Spectral properties and self-reduction of Eu³⁺ to Eu²⁺ in aluminosilicate oxyfluoride glass

Abstract

Eu-doped aluminosilicate oxyfluoride glass prepared via a melt-quenching method was investigated using X-ray diffraction, absorption spectroscopy, X-ray fluorescence spectrometry, photoluminescence spectroscopy and fluorescence decay curves. We found that the reduction of Eu³⁺ to Eu²⁺ ions occurred in the glass prepared in air. The emission spectra showed that the intensity of 4f⁶5d → 4f⁷ transition of Eu²⁺ ions varied with increasing incident beam wavelength. Meanwhile, the fluorescence lifetimes of Eu³⁺: ⁵D₀ → ⁷F₂ monitored at 617 nm in the glass change with the variation of excitation wavelength. The energy transfer between Eu²⁺ and Eu³⁺ and the emission mechanisms of Eu²⁺ ions in the glass were also discussed.

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1 Introduction

The photoluminescence (PL) of Eu-doped materials has been of extensive interest not only because of the excellent red emission of Eu³⁺ (ascribed to 4f–4f electronic forbidden transitions and the exhibiting of a series of highly bright and sharply narrow emissions),^1–3 but because of the characteristic broadband emission of Eu²⁺ (attributed to inter-configurational 4f⁶5d → 4f⁷(⁸S_7/2) transitions and tuned from near-ultraviolet (UV) to red in different hosts).^4–7 Impressively, due to stronger coupling to lattice vibrations (i.e., the strong interaction of 5d electrons with the local crystal field in vicinity of Eu²⁺), the unique electronic configuration of Eu²⁺ (4f⁷-4f⁶5d) makes its luminescent properties sensitive to the coordination environment.^8,9 This is why the emission of Eu²⁺ could present colorful emissions, which has recently made it a perfect candidate for color-tunable phosphors, such as the blue phosphor Mg₂Al₄Si₅O₁₈:Eu²⁺,¹⁰ the green phosphor Rb₃M(Li₃SiO₄)₄:Eu²⁺ (M = Rb or Na),¹¹ the green-to-yellowish-orange phosphor Cs₂MP₂O₇:Eu²⁺ (M = Ba, Sr, and Ca),¹² the red phosphor CaS: Eu,¹³ and so on. Thus, Eu²⁺-doped phosphors are of great technological interest for applications in white LED, optical thermometers, backlight displays, and X-ray storage, etc.^10,11,14

Aluminosilicate oxyfluoride glasses and glass-ceramics containing two kinds of anions (O²⁻ and F⁻) with different valence electrons and different degrees of polarization is another kind of promising host for RE ions because they combine the advantages of the high mechanical strength of oxide glass and the low phonon energy of fluoride glass.^14–16 Still, instead of the high-performance narrow-band emissions in Eu²⁺-activated phosphors for some special applications (e.g., fluorescent illumination and X-ray intensifying panels),^17,18 divalent europium ions generally emit only a broad band blue emission in vitreous hosts.^19,20 In other words, the amorphous structure of glass, which resulting in a higher phonon energy, enhances the overlap of electronic orbit among energy levels in 4f⁶5d configuration, i.e., the broad blue emission originates in the minor splitting energy of Eu 5d-band. However, the broad band emission of Eu²⁺ ions precisely makes it possible to generate tunable smooth spectra, especially warm white light-emitting, in single rare earth doped glass materials by regulating the emission intensity ratio between Eu³⁺ and Eu²⁺.^21,22 In addition, differing from the reduction process in phosphors synthesis (i.e., in a strongly reducing atmosphere, e.g., H₂/N₂, etc.), Eu³⁺ → Eu²⁺ conversion in glass melting is fulfilled in air by adding a reducing agent into the glass composition (e.g., Al) or adjusting the optical basicity (Λ_th) of the glass matrix (i.e., to make the Λ_th value of base glass less than 0.585 by optimizing glass composition.).^3,12,23–25 Obviously, the reduction routes implemented in Eu-doped glasses are relatively convenient. All these mean that Eu-doped oxyfluoride glass materials have promising prospects in the quality lighting field.

In the present study, we developed an Eu-doped glass composition within the system SiO₂–Al₂O₃–BaF₂–Na₂O containing both Eu³⁺ and Eu²⁺ ions, which was synthesized by partially reducing Eu³⁺ ions without using a reducing atmosphere and/or any reductant during the melting process. We performed multispectral excitation measurements (at the incident wavelength (λ_ex) ranging from 320 nm to 360 nm with an interval of 5 nm) and structure analysis to study the emission properties of Eu²⁺ ions in the glass sample. The energy transfer from Eu²⁺ to Eu³⁺ and the emission mechanisms of Eu²⁺ ions in the glass sample were discussed.

2 Experimental

Aluminosilicate oxyfluoride glass with the nominal molar composition of 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O:1Eu was synthesized by a melt-quenching method, using SiO₂ (99.99%, Aladdin), Al₂O₃ (99.99%, Aladdin), BaF₂ (99.99%, Aladdin), Na₂CO₃ (99.99%, Aladdin), and Eu₂O₃ (99.99%, Aladdin) as raw materials. All the chemicals were used as received without further purification. Accurately weighed 10 g per batch of raw materials were fully mixed and melted in a covered alumina crucible at 1723 K for 1 h. The melt was cast quickly into a metal mold with cover on copper-alloy plate of 573 K and finally cooled down to room temperature in air to obtain glass sample. It should be noted that the melting crucible for melting the glass batch was covered by another bigger alumina crucible to protect local atmosphere, i.e., to reduce the volatilization loss of fluoride.^26,27 The obtained glass sample was further polished to 1.5 mm in thickness for further optical measurements after annealed 10 h at 520 °C.

The amorphous characteristic of sample was determined with an X-ray diffraction (XRD, Shimadzu LabX XRD-6100) with Cu/Kα (λ = 1.540598 Å) by short scanning in the 2θ range of 10–70° (2θ) at a scan rate of 6 ° min⁻¹. The UV/VIS/NIR absorption spectrum was recorded in the wavelength range from 200 to 450 nm using a Model U-4100 Spectrophotometer. The final product composition was measured using the X-ray Fluorescence Spectrometer (ZSX Primus II, Rigaku, JPN). The photoluminescence and decay curves were performed in FLS980 fluorescence spectrometer (FLS980, Edinburgh Instruments, UK) equipped with a Xe-lamp and μs flash lamp as excitation sources. All spectroscopic measurements were performed at room temperature.

3 Results and discussion

The X-ray diffraction pattern of the Eu-doped aluminosilicate oxyfluoride glass is shown in Fig. 1A. The characteristic amorphous halo (i.e., a broad hump located at 27° (2θ)) and no diffraction peaks (i.e., no crystalline phases precipitated from the sample) are observed in the spectrum. Fig. 1B shows the absorption spectrum of the Eu-doped as-prepared glass. It can be seen from this figure, a broad absorption band peaking at 275 nm and two absorption shoulders on the low-energy side, decomposed into three fitting bands using Gaussian function, are observed. The most remarkable band, peaking at 275 nm, is assigned to aluminosilicate oxyfluoride glass host absorption. The other two weaker absorption peaks could be attributed to the electronic transition of 4f⁷ → 4f⁶5d, i.e., the 5d orbital is split into two components t_2g (318 nm) and e_g (389 nm) by the crystal-field strength of the coordinates in the vicinity of Eu²⁺ ions.²⁸ In addition, the Eu³⁺ also shows an absorption peak in the range 388–396 nm, whereas the absorption due to the Eu³⁺: f–f transitions in aluminosilicate glass is obscure owing to their small absorption coefficients.²⁸ It is noteworthy that, differing from the absorption spectra in Eu-doped phosphors (i.e., where the absorption band of host and those of Eu²⁺ ions were obviously separated),^12,29 the absorption bands of host and Eu²⁺ ions are closely overlapping in the aluminosilicate oxyfluoride glass. In addition, this absorption band is not in agreement with Eu²⁺ absorption spectra in oxide aluminosilicate glasses, where a broad absorption band considered to contain only the e_g and t_2g splitting components of 5d orbitals of Eu²⁺ ions were observed, and believed that the sites of cubic symmetry with 8- and 12-fold coordination resulted in a splitting to e_g component at lower energy and t_2g component at higher energy.^28,30 This result indicates that the phonon energy caused by lattice vibration of the aluminosilicate oxyfluoride host glass has extended into the bandgap of Eu²⁺ ions,^31,32 which means the 5d energy level of the doped Eu²⁺ ions should be widen. Furthermore, IR spectra also manifests the multiple broad-band absorption peaks (Fig. 1C. 950 cm⁻¹: Si–O–Si, 690 cm⁻¹: Al–O, and 440 cm⁻¹: Si–O–Si and O–Si–O, etc.), meaning various energies (often referred to collectively as ‘phonon energy’) contained in glass. The broad bands will benefit the overlap of electronic orbit among energy levels in 4f⁶5d configuration of Eu²⁺ ions. This predicts that, compared to the emission spectra of Eu²⁺ in above-mentioned hosts, especially the phosphor hosts, a broader emission band of Eu²⁺ will be observed in the studied host glass, i.e., the 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O glass.

Fig. 1 XRD patterns spectrum (A), optical absorption spectrum (B) and IR spectra (C) for the Eu-doped 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O glass. The broad absorption band in (B) contains two shoulders, indicating it could be deconvoluted into three Gaussian functions. IR spectra in (C) manifests multiple broad-band absorption peaks.

Fig. 2 exhibits the excitation spectra of Eu²⁺ and Eu³⁺ ions for Eu-doped aluminosilicate oxyfluoride glass. We can see from this figure that the 5d band of Eu²⁺ is observed in the excitation spectrum monitored at 471 nm. Furthermore, the broad excitation band (i.e., the energy transfer Eu²⁺: ⁸S_7/2 → 5d) can be fitted into two bands by Gaussian function, indicating that the 5d orbital of Eu²⁺ ions in the studied glass system is split to two components t_2g and e_g by its surrounding crystal field. At the same time, one can observe in this figure, two weaker excitation bands around 320 nm (just showing half peak because of a severe disturbance by the frequency-doubled excitation peak of FLS1000 fluorescence spectrometer used in this study) and 370 nm can also be distinguished in the excitation spectrum of Eu³⁺ ions. This indicates the possibility that energy transfer (ET) from Eu²⁺ ions 5d level to the Eu³⁺ ions 4f levels. Indeed, as Malchukova proved,²⁰ the overlap between the broad emission of Eu²⁺ ions and the various excitation bands of Eu³⁺ ions is observed in Fig. 3.

Fig. 2 Excitation spectra for Eu²⁺ ions (red line) and Eu³⁺ ions (green line) monitored at 471 and 612 nm, respectively, measured for the Eu-doped 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O glass. The broad excitation band was deconvoluted to two Gaussian functions (dotted red lines). The excitation spectrum and its two Gaussian fitting bands were magnified 50 times in intensity (for interpretation of the references to color in this figure legend, the reader referred to the web version of this article).

Fig. 3 Emission spectra for Eu²⁺ ions in dependence on excitation wavelength from 320 nm to 360 nm with an interval of 5 nm and excitation spectrum for Eu³⁺ ions (purple line) monitored at 612 nm, respectively, measured for Eu-doped 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O as-prepared glass. The emission spectra bands at the wavelength range from 350 to 560 nm are magnified 5 times in intensity and show “holes” corresponding to Eu³⁺ ions absorption. Excitation spectrum of Eu³⁺ ions is vertically shifted to make comparison (for interpretation of the references to color in this figure legend, the reader referred to the web version of this article).

The down conversion luminescence observed in the Eu-doped 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O as-prepared glass can be attributed to both Eu³⁺ and Eu²⁺ ions, as shown in Fig. 3. This result confirms therefore that trivalent europium ions have been partially reduced to divalent europium in this studied aluminosilicate oxyfluoride glass composition synthesized in air at high temperature. In other words, the broad blue-green emission with maximum at 470 nm undoubtedly belongs to the transition 4f⁶5d → 4f⁷ of Eu²⁺ ions. However, in this work, no reducing atmosphere or agent is employed to promote the reduction of Eu³⁺ ions (Eu₂O₃ as the precursor material). Thus, the existence of Eu²⁺ ions in the present aluminosilicate oxyfluoride glass system obtained in normal atmospheric condition can be explained adopting optical basicity model proposed by Duffy and Ingram characterizing the glasses based on acid–base property.³³ Unlike the classical principle to elucidate the synthesis mechanism for europium monoxide (EuO) via calculating various thermodynamics parameters (e.g., Gibbs free energy, configurational entropy, etc.) and analyzing the effect of high temperature on the reduction of Eu³⁺ → Eu²⁺,^34,35 the optical basicity theory is fundamentally correlated to the chemical bonding in a solid and to the optical properties of a material through the polarizability of electron clouds around atoms (ions) by electromagnetic waves. Optical basicity of a glass is assumed to represent the average oxygen environment throughout the material. The optical property of glasses, e.g., the reduction–oxidation of cations, can be used to predict through optical basicity concepts.^36,37

From the optical basicity concept, as the optical basicity of glass matrix is below a certain critical value (i.e., critical value of 0.585), it favors the transformation of higher oxidation state to lower oxidation state for multivalent cation europium.²⁵ Considering equivalent mole fractions of constituent oxide or fluoride that contributes to the overall glass stoichiometry and their individual reported values of basicity along with number of oxygen or fluorine atoms, the optical basicity (Λ_th) of the glass matrix is calculated using Duffy's empirical formula:

where, q_i is the number of oxygen or fluorine atoms in the ith component oxide or fluoride, x_i is the ith component oxide or fluoride mole fraction, and Λ_i is the optical basicity of ith oxide or fluorides. Herein, fluorine in the glass composition can be volatile during the preparation of glass at high temperature, leading the final product composition being different from the nominal one as designed.³⁸ Thus, we used the X-ray fluorescence spectrometer to measure the glass matrix composition of the obtained sample. The obtained base glass composition is 37.66SiO₂–27.44Al₂O₃–17.34BaF₂–3.86BaO–13.70Na₂O (in mol%).

Based on the previous reports,^39,40 the optical basicity values of oxide or fluoride are listed as the following: Λ(SiO₂) = 0.48, Λ(Al₂O₃) = 0.6, Λ(BaF₂) = 0.50, Λ(BaO) = 1.15, Λ(Na₂O) = 1.15. Using the Duffy's empirical formula, the optical basicity of the base glass is calculated to be 0.5864, which is very close to the critical optical basicity (Λ_th = 0.585). According to the theory proposed by Duffy, a higher optical basicity favors a higher valence state of the multivalence metal ions.⁴¹ This implies that it is more propitious to generation a lower valence ion, i.e., Eu²⁺, at lower optical basicity in this work. In addition, Z. Y. Lin et al. reported the self-reduction of Eu³⁺ to Eu²⁺ in a kind of aluminosilicate glass with the optical basicity of 0.59.¹⁴ They proposed that the structures of aluminosilicate glass play an important role in the reduction of Eu³⁺ ions, which changes the critical optical basicity value under the special chemical environment. In the present study, the base glass composition also contains SiO₂ and Al₂O₃. Thus, the formation of Eu²⁺ is realized in the present glass.

Seen from Fig. 3, the emission intensity of Eu²⁺ decreases slightly in turn with the increasing excitation wavelength from 320 to 360 nm (i.e., the wavelength range between the fitting peaks of t_2g and e_g for the 5d energy level of Eu²⁺ ions). It can be attributed to the reduced energy transfer capacity of ⁸S_7/2 level, directly leading to a reducing luminescence efficiency, as the incident light deviates from the peak of excitation band for Eu²⁺ ions monitored at 471 nm. However, the emission intensity of Eu³⁺ ions in the red wavelength range shows a variation trend, i.e., it decreases before increases with the increasing incident light wavelength in the studied excitation wavelength range (320–360 nm). Despite the possibility of energy transfer from Eu²⁺ to Eu³⁺ exists, as mentioned above, the emission of Eu²⁺ ions is so weak that the transferred energy is faint. Thus, the intenser luminescence of Eu³⁺ excited under 320, 325 and 360 nm, respectively, can be ascribe to the notable overlap between incident to the excitation transition of Eu³⁺: ⁷F₀ → ⁵H_5,6,7 and ⁷F_0,1 → ⁵D₄. Whereas, the rest weaker luminescence of Eu³⁺ may be mainly related to the ET from Eu²⁺ to Eu³⁺ ions, especially the emission spectra under 335–355 nm excitation, because that no excitation band is shown in this wavelength range for Eu³⁺ ions shown as in Fig. 2. Moreover, this result falls in line with the energy transfer hypothesis in the section of excitation analysis, i.e., the ET from Eu²⁺ ions 5d level to the Eu³⁺ ions 4f levels.

Fluorescence decay analysis is very useful for understanding the mechanism of rare earth luminescence, e.g., energy transfer and quenching behavior etc. Fig. 4 shows the decay curves of Eu³⁺ (monitored at λ_em = 617 nm) in the aluminosilicate oxyfluoride as-prepared glass under multispectral excitation at 320, 325, 330, 335, 340, 345, 350, 355, 360 nm, respectively. All the decay curves are fitted by a double exponential decay function with time constant parameters (ExpDec2) in Origin software using the below given expression:⁴²

whereI(t) is the emission intensity at time t. A₁ and A₂ are the decay constants. τ₁ and τ₂ are the lifetimes of the two channels contributing to the decay processes. The average fluorescence lifetime (τ) is estimated as follows:

Fig. 4 Room-temperature luminescence decay curves of the emission Eu³⁺: ⁵D₀ → ⁷F₂ at 617 nm excited by multispectral from 320 to 360 nm with an interval of 5 nm. All curves are normalized to make comparison. Inset: the dependence of decay lifetimes (τ) on excitation wavelength (λ_ex). The red spheres in the inset refer to the fluorescence lifetimes of Eu³⁺: ⁵D₀ in the glass sample. The dashed curves are guide for eyes to show the nonmonotonic change of τ with increasing λ_ex.

Interestingly, τ evolves non-monotonically with λ_ex as seen in the inset of Fig. 4. With increasing the excitation light wavelength, the fluorescence time first increases from 2.2 to 3.1 μs, and then decreases to 2.0 μs. This result confirms therefore that the emission mechanism of Eu³⁺ ions in the studied aluminosilicate oxyfluoride as-prepared glass changes with incident beam, i.e., the faster decay (e.g., excited at 320 and 325 nm) may relate to excited state absorption, and the slower decay (e.g., excited at 355 and 360 nm) can be put down to energy transfer. This implies that the excitation process is faster than the energy transfer for Eu³⁺ ions in the studied glass system. The mechanism of energy transfer from Eu²⁺: 5d to the Eu³⁺: 4f is similar to that described in^20,28, i.e., the excited in 4f⁶5d (e_g) state of Eu²⁺ ions transfers the energy by means of non-radiative transition 4f⁶5d (e_g) → ⁸S_7/2 to the ground state resulting in the excitation of ⁵D₀ level of Eu³⁺ ions. We also observed another evidence for the energy transfer from Eu²⁺ to Eu³⁺ ions by analyzing the excitation spectrum (Fig. 2) and emission spectra (Fig. 3) of Eu²⁺, i.e., the discontinuous peaks, especially at 320 nm in the excitation band monitored at 471 nm. This type of effect where band emission is suppressed only at the position of RE absorption lines is known to be characteristic of resonant radiative energy transfer, i.e., the reabsorption mechanism.^43,44 Significantly, being inconsistent with the results of Jiménez et al.,⁴⁴ the “holes” on the emission bands of Eu²⁺ don't directly correspond the Eu³⁺ ions absorption peaks on its excitation band, as shown in Fig. 3. The possible reason may be the strong phonon energy of the aluminosilicate oxyfluoride host glass as observed in the above-mentioned absorption and IR analyses, i.e., implying the occurrence of multi-phonon relaxation.

The effect of varying incident beam wavelength (λ_ex from 320 to 360 nm) on the luminescence color in Eu-doped aluminosilicate oxyfluoride as-prepared glass is displayed in Fig. 5. The luminescence color can be finely tuned by changing the excitation wavelength, corresponding CIE coordinates vary from yellowish pink (0.5422, 0.3369), warm white light (0.3845, 0.2895), to greenish blue (0.2016, 0.247).

Fig. 5 Dependence of CIE chromaticity diagram on excitation wavelength for the Eu-doped 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O glass. The numbers 1 to 9 represent the chromaticity coordinates under 320 to 360 nm excitation (for interpretation of the references to color in this figure legend, the reader referred to the web version of this article).

Surprisingly, we observed the heterogeneous distribution of Eu²⁺ ions in the studied Eu-doped glass system by analyzing the emission spectra of the other side of sample shown as in Fig. 6, i.e., a photoluminescence increases before decreases with the increasing incident light wavelength from 330–360 nm which is contrary to that shown in Fig. 3. The phenomenon may be related to the variation of the local environment of Eu²⁺ ions. In other words, the results indicate that the phase separation has occurred in the studied glass. That is that the rich fluorine phase with a lower optical basicity has emerged from the interpenetrating oxide phase. As mentioned above, the low optical basicity promoted the reduction from Eu³⁺ to Eu²⁺, i.e., the concentration of Eu²⁺ in rich fluoride is higher than that in dominant oxide phase. This is why the intensity of Eu²⁺ in Fig. 6 is obviously stronger than those in Fig. 3. The high symmetry properties of fluoride increase the splitting degree. Thus, the luminescence mechanism of Eu²⁺ ions under 330–360 nm excitation also changes along with it.

Fig. 6 Emission spectra for Eu²⁺ ions in dependence on excitation wavelength from 330 nm to 360 nm with an interval of 5 nm, respectively, measured for the Eu-doped 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O glass (for interpretation of the references to color in this figure legend, the reader referred to the web version of this article).

As using 330, 335, and 340 nm to excite the sample, the component of 5d level captured transition electron is e_g state, and the larger the wavelength of incident light, the closer the incident excitation energy is to the peak of e_g state. Thus, the emission intensity of Eu²⁺ ions increases with the enhancing excitation wavelength. But, as the incident light wavelength increases further, excitation electrons jump into t_2g state. Then, as mentioned above, the incident light from 340 to 360 nm deviates from the peak of excitation band. So, the emission intensity of Eu²⁺ ions decreases with the enhancing excitation wavelength. The increase in relative concentration of Eu²⁺ ions leads to richer white light emission shown as in Fig. 7. These results indicate that, just by Eu-individually-doped aluminosilicate oxyfluoride glass, various color emissions can be achieved, and the sample has extensive application prospect in solid-state light devices.

Fig. 7 Dependence of CIE chromaticity diagram on excitation wavelength for the Eu-doped 40SiO₂–25Al₂O₃–20BaF₂–15Na₂O glass. The numbers 1 to 7 represent the chromaticity coordinates under 330 to 360 nm excitation (for interpretation of the references to color in this figure legend, the reader referred to the web version of this article).

4 Conclusions

Eu-doped aluminosilicate oxyfluoride glasses have been prepared by the melt-quenching method. In the glass prepared in the air condition, the reduction of Eu³⁺ to Eu²⁺ ions occurred in the air condition. It is confirmed that the presence of significant amount fluoride in the glass composition is critical to promote this reduction process due to it causes the decrease of optical basicity for the studied glass system. With the increasing incident light wavelength (in the studied excitation wavelength range 320–360 nm), the emission intensity of Eu³⁺ ions decreases before increases in the red wavelength range. At the same time, it was noticed that the fluorescence time first increases before decreases. In addition, the heterogeneous distribution of Eu²⁺ ions due to phase separation is observed in the studied luminescence glass system. The results indicate that the spectral properties depend on the excitation mode. The colorful emission of Eu-individual-doped aluminosilicate oxyfluoride glasses may bring potential applications in light-emitting diode systems, three-dimensional displays, etc.

Author contributions

Lei Liu: investigation, writing – original draft. Xianying Shao: investigation, writing – original draft. Zhenyuan Zhang: investigation, original draft. Jiayu Liu: investigation, original draft. Yuebo Hu: supervision, data curation, methodology, investigation, writing – original draft – review & editing, validation. Chaofeng Zhu: supervision, data curation, methodology, writing – review & editing, validation.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 61368007), the Natural Science Foundation of Shandong Province (Grant No. ZR2019MEM053).

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