Investigation on optical temperature sensing behaviour for Y4.67Si3O13:Tm3+,Yb3+ phosphors based on upconversion luminescence

Guibin Chen; Jia Zhang

doi:10.1364/OME.8.001841

1. Introduction

As is known, temperature is one of fundamental parameters of thermodynamics in scientific research, industrial manufacturing, life activities, and so on [1]. Conventional methods for temperature measurements usually utilize liquid-filled thermometers, thermocouples and thermistors, which are not applicable for the objects below 10 μm size [2]. Hence, in recent years, temperature sensing by employing up-conversion (UC) luminescence of trivalent rare earth (RE) ions has been paid much attention. This method is based on the temperature dependence of the fluorescence intensity ratio (FIR) of two thermally coupled energy levels, and thus can provide a non-contact temperature measurement [3]. This non-contact FIR technique could overcome some limitations of spatial resolution and accuracy of detection [4]. It can also prevent errors in measurements arising from power fluctuations of the excitation source, variations on the concentration of luminescent particles and inhomogeneities, since the thermally coupled levels follow the Boltzmann distribution law [5]. To date, several RE ions with UC luminescence have been utilized for the FIR technique, for instance, Er³⁺, Ho³⁺, and Tm³⁺ [6–8]. However, to enhance the luminescence of these ions, Yb with a larger absorption cross section at near infrared (NIR) usually acts as a photosensitizer because the Er³⁺/Ho³⁺/Tm³⁺ ions have small absorption cross section or lack of matched energy levels with 980 nm photon [9].

It is known that fluoride-based phosphors usually show high luminous efficiency due to the low phonon energy. Unfortunately, the fluoride hosts are sensitive to oxygen surface contamination, so the luminescence properties may be influenced and the application could be limited [10]. On the other hand, the fluoride source is harmful and easily causes environmental pollution in the preparation. Instead, the oxide compounds could demonstrate high chemical stability, easy preparation, and environment-friendliness. In this work, to explore new UC phosphors for optical temperature sensing, the Y_4.67Si₃O₁₃ compound was employed as the host material and the Tm³⁺-Yb³⁺ luminescent ions were codoped. The UC luminescence properties and the sensor sensitivity were evaluated.

2. Experimental

The Y_{4.67(0.995-z)}Tm_4.67*(0.5%)Yb_4.67xSi₃O₁₃ (Abbreviated as YSO:0.5%Tm³⁺,xYb³⁺, 5% ≤ x ≤ 20%) samples were prepared by the solid-state reaction method. The starting materials were SiO₂ (99%), Y₂O₃ (99.99%), Yb₂O₃ (99.99%) and Tm₂O₃ (99.99%). 3wt% Li₂CO₃ (99%) was used as the flux. Stoichiometric amounts of the raw materials were thoroughly mixed and ground together in an agate mortar, and then calcined at 1200 °C for 3 h in air.

The phase purity was determined by an ARL X'TRA powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 35 mA. The morphology was inspected by field emission scanning electron microscope (FESEM, FEI, Quanta FEG). The UC photoluminescence spectra were recorded on an EI-FS5 fluorescence spectrophotometer equipped with a 980 nm laser diode (LD). The temperature dependent measurement was also carried out by the EI-FS5 fluorescence spectrophotometer. The sample was mounted on a heating device, the temperature of which could vary from room temperature to 573 K with the step of 0.1 K. The spot size of 980 nm laser was about 1x4 mm², and the pump power was set as 249 mW. The sample was maintained for about 30 seconds at the set temperature before measuring spectra.

3. Results and discussion

Figure 1(a) presents the XRD patterns of the YSO:0.5%Tm³⁺,xYb³⁺ (5% ≤ x ≤ 20%) samples. All the diffraction peaks could be indexed to the hexagonal-structured YSO compound (JCPDS Card No. 30-1457). When the RE ions are incorporated in the YSO host, no obvious diffraction peaks from any secondary phase appear, indicating that the as-prepared samples are single-phase. Figure 1(b) shows the SEM image of the typical YSO:0.5%Tm³⁺,10%Yb³⁺ sample. It can be found the particle size and shape are inhomogeneous, which agrees with the characteristics of the samples prepared by solid-state reaction.

Fig. 1 (a) XRD patterns of YSO:0.5%Tm³⁺,xYb³⁺ (5% ≤ x ≤ 20%); (b) SEM image of YSO:0.5%Tm³⁺,10%Yb³⁺.

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Figure 2(a) shows the emission spectra of the YSO:0.5%Tm³⁺,xYb³⁺ (5% ≤ x ≤ 20%) samples upon 980 nm excitation. To clearly observe the emission spectra around 650 nm, Fig. 2(b) presents the enlarged spectra. The strongest emission peaks are found at 789 nm, which could be attributed to the ³H₄-³H₆ transition of Tm³⁺ [11]. In the visible region, three obvious emissions around 473, 650 and 695 nm appear, which can be ascribed to the ¹G₄-³H₆, ¹G₄-³F₄ and ³F_2/3-³H₆ transitions of Tm³⁺, respectively [8]. Moreover, very weak emission peaks at 365 nm belonging to the ¹D₂-³H₆ transition of Tm³⁺ are also observed in the enlarged emission spectra. The optimal Yb³⁺ concentration for the visible (650 nm) and near ultraviolet (365 nm) emissions is for x = 10%. But the intensities of the emissions at 695 and 789 nm exhibit continuous increase with increasing Yb³⁺ concentration. This observation can be interpreted via the energy level diagram of Tm³⁺-Yb³⁺ ions in Fig. 2(c). The detailed UC energy transfer (ET) processes between Tm³⁺ and Yb³⁺ have been introduced in the previous reference [12]. The populations on ³F_2/3 and ³H₄ levels undergo two-photon process, which are much easier than those on ¹G₄ and ¹D₂ levels of three- and four-photon process respectively. As a result, the electrons pumped on ³F_2/3 and ³H₄ levels increase faster than those on ¹G₄ and ¹D₂ levels based on the ET from Yb³⁺ to Tm³⁺ when the Yb³⁺ concentration is increased, resulting in the higher quenching concentration for the emissions at 695 and 789 nm. To evaluate the luminescence intensity of the present phosphor, the emission spectra of YSO:Tm³⁺,Yb³⁺ and NaYF₄:Tm³⁺,Yb³⁺ under 980 nm excitation are shown in Fig. 2(d). The integrated intensity of YSO:Tm³⁺,Yb³⁺ is about 10.2% of that of NaYF₄:Tm³⁺,Yb³⁺. Hence, the luminous efficiency of YSO:Tm³⁺,Yb³⁺ is relatively low, which is owing to the high phonon energy in oxide hosts.

Fig. 2 (a) Emission spectra of YSO:0.5%Tm³⁺,xYb³⁺ (5% ≤ x ≤ 20%) upon 980 nm excitation; (b) enlarged emission spectra of YSO:0.5%Tm³⁺,xYb³⁺ (5% ≤ x ≤ 20%) from 620 to 730 nm; (c) energy level diagram of Tm³⁺-Yb³⁺ ions; (d) emission spectra of NaYF₄:Yb³⁺,Tm³⁺ and YSO:Tm³⁺,Yb³⁺.

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To better understand the UC mechanism, the pump power dependence of the UC emissions was measured. Figure 3 presents the emission spectra of the typical YSO:0.5%Tm³⁺,10%Yb³⁺ phosphor upon 980 nm excitation for various pump powers. It can be found that the emission intensity of Tm³⁺ exhibits a continuous increase with the pump power increased. It is known that the number of photons that are required to populate the upper emitting state can be determined by the following relation [11]:

I_{f} \propto P^{n}

where I_f is the fluorescent intensity, P is the pump power, and n is the number of photons required to populate the emitting state. For the main emission peaks at 472 and 789 nm, a plot of lnI_f versus P yields a straight line with slope n (see the inset of Fig. 3), and the n values were obtained to be 2.36 and 1.59, respectively. Thus, the blue and NIR emission peaks are respectively there- and two-photon process, which agree with the previous report [11].

Fig. 3 Emission spectra of YSO:0.5%Tm³⁺,10%Yb³⁺ upon 980 nm excitation for various pump powers, inset shows dependence of UC emission intensities on the excitation power.

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The temperature dependence of the Yb³⁺-Tm³⁺ coped YSO was investigated. In order to reduce the pump-induced heating, the laser power density used in the measurement is about 6.2 W/cm², which is lower than those used in some other UC luminescent materials for temperature sensing, such as Y₂O₃:Tm³⁺,Yb³⁺ (86.7 W/cm²) and YF₃:Yb³⁺,Er³⁺ (20 W/cm²) [13,14]. Figure 4(a) shows the emission spectra of the typical YSO:0.5%Tm³⁺,10%Yb³⁺ phosphor under 980 excitation at various temperatures. Figure 4(b) presents the enlarged spectra in the range from 620 to 730 nm. It can be noticed that the intensities of different emission peaks demonstrate obvious change with temperature. For the Tm³⁺ ion, two kinds of thermally-coupled levels for temperature sensing have been developed. First, the thermally-coupled ³F_2/3 and ³H₄ levels could demonstrate high relative sensitivity due to the large energy gap between the two levels. So, many Tm³⁺-activated UC phosphors by employing these two levels have been reported, such as Y₂O₃:Tm³⁺,Yb³⁺, NaLuF₄:Yb³⁺/Tm³⁺, and LiNbO₃: Tm³⁺,Yb³⁺ [13,15,16]. The ³F_2/3-³H₆ and ³H₄-³H₆ transition intensities as a function of temperature for the YSO:0.5%Tm³⁺,10%Yb³⁺ phosphor are depicted in Fig. 4(c). With increasing temperature, the 789 nm emission intensity shows a gradual decrease, but the 695 nm emission is enhanced gradually till 513 K and starts to decrease beyond this temperature. Thus, one can predict that the FIR of the 695 and 789 nm emissions (I₆₉₅/I₇₈₉) will change with temperature. Second, the blue emission of Tm³⁺ is consist of two transitions, i.e., ¹G_4(a)-³H₆ and ¹G_4(b)-³H₆, where the ¹G_4(a) and ¹G_4(b) are thermally-coupled levels [8]. Thus, this emission peak shape will vary with temperature. To clearly understand this point, the normalized emission spectra in the range of 450-510 nm are represented in Fig. 4(d). It is obvious that the emission intensity around 484 nm shows a gradual decrease and that around 466 nm exhibits a continuous increase with increasing temperature. Correspondingly, the FIR for I₄₆₆/I₄₈₄ will also change with temperature.

Fig. 4 (a) Emission spectra of YSO:0.5%Tm³⁺,10%Yb³⁺ under 980 excitation at various temperatures; (b) enlarged emission spectra of YSO:0.5%Tm³⁺,10%Yb³⁺ from 620 to 730 nm; (c) relative intensities of 695 and 789 nm emissions as a function of temperature; (d) normalized emission spectra in the range of 450-510 nm for YSO:0.5%Tm³⁺,10%Yb³⁺ under 980 excitation at various temperatures.

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For the thermally-coupled levels, it is known that the relative population follows the Boltzmann distribution, which can be described as [13]

R = I_{u p p e r} / I_{l o w e r} = N \exp (\frac{- Δ E}{K T})

where

R = I_{u p p e r} / I_{l o w e r}

is the FIR of the emissions from upper and lower thermally-coupled levels to the ground state level, ΔE is the energy gap between the thermally-coupled levels, K = 0.695 K⁻¹cm⁻¹ is the Boltzmann constant, T is absolute temperature, and N is the proportionality constant. Figure 5(a) and (b) show the dependences of I₆₉₅/I₇₈₉ (integrated in the ranges of 670-730 and 755-840 nm, respectively) and I₄₆₆/I₄₈₄ (integrated in the ranges of 448-473 and 473-505 nm, respectively) for the typical YSO:0.5%Tm³⁺,10%Yb³⁺ phosphor on the absolute temperature, respectively. The experimental data for both I₆₉₅/I₇₈₉ and I₄₆₆/I₄₈₄ can be fitted by Eq. (2), that is,

I_{695} / I_{789} = 1.69 \exp (- 2051.6 / T)

and

I_{466} / I_{484} = 3.54 \exp (- 412.6 / T)

. Hence, the ΔE values for ³F_2/3/³H₄ and ¹G_4(a)/¹G_4(a) thermally-coupled levels can be obtained to be 1425.9 and 286.8 cm⁻¹, respectively. For the temperature sensing application, two very important parameters of the absolute (S_A) and relative (S_R) sensitivities can be calculated by the following formulas [17,18]:

S_{A} = \frac{d (R)}{d (T)} = R \frac{Δ E}{K T^{2}}

S_{R} = | \frac{1}{R} \frac{d (R)}{d (T)} | = \frac{Δ E}{K T^{2}}

The S_A and S_R as a function of the temperature from 293 to 553 K for I₆₉₅/I₇₈₉ and I₄₆₆/I₄₈₄ of the YSO:0.5%Tm³⁺,10%Yb³⁺ phosphor are shown in Fig. 5(c) and (d), respectively. For I₆₉₅/I₇₈₉, the S_A value increases from

0.37 \times 10^{- 4}

to

2.78 \times 10^{- 4}

K⁻¹ but the S_R value decreases from 2.39% to 0.67% K⁻¹ with the temperature ranging from 273 to 553 K, respectively. Both the S_A and S_R values of the present samples are higher than those of the KLuF₄:Tm³⁺,Yb³⁺ phosphor (the maximum

1.451 \times 10^{- 4}

for S_A and

1249.85 / T^{2}

K⁻¹ for S_R [8]). For I₄₆₆/I₄₈₄, the S_A value decreases from

4.16 \times 10^{- 3}

to

2.26 \times 10^{- 3}

K⁻¹ and the S_R value decreases from 0.48% to 0.15% K⁻¹ with temperature ranging from 273 to 553 K, respectively. The YSO:0.5%Tm³⁺,10%Yb³⁺ phosphor shows a larger S_A but a lower S_R than those of KLuF₄:Tm³⁺,Yb³⁺ (the maximum

2.101 \times 10^{- 3}

for S_A and

528.714 / T^{2}

K⁻¹ for S_R [8]). Based on the above results, the FIR of I₆₉₅/I₇₈₉ demonstrates a higher relative sensitivity but a lower absolute sensitivity compared with the FIR of I₄₆₆/I₄₈₄ in the YSO:0.5%Tm³⁺,10%Yb³⁺ phosphor.

Fig. 5 Dependence of (a) I₆₉₅/I₇₈₉ and (b) I₄₆₆/I₄₈₄ on the absolute temperature; absolute (S_A) and relative (S_R) sensitivities as a function of the temperature from 293 to 553 K for (c) I₆₉₅/I₇₈₉ and (d) I₄₆₆/I_484.

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To evaluate the repeatability of measurement, the relative standard deviation (RSD) was employed, expressed as

R S D = \frac{1}{\bar{T}} \sqrt{\frac{\sum_{i = 1}^{n} {(T_{i} - \bar{T})}^{2}}{n - 1}} \times 100 %

where T_i is the obtained temperature for the i-th measurement, n is the number of iterations,

\bar{T}

is the mean value of temperature obtained in all the measurements. Figure 6(a) presents the RSD for the obtained temperature by using I₆₉₅/I₇₈₉ and I₄₆₆/I₄₈₄ as a function of the absolute temperature, which were based on the temperature-dependence for the YSO:0.5%Tm³⁺,10%Yb³⁺ sample obtained by repeating the preparation for five times. It can be found the RSD values fluctuate as the temperature rises for both cases, but the RSD for I₆₉₅/I₇₈₉ is lower, indicating that the FIR technique using I₆₉₅/I₇₈₉ shows a better repeatability of measurement. Figure 6(b) presents the temperature-induced switching of I₆₉₅/I₇₈₉ and I₄₆₆/I₄₈₄ (alternating between 293 and 553 K), respectively. Both the FIRs are repeatable and reversible after several cycling processes. Although the temperature sensing behavior of Tm³⁺ in the YSO host was evaluated by different thermally-coupled levels, two issues must interpreted here. First, the red ³F_2/3-³H₆ transition of Tm³⁺ shows a much lower intensity compared with the NIR ³H₄-³H₆ emission. This is the disadvantage to provide accurate data for temperature sensing. So further work is still needed to enhance the red emission of Tm³⁺ by some effect strategies, because this FIR is attracting owing to the large relative sensitivity. Second, there is a large spectral overlap for the ¹G_4(a)-³H₆ and ¹G_4(b)-³H₆ emissions, which is disadvantageous to the accurate measurement. However, it is still a challenge to overcome this problem currently due to the small energy gap between ¹G_4(a) and ¹G_4(b) levels of Tm³⁺.

Fig. 6 (a) RSD for the obtained temperature as a function of absolute temperature by using I₆₉₅/I₇₈₉ and I₄₆₆/I₄₈₄; (b) temperature-induced switching of I₆₉₅/I₇₈₉ and I₄₆₆/I₄₈₄ (alternating between 293 and 553 K).

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To evaluate the thermal stability of luminescence for Tm³⁺-Yb³⁺ codoped YSO, the typical ¹G₄-³H₆ emission was studied. Figure 7(a) depicts the relative integrated (from 448 to 510 nm) intensity (the intensity at 293 K has been set to 1) of the ¹G₄-³H₆ emission as a function of absolute temperature, which is based on the emission spectra in Fig. 4(a). It can be found that the intensity shows a gradual decrease with increasing temperature. Generally, the emission intensity of phosphors at 423 K with respect to that at room temperature is used to assess the thermal stability [19]. In Fig. 7(a), a decay of 24% for this phosphor at 423 K is observed, which indicates the present phosphor shows a relatively good thermal stability. The activation energy for the thermal quenching can also support this point, which could be obtained by the Arrhenius equation [20]:

I_{T} = \frac{I_{0}}{1 + A \cdot \exp (- \frac{Δ E}{k T})}

where I₀ is the initial emission intensity, I_T is the intensity at different temperatures, ΔE is activation energy of thermal quenching, A is a constant for a certain host, and k is the Boltzmann constant (8.629 × 10⁻⁵ eV). Figure 7(b) displays a plot of ln(I₀/I_T −1) versus 1/(kT). By linear fitting, the ΔE was obtained to be 0.27 eV. Thus, this sample shows a relatively large activation energy compared with some other oxide-based phosphors, such as Ca₃Si₄Cl₂:Eu²⁺ (0.159 eV) and Ba₃GdK(PO₄)₃F:Tb³⁺,Eu³⁺ (0.253 and 0.137 eV for Tb³⁺ and Eu³⁺ emissions) [21,22].

Fig. 7 (a) Relative intensity of Tm^{3+ 1}G₄-³H₆ emission as a function of absolute temperature; (b) ln(I/I_T −1) versus 1/(kT) plot as well as the calculated E_a value.

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4. Conclusions

In this paper, new UC phosphors were developed by the solid-state reaction method, and their UC luminescence was studied for temperature sensing. The strongest emission peak of Tm³⁺ in the YSO host was found at 789 nm, ascribed to the ³H₄-³H₆ transition. The blue emission belonging to the ¹G₄-³H₆ transition of Tm³⁺ appeared around 472 nm. The pump-dependence investigation on the YSO:0.5%Tm³⁺,10%Yb³⁺ phosphor reveals that the blue and NIR emissions are three- and two-photon process, respectively. By investigating the temperature-dependence of the YSO:0.5%Tm³⁺,10%Yb³⁺, it indicated that the FIR technique using I₆₉₅/I₇₈₉ showed a better repeatability of measurement based on the RSD calculation. The relative and absolute sensitivities were evaluated and compared for the FIRs of I₆₉₅/I₇₈₉ and I₄₆₆/I₄₈₄. Moreover, relatively high thermal stability was obtained with high activation energy.

Funding

National Natural Science Foundation of China (No. 51602117).

References and links

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Investigation on optical temperature sensing behaviour for Y_4.67Si₃O₁₃:Tm³⁺,Yb³⁺ phosphors based on upconversion luminescence

Abstract

1. Introduction

2. Experimental

3. Results and discussion

4. Conclusions

Funding

References and links

Cited By

Figures (7)

Equations (6)

Optical Materials Express