A method to modify the spectra and optical temperature behaviors of Er3+ doped Sr2CaWO6 phosphors through doping La3+, Y3+, and Al3+ ions is reported. The thermometric parameters such as fluorescence emission intensity, fluorescence intensity ratio of red to green emissions, emission intensity ratios of thermally coupled levels (2H11/2/4S3/2), and temperature sensitivity can be effectively controlled by doping with La3+, Y3+, and Al3+ ions into Er3+ doped Sr2CaWO6. Moreover, the relative temperature sensitivity SR and the absolute temperature sensitivity SA are proved to be dependent on the pump power of 980 nm laser. The sensitivity values of SR in Er3+ doped Sr2CaWO6 increase about 85.2% by doping with 0.15 mol% La3+, when the excitation power is 526 mW/mm2.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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 (2H11/2/4S3/2) of Er3+, the optical temperature sensing performances have been explored in the Yb3+/Er3+ co-doped CaWO4 polycrystalline powder , the Yb3+/ Er3+ co-doped yttrium silicate powders , Er3+-Yb3+ co-doped Y2Ti2O7 phosphors , NaGdF4:Yb3+/Tm3+@Tb3+/Eu3+ core–shell nanoparticles , Yb3+/Er3+/Cr3+ triply doped transparent bulk glass ceramic . 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 . Bai et al. reported that the upconversion emission intensity of ZnO/Er3+ nanocrystals was greatly enhanced by doping Li+ ions . 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+ . Jiang et al. observed that the up-converted green emission can be increased by 20 times by adding Bi3+ ion into Zn2SiO4 host . It is clear that the doping can alter the crystal structure around the rare earth ions, thereby modulating the crystal field . 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 . 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 . 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 . 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.
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 Kα 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 . 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) , 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+.
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.
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 2H11/2→4I15/2 (524 nm), 4S3/2→4I15/2 (552 nm and 564 nm),and 4F9/2 →4I15/2 (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 . The same results are observed in Fig. 6(a) and Fig. 7(a).
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 4S3/2 to 4I15/2 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 (IR/IG) 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 rLa = 1.032 Å, rY = 0.9 Å, rAl = 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 . The forbidden intra-4f electronic transitions are partially allowed with increase of the intra-4f electronic transitions probability of the Er3+ ions . 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 , the fluoresce intensity ratio (FIR) of two thermally coupled levels (2H11/2 and 4S3/2) of Er3+ can be fitted by the Boltzmann distributing law
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 ΔEf and ΔEm is calculated as follows:33]. The ΔEm 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.
Considering the nonradiative relaxation and energy transfer at high temperature, Eq. (1) is modified as36]. 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 . 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 . 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 SR and SA.Eq. (5).
Figure 11 shows temperature dependent sensitivities SR and SA 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 SR curves rises in the low temperature region and decreases in the high temperature region. It means thermally coupled levels (2H11/2/4S3/2) are suitable for monitoring the temperature in the low temperature region. The maximum SR 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 SR value of the Er3+ doped Sr2CaWO6 system are enhanced obviously by doping with La3+ and Y3+ ions in optimum dopant concentrations. The maximum SR 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 SA values decrease with increasing temperature as shown in Fig. 11(d). The SA value of Er3+ doped Sr2CaWO6 is enhanced by doping with La3+ ions. The values of SR and SA indicate that the resulting Er3+-La3+ co-doped Sr2CaWO6 might be a promising candidate for optical temperature sensors.
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 follows36]. Theoretically, the n values of green emissions of Er3+ ions excited by 980 nm are close to 2 . 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.
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 (2H11/2/4S3/2) 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.
The excitation power dependences of SR and SA are illustrated in Fig. 14. One can find that the effect of excitation power on SR varies with doping ions and temperature range. Specially, it can be further found that the values of SR of Er3+-0.15%La3+ co-doped Sr2CaWO6 change with increasing excitation powers. The largest value of SR of Er3+-0.15%La3+ appears at the excitation power of 526 mW/mm2. The values of SR 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 SR are susceptible to excitation powers is attributed to that the populations of thermally coupled 2H11/2/4S3/2 levels can be modified by the heat induced by the high excitation powers [36, 37]. As a result, the FIR of 2H11/2/4S3/2 levels changes a lot.
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 (2H11/2/4S3/2) 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.
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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