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

Abstract

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 S_r 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.

Introduction

Recently, white light emitting diode (LED) technology has attracted much attention in the solid-state lighting industry, due to the advantages of white LEDs including power saving, long lifetime, and environmental benefit^1,7,8. For examples, Das and co-authors reported the controllable white light emission from Dy³⁺-Eu³⁺ co-doped KCaBO₃ phosphor⁶. Laguna reported the shape controlled white light emission from Dy³⁺-Eu³⁺ co-doped CaMoO₄ microarchitectures⁷. Hirai obtained the white light emission from Dy³⁺-Eu³⁺ co-doped Sr₂CeO₄ ⁸. In these works, the white light emission was controlled by changing do** concentration and host types.

It was reported that the temperature was a key parameter to adjust the emission intensity, the fluorescence intensity ratio, and emission color^9,10,11,12. Berry found that the lifetime of ⁵D₀ of Eu³⁺ ion was temperature dependent in Europium Tris(2,2,6,6-tetramethyl-3,5- heptanedionato)⁹. Morgan observed that the homogeneous linewidth of the ⁵D₀ → ⁷F₀ transition of Eu³⁺ was dependent on temperature in amorphous hosts¹⁰. Eckert observed that the phosphorescence decay lifetime of the Dy³⁺-transitions in Dy³⁺: Al₂O₃ showed strong temperature dependency in a temperature range from 1100 to 1500 K¹¹. Zhou reported that the emission intensity ratio of ⁵D₁ to ⁵D₀ of Eu³⁺-doped transparent MF₂ (M = Ba, Ca, Sr) glass ceramics increased with the temperature increase¹². However, the temperature dependent optical property of Dy³⁺-Eu³⁺ co-doped materials has not been studied so far. It is necessary to explore the spectra and energy transfer of Dy³⁺-Eu³⁺ co-doped materials at high temperature.

From the published work on the spectra of Dy³⁺-Eu³⁺ co-doped materials, one can find that it had a little overlap of the excitation spectrum between ⁵D₀ → ⁷F₂ (Eu³⁺) and ⁴F_9/2 → ⁶H_13/2 (Dy³⁺)^{13, 14}. It is necessary to find another ion to sensitize the Dy³⁺ and Eu³⁺ simultaneously. Notably, the charge transfer band of W-O was reported to have the wide absorption band in the ultraviolet range from 200 nm to 300 nm^15,16,17. It may be a promise sensitizer to excite Dy³⁺ and Eu³⁺ simultaneously. Thus, in this work, the optical temperature property of Eu³⁺/Dy³⁺ co-doped SrWO₄ are studied under 266 nm excitation. It is observed that the fluorescence intensity ratio between Eu³⁺ and Dy³⁺ emissions are strongly dependent on the temperature at the temperature range from 11 K to 529 K. The Eu³⁺/Dy³⁺ co-doped SrWO₄ 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 SrWO₄, SrWO₄: 0.4 mol% Eu³⁺, and SrWO₄: x Eu³⁺, 4 mol% Dy³⁺ (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 SrWO₄, 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 Dy³⁺ and Eu³⁺ ions are introduced into the system. Compared with the pure SrWO₄, the diffraction peaks of the Eu³⁺, Dy³⁺ single-doped and Eu³⁺/Dy³⁺co-doped SrWO₄ exhibit a slight shift toward high-angle side, due to substitution of Sr²⁺ (1.26 Å, CN = 8) ions by smaller size Dy³⁺ (1.03 Å, CN = 8) and Eu³⁺ (1.07 Å, CN = 8) ions, which revealing that Dy³⁺ and Eu³⁺ 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 (ų). It can be observed that the value of lattice parameter a (Å) decreases firstly due to substitution of Sr²⁺ ions by smaller size Dy³⁺ and Eu³⁺ ions, and then increases with the increase of Eu³⁺ concentration due to the size differences between the different valence state cations^{20, 21}. The same tendency can be observed in the values of parameter c (Å) and volume (ų). It reveals that Eu³⁺ and Dy³⁺ ions can be easily doped into SrWO₄ lattice, and the lattice can be distorted by the do** ions.

Figure 1

(a) XRD patterns of the as-synthesized SrWO₄, SrWO₄: 0.4 mol% Eu³⁺, and SrWO₄: x Eu³⁺, 4 mol% Dy³⁺ (x = 0, 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%, 1 mol%) phosphors. The standard data of tetragonal SrWO₄ (PDF# 08-0490) is given as a reference; (b) Partially enlarged XRD patterns of the corresponding phosphors (2θ = 27–29°).

Figure 2

(a) Unit cell parameters of a (Å) and (b) c (Å) and (c) unit cell volume (ų) at different Eu³⁺ concentration in tetragonal SrWO₄: x Eu³⁺, 4 mol% Dy³⁺ (x = 0, 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%, 1 mol%).

The scanning electron microscopy (SEM) image of a representative SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ 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) confirms the presence of Sr, W, O, Eu, and Dy elements, and further providing the evidence that Dy³⁺ and Eu³⁺ ions have been successfully doped into the SrWO₄ host lattice.

Figure 3

(a) SEM and (b) EDS images of the Eu³⁺/Dy³⁺ co-doped SrWO₄ phosphor.

The ultraviolet-visible diffuse reflectance spectrum of the SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ in the range of 200–800 nm is shown in Fig. 4. A broad band and several absorption peaks corresponding to the doped ions can be observed. The broad band is located from 200 to 350 nm, corresponding to the O-W ligand-to-metal charge transfer in the WO₄ ²⁻ group^{22, 23}. Four absorption peaks located at 365, 384, 426 and 454 nm can be assigned to the intra 4 f electronic transitions of ⁷F₀ → ⁵D₄ (Eu³⁺), ⁷F₀ → ⁵G₂ (Eu³⁺), ⁶H_15/2 → ⁴G_11/2 (Dy³⁺), and ⁶H_15/2 → ⁴I_15/2 (Dy³⁺), respectively.

Figure 4

The ultraviolet-visible diffuse reflectance spectrum of the SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ phosphor at room temperature.

The PLE spectra of SrWO₄: 4 mol% Dy³⁺, SrWO₄: 0.4 mol% Eu³⁺, and SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ samples are shown in Fig. 5. The PLE spectrum of SrWO₄: 4 mol% Dy³⁺ (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 region can be observed by monitoring at 572 nm. The broad band can be ascribed to the charge transfer from WO₄ ²⁻ group to Dy^{3+  24}, and the seven sharp lines can be ascribed to f–f transitions of Dy³⁺ 4 f configuration, which are ⁶H_15/2 → ⁴K_13/2 (296 nm), ⁶H_15/2 → ⁴K_15/2 (322 nm), ⁶H_15/2 → ⁴M_15/2 (350 nm), ⁶H_15/2 → ⁴P_3/2 (365 nm), ⁶H_15/2 → ⁴M_21/2 (387 nm), ⁶H_15/2 → ⁴G_11/2 (426 nm), and ⁶H_15/2 → 4I_15/2 (450 nm), respectively²⁵. The excitation spectrum of SrWO₄: 0.4 mol% Eu³⁺ is shown in 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 WO₄ ²⁻ group to Eu³⁺ is negligible. Additionally, a series of sharp lines corresponding to the intra 4 f electron transitions of Eu³⁺ ion can also be observed, which are 360 nm (⁷F₀ → ⁵D₄), 380 nm (⁷F₀ → ⁵L₇), 393 nm (⁷F₀ → ⁵L₆), 414 nm (⁷F₀ → ⁵D₃), and 463 nm (⁷F₀ → ⁵D₂), respectively²⁷. Figure 5c shows the excitation spectra of SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ phosphors. When compared with the excitation spectrum of SrWO₄: 4 mol% Dy³⁺ 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 Dy³⁺ is greatly enhanced when Eu³⁺ is introduced. When monitored at 615 nm, the Eu³⁺ excitation intensity decreases compared with the excitation spectrum of SrWO₄: 0.4 mol% Eu³⁺. This may be due to the energy transfer from Eu³⁺ to Dy³⁺. 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 Dy³⁺ and Eu³⁺ ions.

Figure 5

Excitation spectra of (a) SrWO₄: 4 mol% Dy³⁺, (b) SrWO₄: 0.4 mol% Eu³⁺, and (c) SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ phosphors at room temperature.

Figure 6a displays the emission curves of SrWO₄: x Eu³⁺, 4 mol% Dy³⁺ (x = 0, 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%, 1 mol%) phosphors. The emission spectrum of the SrWO₄: 4 mol% Dy³⁺ reveals a strong yellow (572 nm) emission and a blue (485 nm) emission corresponding to the ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2 transition of Dy³⁺ ions, respectively, under the 266 nm excitation²⁸. Two small emission peaks located at 660 and 750 nm are also observed, due to the transitions from ⁴F_9/2 excited state to ⁶H_11/2 and ⁶H_9/2 ground states. And a very weak broad band in the range of 350–550 nm corresponding to the WO₄ ²⁻ emission can be found. One can see that the hypersensitive electric dipole transition ⁴F_9/2 → ⁶H_13/2 at 572 nm dominates the spectrum, which indicates that the Dy³⁺ 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 (⁵D₀ → ⁷F_1,2,3,4) of Eu³⁺ 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 Eu³⁺ concentration as well as the total emissions, as shown in Fig. 6b. The emission intensity of Eu³⁺ (615 nm) increases with increase of the Eu³⁺ 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 effect³⁰. The Dy³⁺ emission (572 nm) intensity increases with the increase of Eu³⁺ concentration and reaches a maximum value at Eu³⁺ concentration of 0.4 mol%, which can be ascribed to the energy transfer from Eu³⁺ to Dy^{3+  31}. With the continuous increasing of Eu³⁺ concentration, the Dy³⁺ emission intensity decreases, which can be attributed to the concentration quenching effect. Focusing on the total emissions intensity, when the do** concentration of Eu³⁺ reaches to 0.8 ml%, the strongest total emission intensity is obtained. Thus, the sample co-doped with 0.4 mol% Eu³⁺ and 4 mol% Dy³⁺ should be selected as the optimum do** concentration to study optical properties at different temperature.

Figure 6

(a) PL emission spectra, (b) Integral intensity of Dy³⁺ (572 nm), Eu³⁺ (615 nm) and total emission, and (c) Calculated lifetimes of ⁴F_9/2 and ⁵D₀ energy levels of SrWO₄: x Eu³⁺, 4 mol% Dy³⁺ (x = 0, 0.2 mol%, 0.4 mol%, 0.6 mol%, 0.8 mol%, 1 mol%) phosphors under 266 nm excitation at room temperature.

The effective lifetimes of ⁴F_9/2 and ⁵D₀ energy levels can be expressed as³²

$${\tau }_{eff}=\frac{\int I(t)tdt}{\int I(t)dt}$$

(1)

where I(t) represents the emission intensity at time t. The decay curves of Dy³⁺ (⁴F_9/2) and Eu³⁺ (⁵D₀) ions at different Eu³⁺ 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 SrWO₄: x Eu³⁺, 4 mol% Dy³⁺ (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 ⁴F_9/2 and ⁵D₀ energy levels can be found with the rise of Eu³⁺ concentration. The Fig. 6c shows the inhomogeneous change of lifetimes of the 572 nm (Dy³⁺) and 615 nm (Eu³⁺) emissions. It means that the energy transfer from charge transfer band of W-O to Dy³⁺ and the energy transfer from charge transfer band of Eu-O to Eu³⁺ as well as energy transfer from Eu³⁺ to Dy³⁺ are different at different Eu³⁺ concentrations.

To further study the temperature-dependent photoluminescence performance, the emission spectra of the SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ 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 Dy³⁺ ions increases with the rise of temperature, while the emission intensity of Eu³⁺ ions decreases. The emission bands of Dy³⁺ ions at 572 nm (⁴F_9/2 → ⁶H_13/2) and Eu³⁺ ions at 615 nm (⁵D₀ → ⁷F₂) were enlarged and shown in Fig. 7b. One can find that the intensity of 572 nm (Dy³⁺) increases with the temperature increase, while the intensity of 615 nm (Eu³⁺) decreases with the temperature increase. It means that the energy transfer from charge transfer bands to Eu³⁺ and Dy³⁺ ions is temperature dependent. The Commission International de L’Eclairage (CIE) diagram (Fig. 7c) shows that the emission color of the SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ sample can be turned from the orange-red to the yellow region with the increase of temperature from 11 K to 529 K.

Figure 7

(a) PL Emission spectra, (b) Temperature-dependent spectra at 572 nm and 615 nm, (c) CIE, (d) Calculated lifetimes of ⁴F_9/2 (Dy³⁺) and ⁵D₀ (Eu³⁺) energy levels of Dy³⁺ and Eu³⁺ ions of the SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ phosphor under 266 nm excitation from 11 K to 529 K.

In order to study the energy transfer among charge transfer bands, Eu³⁺ and Dy³⁺, the decay curves of ⁴F_9/2 and ⁵D₀ 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 ⁴F_9/2 energy level of Dy³⁺ ion increase with the increase of temperature, while the lifetimes of ⁵D₀ energy level of Eu³⁺ ion decrease, demonstrating the different energy transfer rates from charge transfer bands to Dy³⁺ and Eu³⁺ ions³³.

To study the temperature dependence of energy transfer from charge transfer bands to Eu³⁺-Dy³⁺ ions, the dynamic balance rate-equation model for the energy transfer between charge transfer bands and Eu³⁺-Dy³⁺ ions are established in Fig. 8. We supposed ⁷F_J(J = 0, 1, 2, 3, 4, 5, 6), ⁶H_J/2 (J = 15, 13, 11, 9, 7), or ¹B(¹T₂)/¹E(¹T₂)/¹E(¹T₁) energy levels as a same level in the case of the fixed temperature. The energy transfer between Eu³⁺ and WO₄ ^2- is neglected. The corresponding rate equations are as follows:

$$\frac{d{N}_{1}}{dt}={N}_{2}{W}_{21}-{N}_{1}{A}_{10}$$

(2)

$$\frac{d{N}_{2}}{dt}={N}_{3}{W}_{32}-{\beta }_{1}{N}_{2}{N}_{4}-{N}_{2}{W}_{21}$$

(3)

$$\frac{d{N}_{3}}{dt}={\sigma }_{1}{\rho }_{1}N{}_{0}-{N}_{3}{W}_{32}$$

(4)

$$\frac{d{N}_{5}}{dt}={N}_{4}{W}_{45}-{N}_{5}{A}_{56}$$

(5)

$$\frac{d{N}_{8}}{dt}={\sigma }_{2}{\rho }_{2}{N}_{7}-{N}_{8}{A}_{87}-{\beta }_{2}{N}_{8}{N}_{4}$$

(6)

where σ ₁ and σ ₂ are the cross-section of the ground state absorption of ⁷F_J and ¹A₁, ρ ₁ and ρ ₂ are the incident pum** 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 Eu³⁺, (WO₄)²⁻, and Dy³⁺ respectively. β₁ and β₂ correspond to the energy transfer rates from ⁵D₃ and ¹B(¹T₂)/¹E(¹T₂)/¹E(¹T₁) to ⁴I_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.

Figure 8

The mechanism graph of the optical temperature sensing through energy transfer from the charge transfer bands to Dy³⁺ and Eu³⁺ ions under the 266 nm excitation.

By solving the above equations, we have

$$\frac{{N}_{5}}{{N}_{1}}\approx \frac{{W}_{45}{A}_{10}}{{A}_{56}{W}_{21}{\sigma }_{1}{\rho }_{1}{N}_{0}}(\frac{{W}_{21}}{{\beta }_{2}}+\frac{{\beta }_{1}}{{\beta }_{2}^{2}})$$

(7)

The nonradiative relaxation possibility is proportional to ref. 34

$${w}_{ij}\propto {e}^{-\hslash w/kT}$$

(8)

The luminescence intensity of an emission band can be expressed as

$${I}_{ij}=h{v}_{i}{A}_{ij}{N}_{i}$$

(9)

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 state³⁵.

The emission intensity ratio of Dy³⁺ (572 nm) and Eu³⁺ (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

$$FIR=\frac{{I}_{Dy}}{{I}_{Eu}}=\frac{A\exp (-2\hslash \omega /kT)}{1-\exp (-\hslash \omega /kT)}+B$$

(10)

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 constant³⁶. The absolute sensitivity and relative sensitivity can be defined as³⁷

$${S}_{a}=\frac{dR}{dT}=\frac{{\rm{A}}\hslash \omega \exp (-3\hslash \omega /kT)}{k{(1-\exp (-\hslash \omega /kT))}^{2}{T}^{2}}+\frac{{\rm{2A}}\hslash \omega \exp (-2\hslash \omega /kT)}{k(1-\exp (-\hslash \omega /kT)){T}^{2}}$$

(11)

$${S}_{r}=\frac{1}{R}\frac{dR}{dT}=\frac{\frac{{\rm{A}}\hslash \omega \exp (-3\hslash \omega /kT)}{k{(1-\exp (-\hslash \omega /kT))}^{2}{T}^{2}}+\frac{{\rm{2A}}\hslash \omega \exp (-2\hslash \omega /kT)}{k(1-\exp (-\hslash \omega /kT)){T}^{2}}}{B+\frac{A\exp (-2\hslash \omega /kT)}{1-\exp (-\hslash \omega /kT)}}$$

(12)

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 SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ 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 Eu³⁺ doped Gd₂Ti₂O₇ phosphor was 0.015 K^{−1  41}, and in Dy³⁺ doped GdVO₄ 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 Eu³⁺ doped CaGd₂(WO₄)₄ scheelite⁴³ and 0.003 °C⁻¹ in Dy³⁺ doped Y₄Al₂O₉ phosphor⁴⁴. 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 Eu³⁺-Dy³⁺ ions at different temperatures, leading to a significant change in the emission intensity ratio.

Figure 9

(a) Experimental measured and fitted plots of FIR (I₅₇₂/I₆₁₅) versus temperature. (b) Absolute sensitivity S_a and (c) Relative sensitivity S_r versus temperature.

The error analysis of measured and calculated FIR(I₅₇₂/I₆₁₅) 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 Eu³⁺/Dy³⁺ 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.

Figure 10

(a) Experimental measured and calculated plots of FIR (I₅₇₂/I₆₁₅) versus temperature. (b) Measured and calculated absolute sensitivity S_a and (c) relative sensitivity S_r versus temperature.

Conclusions

In this work, a series of Eu³⁺/Dy³⁺ co-doped SrWO₄ 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 Dy³⁺ (572 nm) and Eu³⁺ (615 nm) of the SrWO₄: 0.4 mol% Eu³⁺, 4 mol% Dy³⁺ are investigated in the temperature range from 11 K to 529 K under the 266 nm excitation. The emission intensity ratio of Dy³⁺ and Eu³⁺ 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 Eu³⁺/Dy³⁺ single doped and co-doped SrWO₄ phosphors were prepared by the high-temperature solid-state method. According to the appropriate stoichiometric ratio, the starting materials, SrCO₃ (Aldrich, 99.9%), WO₃ (Aldrich, 99.9%), Eu₂O₃ (Aladdin, 99.99%), and Dy₂O₃ (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).