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Abstract
In this paper, SrMoO4:Er3+ phosphor excited by 976 nm diode laser shows remarkably intense green and relatively weak red upconversion emissions. Subsequently, within the temperature range from 310.8 K to 629.4 K, we tested a temperature-dependent fluorescence intensity ratio originating from the thermally coupled 2H11/2 and 4S3/2 states. It is revealed that SrMoO4:Er3+ phosphor exhibits a high sensitivity of 0.0326 K−1 at 554.0 K, indicating that the phosphor can be regarded as a promising building block for optical temperature sensor.
© 2017 Optical Society of America
1. Introduction
In last decades, the fluorescence intensity ratio (FIR) technique offers a novel route to design and develop versatile optical temperature sensor [1–3].The well-defined scheme is closely associated with the luminescence intensities in principle, which derive from two emitting levels of trivalent lanthanide ions and simultaneously have strong temperature dependence. In particular, the upconversion (UC) emissions of Er3+ at ~520 nm (2H11/2 → 4I15/2) transition and at ~550 nm (4S3/2 → 4I15/2) are of particular interest, as the populations in 2H11/2 and 4S3/2 states follow the Boltzmann Distribution Law when using steady-state excitation [4,5].
On the other hand, the sensitivity of FIR technique is an important parameter for optical temperature sensor. As a result of the substantial reduction of the laser-induced heating effect, lower excitation power density is favorable to improve the sensitivity [6]. And Er3+ ions doped host, which presents a relatively high phonon threshold energy (PTE) level, is also favorable to improve the sensitivity [7]. However, it should be noted that since a relatively high PTE level of the host, the multiphonon non-radiative decay rate of the intermediate state of the involved activator (Er3+ ions) becomes larger, leading to the depressed UC emissions. Meanwhile, it is expected that Er3+ ions doped host with a relatively high PTE level is illuminated with intense excitation power density to boost the UC emission intensity. As analyzed above, there is a contradiction between the excitation power density and PET of the host in terms of the temperature sensitivity. Therefore, it is a great challenge to search for Er3+ doped host with both the relatively high PTE and a relatively high UC efficiency.
SrMoO4 (strontium molybdate) as a host has a wealth of salient features, such as stable chemical and thermal properties, moisture free and a low excitation threshold. More importantly, the corresponding phonon energy is as high as 885 cm–1, which is much more than that of the fluoride [8]. Besides, researches and our group have found that Er3+ doped molybdates show the strong UC emission intensity [9,10], which makes Er3+ doped SrMoO4 suitable for UC-luminescence-based thermometer. Therefore, it is important to study the temperature dependence of FIR in SrMoO4:Er3+ phosphor.
In this work, we prepared SrMoO4:Er3+ phosphor with tetragonal phase via a simple sol–gel process by controlling the molar ratio of Er3+ ions. The UC emissions of SrMoO4:Er3+ phosphor were studied at various temperature under a 976 nm diode laser (LD) illumination with excitation power intensity of 25 W/cm2 for the purpose of temperature sensing. And the effect of thermal quenching of SrMoO4:Er3+ phosphor was discussed.
2. Experimental
The SrMoO4:x mol% Er3+ (x = 0.5, 1, 1.5, 2, 2.5) phosphors were synthesized via a simple sol–gel method. (NH4)6Mo7O24 (AR), Sr(NO3)2 (AR), and Er(NO3)3·6H2O (99.99%) with appropriate stoichiometric were dissolved in deionized water. Citric acid (AR) as complexing agent was dissolved in the precursor solution with a mole ratio of cations (Sr + Mo + Er) to citric acid of 1:1.5. Subsequently, ammonia (AR) was added for adjusting the precursor solution pH to about 7. The resulting solution was vigorously churned for 1 h, and then was dried at 130 °C for 20 h. Finally, the as-prepared sample was further calcined in air at 800 °C for 2 h. Until cooled to room temperature in air, SrMoO4:x mol% Er3+ were pressed to form the thin wafer with a diameter of 10 mm and a thickness of 1 mm. With the adjusting Er3+ ion concentrations, 1 mol% Er3+ doped in SrMoO4 matrix (denoted as SMO:Er) achieves the highest green emission intensity. So the sample was selected as the research object, which attached to an IKA C-MAG HP4 heating stage.
The structural formation of the sample was measured by recording its X-ray diffraction (XRD) pattern using a MSAL XD-2/3 powder diffractometer with graphite monochromatized Cu Ka radiation (λ = 0.15406 nm). A 976 nm continuous wave LD was irradiated the samples. The laser was focused on the samples (spot size of ~1 mm) by a 10 cm focal length lens. The UC spectra were collected at room temperature, by using a spectrometer (Jobin Yvon, iHR 550) with a 1800 g/mm grating (holographic, 400–850 nm) and a photomultiplier tube (R928, Hamamatsu).
3. Results and discussion
3.1 Phase characterization
The XRD pattern of the SMO:Er powder is shown in Fig. 1(a). The diffraction peak positions are in good agreements with those of the standard pattern (JCPDS card No. 08−0482) of SrMoO4 in the scheelite structure with no impurities present, suggesting that Er3+ ions had been uniformly incorporated into the host lattice. The SMO:Er powder has tetragonal symmetry, belonging to the space group I41/a. As shown in Fig. 1(b), each of Mo atoms is surrounded by four equivalent O atoms constituting the MoO42− tetrahedron and each Sr atom shares the corners with eight adjacent O atoms of MoO42− tetrahedral configurations. The mean crystallite size d of sample can be calculated by Scherrer equation: d = 0.89λ/(Bcosθ), where θ, B, and λ are Bragg angle of diffraction peak, full width at half maximum of XRD peaks and the wavelength of X–ray, respectively. The average size of SMO:Er powder is around 45 nm.
3.2 UC luminescence analysis
As shown in Fig. 2, the UC spectra are composed of two band as followed. The strong green band centered at 521 nm and 553 nm associated to the mixed transition 2H11/2 + 4S3/2→4I15/2 and the relatively weak red band centered at 657 nm associated to the 4F9/2→4I15/2 transition of the Er3+ ion. The UC mechanism of SMO:Er phosphor was discussed in our recently paper [10]. Therefore, it is not discussed here. In order to describe the colour of the light emitted from SMO:Er phosphor, the chromaticity coordinates were calculated at different pump power as shown in Fig. 2(b), according to the Commission Interna-tionale de l’Eclairage (CIE) chromaticity diagram. The chromaticity coordinates were found to vary towards the pure green region when increasing the excitation power from 25 W/cm2 (0.369, 0.610) to 192.3 W/cm2 (0.231, 0.727).
Fig. 2 (a) UC spectra and the corresponding photograph of SMO:Er phosphor under LD excitation of 976 nm. (b) CIE chromaticity diagram of SMO:Er phosphor at different excitation powers.
3.3 Thermal effect
The temperature dependence of the emission intensity of Er3+ ions is shown in Fig. 3(a). The emission peaks does not shift with increasing temperature. As shown in Fig. 3(b), IH (IH: the integral intensity of 518–535 nm, 2H11/2→4I15/2) increased with increasing temperature from 310.8 K to 401.8 K, then it gradually reverses with increasing temperature. These results can be attributed to the competition between the thermal agitation and non-radiative relaxation of 2H11/2 levels. IS (IS: the integral intensity of 540–558 nm, 4S3/2→4I15/2) significantly decreased with the increasing temperature from 310.8 K to 629.4 K. The value of IT/I0 also decreased with the increasing temperature (IT and I0: the integral intensity of 518–558 nm at different temperature and at 310.8 K, respectively). According to the multiphonon non-radiative decay rate [11,12]:
where W0(0) and Wm(T) are the multiphonon rate at absolute temperature 0 K and T, respectively, m = △Em/hv is the number of phonons energy hν required to bridge the energy gap △Em between the two states involved, T is the absolute temperature, and k is Boltzmann’s constant. As T rises, the multiphonon non-radiative decay rate Wm(T) increases. Therefore, the thermal effect leads to the quenching of the green emission IT of SMO:Er phosphor. It is noted that the green UC emission intensity is weaken at 629.4 K, due to the thermal quenching effect (see Fig. 3(b)). However, it only goes down toAs sho about 43.8% of that at 310.8 K. It seems that the effect of thermal quenching is not obvious, which makes it suitable for the temperature sensor.Fig. 3 (a) Temperature dependence of the green UC spectra of SMO:Er phosphor under 976 nm excitation. (b) Temperature dependence of IH, IS and IT/I0.
As shown in Fig. 3(b), IH and IS are related to temperature. The population of “thermally coupled pairs” 2H11/2 and 4S3/2 states follows a Boltzmann distribution, and a quasi thermal equilibrium exists between the two states. Therefore, FIR of 2H11/2 and 4S3/2 can be expressed as
where N, ω, g and σ are the number of ions, the angular frequency of fluorescence transitions from the 2H11/2 and 4S3/2 states to 4I15/2 ground-state, the degeneracy, and the emission cross-section, respectively. The pre-exponential factor is given by C = gaσaωa/gbσbωb, and ΔE = E(2H11/2) − E(4S3/2) is the energy gap between two thermally coupled levels.Figure 4(a) shows the natural logarithm plot of the FIR as a function of inverse absolute temperature in the range of 310.8–629.4 K. The experimental data are fitted by straight line with a slope of –1110.8. The data obtained from experiment is well agree with Eq. (2), and the best fit is R(IH/IS) = 66.7exp(–1110.8/T). This demonstrates that the temperature can be measured from the ratio of IH to IS within 310.8–629.4 K. The sensitivity of temperature sensor is given by [1,13]
Fig. 4 (a) Monolog natural logarithm plot of FIR as a function of inverse absolute temperature. (b) FIR as a function of inverse absolute temperature.
The sensitivity of SMO:Er phosphor calculated from Eq. (3) is shown in Fig. 5. The sensitivity is noted to be 0.0220 K−1 at 310.8 K and 0.0320 K−1 at 629.4 K, respectively. It is shown that the sensitivity continuously increasing in a temperature range of 310.8–554.0 K and the maximum sensitivity is 0.0326 K−1 at 554.0 K. It can be observed from Table 1 that the sensitivity of SMO:Er phosphor compares favorably to Er3+ or Er3+/Yb3+ doped other hosts reported in the literature. Other parameters (△E/k, C and temperature range) are also shown in Table 1. This implies that the SMO:Er phosphor could be potentially applied for thermal sensor, even for nanoscale thermal sensor.
Table 1. FIR parameters of Er3+ doped and Er3+/Yb3+ co-doped hosts, SMAX and the temperature range
4. Conclusions
A simple sol–gel method has been employed to prepare the SMO:Er phosphor. Excitation of the phosphor at 976 nm revealed the UC spectra characteristics of Er3+ with the green and red spectral regions. By using the FIR technique, optical thermometry based on the thermally coupled 2H11/2 and 4S3/2 states of Er3+ in SMO:Er phosphor was demonstrated in the range from 310.8 K to 629.4 K. The maximum sensitivity is about 0.0326 K−1 at 554.0 K, which is superior with respect to those reported optical temperature sensors based on the FIR technique. Consequently the SMO:Er phosphor has potential application in making an efficient green phosphor and temperature sensor with high sensitivity.
Funding
National Natural Science Foundation of China (Grant Nos. 11404283, 21401165 and 11604236); Natural Science Foundation of Guangdong Province (Grant Nos. 2014A030307028 and 2014A030307040); Science and Technology Planning Project of Guangdong Province (Grant No. 2016A040403124); Educational Commission of Guangdong Province (Grant No. 2015KQNCX093); Doctoral Scientific Research Foundation of Lingnan Normal University (ZL1608).
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