Controlling optical temperature behaviors of Er3+ doped Sr2CaWO6 through doping and changing excitation powers

Yemin Wang; Yuanjian Liu; Jianping Shen; Xiangfu Wang; Xiaohong Yan

doi:10.1364/OME.8.001926

1. Introduction

Lanthanide-doped fluorescent materials with upconversion properties are able to convert near infrared light into visible radiation and made them especially interesting for wide applicability in micro/nano-electronics, integrated photonics and biomedicine [1–8]. Optical temperature sensors based on upconversion emissions of the rare earth ions doped materials have attracted great attention for the outstanding thermal coupling effect [9–13]. Basing on the intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2) of Er³⁺, the optical temperature sensing performances have been explored in the Yb³⁺/Er³⁺ co-doped CaWO₄ polycrystalline powder [14], the Yb³⁺/ Er³⁺ co-doped yttrium silicate powders [15], Er³⁺-Yb³⁺ co-doped Y₂Ti₂O7 phosphors [16], NaGdF₄:Yb³⁺/Tm³⁺@Tb³⁺/Eu³⁺ core–shell nanoparticles [17], Yb³⁺/Er³⁺/Cr³⁺ triply doped transparent bulk glass ceramic [18]. Those results showed that the sensitivity values strongly depend on the properties of host materials [19–21].

It has been verified that the luminescence of rare earth ions can be modulated by changing the local crystal field of the luminescent centers in rare ions host matrix [22]. Bai et al. reported that the upconversion emission intensity of ZnO/Er³⁺ nanocrystals was greatly enhanced by doping Li⁺ ions [23]. Zhao et al. found that doping with Mg²⁺ ions remarkably enhanced the upconversion emissions in β-NaGdF₄: Yb³⁺, Er³⁺, especially the red emission, showing a 9 times enhancement. It was attributed to the fact that the introducing of Mg²⁺ ions decreased the average Ln-F bond length and increased the efficiency of energy-transfer of Yb³⁺-Er³⁺ [24]. Jiang et al. observed that the up-converted green emission can be increased by 20 times by adding Bi³⁺ ion into Zn₂SiO₄ host [25]. It is clear that the doping can alter the crystal structure around the rare earth ions, thereby modulating the crystal field [26]. It is possible to adjust the optical thermometry behaviors of Er³⁺ ions by doping.

Recently, perovskite oxide with the general formula A₂BWO₆ has attracted much attention [27]. In these substances, the WO₆ octahedron shared common vertex or common edge and was considered to be the main structural unit which played a decisive role on the fluorescence, electricity and magnetic properties [27, 28]. Especially, the double-perovskite structure of Sr₂CaWO₆ can be represented as a three-dimensional network of alternating CaO₆ and WO₆ octahedra, with Sr atoms occupying the interstitial spaces [29]. Wang et al. reported that the luminescence properties of Sr₂CaWO₆: Sm³⁺ phosphors were obviously improved by doping Li⁺, Na⁺ or K⁺ ions. It was explained that Sr²⁺ 12-coordination sites were confirmed to be substituted by Sm³⁺ ions and the local chemical environment of Sm³⁺ was changed [30]. However, the optical temperature sensing property based on upconversion luminescence has been not studied. In this work, we report a method to improve the optical thermometry in Er³⁺ doped Sr₂CaWO₆ through doping with La³⁺, Y³⁺, and Al³⁺ ions and changing the excitation powers. It is found that optical temperature sensing is dependent on ions doping and the pump power of 980 nm laser. The sensitivity values are enhanced greatly.

2. Experimental

All starting materials are SrCO₃ (AR), CaCO₃ (AR), WO₃ (99.99%), Er₂O₃ (99.99%), Y₂O₃ (99.99%), La₂O₃ (99.99%) and Al₂O₃ (AR). All the chemicals were used as received without any further purification.

The solid-state reaction method is used to synthesize Sr₂CaWO₆: Er³⁺ and Sr₂CaWO₆: Er³⁺/Mⁿ⁺ (Mⁿ⁺ = La³⁺, Y³⁺ and Al³⁺) phosphors. The molar ratio of Sr₂CaWO₆: Er³⁺ sample is as follows: 2SrCO₃-0.98CaCO₃-WO₃-0.01Er₂O₃. The molar ratios of Sr₂CaWO₆: Er³⁺/La³⁺ samples are as follows: 2SrCO₃-(0.98-x)CaCO₃-WO₃-0.01Er₂O₃-(x/2)La₂O₃, x = 0.01, 0.03, 0.05, 0.07, 0.10 and 0.15; the molar ratios of Sr₂CaWO₆: Er³⁺/Y³⁺ samples are as follows: 2SrCO₃-(0.98-y)CaCO₃-WO₃-0.01Er₂O₃-(y/2)Y₂O₃, y = 0.01, 0.03, 0.05, 0.07, 0.10 and 0.15. The molar ratios of Sr₂CaWO₆: Er³⁺/Al³⁺ samples are as follows: 2SrCO₃-(0.98-z)CaCO₃- WO₃-0.01Er₂O₃-(z/2)Y₂O₃, z = 0.01, 0.03, 0.05, 0.07, 0.10 and 0.15. The starting materials were entirely mixed and ground with alcohol for 1 hour. The powder was subsequently sintered in a furnace for 5 h at 850°C. Then, when the powder cooled down slowly to room temperature, each sample was grinded for 30 minutes and sintered in a furnace for 12 h at 1250°C. Finally, a series of the required samples in the form of white powder were obtained after cooling down naturally.

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. Luminescence spectra were obtained by the Acton SpectraPro Sp-2300 Spectrophotometer with a photomultiplier tube equipped with 980 nm laser as the excitation sources. Different temperature spectra were obtained by using an INSTEC HCS302 Hot and Cold System.

3. Results and discussion

Figure 1(a) presents the XRD patterns of as-synthesized Er³⁺-La³⁺ doped Sr₂CaWO₆ with the concentration of the La³⁺ from 0 to 0.15 mol%. The position and relative intensity of all the diffraction peaks are in agreement with the PDF card (No. 76-1983), which is readily indexed to pure Sr₂CaWO₆ [27]. The sharp and well defined diffraction peaks in the patterns suggest that the synthesized phosphors are well crystallized. It can be observed that the diffraction peaks of Er³⁺-La³⁺ doped Sr₂CaWO₆ shifted toward higher angles compared with the standard pattern. This can be attributed to the replacement of Ca²⁺ ions (six coordination Ca²⁺, r = 1.00 Å) with La³⁺ ions (six coordination La³⁺, r = 1.032 Å). The same results can be observed in Fig. 2(a) and Fig. 3(a), which can be attributed to the smaller radius of Y³⁺ (six coordination Y³⁺, r = 0.9 Å) and Al³⁺ (six coordination Al³⁺, r = 0.535 Å) ions. The raw materials could not be detected in the as-synthesized samples confirming that Er³⁺, La³⁺, Y³⁺ and Al³⁺ are doped into the Sr₂CaWO₆ lattices. The cell parameters a, b, c and v of Er³⁺-La³⁺ doped Sr₂CaWO₆ are calculated in terms of the X-ray diffraction data. As is shown in Fig. 1(b), compared with the pure Sr₂CaWO₆ crystals (a = 8.2033 Å, b = 5.7676 Å, c = 5.8489 Å) [28], the cell parameters of Er³⁺ doped Sr₂CaWO₆ reduced to a = 8.1731 Å, b = 5.7361 Å, c = 5.8334 Å, further confirming the substitution of Ca²⁺. As the increase of the concentration of the La³⁺ ions, the lattice parameters a, b, c and v present a growing trend. However, the lattice parameters of Er³⁺-Y³⁺ doped Sr₂CaWO₆ and Er³⁺-Al³⁺ doped Sr₂CaWO₆ present different trend, indicating that different ions on the lattice parameters are not the same. The results show that the modulation of crystal structure can be induced by doping of La³⁺, Y³⁺ and Al³⁺.

Fig. 1 (a) XRD patterns and (b) Unit cell parameters a (Å), b (Å), c (Å) and v (Å³) vs of Sr₂CaWO₆ doped with Er³⁺ and La³⁺.

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Fig. 2 (a) XRD patterns and (b) Unit cell parameters a (Å), b (Å), c (Å) and v (Å³) vs of Sr₂CaWO₆ doped with Er³⁺ and Y³⁺.

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Fig. 3 (a) XRD patterns and (b) Unit cell parameters a (Å), b (Å), c (Å) and v (Å³) vs of Sr₂CaWO₆ doped with Er³⁺ and Al³⁺.

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Figure 4 shows the crystal structure and atomic arrangement along the a-axis direction in Sr₂CaWO₆ which has orthorhombic symmetry (ICSD#36460) with octahedral quadrilateral biconical crystals [27, 31]. The orthorhombic Sr₂CaWO₆ belongs the Pmm2 space group, and contains 2 formula units in each unit cell. In the unit cell of Sr₂CaWO₆, there are 1b, 1c Ca and 1a, 1d W cation sites. The Ca²⁺ and W⁶⁺ ions in the three-dimensional network are ordered in such a way that every WO₆ octahedron has only six CaO₆ octahedra as nearest neighbors and the Sr²⁺ cation which has 12 coordination numbers are located in the spaces between them. The La, Y and Al ions occupy the Ca cation sites in the Sr₂CaWO₆ lattice.

Fig. 4 The schematic views of unit cell of Sr₂CaWO₆ structure along a-direction.

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The upconversion emission spectra of Sr₂CaWO₆: Er³⁺, La³⁺ excited by 980 nm infrared radiation is shown in Fig. 5(a). Four emission bands of Er³⁺ are observed at 524, 552, 564 and 660 nm due to the ²H_11/2→⁴I_15/2 (524 nm), ⁴S_3/2→⁴I_15/2 (552 nm and 564 nm),and ⁴F_9/2 →⁴I_15/2 (660 nm) transitions, respectively. No spectral shifts of the emission bands occur for various La³⁺ concentrations in Sr₂CaWO₆: Er³⁺/La³⁺. It can be seen that two emission bands appear from 540 nm to 600 nm. The occurrence of emission bands is explained on the basis of the calculations of the crystal-field splitting [32]. The same results are observed in Fig. 6(a) and Fig. 7(a).

Fig. 5 La³⁺ concentration dependent (a) upconversion spectra, (b) the intensity ratios of the 552 nm to 564 nm emissions of Sr₂CaWO₆: Er³⁺, La³⁺, (c) total emission intensity, and (d) the intensity ratios of the red to green emissions of Sr₂CaWO₆: Er³⁺, La³⁺.

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Fig. 6 Y³⁺ concentration dependent (a) upconversion spectra, (b) the intensity ratios of the 552 nm to 564 nm emissions of Sr₂CaWO₆: Er³⁺, Y³⁺, (c) total emission intensity, and (d) the intensity ratios of the red to green emissions of Sr₂CaWO₆: Er³⁺, Y³⁺.

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Fig. 7 Al³⁺ concentration dependent (a) upconversion spectra, (b) the intensity ratios of the 552 nm to 564 nm emissions of Sr₂CaWO₆: Er³⁺, Al³⁺, (c) total emission intensity, and (d) the intensity ratios of the red to green emissions of Sr₂CaWO₆: Er³⁺, Al³⁺.

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The intensity ratios of the 552 nm to 564 nm emissions of Sr₂CaWO₆: Er³⁺/La³⁺ and Sr₂CaWO₆: Er³⁺/Y³⁺ both present a growing trend with the concentration of La³⁺ and Y³⁺ from 0.01 to 0.15 mol%, as shown in Fig. 5(b) and Fig. 6(b). The dopings of La³⁺ and Y³⁺ modulate the local crystal field of Er³⁺ and change the emission from ⁴S_3/2 to ⁴I_15/2 consequently. The opposite results are observed in Sr₂CaWO₆: Er³⁺/Al³⁺, as shown in Fig. 7(b). The total emission intensity increases first and then decreases at low La³⁺ concentration and increases at high La³⁺ concentration, as shown in Fig. 5(c). The similar results are observed in Fig. 6(c) and Fig. 7(c). Figure 5(d) shows the intensity ratio of the red/green emissions (I_R/I_G) which is changed by changing La³⁺ concentration and the ratio reaches a maximum at 0.01 mol%. The CIE chromaticity in the inset of Fig. 5(d) shows that the emission color is tunable from yellow to green with increasing La³⁺ concentration. The CIE chromaticity in the inset of Fig. 6(d) shows that the emission color is yellow at low Y³⁺ concentration and green at high Y³⁺ concentration. However, the CIE chromaticity in the inset of Fig. 7(d) shows that the emission color is tunable from green to yellow with increasing Al³⁺ concentration. The different spectrum modulation effects of Sr₂CaWO₆: Er³⁺ by co-doping with Y³⁺ and Al³⁺ ions can be explained that the adjustments of three kinds of dopant ions (La³⁺, Y³⁺ and Al³⁺) on the crystal field of Er³⁺ are not the same.

The enhancement of visible emissions induced by doping can be explained as follows: Sr₂CaWO₆ has octahedral quadrilateral biconical structure for which the Ca ion is coordinated with 6 oxygen ions in the Sr₂CaWO₆ lattice, as shown in Fig. 4. Er³⁺ (six coordination r = 0.89 Å) as an optical active center has a local structure with the ErO₆ unit when the Er³⁺ is doped in Sr₂CaWO₆ lattice (six coordination Ca²⁺, r = 1.00 Å). The La³⁺, Y³⁺ and Al³⁺ ions (6 coordination r_La = 1.032 Å, r_Y = 0.9 Å, r_Al = 0.535 Å) substitute for the Ca²⁺ sites in Sr₂CaWO₆: Er³⁺ leading to the distortion of the Er-O bonds. As a result, the local symmetry of the crystal field around Er³⁺ lowers with the change of Coulomb interaction [38]. The forbidden intra-4f electronic transitions are partially allowed with increase of the intra-4f electronic transitions probability of the Er³⁺ ions [39]. Thus, the visible emissions can be enhanced by changing the local environments of the Er³⁺ ions by doping with La³⁺, Y³⁺ and Al³⁺ ions.

According to published works [13], the fluoresce intensity ratio (FIR) of two thermally coupled levels (²H_11/2 and ⁴S_3/2) of Er³⁺ can be fitted by the Boltzmann distributing law

F I R = A e^{- Δ E / K T}

where A is a fitting constant that depends on the experimental system and intrinsic spectroscopic parameters, ΔE is the energy difference between thermally coupled levels, k is the Boltzmann constant, and T is the absolute temperature. The relative sensitivity S_R and the absolute sensitivity S_A are defined as

S_{R} = \frac{d F I R}{d T} = F I R \frac{Δ E}{k T^{2}}

S_{A} = \frac{1}{F I R} \frac{d F I R}{d T} = \frac{Δ E}{K T^{2}}

Figure 8 shows the FIR between 524 nm and 552 nm emissions of Er³⁺ as a function of temperature for different doping systems. The solid lines in Fig. 8 are obtained by fitting the experimental data through the Eq. (1). It can be observed obviously that all the data are well fitted to Eq. (1) and the fit results are dependent strongly on the types and concentrations of co-dopant ions. The fit results of Er³⁺-Mⁿ⁺ (Mⁿ⁺ = La³⁺, Y³⁺ and Al³⁺) co-doped Sr₂CaWO₆ are different from those of Er³⁺ doped Sr₂CaWO₆. According to the Eq. (1), the values of ΔE can be calculated from the fit results at different temperatures. It can be seen from Fig. 8 that the values of ΔE are dependent strongly on the types of dopant ions corresponding to different fitting formulas. The change of temperature does not cause shift of the peaks of green emissions at 524 and 552 nm of any sample. In theory, the values of ΔE obtained from the fitting formula should be consistent with those obtained from the spectrum in Fig. 5, 6 and 7. However, as can be seen from Fig. 9, there is an error between the fitting value and the experimental value and the error changes with the types and concentrations of the dopant ions. The error δ between ΔE_f and ΔE_m is calculated as follows:

δ = | Δ E_{f} - Δ E_{m} | / Δ E_{m}

where ΔE_f is the value of ΔE obtained from the fitting formula, and ΔE_m is experimental value of ΔE from the spectrum [33]. The ΔE_m is 859.39 cm⁻¹ calculated from the spectra in Fig. 5. Figure 9 shows that the calculated errors δ for the Er³⁺doped and Er³⁺-Mⁿ⁺ (Mⁿ⁺ = La³⁺, Y³⁺ and Al³⁺) co-doped Sr₂CaWO₆ by using Eq. (4). The values of error δ are dependent strongly on the types and concentrations of co-doping ions. The minimum value of error δ excesses 35%, which means that the δ is not to be ignored. It means that the Eq. (1) is not suitable to be as an ideal fitting formula at high temperature, since that the Eq. (1) ignored nonradiative relaxation and energy transfer at high temperature [34, 35]. It means that the Eq. (1) should be corrected to calculate the optical temperature sensitivity at high temperature.

Fig. 8 Temperature dependent FIR of 524 nm and 552 nm emissions of (a) Er³⁺ doped, (b) Er³⁺-La³⁺ co-doped, (c) Er³⁺-Y³⁺ co-doped, and (d) Er³⁺-Al³⁺ co-doped Sr₂CaWO₆.

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Fig. 9 Doping dependent δ values in (a) Er³⁺ doped Sr₂CaWO₆, (b, c, d, e, f, g) Er³⁺-xLa³⁺ (x = 0.01, 0.03, 0.05, 0.07, 0.10 and 0.15mol%) co-doped Sr₂CaWO₆, (h, i, j, k, l, m) Er³⁺-yY³⁺ (y = 0.01, 0.03, 0.05, 0.07, 0.10 and 0.15mol%) co-doped Sr₂CaWO₆, and (n, o, p, q, r, s) Er³⁺-zAl³⁺ (z = 0.01, 0.03, 0.05, 0.07, 0.10 and 0.15mol%) co-doped Sr₂CaWO₆.

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Considering the nonradiative relaxation and energy transfer at high temperature, Eq. (1) is modified as

L n F I R = - a / T + b

where a is constant dependent on the materials [36]. The b is a correction term for the comprehensive population of thermally coupled energy levels induced by not only the thermal population but also the nonradiative relaxation and energy transfer [37]. Figure 10 shows the temperature dependent FIR of 524 and 552 nm green emissions in Sr₂CaWO₆ doped with different ions. It can be seen from the Fig. 10(a) that the experimental values for Er³⁺ doped Sr₂CaWO₆ generally show a downward trend, but not a linear mode as reported [33]. However, it can be fitted quite well by two linear functions by dividing the plot into two parts (below and over 384K). The division of the temperature range is determined by the concentration and type of the dopant ions, which is calculated by the intersection of two LnFIR lines, but almost all the ΔE values get larger with the temperature increase. It can be observed that the slopes of the fit results are dependent on the types and concentrations of dopant ions. It means that the FIR can be controlled by co-doping La³⁺, Y³⁺ and Al³⁺ ions. Moreover, compared with values of ΔE/k from Eq. (1) as shown in Fig. 8, all the values of a in Fig. 10 change a lot. It suggests that it is not accurate to use the ΔE calculated in Fig. 8 to evaluate the S_R and S_A.

Fig. 10 Temperature dependent relationship between the LnFIR and 1/T of (a) Er³⁺ doped, (b) Er³⁺-La³⁺ co-doped, (c) Er³⁺-Y³⁺ co-doped, and (d) Er³⁺-Al³⁺ co-doped Sr₂CaWO₆.

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After the Eq. (1) is modified as Eq. (5), the relative sensitivity S_R and the absolute sensitivity S_A are calculated as follows

S_{R} = \frac{d F I R}{d T} = \frac{a}{T^{2}} e^{\frac{b T - a}{T}}

S_{A} = \frac{1}{F I R} \frac{d F I R}{d T} = \frac{a}{T^{2}}

where a and b are from Eq. (5).

Figure 11 shows temperature dependent sensitivities S_R and S_A of Sr₂CaWO₆ doped with different ions. According to two linear functions of LnFIR, the sensitivity is also divided into two sections. It can be seen that all the sensitivity S_R curves rises in the low temperature region and decreases in the high temperature region. It means thermally coupled levels (²H_11/2/⁴S_3/2) are suitable for monitoring the temperature in the low temperature region. The maximum S_R value of all the co-doped Sr₂CaWO₆ systems changes non-monotonically with different types and concentration of dopant ions. As shown in Fig. 11(a) and (b), the maximum S_R value of the Er³⁺ doped Sr₂CaWO₆ system are enhanced obviously by doping with La³⁺ and Y³⁺ ions in optimum dopant concentrations. The maximum S_R value of Er³⁺ doped Sr₂CaWO₆ is estimated to be 0.0027 K⁻¹ at 241 K, while 0.0049 K⁻¹ at 423 K for Er³⁺- La³⁺ (0.15 mol%), 0.0040 K⁻¹ at 314 K for Er³⁺- Y³⁺ (0.15 mol%), and 0.0027 K⁻¹ at 229 K for Er³⁺-Al³⁺ (0.05 mol%) systems. The corresponding S_A values decrease with increasing temperature as shown in Fig. 11(d). The S_A value of Er³⁺ doped Sr₂CaWO₆ is enhanced by doping with La³⁺ ions. The values of S_R and S_A indicate that the resulting Er³⁺-La³⁺ co-doped Sr₂CaWO₆ might be a promising candidate for optical temperature sensors.

Fig. 11 Optical temperature sensitivities S_R and S_A as a function of temperature for various dopant ions. (a) Er³⁺-La³⁺, (b) Er³⁺-Y³⁺, and (c) Er³⁺-Al³⁺ co-doped Sr₂CaWO₆. (d) Temperature dependent S_A.

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It is vital for optical temperature sensors to study the thermal stability of emission bands. It is well known that the upconversion emission intensity I and excitation power P is expressed as follows

I \propto P^{n}

where n is the number of photons absorbed to pump the population in particular level in upconversion process [36]. Theoretically, the n values of green emissions of Er³⁺ ions excited by 980 nm are close to 2 [35]. In order to investigate the thermal stability of green upconversion emission, the evolution between green emission intensity and pump power is measured. The temperature dependent log–log plots of emission intensity and pump power for green emissions are shown in Fig. 12. The slopes of the fit results for green emissions of Er³⁺ doped Sr₂CaWO₆ depend on temperature, and show that the upconversion process involves two photons to contribute green emissions. Compared with the slopes of Er³⁺ doped Sr₂CaWO₆, those of the best-fit results for green emissions of Er³⁺-La³⁺ co-doped Sr₂CaWO₆ are closer to 2 than those values in Er³⁺-Y³⁺ co-doped Sr₂CaWO₆ and Er³⁺-Al³⁺ co-doped Sr₂CaWO₆. It means that Er³⁺-La³⁺ co-doped Sr₂CaWO₆ may has high thermal stability in emission bands than Er³⁺ doped Sr₂CaWO₆.

Fig. 12 Log–log plots of intensity and pumping power for green emission in (a) Er³⁺ doped Sr₂CaWO₆, (b) Er³⁺-0.15mol%La³⁺ doped Sr₂CaWO₆, (c) Er³⁺-0.15mol%Y³⁺ doped Sr₂CaWO₆, (d)Er³⁺-0.05%Al³⁺ doped Sr₂CaWO₆ at different temperatures.

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The LnFIR as a function of 1/T for various excitation powers are illustrated in Fig. 13. The LnFIR can be fitted quite well by two linear functions by dividing the plot into two parts. The slopes of Er³⁺-0.15%La³⁺ co-doped Sr₂CaWO₆ at low excitation powers are close to those at high excitation powers. The similar result is shown in the Er³⁺-0.15%Y³⁺ co-doped Sr₂CaWO₆. It means that the FIR of thermally coupled levels (²H_11/2/⁴S_3/2) is steady to excitation powers after the doping of La³⁺ and Y³⁺. However, the slopes of Er³⁺ doped Sr₂CaWO₆ and Er³⁺-0.05%Al³⁺ co-doped Sr₂CaWO₆ are greatly susceptible to excitation powers.

Fig. 13 The LnFIR as a function of 1/T for various excitation powers for (a) Er³⁺ doped Sr₂CaWO₆, (b) Er³⁺-0.15%La³⁺ doped Sr₂CaWO₆, (c) Er³⁺-0.15%Y³⁺ doped Sr₂CaWO₆ and (d) Er³⁺-0.05%Al³⁺ doped Sr₂CaWO₆.

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The excitation power dependences of S_R and S_A are illustrated in Fig. 14. One can find that the effect of excitation power on S_R varies with doping ions and temperature range. Specially, it can be further found that the values of S_R of Er³⁺-0.15%La³⁺ co-doped Sr₂CaWO₆ change with increasing excitation powers. The largest value of S_R of Er³⁺-0.15%La³⁺ appears at the excitation power of 526 mW/mm². The values of S_R of Er³⁺ doped Sr₂CaWO₆ and Er³⁺-Al³⁺/Y³⁺ co-doped Sr₂CaWO₆ are more susceptible to excitation powers than those of Er³⁺-0.15%La³⁺ co-doped Sr₂CaWO₆. Thus, the Er³⁺-0.15%La³⁺ co-doped Sr₂CaWO₆ is a better candidate for optical temperature sensors by considering the stabilities induced by temperature and excitation powers. The fact that the values of S_R are susceptible to excitation powers is attributed to that the populations of thermally coupled ²H_11/2/⁴S_3/2 levels can be modified by the heat induced by the high excitation powers [36, 37]. As a result, the FIR of ²H_11/2/⁴S_3/2 levels changes a lot.

Fig. 14 Excitation powers dependent S_R and S_A of Er³⁺ doped Sr₂CaWO₆ and Er³⁺- La³⁺, Y³⁺ and Al³⁺ doped Sr₂CaWO₆.

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4. Conclusion

In this work, a series of Er³⁺ doped and Er³⁺-Mⁿ⁺ (M ⁿ⁺ = La³⁺, Y³⁺ and Al³⁺) co-doped Sr₂CaWO₆ phosphors were synthesized by using the high-temperature solid-state reaction method. The structural property of resulting powder is investigated by the X-ray diffraction. The upconversion emission intensity, green-to-red emission intensity ratio, fluorescence color of the Er³⁺ doped sample are controlled by doping La³⁺, Y³⁺ and Al³⁺ ions. The fluorescence intensity ratios of thermally coupled levels (²H_11/2/⁴S_3/2) and optical temperature sensitivity are observed to be dependent on dopant ions and excitation powers. It is found that the temperature sensitivity of Sr₂CaWO₆: 0.02%Er³⁺,0.15%La³⁺ can reach a maximum relative sensitivity of 0.0050 K⁻¹ at 439 K and absolute sensitivity of 846.40/T², which is much higher than the reported temperature sensors based on Er³⁺ green luminescence. This work opens a new method to increase the optical temperature sensitivity of rare earth ions doped materials.

Funding

National Natural Science Foundation of China (NSFC) (11404171, 61372045); the Six Categories of Summit Talents of Jiangsu Province of China (2014-XCL-021); Jiangsu Natural Science Foundation for Excellent Young Scholar (BK20170101); The Scientific Research Foundation of Nanjing University of Posts and Telecommunications (NY215174, NY217037, NY218015); Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0753).

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Controlling optical temperature behaviors of Er³⁺ doped Sr₂CaWO₆ through doping and changing excitation powers

Abstract

1. Introduction

2. Experimental

3. Results and discussion

4. Conclusion

Funding

References and links

Cited By

Figures (14)

Equations (8)

Optical Materials Express