechanism on optical thermal sensing through Yb³⁺-Tm³⁺ energy transfer under 980 nm excitation is proposed in Fig. 7. If the temperature around NaYbF₄:Tm³⁺/SiO₂ core-shell micro-particles is low and keeps a constant, the population of the ¹D₂ level is achieved by four successive transfer processes from Yb³⁺ to Tm³⁺ ions under 980 nm excitation. The energy transfer processes from the ²F_5/2 of Yb³⁺ to ³H ₅, ¹G₄ and ¹D₂ levels of Tm³⁺ are non-resonant and add the amount of energy that has to be bridged producing phonons. Since the large energy mismatch (3516 cm⁻¹) between ¹G₄→¹D₂ (Tm³⁺) and ²F_5/2→²F_7/2 (Yb³⁺) levels, the process ³F₃ (Tm³⁺) + ³H₄ (Tm³⁺) → ³H₆ (Tm³⁺) + ¹D₂ (Tm³⁺) may alternatively play the most important role in populating ¹D₂ [32,33]. As a result, the ¹D₂, ¹G₄, ³F₂, and ³H ₄ levels are populated, and five emission peaks at 365 nm, 450 nm, 478 nm, 697 nm, and 798 nm are originated from the ¹D₂→³H₆ (365 nm), ¹D₂→³F₄ (450 nm), ¹G₄→³H₆ (478 nm), ³F₂→³H₆ (697 nm), and ³H₄→³H₆ (798 nm) transitions of Tm³⁺ ions, respectively. With increase of temperature, most of ions on ³H ₄ levels are excited onto upper ³F₃ and ³F₂ levels through thermal population [19]. The intensity 798 nm infrared emissions decreases slowly and the intensity 697 nm red emissions increases slowly with increase of temperature. As a result, the intensity ratio R between 697 nm and 798 nm emissions increases with increase of temperature, which agrees with the experimental data in Fig. 6(a). It means that temperature induced population re-distributions occurs, if the NaYbF₄:Tm³⁺/SiO₂ core-shell micro- particles is heated slowly. As for the ¹D₂ and ¹G₄ levels, the nonradiative resonant cross relaxation energy transfer ¹D₂ (Tm³⁺) + ³H₆ (Tm³⁺) → ¹G₄ (Tm³⁺) + ³F₄ (Tm³⁺) may be a main mechanism to depopulate the ¹D₂ level and populate ¹G₄ level at low temperature (100 K~400 K). At high temperature (400 K~700 K), cross-relaxation energy transfer ³F₄ (Tm³⁺) + ¹G₄ (Tm³⁺) → ³H₆ (Tm³⁺) + ¹D₂ (Tm³⁺) may be a main mechanism to populate the ¹D₂ level and depopulate ¹G₄ level.

Fig. 7 The mechanism on optical thermal sensing through Yb³⁺-Tm³⁺ energy transfer under 980 nm excitation

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

P C A = \frac{I_{U}}{I_{U} + I_{L}},

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 Eqs. (2) and Eqs. (3) the PCA can be expressed as

P C A \approx \frac{A}{A + e^{Δ E / k_{B} T}},

where A is a fitting constant in Fig. 6, ΔE is the fitting energy difference between thermally coupled levels, k_B is the Boltzmann constant, and T is the absolute temperature. Figure 8 shows the temperature dependent PCA of ¹D₂ /¹G₄ and ³F₂/³H₄ 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 400 K, the PCA is very low. When the temperature is higher than 400 K, the PCA increases sharply, and the ¹D₂ /¹G₄ thermally coupled levels exhibit the higher PCA.

Fig. 8 Temperature dependent PCA of (a) ¹D₂ and ¹G₄ and (b) ³F₂ and ³H₄ thermally coupled levels.

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Considering practical applications, it is also necessary to understand the change of sensitivity with temperature. The sensitivity of optical thermometry is the rate of change in the R in response to the variation of temperature [19]. To allow the comparison between the sensitivities obtained from different thermally coupled levels, the relative sensitivity S_R and the absolute sensitivity S_A are defined as

S_{R} = \frac{d R}{d T} = R \frac{Δ E}{K_{B} T^{2}},

S_{A} = \frac{1}{R} \frac{d R}{d T} = \frac{Δ E}{K_{B} T^{2}}

where the ΔE is the energy difference between thermally coupled levels, k_B is the Boltzmann constant, T is the absolute temperature, and R is the luminescence ratio between the two thermally coupled levels [19,20]. Equation (6) suggests that using pairs of levels with larger energy differences are in favor of higher sensitivity. If energy difference is too large, there won’t be any thermalization at low or room temperature. The absolute sensitivity S_A decreases with the increase of temperature, shown in Fig. 9(a). The relative sensitivity S_R firstly sharply decreases and then slowly increases with the increase of temperature, a minimum S_R = 5.4 × 10⁻⁴ existing at T = 430 K, shown in Fig. 9(b). It means that NaYbF₄:Tm³⁺/SiO₂ core-shell micro-particles are sensitive in low temperature area.

Fig. 9 (a) Sensor sensitivity S_A and (b) relative sensor sensitivity S_R as a function of the temperature for excitation at 980 nm.

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4. Conclusion

NaYbF₄:Tm³⁺@SiO₂ core-shell micro-particles were synthesized by combining hydrothermal process with ultrasonic coating. The optical thermometry in the rage of 100~700 K is presented using the luminescence intensity ratio technique of adjacent thermally coupled ¹D₂ /¹G₄ and ³F₂ /³H₄ levels under 980 nm excitation. The temperature modification of the crystal field around the Tm ions is observed through analyzing the temperature variation of the effective bandwidth. The minimum sensitivity and revolution for the optical low-temperature sensor based on NaYbF₄:Tm³⁺@SiO₂ core-shell micro-particles are 5.4 × 10⁻⁴ K⁻¹ at 430 K. Temperature dependent population re-distribution ability from ¹D₂ /¹G₄ thermally coupled levels reaches more than 80% at higher temperature range. This work offers a new method to achieve optical temperature sensing with core-shell micro-particles.

Acknowledgments

This work was supported by National Natural Science Foundation of China (NSFC51032002, 11247033, 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).

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Optical temperature sensing of NaYbF₄: Tm³⁺ @ SiO₂ core-shell micro-particles induced by infrared excitation

Abstract

1. Introduction

2. Experimental

3. Results and discussion

4. Conclusion

Acknowledgments

References and links

Cited By

Figures (9)

Equations (6)

Optics Express