1. Introduction

The development of rare earth ions doped phosphors has undergone a fast growth period in recent decades due to their unique luminescent characteristics and irreplaceable roles in a wide range of applications, such as plasma display panels (PDPs), field emission displays (FEDs), white light-emitting diodes (w-LEDs), X-ray imaging scintillators, and so on [1]. PDPs have been regarded as a commercial available display in the field of flat panel displays (FPDs), due to the advantages of thin panel, a slight weight, an extensive visual angle, a large screen size, high-speed response with emissive, high contrast, more saturated colors, etc [2–4]. Recently, along with the rapid development of three-dimensional (3D) displays, the fast-decay phosphors are imminently required due to the short fluorescence decay time (within about 2 ms) which could avoid the cross-talk for 3D displays [5, 6]. For 3D PDPs, the conventional red phosphor (Y,Gd)BO₃ has slow decay time, low luminous efficiency and poor color purity, so it is urgent to find new red phosphors with higher luminous efficiency and fast decay [7]. As a promising FPD, FEDs have attracted more and more attention due to their high brightness, high efficiency with low power consumption, and a wide working temperature range [8–10]. Compared with the cathode-ray tube screens (CRTs), FEDs operate at lower voltage (≤ 10 kV) and higher current density (10-100 μA/cm²). This infers it is important to develop phosphors for FEDs with high efficiency at low voltages, high resistance to current saturation and long service time [11–13]. Sulfide phosphors such as ZnS: Cu, Al and SrGa₂S₄: Ce³⁺ usually show higher luminance by electron beam excitation [14]. However, the sulfide phosphors are unstable under electron bombard and would contaminate emission tips and shorten the device lifetime [15]. Therefore, much attention has been paid to the oxide-based phosphors due to the higher stability [16, 17]. Recently, Y₂SiO₅: Ce³⁺ and YNbO₄: Bi³⁺ have been widely investigated. However, they show low efficiencies under electron beam bombard due to their insulative performance [18, 19], which would limit their application in FEDs. For these reasons, it is required to develop novel oxide-based phosphors with suitable electrical conductivity to improve the cathodoluminescent (CL) properties. In the field of solid state lighting, w-LEDs have attracted much attention due to their high efficiency, compactness, long operational lifetime, and resultant energy saving [20]. To generate white light, single phased multi-color-emitting phosphor for ultraviolet (UV)-pumped LEDs can avoid the problems of poor color rendition, thermal quenching, narrow visible range and reabsorption caused by type of di-chromatic (yellow-blue) white LEDs and tri-chromatic white LEDs by combining the near-ultraviolet (n-UV) or (UV) LEDs with tricolor (red, green and blue) phosphors [21]. Despite the more mature development of visible InGaN-based LEDs [22], significant advances have been reported in the fields of high-AlGaN quantum wells for deep UV and mid-UV emitters [23–27]. Therefore, it is significant to develop novel single phased multi-color-emitting phosphors for deep or mid UV-pumped w-LEDs.

Tungstates have been widely used as luminescence hosts owing to the good self-activating luminescent properties, thermal and chemical stability [28]. As well known, rare earth ions play an important role in luminescence materials and rare earth activated tungstates have been used in many fields, such as up-converted red laser [29], FEDs [30] and w-LEDs [31]. Moreover, the partially filled 4f electrons of rare earth ions are well screened by the 5s and 5p electrons, which results in the luminescence spectra from the 4f→4f transition are constituted with sharp lines [32, 33]. Based on the 4f→4f transition, Eu³⁺, Sm³⁺ and Dy³⁺ are usually used in lighting and display fields [34].

To the best of our knowledge, there are rare results about the electronic structures of Y₂WO₆ and the luminescence performance of Y₂WO₆: Ln³⁺ under vacuum ultra violet (VUV), UV and electron beam excitation. So in this work, the electronic structure of Y₂WO₆ were carried out with density functional theory (DFT) and performed with the CASTEP code. The photoluminescence (PL) and CL properties were investigated in detail, for basic research and potential application in 3D PDPs, FEDs and LEDs.

2. Experimental

2.1 Materials and synthesis

A series of Y₂WO₆: xSm³⁺ (x = 0.005-0.07), Y₂WO₆: yDy³⁺ (y = 0.005-0.07) and Y₂WO₆: zEu³⁺ (z = 0.01, 0.06) were synthesized by conventional solid state reaction method at high temperature, starting from Y₂O₃ (99.99%), WO₃ (99%), Eu₂O₃ (99.99%), Dy₂O₃ (99.99%) and Sm₂O₃ (99.99%). The raw materials were weighed at stoichiometric ratio and thoroughly mixed on an agate mortar by using ethanol. Then the mixture was put to the alundum crucible, then heated up to 1100 °C and kept for 6 h at atmosphere. Finally, the final samples were cooled down to room temperature.

2.2 Measurements and characterization

The phase identification was determined by using a Rigaku D/MAX-2400 powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.54178 Å) operating at 40 kV and 60 mA. The 2θ ranges from 10° to 80° with the step size of 0.02°. Diffuse reflection spectra (DRS) were obtained by an ultraviolet-visible (UV-Vis) spectrophotometer (PE lambda950) using BaSO₄ as a reference. The Fluorlog-3 spectrofluorometer equipped with 450 W xenon lamp (Horiba JobinYvon) was employed to measure the UV PL spectra. The VUV PL spectra and the decay curves were measured by using an FLS-920T fluorescence spectrophotometer equipped with a 450 W Xe light source. The CL properties of the samples were obtained using a modified Mp-Micro-S instrument. And the electrical characterizations for the phosphor were recorded at room temperature in air with a Keithley 4200 and a Micromanipulator 6150 probe station.

3. Results and discussion

3.1 Rietveld refinement

In order to obtain the positions of atoms in a primitive cell, the Y₂WO₆ was refined using the Maud refinement program by the Rietveld method. The refinement finally converged to Rw = 10.15% and sig = 1.46. Figure 1(a) presents the experimental values, calculated values, peak positions and difference results of the XRD refinement. Y₂WO₆ has a crystalline monoclinic structure with space group P2/c (13), and shows three different sites for Y³⁺: 2e (eight-coordinated), 2f (eight-coordinated) and 4g (seven-coordinated). Considering the similar radius and valence, the Ln³⁺ prefer to occupy Y³⁺ sites rather than W⁶⁺ sites. From Fig. 1(b), it can be seen that all the Y³⁺ are located in noncentrosymmetric sites, which would have strong effect on their luminescence properties and will be discussed later. The refinement result and the atoms coordinate are shown in Table 1.Figure 7 in the Appendix shows the XRD patterns of Y₂WO₆: Ln³⁺. It can be seen that all the diffraction peaks can be well indexed to standard data of Y₂WO₆ (PDF#73-0118), implying Ln³⁺ have successfully entered into the host without changing the crystal structure.

 

Fig. 1 (a) XRD refinement of Y₂WO₆ host (b) the coordinate environment of Y.

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Table 1. Crystal data and Structural Parameters of Y₂WO₆ from Rietveld refinement.

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3.2 Band structure analysis

Figure 2(a)-2(c) exhibit the DFT calculations of Y₂WO₆ dependent on crystal structure refinement. The calculations of Y₂WO₆ were performed with the CASTEP code. The generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) was chosen for the theoretical basis of density function [35]. The electronic structures and total density of states (TDOS) of Y₂WO₆ are shown in Fig. 2(a). It is indicated that the conduction band (CB) minimum and the valence band (VB) maximum are at D and B points of the Brillouin zone, respectively, suggesting that Y₂WO₆ is an indirect band gap material and the band gap is calculated to be about 3.139 eV. It is obvious that the valence band is mainly comprised of O (2p), and the Y (4d) levels have weak effect on the valence band. The conduction band mainly originates from O (2p) and W (5d) levels. The DRS of Y₂WO₆ is shown in Fig. 2(d), the absorb band at around 325 nm could be ascribed to the absorption of the WO₆^6- complex. In order to calculate the optical band gap of Y₂WO₆, the inset of Fig. 2(d) presents the plot of [F(R)*E]^1/2 versus E, where E is the photo energy and F(R) is the Kubelka–Munk (KM) function [36] with $F(R)={(1-R)}^2/2R$, here, R is the reflectance in the diffuse reflection spectra. The optical band gap of Y₂WO₆ is calculated to be 3.183 eV, which is similar to the value of band gap of Y₂WO₆ calculated by the CASTEP. The narrow band gap shows the Y₂WO₆ would be a suitable host for FED phosphors.

 

Fig. 2 (a) Enlarged band structure and density of states (DOS) (b) Brillouin zone (c) total and partial density of states of Y₂WO₆ (d) DRS of Y₂WO₆ (The inset shows the plot of [F(R)*E]^1/2 versus photo energy E).

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3.3 PL properties of Y₂WO₆:Ln³⁺

3.3.1 PL properties of Y₂WO₆:Ln³⁺ under VUV excitation

The photoluminescence excitation (PLE) spectrum of Y₂WO₆: 0.01Eu³⁺ monitored by 612 nm and the PL spectra of Y₂WO₆: 0.01Eu³⁺ and the commercial (Y,Gd)BO₃: Eu³⁺ (GYBE) (type: KX-504A) are shown in Fig. 3(a). The excitation spectrum is composed of three broad bands in the ranges of 132-190 nm, 190-266 nm and 266-300 nm, which could be ascribed to the charge transfer band (CTB) of O^2--Y³⁺ [37], O^2--Eu³⁺ [38] and O^2--W⁶⁺ [39], respectively. The strong absorption ranged from 132 to 190 nm shows that Y₂WO₆: Eu³⁺ may have potential to be used in 3D PDPs. Y₂WO₆: Eu³⁺ phosphor shows a bright red emission with dominated peak at 612 nm due to ⁵D₀-⁷F₂ electron transition of Eu³⁺, which indicates that Eu³⁺ occupy noncentrosymmetric sites [40]. The PL spectrum properties are consistent with the XRD refinement results. Generally, the phosphors with narrow band gap are not suitable for PDP application. However, when compared with the commercial phosphor GYBE, the emission intensity of Y₂WO₆: Eu³⁺ is measured as high as 66% of GYBE, which infers that Y₂WO₆ may be a good host excited by VUV light. Hence, the PL properties of Y₂WO₆: 0.01Dy³⁺ and Y₂WO₆: 0.01Sm³⁺ excited at 147 nm are investigated and illustrated in Fig. 3(b) and 3(c), respectively. For PLE spectrum of Y₂WO₆: 0.01Dy³⁺, the former broad band from 150 to 200 nm is attributed to the overlap between CTB of O^2--Y³⁺ and O^2--Dy³⁺ [41], and the latter band is due to the CTB of O^2--W⁶⁺. For Y₂WO₆: 0.01Sm³⁺, the three bands in the range of 135-198 nm, 198-235 nm and 235-300 nm are ascribed to the CTB of O^2--Y³⁺ and f→d transition of Sm³⁺, CTB of Sm³⁺ [37, 41] and CTB of O^2--W⁶⁺, respectively. The Y₂WO₆: 0.01Dy³⁺ and Y₂WO₆: 0.01Sm³⁺ phosphors show characteristic emissions ascribed to the ⁴F_9/2 to ⁶H_J/2 (J = 15, 13) transitions of Dy³⁺ and ⁴G_5/2 to ⁶H_J/2 (J = 5, 7, 9) transitions of Sm³⁺, respectively. The broad band centered at about 450 nm is due to the transition of ³T_1u to ¹A_1g level of WO₆^6- complex. The Commission International de l’Eclairage (CIE) coordinates of Y₂WO₆: 0.01Eu³⁺, Y₂WO₆: 0.01Dy³⁺ and Y₂WO₆: 0.01Sm³⁺ are calculated to be (0.65, 0.35), (0.26, 0.22) and (0.32, 0.21), respectively, as shown in Fig. 3(d). It can be seen that Y₂WO₆: 0.01Dy³⁺ and Y₂WO₆: 0.01Sm³⁺ show white light emission, which reveals that they could be used for Hg-free fluorescent lamps and the Y₂WO₆: Eu³⁺ may be potential to be applied as red 3D PDPs phosphor.

 

Fig. 3 (a) PLE and PL spectra of Y₂WO₆:0.01Eu³⁺ (black lines) and PL spectra of commercial GYBE (red lines); PLE and PL spectra of (b) Y₂WO₆:0.01Dy³⁺ and (c) Y₂WO₆:0.01Sm³; (d) the CIE coordinates of Y₂WO₆:Eu³⁺ (red star), Y₂WO₆:Dy³⁺ (blue star) and Y₂WO₆:Sm³⁺ (purple star) under 147 nm excitation.

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3.3.2 PL properties of undoped and Ln³⁺ doped Y₂WO₆ under UV excitation

In order to investigate the energy transfer between the host to Ln³⁺ (Ln = Dy, Sm), the PL properties under UV excitation were investigated. Figures 4(a) and 4(b) present the PLE and PL spectra of undoped and Ln³⁺ (Ln = Dy, Sm) activated Y₂WO₆ samples. In Fig. 4(a), the PLE spectrum of Y₂WO₆ (black lines) monitored at 461 nm shows a broad band ranging from 240 to 350 nm ascribed to the charge transfer from the p orbital of O^2- to the d orbital of W⁶⁺ within the group of WO₆^6-, which is in agreement with the DRS in Fig. 2(d). In the PL spectrum excited by 319 nm, the broad band peaked at 461 nm is due to the transition of ³T_1u to ¹A_1g level of WO₆^6- complex. In Fig. 4(a) and 4(b), the PLE spectra of Y₂WO₆: 0.05Dy³⁺ (red line) and Y₂WO₆: 0.01Sm³⁺ show a broad band and several narrow lines. It is clear that the broad band is derived from the host absorption and the narrow lines are ascribed to the characteristic f → f transitions of Dy³⁺ and Sm³⁺, respectively. Under 319 nm excitation, the PL spectra of Y₂WO₆: 0.05Dy³⁺ and Y₂WO₆: 0.01Sm³⁺ are both dominated by the host emission and the characteristic emission of Dy³⁺ and Sm³⁺, respectively. For Y₂WO₆: 0.05Dy³⁺, the yellow emission is stronger than the blue one, demonstrating that the Dy³⁺ are located in the noncentrosymmetric sites, which is in agreement with the above results. The CIE coordinates of Y₂WO₆: 0.05Dy³⁺ and Y₂WO₆: 0.01Sm³⁺ are (0.35, 0.37) and (0.36, 0.29), respectively, which are shown in Fig. 8 in the Appendix. The results show that they all emit excellent white light and can be used for w-LEDs.

 

Fig. 4 (a) PLE and PL spectra of Y₂WO₆ (black lines) and Y₂WO₆:0.05Dy³⁺ (red lines); (b) PLE and PL spectra of Y₂WO₆:0.01Sm³⁺; emission spectra of (c) Y₂WO₆:yDy³⁺ and (d) Y₂WO₆:xSm³⁺ with different contents (inset of (a) and (b) show the decay curves of Y₂WO₆, Y₂WO₆:0.05Dy³⁺ and Y₂WO₆:0.03Sm³⁺, respectively).

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The spectra overlaps between the emission of WO₆^6- complex and excitation of Ln³⁺ (Ln = Sm, Dy) are observed in Fig. 4(a) and 4(b). It indicates that energy transfer may occurs between the host and the Ln³⁺ (Ln = Dy, Sm). In order to further investigate energy transfer, a series of Y₂WO₆: xSm³⁺ and Y₂WO₆: yDy³⁺ was synthesized and the PL properties were measured. The emission spectra of the series of Y₂WO₆: yDy³⁺ and Y₂WO₆: xSm³⁺ are illustrated in Fig. 4(c) and 4(d). It is obvious that with the Dy³⁺ and Sm³⁺ contents increasing, the host emission intensity decreases and the PL intensities of Dy³⁺ and Sm³⁺ increase until the Dy³⁺ and Sm³⁺ contents reach to y = 0.05 and x = 0.03, respectively, then decrease with further increasing the contents due to the content quenching. This indicates that the energy transfer exists in both Y₂WO₆: xSm³⁺ and Y₂WO₆: yDy³⁺ phosphors. The decay curves of Y₂WO₆, Y₂WO₆: 0.05Dy³⁺ and Y₂WO₆: 0.03Sm³⁺ excited at 319 nm with emission monitored at 461 nm were measured and shown in inset of Fig. 4(c) and 4(d), respectively. The PL decay curves of Y₂WO₆, Y₂WO₆: 0.05Dy³⁺ and Y₂WO₆: 0.03Sm³⁺ can be well fitted with a second-order exponential decay mode, and the average decay times (τ) can be determined by the formula given in the following [42]:

3.4 CL properties of Y₂WO₆:Ln³⁺

The CL spectra of Y₂WO₆: Eu³⁺, Y₂WO₆: Dy³⁺ and Y₂WO₆: Sm³⁺ under excitation of 5 kV and 70 mA were measured and drawn in Fig. 5(a)-5(c). The small difference between the CL and PL spectra could be due to the different excitation source [45]. Figure 5(d) shows the CIE coordinates of Y₂WO₆: xSm³⁺ (purple stars), Y₂WO₆: yDy³⁺ (blue stars) and Y₂WO₆: zEu³⁺ (red stars) phosphors with the content of activators. It is obvious that the color tunes of Y₂WO₆: Dy³⁺, Y₂WO₆: Sm³⁺ and Y₂WO₆: Eu³⁺ are located in the blue region, white region and red region, respectively.

 

Fig. 5 CL spectra of (a) Y₂WO₆:0.06Eu³+ (b) Y₂WO₆:0.05Dy³+ (c) Y₂WO₆:0.01Sm³+ , and (d) the CIE coordinates of Y₂WO₆:Eu³+ (red stars), Y₂WO₆:Dy³ + (blue stars), Y₂WO₆:Sm³ + (purple stars) phosphors varying with the content of activators; the CL spectra and intensities of Y₂WO₆:0.06Eu³+ phosphor as a function of (e) accelerating voltage and (f) probe current.

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The dependence of the CL spectra and intensities [inset of Fig. 5(e)] of Y₂WO₆: 0.06Eu³⁺ phosphor on accelerating voltage was investigated, as shown in Fig. 5(e). Obviously, the CL intensity gradually increases with applied voltage from 4 to 10 kV, attributed to the deeper penetration of the electrons into the phosphor body. The electron penetration depth can be calculated using the following empirical formula [46]:

A is the atomic or molecular weight of the material, ρ is the bulk density, Z is the atomic number or the number of electrons per molecule in the case of compounds, and E is the energy of the accelerating electron (kV). For Y₂WO₆: 0.06Eu³⁺, ρ = 6.827g/cm³, Z = 200, A = 457.65, the electron penetration depths at 4, 5, 6, 8 and 10 kV are estimated to be 17.6, 39.4, 76.2, 214.8 and 480.3 nm, respectively. The higher the incident electron energy is, the deeper the electron penetration depths will be. For this reason, the more activator ions will be excited, resulting in the increase of the CL intensity.

Figure 5(f) shows the CL intensities of Y₂WO₆: 0.06Eu³⁺ phosphor as a function of probe current under 5 and 8 kV, respectively. Under the excitation of 5 and 8 kV, the CL intensities of Y₂WO₆: 0.06Eu³⁺ increase with increasing probe current until current reaches 90 mA, then decreases with further increasing probe current. The increase in CL intensities with increasing probe current could be attributed to the larger electron-beam current density. When the current reaches 90 mA, the CL intensity dropped sharply owing to the luminous efficiency dropped, so the brightness saturation phenomenon appears. The brightness saturation phenomenon may be due to the ground state depletion, charge build-up effect or the thermal quenching effect or the interaction of these three effects [47]. The power density on the surface of the layer increases with the increasing anode voltage and probe current, which might cause more charge accumulation or increase of temperature and then the CL intensity decreases. The decay curves for Y₂WO₆: 0.06Eu³⁺, Y₂WO₆: 0.06Sm³⁺ and Y₂WO₆: 0.06Dy³⁺ are shown in Fig. 9 in the Appendix and the decay times are calculated to be 905, 776 and 298 μs, respectively, which is slower than Y₂SiO₅ (τ = 25 ns) [48]. The fast decay time activators can reduce the ground state depletion and help overcome saturation [48]. So the ground state depletion would have effect on the brightness saturation phenomenon. Whereas from Fig. 6(a), it is observed that the charge build-up effect would not be the key factors because the CL intensity is almost unchanged under constant electron bombardment. From Fig. 6(b), it can be seen that the emission intensity at 200 °C drops to 70% of the initial value. So the thermal quenching effect is an important factor for decrease of CL intensity. In conclusion, the ground state depletion and thermal quenching effect play a key role in the decrease of CL intensity, and the charge build-up effect has weak effect on the decrease of CL intensity.

 

Fig. 6 (a) Degradation properties of Y₂WO₆:Eu³⁺, Y₂WO₆:Dy³⁺, Y₂WO₆:Sm³⁺ phosphors under constant electron beam bombardment (voltage = 5 kV, probe current = 70 mA). (The inset figure shows the CIE coordinates of Y₂WO₆:Eu³⁺ (red stars), Y₂WO₆:Dy³⁺ (blue stars), Y₂WO₆:Sm³⁺ (purple stars) phosphors under constant electron bombardment). (b) The PL spectra changing with the temperature. (c) and (d) the I-V characteristics measured from Y₂O₃ and Y₂WO₆ phosphors.

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The degradation property of FEDs phosphor is related to its working life and is an important factor for FEDs application. Thus the degradation behaviors of Y₂WO₆: Eu³⁺, Y₂WO₆: Dy³⁺ and Y₂WO₆: Sm³⁺ were measured under low voltage electron beam excitation. Figure 6(a) shows the degradation properties of these samples under constant electron beam bombardment. The voltage is fixed at 5 kV and the probe current is 70 mA. It is clear that the CL intensity drops slightly with prolonging the electron bombardment time. After the continuous electron radiation for 2 h, the CL intensities of Y₂WO₆: Eu³⁺, Y₂WO₆: Dy³⁺ and Y₂WO₆: Sm³⁺ dropped to 88.3%, 97% and 97.6% of the initial values, respectively. However, under the same condition, the CL intensity of Y₂O₃: Eu³⁺ (in the Fig. 10 in the Appendix) falls to 84% of the initial value after electron bombards the sample for 2 h. Therefore, the Y₂WO₆ shows the better degradation property than Y₂O₃. This degradation of CL intensity could be due to the accumulation of carbon at the surface during the electron bombardment [49], which will prevent low energy electrons from reaching the phosphor grains and also exacerbate surface charging, and thus lower the CL intensity. Therefore, the electrical conductivity is also an influence factor for the degradation property of phosphor. So the electrical conductivities of Y₂WO₆ and Y₂O₃ (for comparison) were measured and shown in Fig. 6(c) and 6(d). It is obvious that the Y₂WO₆ has a better electrical conductivity than Y₂O₃, which consists with that Y₂WO₆ has a better degradation property than Y₂O₃. In addition, permanent damage to the phosphor occurs after continuous electron radiation, which is another reason for the decrease of the CL intensity. The inset of Fig. 6(a) shows the CIE coordinates of these samples under constant electron beam bombardment, and the red, blue and purple stars represent Y₂WO₆: Eu³⁺, Y₂WO₆: Dy³⁺ and Y₂WO₆: Sm³⁺, respectively. It is obvious that the CIE coordinates of these samples are almost unchanged. The CIE coordinates of Y₂WO₆: Eu³⁺ maintain at (0.48, 0.29), while Y₂WO₆: Dy³⁺ vary a narrow region between (0.20, 0.22) and (0.20, 0.23), along with Y₂WO₆: Sm³⁺ (0.30, 0.26) and (0.31, 0.27). In conclusion, Y₂WO₆: Eu³⁺ and Y₂WO₆: Dy³⁺ are suitable red and blue phosphors for FEDs application, and Y₂WO₆: Sm³⁺ may have potential applications as backlights in display devices.

4. Conclusion

A series of Y₂WO₆: Ln³⁺ (Ln = Eu, Dy, Sm) phosphors were prepared by the solid state reaction. Under 319 nm excitation, the efficient energy transfer between the WO₆^6- and Ln³⁺ was proved and the maximal energy transfer efficiencies in Y₂WO₆: Sm³⁺ and Y₂WO₆: Dy³⁺ were calculated to be 89% and 69%, respectively. What’s more, the Y₂WO₆: Sm³⁺ and Y₂WO₆: Dy³⁺ can show good white light excited by 319 nm. When excited by 147 nm, Y₂WO₆: Eu³⁺ shows bright red emission and the emission intensity can reach 66% of GYBE. For Y₂WO₆: Ln³⁺ (Ln = Sm, Dy), they exhibited white light emission with CIE coordinates (0.32, 0.21) and (0.26, 0.22), respectively. At the same time, when excited by electron beam, Y₂WO₆: Eu³⁺ also show red light emission, and the CIE coordinates of Y₂WO₆: Sm³⁺ and Y₂WO₆: Dy³⁺ are located in white and blue region, respectively. Furthermore, Y₂WO₆ reveals better electrical conductivities than Y₂O₃, resulting in better degradation property. In summary, Y₂WO₆: Eu³⁺ can serve as a potential red phosphor for 3D PDPs and Y₂WO₆:Ln³⁺ (Ln = Eu, Sm, Dy) have potential for using in the LEDs and FEDs.

5. Appendix section

 

Fig. 7 XRD patterns of Y₂WO₆:Ln³⁺ (Ln = Eu, Sm, Dy).

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Fig. 8 The CIE coordinates of Y₂WO₆:0.05Dy³⁺ (blue star) and Y₂WO₆:0.01Sm³⁺ (purple star) under 319 nm excitation.

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Fig. 9 Decay curves of Y₂WO₆:0.06Eu³⁺, Y₂WO₆:0.05Dy³⁺ and Y₂WO₆:0.03Sm³⁺

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Fig. 10 The degradation properties of Y₂O₃.

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Acknowledgments

This work was financially supported by the National Science Foundation for Distinguished Young Scholars (No. 50925206), and the Research Fund for the Doctoral Program of Higher Education (No. 20120211130003).

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