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Abstract
Rare-earth (RE) ion doped up-conversion luminescence (UCL) materials with high temperature sensing sensitivity (S) have attracted immense interest in optical temperature sensors. Here, research on Er-doped strontium tungstate (SWOE-x) phosphors with multiphase structures for optical temperature sensing applications was first proposed. With Sr/W ratio increasing, the samples’ structures and UCL features are divided into three types: Sr3WO6 types, Sr2WO5 types, and SrWO4 types. The influence of the Sr/W ratio in Er-doped strontium tungstates on temperature sensing properties has been investigated via the fluorescence intensity ratio (FIR) technique. The experimental results demonstrated that all of the samples exhibited excellent chemical stability and the S values were reliable. Especially, the maximal S is determined to be 12.75 × 10−3 K−1 at 523K with a broad temperature range from 83K to 563K in SWOE-0.6. These results indicate that Er-doped strontium tungstate multiphase phosphors are promising candidates for optical temperature sensors with high sensitivity and broad temperature range.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, rare-earth (RE) ion doped materials with up-conversion luminescence (UCL) has attracted immense attention due to their potential applications in biolabels, optical amplifiers, three-dimensional color displays, solid state lasers and photovoltaics, as well as optical temperature sensors [1–10]. Particularly, high temperature sensing sensitivity (S) is a key parameter for application in optical temperature sensor [11–13]. Optical temperature sensors based on the fluorescence intensity ratio (FIR) technique which capitalizes on the temperature-dependent luminescence intensities of two thermally coupled levels of RE ions have been widely accepted in recent years [14, 15]. Compared with other temperature measuring methods, FIR technique has potential application prospect in harsh environment, e.g. electrical power stations, oil refineries, coal mines because of its independence of measurement condition, e.g. fluorescence loss, electromagnetic compatibility problems and fluctuations of exciting light [16, 17]. The stability of S, the width of temperature range and luminescent intensity are other important factors which cannot be ignored. Since the luminescent intensity should be high enough under a relatively lower excitation power, so that the sensitivity (S) obtained from FIR technique can be reliable.
Generally, for phosphors doped with certain RE, their luminescent intensity, peak positions and shape of up-conversion (UC) emissions are mainly affected by host lattice, lattice sites and surroundings of luminescence centers [18, 19]. As one of the factors, host lattice that works on luminescent properties is closely related to covalent bond effect and crystal field effect [20, 21]. For this reason, it is essential to find a good host that can provide suitable crystal field environment which is in favor of UCL and temperature sensing application.
Tungstates which are known to be excellent sensitizers for rare earth luminescence has been widely studied as phosphors and scintillators, especially as excellent hosts for UCL and optical temperature sensing because of their chemical stability, relatively low phonon energy and the self-activating ability [22, 23]. In tungstate-based phosphors, the most investigated compounds are SrWO4 and MgWO4, as well as ZnWO4 and CaWO4. It is found that fairly high S have been realized by co-doping RE ions in tungstate based hosts, such as CaWO4, SrWO4, ZnWO4, MgWO4 and La2(WO4)3 [11, 23–26, 30], for which the details are listed later in Table 2. As one sort of strontium tungstates, Sr3WO6 has been known for several decades for its cation lattice being based on the double perovskite (Sr2SrWO6). Red light emission with high color purity of Sr3WO6:Eu3+ phosphor was well investigated by F. M. Emen et al. [31]. Another sort of strontium tungstates Sr2WO5 has a structure consisting of “infinite chains of tungsten-oxygen octahedra connected by their vertices”. Recently, Stephan G. Jantz et al. presented the first spectroscopic results on assessing the suitability of Sr2WO5 to act as host structures for sensitized phosphors [32].
In this study, we prepared the Er-doped strontium tungstate multiphase phosphors via a solid-state reaction method. The materials design is expected to modify the host and improve the luminescence performances. Moreover, the effect of the temperature and the pump power dependence on the UC emissions and n values has also been studied. Furthermore, to demonstrate the reliability and repeatability of the phosphors for practical use, the R values after 3 cycles of alternate cold and hot tests have also been investigated. The experiment results indicate that up-conversion luminescence and temperature sensing properties are indeed improved. The Sr/W ratio in Er-doped strontium tungstate phosphors has an effect on crystalline phases, UC emissions and temperature sensing properties. It is worthy to note that the maximal value of S is 12.75 × 10−3 K−1 at 523K in SWOE-0.6 with a broad temperature range from 83K to 563K (∆T = 480K). These results indicate that Er-doped strontium tungstate multiphase phosphors have great potential applications in high sensitivity optical temperature sensors.
2. Experimental
Er-doped strontium tungstates were designed as (1-x)SrCO3-xWO3:0.01Er (x = 0.2, 0.25, 0.333, 0.4, 0.5, 0.6) and were synthesized by a solid state reaction method, the samples denoted as SWOE-x. Stoichiometric amounts of raw materials SrCO3 (99.9%), WO3 (99.8%) and Er2O3 (99.9%) were thoroughly mixed in ethanol. The homogeneous mixture was then calcined at 800 °C~900 °C for 4 h in a muffle furnace. After calcination, 10 wt.% polyvinyl alcohol (PVA) binder was uniformly added into the reground powders. Then they were pressed into disk-shaped pellets of 10 mm in diameter and 2 mm in thickness. Finally, samples were sintered at temperatures of 1300 °C~1500 °C for 4 h in an alumina crucible in air.
The structure phase of the samples were analyzed using an x-ray diffractometer (XRD, Bruker D8 Advanced, Germany) with Cu Kα1 radiation (λ = 0.154056 nm), tube voltage 40 kV, and tube current 40mA, in 2θ angle ranging from 10° to 80° at room temperature, with a scanning speed of 5°/min. Reflectance spectra in the range of 200-2600 nm were measured with a U-4100 Spectrophotometer. UCL spectra were obtained using a 980 nm laser diode (HJZ980–100) to excite the surface of ceramic samples. The photons emitted from the samples were detected by a fluorescence spectrophotometer (F-7000, Hitachi High-Technologies Corporation, Tokyo, Japan). The laser beam was coupled into an optical fiber, collimated and then focused to the sample. For temperature sensing measurement, the sample was placed on a heating stage controlled by a TP94 temperature controller (Linkam Scientific Instruments Ltd, Surrey, UK) with a heating rate of 10 K/min.
3. Results and discussion
Figure 1(a) shows the XRD patterns of SWOE-x phosphors. It is obvious that the multiphase materials have been obtained by changing WO3 content, namely Sr/W ratio. The crystalline phases of samples vary with Sr/W ratio as well as their content and the XRD patterns matched well with the standard JCPDS cards. As can be observed in Fig. 1(a), for SWOE-0.2 and SWOE-0.25, the primary crystalline phase Sr3WO6 was obtained as well as small amounts of secondary phase Sr2WO5. When x was above 0.25, SrWO4 would appear. For SWOE-0.333 and SWOE-0.4, Sr3WO6, Sr2WO5 and SrWO4 were obtained simultaneously and their primary phases are Sr2WO5 and SrWO4 respectively. Additionally, for SWOE-0.5, the pure phase SrWO4 was obtained and as a comparison, for SWOE-0.6, the primary crystalline phase is SrWO4 with a slice of WO3. Therefore, to prove the uniformity, SWOE-0.6 was selected as a representative. As can be seen in Fig. 1(b), the shape of the phosphor was granulous and regular. The composition of Sr, O, W and Er was confirmed by element mapping, as displayed in Fig. 1(c). Note that all of the elements were evenly distributed in the phosphor.
Fig. 1 (a) XRD patterns of SWOE-x samples. The standard data for Sr3WO6, Sr2WO5 and SrWO4 are shown as the reference. (b) FESEM image and (c) element mapping images of SWOE-0.6.
In order to facilitate understanding the effect of crystalline phases on up-conversion luminescence properties, the relative phase contents of SWOE-x samples by pseudo-quantitative analysis of a specific phase are shown in Fig. 2. It is estimated from the measured XRD patterns by using Eq. (1):, where C is the relative phase content; I(phase) is the selected peak intensity from the peak position that consists of only one phase; and I(total) is the summation of all selected peaks intensity [27].
Fig. 2 Relative contents of all crystalline phases of SWOE-x samples as a function of WO3 content x.
The primary crystalline phase Sr3WO6 and the secondary phase Sr2WO5 appear at x = 0.2. With the increase of WO3 content up to x = 0.25, the Sr3WO6 content has a little increase then gradually decreases until to zero when x = 0.5. And at the same time, the Sr2WO5 content increases sharply and reaches maximum at x = 0.333 and then decreases rapidly disappearing with Sr3WO6. For the SrWO4 content, since the SrWO4 phase appeared at x = 0.333, it always increases with the WO3 content increasing. Combining with above analysis in Fig. 1, it indicates that different luminescent host can be obtained with different Sr/W ratio and it will have an effect on UC emission and temperature sensing properties eventually.
In order to further investigate the influence of Sr/W ratio in Er-doped strontium tungstate multiphase phosphors on UC emission, the UC emission spectra of SWOE-x samples were obtained, as shown in Fig. 3(a). It was observed that the emission peaks and intensity were largely influenced by Sr/W ratio. Firstly, according to the emission peak positions and crystalline phases, with increasing Sr/W ratio the samples could be divided into three kinds, as listed in Table 1. Secondly, it was obvious that the UC emission intensity of SWOE-0.25 was the strongest, which was much larger than that of others. Combined with the XRD patterns, it implies that the intensity, peak positions and shapes of UC emissions are largely affected by different host that can give different crystal field environments for certain RE, namely, luminescence center. Figure 3(b) shows the CIE chromaticity diagram of UC emission for SWOE-x samples. When x was below 0.4, the color of UC emission is green and with x increasing, it is red-shifted. Subsequently, it is blue-shifted (x = 0.4 and 0.5) and eventually it changes back to green again. It shows that by changing Sr/W ratio, SWOE-x phosphors are capable of providing different host and generating various UC spectra and the color can be tunable. To better understand the existence of different UC peak positions, the reflectance spectrums of SWOE-x (x = 0.25, 0.333, 0.5) samples from UV, Vis to NIR are shown in Fig. 3(c). The positions of these reflectance bands are almost identical while there are some differences among them in the shape, width and intensity. The broad reflectance bands at around 524-538 nm, 538-584 nm are the overlap of the 4I15/2→2H11/2 and 4I15/2→4S3/2 transitions of Er3+ ion, the band at 665 nm belong to transitions of Er3+ ion from4I15/2 to 4F9/2.
Fig. 3 (a) The UC emission spectra of SWOE-x samples, the inset shows the enlarged up-conversion emission spectra in 600nm-700nm. (b) CIE chromaticity diagram of UC emission for SWOE-x samples. (c) Reflectance spectrums of SWOE-x (x = 0.25, 0.333, 0.5) samples in the wavelength region from 200 to 1700 nm at room temperature.
Table 1. The comparison of UC peak positions and crystalline phases for SWOE-x samples.
To better seize the UC mechanisms, UC emission spectra were obtained under different pump power from 30mW to 70mW, as shown in Fig. 4(a). Apparently, with the increase of excitation power, all the detected UCL intensity of SWOE-x samples were enhanced step by step. It is known that in an unsaturated UC process, the number of photons (n) which are used to populate the upper UC emitting state can be obtained by the following relation:, where I is the UCL intensity and P is the pump laser power [26, 28]. Figure 4(b) shows CIE chromaticity diagram of UC emissions for SWOE-x samples under different pump power from 30mW to 70mW. For SWOE-0.2, 0.25 and 0.4, colourcoordinates were almost constant while they shifted largely along y-axis for SWOE-0.5 and 0.6. However, for SWOE-0.333, the colourcoordinates didn’t change as much as SWOE-0.5 and 0.6. It is indicating that the sensitivity of phosphors to pump power with different Sr/W ratio makes a large difference.
Fig. 4 (a) The UC emission spectra and (b) CIE chromaticity diagram of UC emissions for SWOE-x samples under different pump power from 30mW to 70mW.
Figure 5 shows the relationship between the values of n and temperature for SWOE-x samples. Under a 980 nm excitation, the n values change regularly with temperature increasing. For SWOE-0.2 and 0.25, they decreased firstly and then increased with temperature, while for SWOE-0.333 and 0.4 they declined after an initial rise. Besides, for SWOE-0.5 and 0.6, they went up after dropping and tend to be stable. The n values of SWOE-0.333 and 0.4 at 50 ̊C, 90 ̊C are much larger than 2 manifesting that three-photon process may be involved in the UC emissions, while those of others are less than or very close to 2 indicating the two-photon process are involved. This is mainly attributed to that three crystalline phases are coexistent in SWOE-0.333 and 0.4 while others only have one or two crystalline phases. It means that the crystal field around Er3+ in SWOE-0.333 and 0.4 are very different to others, which have massive influence on luminescent properties. This phenomenon is also consistent with the result that the samples can be divided into three kinds according to the UC emission peak positions, intensity and crystalline phases.
Fig. 5 The n values relative to temperature (20 ̊C, 50 ̊C, 90 ̊C and 130 ̊C) for SWOE-x samples under different pump power from 30mW to 70mW.
To demonstrate the possible application of SWOE-x phosphors, the optical temperature sensing performance was investigated, as shown in Fig. 6. At 83K, there were no peaks of UC emission spectrum at about 525 nm. Afterwards they gradually emerged and meanwhile their luminescent intensity steadily increased with the increase of temperature. This phenomenon could be attributed to the low energy gap (about 736 cm −1) between the levels 2H11/2 and 4S3/2 of Er3+ causing the state of 2H11/2 populated effectively from 4S3/2 by a thermalization process [29-30, 33]. Therefore, the relative variation in intensity between the two thermally coupled levels could guarantee a quasi-thermal equilibrium, according to the Boltzmann distribution. Conversely, the luminescent intensity of the UC band at about 550 nm and 560nm gradually decreased (Fig. 6(a)). This generally due to the enhancements of non-radiative relaxation [1]. As is known, the fluorescence intensity ratio of IH/IS (R) and sensitivity (S) are related to temperature, which can be expressed as follows: , , where A is the pre-exponential factor, ∆E is the energy separation between the two thermally coupled levels, k is the Boltzmann constant, and T is the temperature in Kelvin [2, 33-34]. The experimental data are well fitted to R and S equations, generating parameters of A and ∆E as listed in Table 2. For comparison, the optical thermometry parameters in other tungstate based hosts are also listed in Table 2. For practical applications of optical thermometry, S and temperature range are the most significant parameters to be considered. Apparently, the optical thermometry parameters of SWOE-x phosphors varied with x. It is worthy to note that, the maximal value of S is 12.75 × 10−3 K−1 at 523K in SWOE-0.6 with a broad temperature range from 83K to 563K (∆T = 480K). It means that this phosphor has a good prospect in high temperature sensor application. Furthermore, the relationship between ∆E and S with the increase of x is shown in the Fig. 7. It is interesting that with x increasing, ∆E first decreased slightly and then increased to the maximum value (1173.427 cm−1) at x = 0.333, afterwards it dropped to the minimum value (611.4218 cm−1) at x = 0.5 after a slight decrease and at last it picked up slightly again with the value 738.28 cm−1 at x = 0.6. However, the variation trend of S with x was absolutely inverse when the temperature was lower than 463K. As the temperature goes up, the values of S at x = 0.6 increase steadily until it is larger than that at x = 0.5.
Fig. 6 Temperature sensing properties of SWOE-x (x = 0.25 and 0.6) samples: (a) UC emission spectrum at different temperatures, (b) FIR relative to temperature. The inset shows the temperature dependence of S.
Table 2. Optical thermometry parameters in tungstate based hosts with Er3+ as activator under a 980 nm excitation.
To demonstrate the reliability and repeatability of SWOE-x phosphors for practical use, the R values after 3 cycles of alternate cold and hot test were investigated and discussed. Figure 8 shows the R values obtained after repeated 3 cycles from low temperature to high temperature and recorded at 83K, 283K and 483K, respectively. For all the SWOE-x samples, the R values are almost invariably and nearly as same as those in Fig. 6(b), indicating that SWOE-x samples exhibit excellent chemical stability and the S values are reliability. Therefore, SWOE-x phosphors can be applicable for temperature sensing application with a high sensor sensitivity.
Fig. 8 The R values obtained after 3 cycles of alternate cold and hot test at 83K, 283K and 483K, respectively.
4. Conclusions
In summary, the Er-doped strontium tungstate phosphors have been successfully prepared by a solid state reaction method. XRD study verifies that different composite hosts can be obtained by tuning Sr/W ratio. The influence of the Sr/W ratio in Er-doped strontium tungstates on the UC emissions and temperature sensing property has been investigated. With increasing Sr/W ratio, the samples’ structures and UC features are divided into three kinds, Sr3WO6 types and composites for SWOE-0.2 and 0.25, Sr2WO5 types and composites for SWOE-0.333 and 0.4, and SrWO4 types for SWOE-0.5 and 0.6. Meanwhile, the maximal S is determined to be 12.75 × 10−3 K−1 at 523K with a broad temperature range from 83K to 563K in SWOE-0.6. In addition, the R values after 3 cycles of alternate cold and hot test are almost invariably, indicating that the samples exhibit excellent chemical stability and the S values are reliable. From these results, the Er-doped strontium tungstate phosphors could have promising applicability in optical temperature sensors with high sensitivity and broad temperature range.
Funding
National Natural Science Foundation of China (Grant no. 51572195).
Acknowledgments
The authors thank all their colleagues for their precious discussions and technical support during the experiments and valuable discussions.
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