1. Introduction

Solid-state lighting (SSL) based on light-emitting diodes (LEDs) is an emerging technology, which will provide significant energy savings, important environmental benefits, and dramatically new ways to utilize and control light [1,2]. LEDs based on group III-nitride materials are predicted to be key components in next-generation lighting and display systems [3–5]. In the past years, semiconductor techniques have gain great developments for LEDs applications in SSL [6–8].

Rare earth (RE) ions activated materials have been widely investigated for applications in phosphor-converted white LEDs (pc-WLEDs) [9–12]. The most common pc-WLED is made using an In_xGa_1−xN blue LED chip combined with a YAG:Ce³⁺ coating. The well-known weakness is the deficiency in the red region leading to low CRIs (color rendering index) and high CCTs (correlated color temperature). Therefore, it is still necessary to develop new red-emitting phosphors with good color purity and high absorption in near UV or blue wavelength region. Eu³⁺-doped compounds have been widely reported as the potential candidates, especially tungstates and molybdates [10]. However, the excitation from CTB usually lies in 250–350 nm. This strongly limits its excitation in the near-UV (380–410 nm) and blue (440–470 nm) range. For a practical application, it is necessary to shift CTB from UV to near-UV and preferably blue region.

It has been reported that wavelength positions of CTB from W/MoO_x groups depend on coordination number (CN) x [13]. Dutta et al [10] gave a summary on CTB extension methodologies from the CTB characteristics of tungstates and molybdates including tetragonal, hexagonal, monoclinic, orthorhombic, cubic, and triclinic structures. CNs of Mo/W in the hosts can shift CTB to longer wavelengths. A cubic crystalline phase with MoO₆ groups could provide the longer CTB wavelength. It is possible to extend CTB energy from UV to near-UV or blue region by choosing an appropriate crystal structure and CN environment in a tungstates or molybdates (W⁶⁺/Mo⁶⁺).

NaLa₄Mo₃O₁₅F was selected to develop a new phosphor due to its cubic structure with a space group of Pn3n (222). Figure 1 is the schematic views along [001]-direction modelled on the coordinate data reported by Faurie [14]. The cell parameters are a = b = c = 11.333 Å, V = 1455.575 ų, and Z = 4. The lattice consists MoO₆ linked by La³⁺ and Na⁺ ions. The linkages between the bands running along [001] are provided by cations. There are two anion sites noted as M1 with Wyckoff position 48i (0.94O + 0.06F) and M2 (16f) (0.94O + 0.06F). Cation sites occupied by Mo⁶⁺ (12d) and (0.8La + 0.2Na) (12e) locate on (0, 1/4, 1/4) and (0, 3/4, 1/4), respectively. M4 (0.8La + 0.2Na) (8c) sites are more irregularly coordinated.

 

Fig. 1 The sketch map of NaLa₄[Mo₃O₁₅]F structure along [001] direction. M1 = 0.94O + 0.06F; M2 = 0.94O + 0.06F;M3 = (0.8La + 0.2Na) (12e) + Mo (12d); M4 = 0.8La + 0.2Na.

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Eu³⁺-doped NaLa₄Mo₃O₁₅F were prepared by typical solid-state method. The structure was investigated by powder XRD and SEM measurements. The photoluminescence excitation and emission spectra were measured and compared with that of the red-emitting phosphor Y₂O₂S:Eu³⁺. The luminescence mechanisms, CTB shift and activation energy for thermal quenching were discussed.

2. Experimental

NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.1-1.0) were prepared by solid state synthesis. The raw materials are high-purity (NH₄)₆Mo₇O₂₄.4H₂O, NaF, La₂O₃, and Eu₂O₃. 50-200% excess of NaF was added in each of the reaction chemicals. For example, to prepare NaLa_3.6Eu_0.4[Mo₃O₁₅]F, NaF (0.21 g), La₂O₃ (1.466 g), Eu₂O₃ (0.176 g), (NH₄)₆Mo₇O₂₄.4H₂O (1.324 g) were weighted according to the mole ratio of 2:3.6:0.4:3. Firstly the mixture was ground together in an agate mortar for enough time, which was then heated up to 350 °C and kept at this temperature for 2-4 h in a crucible with a cover. After a second homogenization in the mortar, the samples were heated up to 780 °C for 5-10 h in a crucible with a cover.

The phase purity was checked by XRD collected on a Rigaku D/Max diffractometer operating at 40 kV, 30 mA with Bragg-Brentano geometry using Cu Kα radiation. SEM was obtained using a JEOL, JSM-6360 LA instrument. The static PL excitation and emission spectra were recorded on a Perkin-Elmer LS-50B luminescence spectrometer. 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 the quantum yield measurement software.

3. Results and Discussions

3.1 Crystal phase formation

XRD patterns are shown in Fig. 2(a).All the samples have similar patterns, which can be well indexed to the cubic structure. No impurity lines were observed in the XRD patterns. The patterns were fitted by Jade. 5.0 program. The dependences of lattice parameter a and the unit cell volumes V on Eu³⁺ doping are presented in Fig. 2(b). It is obvious that the structure parameters become smaller with increasing Eu-doping. This can be clearly seen from the high degree-shift of the patterns inset Fig. 2(b). In NaLa₄Mo₃O₁₅F, Eu³⁺ ions prefer to occupy La³⁺ sites due to the same charge and similar ionic radius of 0.947 Å and 1.032 Å, respectively. The smaller ions Eu³⁺ for La³⁺ sites induce the shrink of the lattices.

 

Fig. 2 (a) the selected XRD patterns of NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.1, 0.3, 0.5, 0.6, 0.7, 0.8, 1.0). (b) The dependence of lattice parameter a and the unit cell volume V on Eu³⁺ doping. Inset is the normalized enlargements of XRD peak (222).

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The dependence of the cell parameters on Eu-doping (x = 0.1-1.0) is nonlinear. This does not follow Vegard's law, which is an approximate empirical rule that unit cell parameters should vary linearly with composition for a continuous substitutional solid solution [15]. In Fig. 2(b) there seems to be a transition around x = 0.5 between two regions that can follow Vegard's law. It can be tentatively suggested that the Eu³⁺ ions might doped into a second site at higher (or lower) concentrations.

Figure 3 shows the representative SEM micrograph of NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7). The phosphor contains well crystallized particles with small ball morphology. All particles are lightly aggregated and have an average particle size of 2-5 μm, which is suitable for the solid-lighting devices.

 

Fig. 3 The typical SEM micrograph of NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7).

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3.2 Photoluminescence emission spectra

Figure 4 shows the spectra of NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.1, 0.7) and Y₂O₂S:0.05Eu³⁺. The luminescence display typical emission lines from the ⁵D₀→⁷F_J (J = 0-4) transitions of the Eu³⁺ ions. A distinct luminescence property of NaLa_4-4xEu_4x[Mo₃O₁₅]F is the highest intensity of ⁵D₀→⁷F₂ transition. The main emission line of Y₂O₂S:Eu³⁺ is at 627 nm from ⁵D₀→⁷F₂. Under the same measurement conditions (λ_ex = 395nm), the intensity of NaLa_4-4xEu_4x[Mo₃O₁₅]F are about 19.2 (x = 0.7) and 8.4 (x = 0.1) times higher than that of Y₂O₂S:0.05Eu³⁺, respectively.

 

Fig. 4 the luminescence spectra of NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7, λ_ex = 395nm) (a), NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7, λ_ex = 465nm) (b), and NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.1, λ_ex = 395 nm) (c) compared with Y₂O₂S:0.05Eu³⁺ (λ_ex = 395nm) at 300 K. I_PL denotes the integrated area of the spectrum. Inset is emission intensity as a function of Eu³⁺ doping in NaLa_4-4xEu_4x[Mo₃O₁₅]F.

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In the case of UV excitation, the energy can be transferred from MoO₆^6- groups to Eu³⁺ via either an exchange or super exchange mechanism, which depends on Mo-O-Eu bond angle. Usually in Eu³⁺-doped materials with highly charged transition metal ion (M) complexes such as tungstates, molybdates, a 180° angle of M-O-Eu would maximize wave-function overlap and enhances the energy transfer efficiency [16]. In NaLa_4-4xEu_4x[Mo₃O₁₅]F a Mo-O-Eu angle of 180° could provide an efficient MoO₆→Eu³⁺ energy transfer. Inset Fig. 4 is the relationship between Eu³⁺ concentration (x) and the intensity. The luminescence increases with the increase of Eu³⁺ doping and the summit appears at x = 0.7. However, the luminescence intensity decreases when x>0.7. This is a high value for Eu³⁺-doped phosphor. It is difficult to get a concentration quenching between Eu³⁺ ions that are separated by octahedral [LaO₆] and [NaO₆] in NaLa_4-4xEu_4xMo₃O₁₅F lattices.

CIE chromaticity coordinates are listed in Table 1, which are closer to NTSC (National Television System Committee) standard (x = 0.67, y = 0.33) than that of Y₂O₂S:0.05Eu³⁺ (0.647, 0.343). The QEs of NaLa_4-4xEu_4x[Mo₃O₁₅]F are also listed in Table 1. The maximum QE value is 72.5% for NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7). The results demonstrate that QEs of NaLa_4-4xEu_4x[Mo₃O₁₅]F are higher than the reported red-emitting phosphors such as Gd₆MoO₁₂:0.25Eu³⁺ nano-powder (62%, λ_ex = 395 nm) [17], SrY_0.7Eu_1.4(MoO₄)₄ (48%, λ_ex = 395 nm) [18], commercial Y₂O₂S:Eu³⁺ (35%, λ_ex = 317 nm) [19].

Table 1. the PL QEs and CIE color coordinates of novel red-emitting NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.1-1.0).

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3.3 The excitation spectra

The excitation spectra of NaLa_4-4xEu_4x[Mo₃O₁₅]F and Y₂O₂S:0.05Eu³⁺ are shown in Fig. 5(a), which were obtained by monitoring ⁵D₀→⁷F₂ transitions of Eu³⁺ at 613 and 627 nm, respectively. The characteristic sharp lines belong to 4f→4f transitions of Eu³⁺ ions. The strong band (250-350 nm) in Y₂O₂S:Eu³⁺ corresponds to the CT transitions of Eu³⁺-O^2- and Eu³⁺-S²⁻. The absorption in the near-UV or blue region is very weak. However, NaLa_4-4xEu_4x[Mo₃O₁₅]F has very efficient excitation in the near-UV and blue region. This indicates that NaLa_4-4xEu_4x[Mo₃O₁₅]F matches well with wavelength of near-UV or blue LED chips in phosphor-converted pc-WLEDs.

 

Fig. 5 (a): The excitation spectra of NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.1, 0.7) (λ_em = 615nm) and Y₂O₂S:0.05Eu³⁺ (λ_em = 627nm). E_ct is obtained by extrapolating the tangent line of excitation band edges to zero. (b): The dependence of excitation band edge energy E_ct on doping levels.

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CTB in Eu³⁺-doped molybdates can be created by three primary mechanisms [10]. The first contribution is a charge transfer from O²⁻ ligand to Mo⁶⁺ ions [20], i.e., inter-band energy formed by the 4d orbitals of Mo⁶⁺ (bottom of conduction band) to 2p orbitals of O^2- (top of valence band). The second contribution is an inter-valence charge transfer (IVCT) states [21], i.e., electron transition from ground state 4f of Eu³⁺ to Mo⁶⁺ [20]. The third one is electron transfer from the neighboring O²⁻ to Eu³⁺, which is not seen in the excitation spectrum since it overlaps with the CTB position of O²⁻-Mo^6-. Usually in a Eu³⁺-doped molydate, CTB is mainly attributed to O²⁻→Mo⁶⁺ and O²⁻→Eu³⁺ transitions [22].

Usually excitation bands of Eu³⁺-doped tungstates or molybdates are located in 250-300 nm, which is not useful for a pc-WLED [23]. However, the wavelength of CTB in NaLa_4-4xEu_4x[Mo₃O₁₅]F can reach to 455 nm. This is longer than the reported molybdates with tetragonal, hexagonal, monoclinic, orthorhombic, and triclinic structures [10]. Firstly, this is due to the cubic crystalline phase with MoO₆ absorbing groups (high coordination number) in NaLa_4-4xEu_4x[Mo₃O₁₅]F. Mo⁶⁺-O²⁻ CTB edge of NaLa_4-4xEu_4x[Mo₃O₁₅]F shifts to near-UV and blue region (380-455 nm), making the phosphor suitable for near-UV or blue LEDs. Secondly, the existence of F^- ions in the lattices could give an effect on the red-shift of CTB. Fluorine atoms possess the largest electronegative and exhibit strongest attractive electron ability. The incorporation of F^- ions in a host lattices will make electronic cloud more intensive, which will compete with O²⁻ ions, resulting in a weak ionicity of Mo-O bond and a intense covalency of Mo-O bond in F⁻-containing host.

As seen in Fig. 5(a), the excitation spectrum of NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.1) comprises a strong CTB and a weak peaks from Eu³⁺ ions. However, NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7) shows a great red-shift of CTB extension to 455 nm and the f-f transition from ⁷F₀→⁵L₆ transition can be hardly observed overlapping with strong CTB. In order to further discuss this effect, E_ct was denoted in Fig. 5(a) by extrapolating the tangent line of CTB edges to zero. Figure 5(b) shows the dependence of E_ct on Eu³⁺-doping. It can be observed that with the increase of Eu³⁺-doping the CTBs show an obvious red-shift (the longer wavelength side). It has been suggested that the energy of CTB transition is dependent upon M⁶⁺-O²⁻ (M = Mo,W) bond at a given coordination number [10]. Thus, it is expected that the lattices shrink shown in Fig. 2(b) could be responsible for red-shift of the CTB energy. However, the detailed mechanism should be continued in the next work.

3.4 The thermal stability

The luminescence intensities of NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7) at different temperature were plotted in Fig. 6.The intensity becomes weak with increasing temperature to 160 °C. The intensity at 150 °C is about 80% of the initial value at 20 °C. The temperature-dependent intensity can be described by a modified Arrhenius equation as following:

 

Fig. 6 the temperature dependence of the integrated intensity in NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7) normalized with respect to the value at 20 °C, inset shows the activation energy of the thermal quenching fitted in Eq. (1).

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The thermal stability of Eu³⁺-doped NaLa₄[Mo₃O₁₅]F can be explained by two aspects; firstly, Eu³⁺ ions are located only in M3 sites in the lattices and separated by NaO₆ and MoO₆, the probability of energy transfer or energy migration among Eu³⁺ ions is small. Secondly, due to the steric effect caused by small F⁻ ions and the large electronegativity difference between the metal cation and ligands, fluorides tend to prefer octahedral coordination. Iconicity of Mo-O bond is weakened and the covalency of Mo-O bond is increased in F⁻/Mo-containing host and thus affects the overall thermal stability by increasing the phonon energy.

5. Conclusions

In summary, NaLa_4-4xEu_4x[Mo₃O₁₅]F were prepared by solid-state reaction. Under the excitation of near UV or blue light, it presents bright red luminescence with CIE color coordinates of (0.66, 0.32) close to the NTSC standard. The optimal level of Eu³⁺ doping is x = 0.7. The maximum QE 72.5% was obtained in NaLa_4-4xEu_4x[Mo₃O₁₅]F (x = 0.7). A marked property of this phosphor is that with increasing Eu³⁺-doping the CTB shows an obvious red-shift to blue wavelength region, which can reach to 455 nm. This matches well with wavelength of near-UV or blue LED chips. The thermal activation energy E_a is 0.408 eV. The obtained results have demonstrated the potentiality of NaLa_4-4xEu_4x[Mo₃O₁₅]F for near-UV/blue GaN-based white LEDs. The improvements of luminescence properties of NaLa₄Mo₃O₁₅F are due to the cubic crystalline phase with MoO₆ groups and incorporation of F^- ions in the lattices.