Abstract: Eu3+-doped NaLa4Mo3O15F was prepared by solid-state method and characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM). The luminescence properties such as the excitation and emission spectra, the internal quantum efficiency (QE) and the thermal stability were investigated. The optimal level of Eu3+-doping was determined by the luminescence intensities and QEs. The phosphor exhibits a bright red luminescence at 615 nm corresponding to the electric dipole transition 5D0→7F2, which is more efficient than the commercial phosphor of Y2O2S:Eu3+. With the increase of Eu3+-doping the charge transfer band (CTB) shows an obvious red-shift, which presents a longer wavelength than any other reported Eu3+-doped molydates with non-cubic structures. NaLa4Mo3O15F:Eu3+ has excellent properties such as the high quantum efficiency excited in near-UV and blue wavelength, the high thermal stability, etc. This is benefited from its structural characteristics: the cubic crystalline phase with MoO6 groups and the incorporation of F- ions in the host lattices.
© 2014 Optical Society of America
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 InxGa1−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 . 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/MoOx groups depend on coordination number (CN) x . Dutta et al  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 -direction modelled on the coordinate data reported by Faurie . The cell parameters are a = b = c = 11.333 Å, V = 1455.575 Å3, and Z = 4. The lattice consists MoO6 linked by La3+ and Na+ ions. The linkages between the bands running along  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.
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.
NaLa4-4xEu4x[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 NaLa3.6Eu0.4[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.
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 . 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 NaLa4-4xEu4x[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.
3.2 Photoluminescence emission spectra
Figure 4 shows the spectra of NaLa4-4xEu4x[Mo3O15]F (x = 0.1, 0.7) and Y2O2S:0.05Eu3+. The luminescence display typical emission lines from the 5D0→7FJ (J = 0-4) transitions of the Eu3+ ions. A distinct luminescence property of NaLa4-4xEu4x[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 (λex = 395nm), the intensity of NaLa4-4xEu4x[Mo3O15]F are about 19.2 (x = 0.7) and 8.4 (x = 0.1) times higher than that of Y2O2S:0.05Eu3+, respectively.
In the case of UV excitation, the energy can be transferred from MoO66- 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 . In NaLa4-4xEu4x[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 NaLa4-4xEu4xMo3O15F 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 NaLa4-4xEu4x[Mo3O15]F are also listed in Table 1. The maximum QE value is 72.5% for NaLa4-4xEu4x[Mo3O15]F (x = 0.7). The results demonstrate that QEs of NaLa4-4xEu4x[Mo3O15]F are higher than the reported red-emitting phosphors such as Gd6MoO12:0.25Eu3+ nano-powder (62%, λex = 395 nm) , SrY0.7Eu1.4(MoO4)4 (48%, λex = 395 nm) , commercial Y2O2S:Eu3+ (35%, λex = 317 nm) .
3.3 The excitation spectra
The excitation spectra of NaLa4-4xEu4x[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+-O2- and Eu3+-S2−. The absorption in the near-UV or blue region is very weak. However, NaLa4-4xEu4x[Mo3O15]F has very efficient excitation in the near-UV and blue region. This indicates that NaLa4-4xEu4x[Mo3O15]F matches well with wavelength of near-UV or blue LED chips in phosphor-converted pc-WLEDs.
CTB in Eu3+-doped molybdates can be created by three primary mechanisms . The first contribution is a charge transfer from O2− ligand to Mo6+ ions , i.e., inter-band energy formed by the 4d orbitals of Mo6+ (bottom of conduction band) to 2p orbitals of O2- (top of valence band). The second contribution is an inter-valence charge transfer (IVCT) states , i.e., electron transition from ground state 4f of Eu3+ to Mo6+ . 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−-Mo6-. Usually in a Eu3+-doped molydate, CTB is mainly attributed to O2−→Mo6+ and O2−→Eu3+ transitions .
Usually excitation bands of Eu3+-doped tungstates or molybdates are located in 250-300 nm, which is not useful for a pc-WLED . However, the wavelength of CTB in NaLa4-4xEu4x[Mo3O15]F can reach to 455 nm. This is longer than the reported molybdates with tetragonal, hexagonal, monoclinic, orthorhombic, and triclinic structures . Firstly, this is due to the cubic crystalline phase with MoO6 absorbing groups (high coordination number) in NaLa4-4xEu4x[Mo3O15]F. Mo6+-O2− CTB edge of NaLa4-4xEu4x[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 NaLa4-4xEu4x[Mo3O15]F (x = 0.1) comprises a strong CTB and a weak peaks from Eu3+ ions. However, NaLa4-4xEu4x[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, Ect was denoted in Fig. 5(a) by extrapolating the tangent line of CTB edges to zero. Figure 5(b) shows the dependence of Ect 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 . 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 NaLa4-4xEu4x[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 plots of ln[(I0/IT)-1] versus 1000/T, which are linear with slope of −4.73. According to Eq. (1), ΔE was calculated to be 0.408eV, which is higher than those of the reported red-emitting phosphors such as CaLa2(MoO4)4:Sm3+,Eu3+(0.13eV) , Li3Ba2Gd3-x(MoO4)8:Eu3+(0.283 eV) , Ca9Eu2W4O24 (0.3 eV) , Ca2GdF4PO4:Eu3+ (0.375 eV) .
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.
In summary, NaLa4-4xEu4x[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 NaLa4-4xEu4x[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 Ea is 0.408 eV. The obtained results have demonstrated the potentiality of NaLa4-4xEu4x[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.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013-R1A1A2009154).
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