A novel optical thermometry based on the energy transfer from charge transfer band to Eu3+-Dy3+ ions

Optical thermometry based on the up-conversion intensity ratio of thermally coupled levels of rare earth ions has been widely studied to achieve an inaccessible temperature measurement in submicron scale. In this work, a novel optical temperature sensing strategy based on the energy transfer from charge transfer bands of W-O and Eu-O to Eu3+-Dy3+ ions is proposed. A series of Eu3+/Dy3+ co-doped SrWO4 is synthesized by the conventional high-temperature solid-state method. It is found that the emission spectra, emission intensity ratio of Dy3+ (572 nm) and Eu3+ (615 nm), fluorescence color, lifetime decay curves of Dy3+ (572 nm) and Eu3+ (615 nm), and relative and absolute sensitivities of Eu3+/Dy3+ co-doped SrWO4 are temperature dependent under the 266 nm excitation in the temperature range from 11 K to 529 K. The emission intensity ratio of Dy3+ (572 nm) and Eu3+ (615 nm) ions exhibits exponentially relation to the temperature due to the different energy transfer from the charge transfer bands of W-O and Eu-O to Dy3+ and Eu3+ ions. In this host, the maximum relative sensitivity Sr can be reached at 1.71% K−1, being higher than those previously reported material. It opens a new route to obtain optical thermometry with high sensitivity through using down-conversion fluorescence under ultraviolet excitation.

co-doped SrWO 4 are studied under 266 nm excitation. It is observed that the fluorescence intensity ratio between Eu 3+ and Dy 3+ emissions are strongly dependent on the temperature at the temperature range from 11 K to 529 K. The Eu 3+ /Dy 3+ co-doped SrWO 4 phosphors are proved as an excellent materials used for optical thermometry, due to its maximum value of S r as high as 1.71% K −1 .

Results
The X-ray diffraction (XRD) patterns of the SrWO 4 , SrWO 4 : 0.4 mol% Eu 3+ , and SrWO 4 : x Eu 3+ , 4 mol% Dy 3+ (x = 0, 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%, 1 mol%) samples synthesized by high-temperature solid-state reaction method are shown in Fig. 1. The peaks of all the products can be easily indexed to tetragonal system of SrWO 4 , which has a I41/a space group (PDF# 08-0490, unit cell parameters: a = b = 5.416 Å, c = 11.95 Å). No trace of impurity peaks can be found when Dy 3+ and Eu 3+ ions are introduced into the system. Compared with the pure SrWO 4 , the diffraction peaks of the Eu 3+ , Dy 3+ single-doped and Eu 3+ /Dy 3+ co-doped SrWO 4 exhibit a slight shift toward high-angle side, due to substitution of Sr 2+ (1.26 Å, CN = 8) ions by smaller size Dy 3+ (1.03 Å, CN = 8) and Eu 3+ (1.07 Å, CN = 8) ions, which revealing that Dy 3+ and Eu 3+ ions have been successfully doped into the system 18,19 . Figure 2 shows the unit cell parameters of a (Å) and c (Å) as well as unit cell volume (Å 3 ). It can be observed that the value of lattice parameter a (Å) decreases firstly due to substitution of Sr 2+ ions by smaller size Dy 3+ and Eu 3+ ions, and then increases with the increase of Eu 3+ concentration due to the size  x Eu 3+ , 4 mol% Dy 3+ (x = 0, 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%, 1 mol%). differences between the different valence state cations 20,21 . The same tendency can be observed in the values of parameter c (Å) and volume (Å 3 ). It reveals that Eu 3+ and Dy 3+ ions can be easily doped into SrWO 4 lattice, and the lattice can be distorted by the doping ions.
The scanning electron microscopy (SEM) image of a representative SrWO 4 : 0.4 mol% Eu 3+ , 4 mol% Dy 3+ sample is shown in Fig. 3a, exhibiting sphere-like morphology with a particle size of about 1 µm. The energy dispersive spectrometer (EDS) spectrum (Fig. 3b) (Fig. 5a) illustrates a broad charge transfer band centered at 247 nm from 200 nm to 280 nm and a series of sharp lines extended to visible    Fig. 5b. Monitored at 615 nm, an intense broad band can be found in the range of 250-320 nm, which is due to the Eu-O charge transfer transition 24,26 . While in the range of 200-250 nm, no obvious band can be found, indicating the energy transfer from WO 4 2− group to Eu 3+ is negligible. Additionally, a series of sharp lines corresponding to the intra 4 f electron transitions of Eu 3+ ion can also be observed, which are 360 nm ( 7 F 0 → 5 D 4 ), 380 nm ( 7 F 0 → 5 L 7 ), 393 nm ( 7 F 0 → 5 L 6 ), 414 nm ( 7 F 0 → 5 D 3 ), and 463 nm ( 7 F 0 → 5 D 2 ), respectively 27 . Figure 5c shows the excitation spectra of SrWO 4 : 0.4 mol% Eu 3+ , 4 mol% Dy 3+ phosphors. When compared with the excitation spectrum of SrWO 4 : 4 mol% Dy 3+ by monitoring at 572 nm, the position of broad band and the excitation peaks in both the spectra can be matched well with each other. Nevertheless, the excitation intensity of Dy 3+ is greatly enhanced when Eu 3+ is introduced. When monitored at 615 nm, the Eu 3+ excitation intensity decreases compared with the excitation spectrum of SrWO 4 : 0.4 mol% Eu 3+ . This may be due to the energy transfer from Eu 3+ to Dy 3+ . The apparent overlap of charge transfer band centered at about 266 nm can also be observed. Hence, 266 nm pulsed laser is selected as the excitation light source to excite Dy 3+ and Eu 3+ ions. Figure 6a displays the emission curves of SrWO 4 : x Eu 3+ , 4 mol% Dy 3+ (x = 0, 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%, 1 mol%) phosphors. The emission spectrum of the SrWO 4 : 4 mol% Dy 3+ reveals a strong yellow (572 nm) emission and a blue (485 nm) emission corresponding to the 4 F 9/2 → 6 H 13/2 and 4 F 9/2 → 6 H 15/2 transition of Dy 3+ ions, respectively, under the 266 nm excitation 28 . Two small emission peaks located at 660 and 750 nm are also observed, due to the transitions from 4 F 9/2 excited state to 6 H 11/2 and 6 H 9/2 ground states. And a very weak broad band in the range of 350-550 nm corresponding to the WO 4 2− emission can be found. One can see that the hypersensitive electric dipole transition 4 F 9/2 → 6 H 13/2 at 572 nm dominates the spectrum, which indicates that the Dy 3+ ions are placed at the sites of non-inversion symmetry 5,29 . Four new emission peaks at 590, 615, 650 and 700 nm appear, due to the f-f transitions ( 5 D 0 → 7 F 1,2,3,4 ) of Eu 3+ ions, along with the characteristic transitions of Dy 3+ 13 . The integral intensity of 572 nm and 615 nm emissions is calculated as a function of Eu 3+ concentration as well as the total emissions, as shown in Fig. 6b. The emission intensity of Eu 3+ (615 nm) increases with increase of the Eu 3+ concentration from 0.2 mol% to 0.8 mol%, and then decreases when the concentration further increases above 0.8 mol% due to the concentration quenching effect 30 . The Dy 3+ emission (572 nm) intensity increases with the increase of Eu 3+ concentration and reaches a maximum value at Eu 3+ concentration of 0.4 mol%, which can be ascribed to the energy transfer from Eu 3+ to Dy 3+ 31 . With the continuous increasing of Eu 3+ concentration, the Dy 3+ emission intensity decreases, which can be attributed to the concentration quenching effect. Focusing on the total emissions intensity, when the doping concentration of Eu 3+ reaches to 0.8 ml%, the strongest total emission intensity is obtained. Thus, the sample co-doped with 0.4 mol% Eu 3+ and 4 mol% Dy 3+ should be selected as the optimum doping concentration to study optical properties at different temperature.
The effective lifetimes of 4 F 9/2 and 5 D 0 energy levels can be expressed as 32 where I(t) represents the emission intensity at time t. The decay curves of Dy 3+ ( 4 F 9/2 ) and Eu 3+ ( 5 D 0 ) ions at different Eu 3+ concentration were recorded by monitoring at 572 nm and 615 nm, respectively. The decay curves support the existence of energy transfer progress for doped and co-doped samples. The values of lifetimes of SrWO 4 : x Eu 3+ , 4 mol% Dy 3+ (x = 0, 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%, 1 mol%) phosphors were calculated by using equation (1), in Fig. 6c. The decreasing tendency of lifetimes of both 4 F 9/2 and 5 D 0 energy levels can be found with the rise of Eu 3+ concentration. The Fig. 6c  To further study the temperature-dependent photoluminescence performance, the emission spectra of the SrWO 4 : 0.4 mol% Eu 3+ , 4 mol% Dy 3+ samples are investigated in the temperature range from 11 K to 592 K, as shown in Fig. 7a. One can see that the emission intensity of Dy 3+ ions increases with the rise of temperature, while the emission intensity of Eu 3+ ions decreases. The emission bands of Dy 3+ ions at 572 nm ( 4 F 9/2 → 6 H 13/2 ) and Eu 3+ ions at 615 nm ( 5 D 0 → 7 F 2 ) were enlarged and shown in Fig. 7b. One can find that the intensity of 572 nm (Dy 3+ ) increases with the temperature increase, while the intensity of 615 nm (Eu 3+ ) decreases with the temperature increase. It means that the energy transfer from charge transfer bands to Eu 3+ and Dy 3+ ions is temperature dependent. The Commission International de L'Eclairage (CIE) diagram (Fig. 7c) shows that the emission color of the SrWO 4 : 0.4 mol% Eu 3+ , 4 mol% Dy 3+ sample can be turned from the orange-red to the yellow region with the increase of temperature from 11 K to 529 K.
In order to study the energy transfer among charge transfer bands, Eu 3+ and Dy 3+ , the decay curves of 4 F 9/2 and 5 D 0 energy levels at different temperature were measured by monitoring at 572 nm and 615 nm, respectively, and calculated by using equation (1). The values of the effective lifetimes are shown in Fig. 7d. It can be found that the lifetimes of 4 F 9/2 energy level of Dy 3+ ion increase with the increase of temperature, while the lifetimes of 5 D 0 energy level of Eu 3+ ion decrease, demonstrating the different energy transfer rates from charge transfer bands to Dy 3+ and Eu 3+ ions 33 .
To study the temperature dependence of energy transfer from charge transfer bands to Eu 3+ -Dy 3+ ions, the dynamic balance rate-equation model for the energy transfer between charge transfer bands and Eu 3+ -Dy 3+ ions are established in Fig. 8. We supposed 7 F J (J = 0, 1, 2, 3, 4, 5, 6), 6 H J/2 (J = 15, 13,11,9,7), or 1 B( 1 T 2 )/ 1 E( 1 T 2 )/ 1 E( 1 T 1 ) energy levels as a same level in the case of the fixed temperature. The energy transfer between Eu 3+ and WO 4 2is neglected. The corresponding rate equations are as follows: where σ 1 and σ 2 are the cross-section of the ground state absorption of 7 F J and 1 A 1 , ρ 1 and ρ 2 are the incident pumping power density, N 0 , N 1 , N 2 , N 3 , N 4 , N 5 , N 6 , N 7 , and N 8 are the population densities of the levels of Eu 3+ , (WO 4 ) 2− , and Dy 3+ respectively. β 1 and β 2 correspond to the energy transfer rates from 5 D 3 and 1 B( 1 T 2 )/ 1 E( 1 T 2 )/ 1 E( 1 T 1 ) to 4 I 15/2 , respectively. The terms of W ij represent the nonradiative decay rates between the levels i and j, A ij is the radiative transition rates between the levels i and j. By solving the above equations, we have  The nonradiative relaxation possibility is proportional to ref. 34 The luminescence intensity of an emission band can be expressed as where hν i is transition energy per photon, A ij is spontaneous radiative emission probability from an i state to a j state, and N i is the state population of the i state 35 . The emission intensity ratio of Dy 3+ (572 nm) and Eu 3+ (615 nm) ions, defined as FIR (I Dy /I Eu ), is adopted to study the temperature-dependent photoluminescence property. Combining with above equations, the FIR (I Dy / I Eu ) can be fitted as Dy Eu where A is the fitting constant that depends on the experimental system and intrinsic spectroscopic parameter; ħω is the phonon energy; and k is a Boltzmann constant 36 . The absolute sensitivity and relative sensitivity can be defined as 37 (1 exp( / )) exp( 2 / ) 1 exp( / ) 2 2 2 As displayed in Fig. 9a, the FIR data could be exponentially fitted by the equation (10) from 11 K to 529 K. The parameters A, B and ħω can be determined to be 3250.7, 0.55 and 903.8 cm −1 for the SrWO 4 : 0.4 mol% Eu 3+ , 4 mol% Dy 3+ sample by using the fitting equation. The fitted phonon energy of 903.8 cm −1 is closed to the literature reported of 917.7 cm −1 38 . The error of the fitted phonon energy is about 1.5%. On the basis of the equations (11) and (12), the absolute sensitivity S a and relative sensitivity S r are calculated and shown in Fig. 9b,c. One can see that the absolute sensitivity is as high as 0.27 K −1 at 529 K. It is much higher than the literature reported 39,40 . For example, the absolute sensitivity in Eu 3+ doped Gd 2 Ti 2 O 7 phosphor was 0.015 K −1 41 , and in Dy 3+ doped GdVO 4 phosphor was 0.01 K −1 42 . The maximum relative sensitivity of 1.71% K −1 is obtained at 335 K. It is higher than the reported phosphors, 0.014 K −1 in Eu 3+ doped CaGd 2 (WO 4 ) 4 scheelite 43 and 0.003 °C −1 in Dy 3+ doped Y 4 Al 2 O 9 phosphor 44 . The improvement of both the relative sensitivity and absolute sensitivity of this material may be owing to different energy transfer ratio from charge transfer bands to Eu 3+ -Dy 3+ ions at different temperatures, leading to a significant change in the emission intensity ratio.
The error analysis of measured and calculated FIR(I 572 /I 615 ) is shown in Fig. 10a. One can see that the measured and the calculated FIR match well at low temperature, while the error appears at high temperature more than 400 K. The error may originated from the active nonradiative relaxation and energy transfer between Eu 3+ /Dy 3+ ions and host 39,45 . Notably, this error affects little on the values of S a and S r , as shown in Fig. 10b,c.

Conclusions
In this work, a series of Eu 3+ /Dy 3+ co-doped SrWO 4 phosphors were prepared by the high-temperature solid-state method. The structural property was studied by the X-Ray diffraction. The emission intensity, fluorescence color, and lifetimes of Dy 3+ (572 nm) and Eu 3+ (615 nm) of the SrWO 4 : 0.4 mol% Eu 3+ , 4 mol% Dy 3+ are investigated in the temperature range from 11 K to 529 K under the 266 nm excitation. The emission intensity ratio of Dy 3+ and Eu 3+ ions was found to be temperature dependent. The maximum value of S r can be reached 1.71% K −1 at 335 K, being higher than those previously reported material. This work opens a new route to obtain optical thermometry with high sensitivity through using down-conversion fluorescence under ultraviolet excitation.

Methods
A series of Eu 3+ /Dy 3+ single doped and co-doped SrWO 4 phosphors were prepared by the high-temperature solid-state method. According to the appropriate stoichiometric ratio, the starting materials, SrCO 3 (Aldrich, 99.9%), WO 3 (Aldrich, 99.9%), Eu 2 O 3 (Aladdin, 99.99%), and Dy 2 O 3 (Aldrich, 99.99%) were weighted and ground thoroughly in an agate mortar for 30 minutes with ethanol. Then the homogenous mixture was collected into a crucible and sintered at 1000 °C for 4 hours. After cooling to the room temperature, the obtained white samples were ground to powder for further investigation.
The obtained products were characterized by X-ray diffraction (XRD) using a Philips X'Pert MPD (Philips, Netherlands) X-ray diffractometer at 40 kV and 30 mA. All patterns are recorded in the range of 10-90° with a step size of Δ2θ = 0.02. The morphology, particle size and energy dispersive spectrometer (EDS) of the phosphor are characterized by scanning electron microscope (SEM) system (JSM-6490, JEOL Company). The ultraviolet-visible diffuse reflectance spectrum is recorded using a V-670 (JASCO) UV-vis spectrophotometer. The photoluminescence excitation (PLE) spectra are recorded by a Pjoton Technology International (PTI, USA) fluorimeter with a 60 W Xe-arc lamp as the excitation light source at room temperature. The photoluminescence (PL) spectra and decay lifetimes are collected by a 266 nm-pulsed laser with a pulse width of 5 ns and a repetition rate of 10 Hz (Spectron Laser Sys. SL802G).