1. Introduction

The luminescence of Eu³⁺ is characterized by sharp peaks from electronic transitions of ⁵D₀ to ⁷F_J (J = 0-6) states. The usual emission at 610–615 nm is the best choice for red fluorescent light source. Eu³⁺-doped inorganics have been widely developed as efficient red-emitting phosphors [1,2]. It is well known that the oscillation strength of Eu³⁺ in UV-near UV region is very low (<10⁻⁶) due to f–f forbidden transitions. So direct absorption of excitation wavelength in visible region by Eu³⁺ will be weak. It is important to choose an appropriate host matrix, in which the energy can transfer from host to Eu³⁺ ions. The best situation is that excitation from charge transfer state (CTS) lies in near-UV (360–420 nm) and blue range (430–475 nm). Molybdates have been confirmed to be one of the best candidates, which can efficiently absorb UV-near light through CTS in MoO_x groups and easily transfer energy to Eu³⁺ for red emission.

Eu³⁺-doped CdY₄Mo₃O₁₆ was chosen to develop a new red-emitting phosphor. The dominated motivate is that this molybdate matches for the suggested criterion: CdY₄Mo₃O₁₆ has a cubic molybdate structure with the space group Pn-3n [3]. The structure is a derivative of a fluorite-type, in which some anions are displaced to provide the tetrahedral environments for Mo⁶⁺. Under excitation with UV light, broad and intense CT absorption in near UV or blue wavelength region are expected.

To preperate phosphors, some methods have been applied such as solid state reaction, single crystal growth, coprecipitation, sol–gel method and so on. The sol-gel synthesis is considered one of the most practical methods due to its simplicity, ultrafine powders at low temperature without complex processes, better chemical homogeneity of the product, reproducibility, low cost etc. In this work, Eu³⁺-activated CdY₄Mo₃O₁₆ were synthesized via the sol-gel method. The crystal formations were confirmed by structural refinements. The luminescence were investigated for the excitation and emission spectra, absolute quantum efficiency (QE), thermal stability etc. The phosphor has a possible application for W-LEDs by taking into its luminescence characteristics.

2. Experimental

CdY_4-4xEu_4xMo₃O₁₆ (x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) nanophosphors were prepared by the sol-gel method. The starting materials were stoichiometric Cd(NO₃)₂•4H₂O, Y₂O₃, Eu₂O₃, (NH₄)₆M_O7O₂₄·4H₂O. Firstly, Cd(NO₃)₂•4H₂O, (NH₄)₆M_O7O₂₄·4H₂O were dissolved in aqueous solution (about 80 °C) by adjusting the pH at about 7.0. Secondly, Y₂O₃, Eu₂O₃, were completely dissolved in HNO₃ solutions. Thirdly, the ethylene diamine tetraacetic acid (EDTA) was slowly added into the nitride solutions. The polymeric resin was obtained by adding the ethylene glycol and citric acid under the strong stirring. The gel could be obtained after heating the solution at 100 °C for 2-3 h. Finally CdY_4-4xEu_4xMo₃O₁₆ (x = 0.05-0.6) compounds were synthesized by heating the dry precursor gels at 800 °C for 5 h.

The XRD pattern was measured on a Rigaku D/Max diffractometer at 40 kV, 30 mA with Bragg–Brentano geometry using Cu Kα radiation (λ = 1.5405 Å). The SEM photos were finished on a JEOL, JSM-6360 LA instrument. The photoluminescence excitation (PLE) and emission (PL) spectra were conducted on the luminescence spectrometer (Perkin-Elmer LS-50B) equipped with Monk-Gillieson type monochromators. The xenon discharge lamp was used as an excitation source in the PL measurements. The internal QEs were measured by Edinburgh Instruments FS-920 spectrometer that was equipped with an Edinburgh instruments integrating sphere. The monochromator is connected with CCD sensor and a computer by light guides. QE values were calculated by quantum yield measurement software.

3. Results and discussions

3.1 The Phase Formation and surface characteristics

All the Eu³⁺-doped CdY₄Mo₃O₁₆ have similar XRD patterns, which can be well indexed to the cubic structure. As an example, the structural refinement was completed by the GSAS program. Figure 1 is the experimental and calculated XRD profiles of 50mol % Eu³⁺-doped CdY₄Mo₃O₁₆. The phosphors well crystallize in a cubic space group Pn3n (222) with Z = 4. The unit parameters are a = 11.0391 Å, α = 90°, and V = 1345.24 ų. The dependence of lattice parameter a and cell volume V on the Eu³⁺ doping level is shown in Fig. 1(b). It could be observed that the structural parameters get larger with the increase of Eu³⁺-doping. In CdY₄Mo₃O₁₆, Eu³⁺ ions prefer to occupy Y³⁺ sites. The the lattices expansion was induced by the substitution of smaller ions Y³⁺ (0.9 Å, CN = 6) by Eu³⁺ (0.947 Å, CN = 6) ions. The cell parameters have a linear dependence on the Eu³⁺-doping (x = 0.05-0.6). This relationship is in agreement with the Vegard's law, which empirically concludes that unit cell parameters of a host can linearly vary with the composition for a continuous substitutional solid solution [4].

 

Fig. 1 (a): the structural refinement of CdY₄Mo₃O_16:0.5Eu³⁺. (b): The dependence of lattice a parameter and cell volume V on Eu³⁺ doping.

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The structure sketch map (Fig. 2(a)) of CdY₄Mo₃O₁₆ was drawn by the Diamond Crystal and Molecular Structure Visualization software using the refined structural data. The structure is a derivative of a fluorite-type structure, in which some anions are displaced to provide some tetrahedral environment for the Mo⁶⁺ ions in the lattices. The Mo⁶⁺ cation is located in a 12(d) site; while parts of the Y³⁺ ions are in pure 12(e) sites and the other Y³⁺ ions are statistically arranged with Cd²⁺ in the 8(c) sites. While the O^2- ions stay in the 48(i) sites in this fluorite-type sublattice.¹² There are two kinds of Y³⁺ ions in the deformed cubic coordination (Fig. 2 b). The Y(1) cations fully occupy 48i sites, while Y(2) ions are statistically distributed with Cd²⁺ in the 2a sites in the lattices.

 

Fig. 2 (a): the structural view of CdY₄Mo₃O₁₆ framework along [001] direction; (b): the surrounding for two kinds of cations Y³⁺ ions.

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The morphological characteristics were investigated by SEM images. Figures 3(a) and 3(b) are the typical SEM photos of Eu³⁺-doped CdY₄Mo₃O₁₆ phosphor. The crystalline morphology presents many ball-like particles. The average size estimated by the SEM micrograph is about 100 nm as shown in Fig. 3(b). This SEM characteristic was confirmed by the TEM photo in Fig. 3(c). The HRTEM image of CdY₄Mo₃O₁₆ particles is shown in Fig. 3(d). In the HRTEM image, the clear lattice fringes were observed indicating the high crystallinity of the as-prepared sample. The distance of 0.273 nm between the adjacent fringes is agree well with the distance of the (004) reflection plane of the lattices. In addition, the observed SAED pattern (Fig. 3€) exhibited the cubic symmetric diffraction style matched to CdY₄Mo₃O₁₆.

 

Fig. 3 the typical SEM pictures (a,b), TEM photo (c), HRTEM image (d) and the selected area electron diffraction pattern (e).

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3.2 The optical absorption and the electronic calculation

Figure 4(a) shows the excitation of CdY_4-4xEu_4xMo₃O₁₆, which consists of a broad band and some absorption lines. The first origin of the band (CT1 and CT2) is CT transitions from O²⁻ ions to Mo⁶⁺ ions [5], i.e., interband energy from Mo⁶⁺4d to O^2-2p. The second possible origin is an inter-valence charge transfer (IVCT) states [6], i.e., the electron transitions from 4f states of Eu³⁺ ions to Mo⁶⁺ ions. The third contribution is the electronic transferring from O²⁻ to the neighbor Eu³⁺. This band was not observed on the excitation spectrum because it could overlap with the CTB of O²⁻-Mo^6-. Usually in a Eu³⁺-doped molydate, CTB is mainly attributed to O²⁻→Mo⁶⁺ and O²⁻→Eu³⁺ transitions [7]. The excitation band CT1 at 265 nm can be assigned to CT of Eu³⁺-O²⁻. While the band CT2 is from O²⁻→Mo⁶⁺. The excitation presents the intra-configurational 4f–4f transitions of Eu³⁺ 330 to 500 nm: 394 nm (⁷F₀→⁵L₆), 464 nm (⁷F₀→⁵D₂). It can be concluded that this excitation can matches with the emission of a near-UV or blue LED chip. This is critical for phosphor-converted W-LEDs.

 

Fig. 4 (a): The excitation spectra of CdY_4-4xEu_4xMo₃O₁₆ (x = 0.1, 0.5) (λ_em = 615 nm). E_cut-off is the wavelength at the intersection between x-axle and tangent line of excitation edge. (b): The dependence of E_cut-off on Eu³⁺ doping.

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To discuss the Eu³⁺ doping effects, E_cut-off was introduced, i.e., wavelength position at the intersection point between the x-axle and the tangent line of excitation edge. The Eu³⁺ lowly doped sample CdY_4-4xEu_4xMo₃O₁₆ (x = 0.1) has the E_cut-off value of around 420 nm. However, the E_cut-off value of the heavily doped phosphor CdY_4-4xEu_4xMo₃O₁₆ (x = 0.5) has a longer wavelength of 457 nm. The relationship of E_cut-off values and the Eu³⁺ doping concentrations is shown in Fig. 4(b). The results indicate that the excitation edge of E_cut-off has great red-shift (to the longer wavelength side) extending to blue wavelength on the increase of Eu³⁺-doping. There is also strong excitaion from the ⁷F₀→⁵D₂ transitions locating in the blue wavelength region. This is greatly valuable for a phosphor-converted W-LEDs because of the excellent match with the near-UV or blue LED chip.

According to the references, the CTB excitation position of Eu³⁺-doped molybdates or tungstates usually locate in the wavelength of 230-350 nm, which is limited for a pc-WLED. However, the maximum CTB excitation wavelength in CdY_4-4xEu_4xMo₃O₁₆ (x = 0.5) is measured to be 457 nm (Fig. 4). As far as we know, this is longer than any one phosphor in the reported Eu³⁺-doped molybdates [2]. This is due to the special structural characteristic of CdY₄Mo₃O₁₆, i.e., the cubic framework lattices, providing the longer CTB wavelength.

Another emission characteristic is that the f-f transitions of Eu³⁺ show dominated intensity, which is stronger than the CTB. It has been reported that Eu³⁺ ions could present efficient intraionic (f-f) transitions and weak interionic transitions (Eu³⁺ and O^2-) when it is subject to a site with a positive effective charge [8]. Following this model, the excitation in CdY₄Mo₃O₁₆:Eu³⁺ can be explained in the specialty of the microstructure. In CdY₄Mo₃O₁₆, there are disordered cation sites (Cd_0.5,Y_0.5)^2.5 polyhedra in the lattices [3]. When Eu³⁺ ions are doped in the lattices, the activators with positive charge will be created leading to the low intensity of CT bands and stronger f-f transitions of Eu³⁺. Similar results have been reported in the other Eu³⁺-activated red-emitting phosphors such as Ca₉R(PO₄)₇ (R = Al, Lu):Eu³⁺ [8].

3.3 The photoluminescence

Figure 5 shows the luminescence of CdY_4-4xEu_4xMo₃O₁₆ (x = 0.05, 0.5). The spectra present the electronic transitions the excited states ⁵D₀ to the ground states ⁷F_J (J = 0, 1, 2, 3, 4) in the 4f⁶ configuration. The upper energy transitions from Eu³⁺ ions such as ⁵D_1,2→⁷F_J (J = 0, 1, 2, 3, 4), were not observed in all the samples. The dominated emission is the electric dipole transition ⁵D₀→⁷F₂ at 611 nm due to the fact that the Eu³⁺ ions in CdY₄Mo₃O₁₆ are subject to the non-inversion centers in the lattice. The value of (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) ratio can measure the site symmetry. A higher value of (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) will be created in a lower crystal field symmetry around Eu³⁺ ions. The value of (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) in CdY_4-4xEu_4xMo₃O₁₆ (x = 0.5) is 3.35. This could bring to a pure color for a red-emitting phosphor. The CIE (Commission Internationale de l’Eclairage) color coordinates of CdY_4-4xEu_4xMo₃O₁₆ (x = 0.5) are calculated from the emission spectrum to be x = 0.665 y = 329. The CIE coordinates are closer to the NTSC (National Television System Committee) standards (x = 0.670, y = 0.330) than the commercial red-emitting Y₂O₂S:Eu³⁺ (x = 0.630, y = 0.350) [9].

 

Fig. 5 The representative emission spectra of CdY_4-4xEu_4xMo₃O₁₆ (x = 0.05, 0.5) under the UV excitation (395 nm). The inset is the dependence of the integrated emission intensities on the doping levels of 0.05-0.6.

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The dependence of the luminescence of CdY_4-4xEu_4xMo₃O₁₆ (x = 0.05-0.6) on the Eu³⁺-doping is displayed inset of Fig. 5. The emission intensity gets stronger with the increases of Eu³⁺-content. However, the luminescence becomes a turnover when the doping level reach at is 50.0 mol %. The luminescence begins to quench when the doping is above 50.0 mol %.

3.4 The luminescence dynamics, stability and efficiency

The representative luminescence spectra at several temperatures are displayed in Fig. 6(a). It can be seen that the luminescence keeps stable from 20 to 125 °C. And it decreases when the temperature is above 150 °C. The luminescence intensity at 150 °C is about 80% of the initial value at 20 °C. Importantly, the emission peaks of the Eu³⁺ ions in CdY₄Mo₃O₁₆ don’t change with the increase of temperature. This indicates that the luminescence color didn’t change with the increase of temperature.

 

Fig. 6 temperature dependent spectra (a) and decay curves (b) of Cd_Y4-4xEu_4xMo₃O₁₆ (x = 0.5). Inset is the thermal quenching fit on the lifetimes.

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The luminescence lifetimes are calculated as a function of temperature and displayed in Fig. 6 (b). The decay time drops at higher temperature from 300 K, presenting a typical temperature quenching behavior. The data is fitted to equation as:

The experimental QEs were measured and are listed in Table 1. The maximum QE measured for CdY_4-4xEu_4xMo₃O₁₆ (x = 0.5) is 67.5% (λ_ex = 395 nm). Althrough this QE value is still lower than some reported phosphors such as CaLa_1.98−y(MoO₄)₄:0.02Dy³⁺, 0.03Eu³⁺ (80%, λ_ex = 353 nm), [10], NaCsGaSi₂O₇:RE³⁺ (RE = 1%Ce, 10%Eu, 25%Tb) (76.72%, λ_ex = 351 nm) [11], etc, however, the value is higher than that of the commercial red-emitting Y₂O₂S:Eu³⁺ (QE = 35%, λ_ex = 317 nm, λ_em = 611 nm) [12]. Moreover, this value is also higher than some red-emitting molybdates such as Gd₆MoO₁₂:0.25Eu³⁺ nano-powder (62%, λ_ex = 395 nm) [13], SrY_0.7Eu_1.4(MoO₄)₄ (48%, λ_ex = 395 nm) [14], etc. The results show that CdY_4-4xEu_4xMo₃O₁₆ (x = 0.05-0.6) could be a potential candidate for lighting and display.

Table 1. the PL QEs and CIE color coordinates of novel red-emitting

View Table

4. Conclusions

In summary, CdY_4-4xEu_4xMo₃O₁₆ (x = 0.05-0.6) prepared by sol-gel method crystallize in ball-like particles with average size of 100 nm. This phosphor displays broad CT transitions from O²⁻ to Mo⁶⁺, which have a great red-shift with increase of Eu³⁺-doping. In CdY_4-4xEu_4xMo₃O₁₆ (x = 0.5), the maximum CTB can reach to 457nm, which is longer than any one phosphor in the reported Eu³⁺-doped molybdates. This is due to the structural characteristic of CdY₄Mo₃O₁₆, i.e., the cubic framework lattices provide the longer CTB wavelength. Another characteristic is that f-f excitation show stronger intensity than the CTB band. This is due to the fact that the Eu³⁺ ions in (Cd_0.5, Y_0.5)^2.5 sites will bring positive effective charges in the lattices. The optimal Eu³⁺ doping in CdY_4-4xEu_4xMo₃O₁₆ is 50 mol%. The maximum QE for CdY_4-4xEu_4xMo₃O₁₆ (x = 0.5) is 67.5%. Moreover, the red-emitting phosphor has a high thermal stability with the activation energy of 0.412 eV. The results have demonstrated the potentiality of the phosphor for near-UV/blue GaN-based white LEDs.