Bright dual-mode green emission and temperature sensing properties in Er³⁺/Yb³⁺ co-doped MgWO₄ phosphor

Abstract

Er³⁺/Yb³⁺ co-doped MgWO₄ phosphor was synthesized by a solid state reaction method and the phase, photoluminescence and temperature sensing properties were analyzed. The phosphor has shown intense visible green emission via up-conversion process on near-infrared (980 nm) excitation and down-conversion process on 379 nm excitation and thus behaves as a dual-mode phosphor. In the up-conversion and down-conversion emission, the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ion portray a temperature dependent behavior and have been used for optical temperature sensor by means of the fluorescence intensity ratio method. Moreover, we compared the curves of S in the continuous temperature range (83–583 K) and the two sectional temperature ranges in up-conversion process. It is found that the phosphor can be operated over a temperature range of 83–583 K with a maximum sensitivity of about 0.0093 K⁻¹ at 583 K. The results indicate that the Er³⁺/Yb³⁺ co-doped MgWO₄ phosphor is a potential candidate to be used in display devices and optical temperature sensors.

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1. Introduction

Dual-mode photoluminescence means that the same material can emit visible light under excitation with both ultraviolet (UV) and near-infrared (NIR) radiation.^1–3 That is to say, the dual-mode luminescent mechanism contains two energy transmission processes of down-conversion (DC) and up-conversion (UC). There are several methods to realize dual-mode luminescence. Firstly, it can be obtained by forming hybrid nanomaterials.^1,4 For example, mixing Gd₂O₃:Yb³⁺/Er³⁺, which exhibits UC, and Eu(DBM)₃Phen, which shows classic luminescence, forms a hybrid material that could be sensitive to two ranges of excitation radiation.⁴ Secondly, dual-mode luminescence can be obtained by formatting core/shell nanostructures. For example, a core with UC properties, which could be composed of Na(Y,Gd)F₄:Yb³⁺/Er³⁺, could be covered with a NaGdF₄:Ce³⁺/Tb³⁺ shell, which shows DC properties.⁵ Thirdly, the luminescence caused by UV irradiation is due to the DC of excited light in the host material and the UC occurs between do pant ions, which occurs in YNbO₄:Yb³⁺/Er³⁺.² Fourthly, dual-mode photoluminescence can also be obtained by selecting two different ions which can be excited by UV and NIR respectively and doping them into one host.⁶ In addition, dual-mode photoluminescence is possible in materials doped with only one type of activator.⁷ As reported in this paper, bright green dual-mode photoluminescence from Er³⁺ ions was achieved by the simultaneous DC and UC in a single material, which was a monoclinic magnesium tungstate MgWO₄ when co-doped with Yb³⁺ ions.

Among the variety of available host matrices, the compounds belonging to the AWO₄ (A = Mg, Ca, Sr, Ba, B = W, Mo) family have been thought to be a kind of potential luminescent materials. The properties of lanthanide doped CaWO₄, CaMoO₄, SrWO₄, SrMoO₄ and BaWO₄ have been extensively investigated.^14,18–22 However, MgWO₄, another compound of this family, is not well explored. MgWO₄ has a wolframite structure which belongs to a low crystal symmetry monoclinic crystal system, with the space group P2/c (13).²³ Meanwhile, MgWO₄, exhibited good chemical and physical stability, is self-activated luminescent materials and has been used as lamp phosphor for a long-term application. The rare earth Eu³⁺ doped MgWO₄ phosphors produces a red emitting through normal fluorescence process (DC).²⁴ However, to the best of our knowledge, there are hardly any reports on dual-mode photoluminescence studies for an MgWO₄ host are available.

For the rare-earth doped UC luminescence materials, one of its potential applications is optical temperature sensor. The optical temperature sensor measured temperature by using a fluorescence intensity ratio (FIR) technique which provides good accuracy and high sensitivity.^8–11 Compared with other temperature measurement techniques, this optical thermometry method can reduce the dependence of testing conditions, such as fluorescence loss and the electromagnetic compatibility problem.^8,12 The FIR technique required two coupled levels of rare earth and the energy separation must be large enough (>200 cm⁻¹) to avoid overlap of two emissions and small enough (<2000 cm⁻¹) to guarantee a population of the upper level in the temperature range of interest.⁸ Because of the special 4f energy levels of Er³⁺ ions, the Er-doped matrix is favorable as temperature sensing fluorescent material.¹³ In general, Yb³⁺ ions are usually used as the co-doped ion to sensitize Er³⁺. To date, several materials have been investigated as the Er³⁺/Yb³⁺ hosts for thermometry application.^2,7,14–17

In this work, the MgWO₄ dual-mode photoluminescence phosphor with Er³⁺ as the activators was prepared, and Yb³⁺ ions were added as the sensitizer due to the efficient energy transfer from Yb³⁺ to Er³⁺. Intense UC and DC green emissions from the samples were obtained under the 980 nm and 379 nm light excitation, respectively. And the optical temperature sensing properties based on the green UC emissions of Er³⁺ ions were also investigated.

2. Experimental

The xEr³⁺/yYb³⁺ co-doped MgWO₄ phosphors (MgWO₄:Er³⁺/Yb³⁺, x : y = 1 : 10) were prepared by a solid state reaction method. Raw materials MgO (99.9%) and WO₃ (99.9%) were mixed at 1 : 1 molar ratio, and x mol% Er₂O₃ (99.99%)–y mol% Yb₂O₃ (99.99%) were added. The initial powders were firstly ground finely in an agate mortar then dried and were calcined at 750 °C for 5 h. After calcinations, the ground powders, added with 10 wt% polyvinyl alcohol (PVA) binder, were pressed into disks shaped pellets of 10 mm in diameter and about 2 mm in thickness. Finally, the samples were sintered at temperatures of 1080 °C for 4 h in an alumina crucible in air.

The crystal structure was identified by an X-ray diffractometer (XRD) (D/MAX 2550, Rigaku, Japan) with Cu Kα1 radiation (λ = 0.154 056 nm), tube voltage 40 kV, and tube current 40 mA. The XRD profiles of the samples were collected in the range 10° ≤ 2θ ≤ 90°, with scanning speed of 5° per min. The luminescence spectra collected from the samples was recorded by using a fluorescence spectro-fluorometer (F-7000, Hitachi, Japan). Excitation and emission spectra were collected for the instrumental response. For UC measurement, a 980 nm laser diode (LD) (HJZ980-100) with keeping the power at 30 mW was used to excite the surface of ceramic samples. The sample temperature was measured by a Pt-100 thermocouple located at a heating stage controlled by a TP94 temperature controller (Linkam Scientific Instruments Ltd, Surrey, UK).

3. Results and discussion

3.1 Crystal structure

The representative XRD patterns for the MgWO4:Er³⁺/Yb³⁺ samples sintered at 1080 °C are shown in Fig. 1(a). All the diffraction peaks can be well indexed by a standard power diffraction file card JCPDS#27-0789 and no other impurity phases were observed, indicating that the Er³⁺ and Yb³⁺ ions had been diffused into the MgWO₄ host lattices. The ionic radii of Er³⁺ and Yb³⁺ (r = 0.89 Å, CN = 6 and r = 0.868 Å, CN = 6) are close to that of Mg²⁺ (r = 0.72 Å, CN = 6) and much larger than that of W (r = 0.60 Å, CN = 6) which indicates that Er³⁺/Yb³⁺ ions are preferably substituted for the Mg sites. It is found that all the XRD peaks shift toward the lower angle side because of the relatively larger ionic radius of Er³⁺ and Yb³⁺ than that of Mg²⁺ leading to an increase in cell volumes [Fig. 1(b)]. Fig. 1(c) shows the SEM micrograph of MgWO₄:Er³⁺/Yb³⁺ (x = 0.05 mol%) sample sintered at 1080 °C for 4 h. It can be seen that the ceramics show rod-like grains, which is typical morphology of tungstate.²⁵

Fig. 1 (a) XRD pattern of the Er³⁺/Yb³⁺ co-doped MgWO₄ samples; (b) the enlarged diffraction peaks of (1̄11), (111) and (020); (c) SEM micrograph of the Er³⁺/Yb³⁺ co-doped MgWO₄ (x = 0.05 mol%) sample.

3.2 Dual mode photoluminescence properties

The dual-mode photoluminescence spectra of MgWO₄:Er³⁺/Yb³⁺ ceramic at room-temperature is shown in Fig. 2. Fig. 2(a) shows the UC emission spectra of the sample under a 980 nm diode laser excitation with the pumping power of 30 mW. The UC luminescent spectra of all the samples exhibit two strong green emission bands: (1) green emission band between 517 nm to 540 nm assigned to ²H_11/2 → ⁴I_15/2 transitions, (2) the intense green emission band between 540 nm and 570 nm attributed to ⁴S_3/2 → ⁴I_15/2 transitions. The observed photoluminescence intensity increases with the x increasing and reaches a maximum at x = 0.05 mol%. It has been observed that the emission intensity of the MgWO₄:Er³⁺/Yb³⁺ (x = 0.05 mol%) sample is very strong that it can be easily observed by the naked eye. Due to concentration-quenching effect,⁷ the green emission intensity decreases at x > 0.05 mol%. As shown in photoluminescence spectra [Fig. 2(a)], the MgWO₄:Er³⁺/Yb³⁺ ceramics also exhibit red emission band located at about 640–680 nm corresponds to the ⁴F_9/2 → ⁴I_15/2 transition. Unlike the green emission bands, the red emission intensity showed a unidirectional increase with the increase of x.

Fig. 2 UC and DC emission spectra at excited by 980 nm (a) and 379 nm (b) light, respectively. The inset of (a) is the plot of ln I versus ln P for the green and red emission of the 0.05 mol% Er³⁺/0.5 mol% Yb³⁺ co-doped MgWO₄ sample and the inset of (b) is the excitation spectrum of the 0.05 mol% Er³⁺/0.5 mol% Yb³⁺ co-doped MgWO₄ sample monitored at 549 nm.

The UC emission mechanism and population processes in Er³⁺/Yb³⁺ co-doped systems are presented in Fig. 3. For the green emissions, the ⁴F_7/2 level of Er³⁺ ions can be populated via ESA (excited state absorption): [⁴I_11/2 + a photon (980 nm) → ⁴F_7/2] and energy transfer processes [²F_5/2(Yb³⁺) + ⁴I_11/2(Er³⁺) → ²F_7/2(Yb³⁺) + ⁴F_7/2(Er³⁺)] as described in UC emission zone of Fig. 3. And then ⁴F_7/2 level relaxes rapidly to the lower levels ²H_11/2 and ⁴S_3/2 with non-radiative transition. The processes produces the green emission corresponding to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions. However, there are two possible channels contributing to the red emission. Firstly, the non-radiative relaxation from ⁴S_3/2 to ⁴F_9/2 can contribute to the red emission (shown as 1 in Fig. 3). Secondly, parts of Er³⁺ at ⁴I_11/2 level relax to the ⁴I_13/2 level, and then the long-lived ⁴I_13/2 level is promoted to ⁴F_9/2 level through the ESA or energy transfer (shown as 2 in Fig. 3). Since the energy gap between ⁴S_3/2/⁴H_11/2 and ⁴F_9/2 is larger than that between ⁴F_7/2 and ⁴S_3/2/⁴H_11/2, non-radiative relaxation probability from ⁴S_3/2/⁴H_11/2 to ⁴F_9/2 is lower than that from ⁴F_7/2 to ⁴S_3/2/⁴H_11/2, resulting in the dominant green luminescence in the sample of x ≤ 0.05 mol%. The intensity of red emission ⁴F_9/2 to ⁴I_15/2 is lower compared with that of the green emission. With the increasing of the Er³⁺/Yb³⁺ content, the Yb³⁺–Er³⁺ and Er³⁺–Er³⁺ distances remarkably decreases, which results in great population of Er^{3+ 4}I_11/2 and ⁴F_7/2 states. The cross-relaxation of ⁴F_7/2 + ⁴I_11/2 − ⁴F_9/2 + ⁴F_9/2 between Er³⁺ ions becomes significant and dominant for realizing the population of the Er³⁺:⁴F_9/2 excited state from which red emission is yielded. So, the intensity of green decrease and the intensity of red emission increase monotonously.

Fig. 3 Schematic diagram of Er³⁺ energy levels for visible UC and DC photoluminescence mechanisms in Er³⁺/Yb³⁺ co-doped MgWO₄ phosphors.

In UC emission, the Ln I–Ln P plot, between the input (excitation) power and emission intensities of the different bands, tells us about the number of incident photons involved in a particular UC transition.²⁶ It is a common method to investigate the physical mechanism of UC process. For the MgWO₄:Er³⁺/Yb³⁺ (x = 0.05 mol%) phosphor (see inset of Fig. 2(a)), the value of the slope for the emission bands at 524 nm, 548 nm and 668 nm were calculated to be 1.90, 1.71 and 1.73, respectively, indicating that a two-photon UC mechanism is dominant in both the green and red emissions.

Fig. 2(b) shows the visible DC luminescence emission spectra of the MgWO₄:Er³⁺/Yb³⁺ phosphors. Similar to the up-conversion spectra, the green emission (⁴S_3/2 → ⁴I_15/2) has the high intensity. The emission intensity increases with the increase of x and reaches a maximum at x = 0.05 mol%, then decreases. However, it is noted that the observed intensity of the red emission (⁴F_9/2 → ⁴I_15/2) is almost zero which is coincident with other Er³⁺/Yb³⁺ luminescence materials.⁷ Under a 379 nm light excitation, Er³⁺ ions are excited directly to the ⁴G_11/2 level (shown in DC emission zone of Fig. 3). Due to the fastest multiphonons relaxation rate, most of the excited Er³⁺ ions relax to the ⁴S_3/2 non-radiative level and then to the ⁴I_15/2 ground radiative level. Because no Er³⁺ ions are excited to the ⁴I_11/2 level, the second approach of red emission will not be occurred. So, the red emission can be obtained only by the multiphonon relaxation from the ⁴F_7/2 level. The population of Er³⁺ ions in the ⁴F_9/2 is very few as well as the intensity of the red emission. The excitation spectrum of the MgWO₄:Er³⁺/Yb³⁺ (x = 0.05 mol%) phosphor, monitored at 549 nm, was shown in the inset of Fig. 2(b). The excitation band can be attributed to the typical f–f transition from ⁴I_15/2 ground state level to the excitation state levels, where the peaks around 379 nm, 408 nm, 450 nm, and 487 nm are corresponded to the ⁴I_15/2 → ⁴G_11/2, ⁴I_15/2 → ²H_9/2, ⁴I_15/2 → ⁴F_5/2, and ⁴I_15/2 → ⁴F_7/2 transitions, respectively. The highest peak occurs at around 379 nm indicating that the most efficient excitation source for MgWO₄:Er³⁺/Yb³⁺ materials should be of 379 nm for the down conversion processes.

3.3 Temperature sensing properties

The temperature sensing properties of the MgWO₄:Er³⁺/Yb³⁺ (x = 0.05 mol%) ceramic based on the UC emission were investigated. Fig. 4(a) shows the UC emission spectra in the wavelength range of 450–750 nm for the MgWO₄:Er³⁺/Yb³⁺ (x = 0.05 mol%) ceramic in the temperature range from 83 to 553 K, where the emission intensity has been normalized at 549 nm. With the increase of temperature, the positions of these two green UC emission bands do not have significant changes. However, the normalized intensity of Er^{3+ 2}H_11/2 level exhibits a remarkable dependence on the temperature, ascribing to the thermal coupling between ²H_11/2 and ⁴S_3/2 levels of Er³⁺ ions. As shown in Fig. 4(b), the intensity of the green UC emission located at about 524 nm is very low when the temperature is below 163 K. It probably because that the populated of ²H_11/2 level from ⁴S_3/2 level was difficult by thermal excitation at low temperature. The energy gap between the levels ²H_11/2 and ⁴S_3/2 of Er³⁺ is about 835 cm⁻¹ which can be obtained from the UC emission spectra.^7,17 The energy separation allows the ²H_11/2 level can be populated from ⁴S_3/2 level by thermal excitation and a quasi-thermal equilibrium, leading to variation in the transitions of ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 with the variable temperature.

Fig. 4 (a) Normalized green UC emissions of Er³⁺/Yb³⁺ co-doped MgWO₄ phosphor at different temperatures; (b) UC emission spectra of Er³⁺/Yb³⁺ co-doped MgWO₄ phosphor at 83 K, 123 K, 163 K and 243 K.

According to the literatures^8,27–30 with the thermalization of population at the two thermally coupled energy levels and ignoring the effects of self-absorption of the fluorescence, the FIR of the emissions from ²H_11/2 → ⁴I_15/2 to ⁴S_3/2 → ⁴I_15/2 can be defined as . Here, A is a constant, ΔE is the energy separation between the ²H_11/2 and ⁴S_3/2 levels, k is the Boltzmann constant, and T is absolute temperature. Curves of FIR versus T measured for the MgWO₄:Er³⁺/Yb³⁺ (x = 0.05 mol%) sample are presented in Fig. 5(a). It can be observed that the FIR value increased gradually with the temperature rising and reached its maximum value at 583 K. The experimental data can be well fitted with the formula and the values of coefficient A and energy gap ΔE are about 21.378 and 899 cm⁻¹, respectively. The fitted value of 899 cm⁻¹ is close to the experiment value (ΔE₀) of 835 cm⁻¹. Three temperature circles have been tested and a good reproducibility was obtained.

Fig. 5 (a) FIR relative to temperature; and (b) sensor sensitivity as a function of temperature for Er³⁺/Yb³⁺ co-doped MgWO₄ in UC process.

The sensitivity (S) is a key parameter in practical applications which reflect the rate of change of FIR in response to the variation of temperature. The value of S can be defined as:⁸ . The corresponding resultant curve is shown in Fig. 5(b). It can be seen that the sensitivity reaches its maximum value of about 0.0093 K⁻¹ at 583 K and no significant decrease was observed with the further increment of the temperature. In order to further analyze the temperature sensing properties of the material in range of low and high temperature. The FIR and sensitivity as a function of temperature for Er³⁺/Yb³⁺ co-doped MgWO₄ phosphor in different temperature ranges were shown in Fig. 6. It can be seen that the value A in the fitting formula is 17.9079 (83–300 K) and 31.693 (300–583 K), respectively. However, the fitted value of A is 21.378 in the temperature range of 83–583 K which between 17.9079 and 31.693. As we all know, the S is proportional to the value of A. Therefore, the value of S, compared with that of in the continuous temperature range from 83 to 583 K, is larger in the high temperature zone and is smaller in the low temperature zone [see Fig. 5(b), 6(c) and (d)]. The maximum of S were obtained at 583 K in the high temperature (300–583 K) and the value is 0.0103 K⁻¹. And the S also is predicted at a higher temperature beyond our tests and shown in Fig. 6(d). Moreover, the error δ, reflecting the different between the fitted ΔE and the experimental value (ΔE₀), was defined as: .³¹ The smaller δ, the more accurate S is. In the continuous temperature range of 83–583 K, the value of fitted ΔE is 900 cm⁻¹. The error δ value is about 7.78%. If we divide the temperature range into two sections, the fitted ΔE value is 804 cm⁻¹ in low temperature range and 1082 cm⁻¹ in high temperature range, respectively. The corresponding δ value is 3.71% and 17.6%, respectively. Based on the above analysis, we think that it is more appropriate for our materials to discuss the sensitivity properties in the continuous temperature range of 83–583 K.

Fig. 6 (a) and (c) FIR and sensitivity as a function of temperature for Er³⁺/Yb³⁺ co-doped MgWO₄ phosphor in the range of low temperature; (b) and (d) FIR and sensitivity as a function of temperature in the range of high temperature in UC process.

For comparison purpose, the temperature sensing properties of MgWO₄:Er³⁺/Yb³⁺ (x = 0.05 mol%) phosphor, which based on the down conversion photoluminescence, are also investigated. As shown in Fig. 7(a) the intensity of two green emissions (524 nm, 549 nm) has changed with temperature. According to the FIR technique, the plots of the S versus absolute temperature were depicted in Fig. 7(b). It can be seen that the law of sensitivity in DC process is similar with that of in UC process. And the maximum sensitivity 0.0081 K⁻¹ was obtained at 583 K in the measuring temperature range.

Fig. 7 (a) FIR relative to temperature; and (b) sensor sensitivity as a function of temperature for Er³⁺/Yb³⁺ co-doped MgWO₄ phosphor in DC process.

In contrast with other materials, there are two advantages in our material. On the one hand, the S is largest in similar temperature range. Such as Er/Ho doped fluoroindate glass (0.0028 K⁻¹),³² Er/Yb doped Na_0.5Bi_0.5TiO₃ ceramics (0.0031 K⁻¹),³³ Yb/Er doped (Ba_0.4Ca_0.6)TiO₃ (0.0033 K⁻¹),⁷ Yb/Er doped Y₂O₃ (0.0044 K⁻¹),³⁴ Yb/Er doped Bi₇Ti₄NbO₂₁ (0.0044 K⁻¹),³⁵ etc., the temperature sensitivity for MgWO₄:Er³⁺/Yb³⁺ is highest in a similar temperature range. And it is also comparable with the maximum sensitivity 0.01498 K⁻¹ which obtained in SrWO₄:Er³⁺/Yb³⁺ phosphor.²⁰ On the other hand, the bright green UC and DC emission under the laser excitation of low power were observed in MgWO₄:Er³⁺/Yb³⁺ phosphor. It is known that the pumping laser would more or less disturb the surface temperature of the sample, and introduce error in temperature detection. The higher the pumping laser power is, the more influence on the temperature of the sample. So, low power laser excitation is beneficial for enhancing the detection accuracy. With wide operating temperature range (83–583 K) and low excitation power (30 mW), the MgWO₄:Er³⁺/Yb³⁺ will be more promising for applications in the temperature sensing.

4. Conclusions

MgWO₄:Er³⁺/Yb³⁺ ceramics were prepared by the conventional solid state reaction method. The sample exhibited bright green emission through UC as well as by DC processes and thus behaves as a dual-mode phosphor. The temperature sensing behaviors of materials were analyzed using the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ions in UC and DC process. The curves of S in the continuous temperature range (83–583 K) and the two sectional temperature ranges were compared in UC process and obtained a maximum sensitivity of 0.0093 K⁻¹ at 583 K⁻¹ in 83–583 K temperature range. Our results of the optical studies indicate that the MgWO₄:Er³⁺/Yb³⁺ phosphor has enough potential to be used in display devices and optical temperature sensors.

Acknowledgements

This work was supported by the National Science Foundation of China (Grant no. 51572195).

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