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Abstract
In this paper, by changing the ligand-to-metal charge transfer (LMCT) band through Mo6+ substitution, the excitation band of our prepared NaLaMg(W,Mo)O6:Eu3+ sample had been extended largely and could match with the InGaN-based LED chips successfully. Diffuse reflection spectra and photoluminescence properties of NaLaMg(W,Mo)O6:Eu3+ had been investigated as a function of W/Mo ratio. Density functional theory calculations gave an insight into the excitation band regulation on the aspect of band structure. The quantum efficiency together with thermal stability of typical prepared samples had been measured and investigated in detail. Moreover, a red LED device fabricated by a 375 nm UV chip with prepared NaLaMgW0.6Mo0.4O6:0.25Eu3+ phosphor had been obtained. Our study suggested that the NaLa0.75Eu0.25MgW0.6Mo0.4O6 phosphor might have potential value in serving as red component for WLEDs.
© 2017 Optical Society of America
1. Introduction
In recent years, white light emitting diodes (WLEDs) had emerged as the next generation solid-state lighting because of their higher conversion efficiency of electricity to visible light over traditional light bulbs. Researchers had made great efforts in attempting different realization modes of white light as well as exploring novel luminescent materials with high luminescence efficiency [1–5]. Many kinds of inorganic compounds had been investigated, such as phosphate, silicate, aluminate, molybdate and tungstate [6–10]. So far, the commercial market is still dominated by WLEDs realized by blue chips coating with yellow phosphors. However, WLEDs with this combination possess low colour rendering index and high correlated colour temperature due to the lack of red phosphors which can improve the lumen equivalency and colour rendering of the obtained white light [11,12]. To have warm white light, WLEDs realized by near UV chips with tricolour phosphors (blue, green and red) were proposed and expected to dominate the market due to their high performance and easily controlled luminescence properties. Considering that red phosphor plays an important role in this kind of WLEDs, it is urgently needed to find UV light excited red phosphors with superior luminescence properties.
The europium is one kind of common and important activator for rare earth ions doped phosphors. As we know that the emission of Eu2+ ions varies in a wide range (n-UV to red region) in different hosts [13]. The strong interaction between ligands and activators will increase the energy splitting of 4f65d1 excited states, resulting in a longer Stokes shift of the Eu2+ ions. In this case, many Oxynitride, nitride and sulfide phosphors doped with Eu2+ ions have been prepared [14–18]. However, these red phosphors either need rigorous synthesis conditions or suffer poor chemical stability. Taking account of the Eu3+ emission (5D0-7F2 transition) in the red region, developing Eu3+-activated phosphors is of great importance due to their superior red colour purity. Many Red emitting phosphors such as, GdVO4:Eu3+, Gd2MoO6:Eu3+, LaNbO4:Eu3+ and NaLa(PO3)4:Eu3+ had been reported [19–22]. In recent years, one kind of tungstate NaREMgWO6 (RE = La, Gd) with double perovskite AA’BB’O6 structure had attracted attention since the host could transfer energy to Eu3+ ions efficiently and result in an intense red emission. The NaREMgWO6 host possessing layered ordering A-site cations and rock-salt ordering B-site cations [23,24]was firstly determined by Sekiya in 1984 [25]. Hou et al studied the photoluminescence properties of Sm3+, Eu3+ and Tb3+ ions in this host [26]. Q. Liu et al studied the delayed concentration quenching mechanism of NaLaMgWO6:Eu3+ [27]. L. Zhang et al studied the enhancement of NaGdMgWO6:Eu3+ emission intensity based on alternative excitation and delayed quenching [28]. Moreover, the emission intensity and colour purity of Eu3+ doped NaLaMgWO6 had been improved by substitution of Li+ for Na+ as well as substitution of Gd3+ for La3+ ions [29,30]. However, for the NaLaMgWO6:Eu3+ sample with intense emission, the main excitation band arising from W-O charge transfer band locates at about 325 nm which cannot be excited by the InGaN-based chips (360–470 nm). In this paper, by changing the ligand-to-metal charge transfer (LMCT) band through Mo6+ substitution, the excitation band of NaLaMgWO6:Eu3+ had been extended largely and can match with the InGaN-based LED chips successfully. The influence of this substitution on band structure and luminescence properties had been investigated systemically by DFT calculation and spectra measurements. Moreover, the performance of prepared NaLa0.75Eu0.25MgW0.6Mo0.4O6 phosphor had been studied and results suggests that the phosphor matching successfully with InGaN chip might have potential value in UV excited WLEDs.
2. Experimental section
2.1 Raw materials and synthesis
Samples with composition of NaLa1-xEuxMg(W1-mMom)O6 (m stands for the Mo6+ content, x stands for the Eu3+ content) were synthesized via common high temperature solid-state reaction using La2O3 (96.0%), MgO (A.R.), (NH4)6Mo7O24·4H2O (A.R.), Na2CO3 (A.R.), (NH4)6W7O24·6H2O (A.R.) and Eu2O3 (99.99%) as starting materials. The doped europium ions are considered to substitute the lanthanum ions due to their same valence states and similar ionic radii. Firstly, the raw materials were weighed according to their corresponding stoichiometric ratio. Then, the raw substances were grinded for 20 minutes to have a thorough mix. After that, the mixtures were sintered at 1200 °C for 3 h in a box furnace to obtain the as-prepared phosphors.
2.2 Characterization techniques
X-ray diffraction (XRD) patterns of the samples were obtained by the D8 Focus diffractometer operated at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15405 nm). The XRD data were collected with scanning rate being 10° min−1 in the range of 10° to 80°. Diffuse reflectance spectra measurement was conducted by Vis/NIR spectrophotometer (Hitachi U-4100) and BaSO4 white plate was used as the standard reference for reflection measurement. The photoluminescence spectra of phosphors were measured on Hitachi F-7000 spectrophotometer with excitation source being 150 W xenon lamp. The decay curves of samples were measured on Digital Oscilloscope (Lecroy Wave Runner 6100, 1 GHz) with excitation source being tunable laser (pulse width = 4 ns, gate = 50 ns). The emission spectra of the phosphor at different temperature were measured on the spectrophotometer (Edinburgh Instrument FLS 920) with temperature controller. The Electroluminescent spectrum and Commission Internationale de l’Eclairage (CIE) chromaticity color coordinates of red LED device were measured by Starspec SSP6612. The PL quantum efficiency of the phosphors was determined on an internal quantum efficiency measurement system (C9920-02, Hamamatsu Photonics K. K., Japan).
2.3 Details of the calculation
The pristine NaLaMgWO6 and Mo-doped NaLaMgW0.6Mo0.4O6 were selected for DFT analysis to determine the crystal and band structures. Considering the inherent error of the self-interaction, GGA always underestimate the band energy gap and some other correct electronic properties [31,32].The meta-GGA + MBJ potential was used to get more accurate band gaps and electronic properties [33]. All the calculations were performed using the Vienna Ab initio Simulation Package (VASP) [34], with projector augmented wave (PAW) potential [35] with a cutoff energy 415 eV for the both structures. The k-points of 2 × 2 × 2 were generated by using Monkhorst-Pack scheme [36]. During the optimization, the lattice parameters and the atomic coordinates were optimized simultaneously. The convergence of the total energy is 1 × 10−5 eV and the ion interatomic forces converge to 0.0001 eV/Å.
3. Results and discussion
3.1 Phase identification and crystal structure
To verify the phase purity of as-prepared phosphors, powder X-ray diffraction measurement was used and the corresponding results were depicted in Fig. 1. From Fig. 1 one can see that with the value of m varying from 0 to 0.4, the diffraction peaks of our phosphors match well with the standard data of NaLaMgWO6 (JCPDS card no. 37-0243), suggesting that our obtained samples are single-phased and the substitution of Mo6+ for W6+ ions do not cause any impurity. Moreover, diffraction peaks of these samples hardly have a shift due to the pretty similar ionic radii between Mo6+ (r = 0.59, CN = 6) and W6+ (r = 0.60, CN = 6) ions. When m increased to 0.5, a diffraction peak appeared at 2θ = 27.42° which can be ascribed to the formation of La2MoO6. The impurity phase became more obvious when m increased to 0.7, indicating that the Mo6+ can substitute for 40% of W6+ site to the greatest extent. According to the report of Sekiya [25], the NaLaMgWO6 has monocline crystal system with space group being P21/m. In the host, A-site cations (Na/Gd) arrange with layered order, while the B-site cations (Mg/W) have long-range rock-salt order. The Mg and W atoms are coordinated with six oxygen atoms forming octahedrons which are further connected by sharing O atoms. The arrangement of cations at A-site can lead to bonding instability which is further compensated by the second-order Jahn–Teller (SOJT) distortion of cations at B-site [23–25,37]. Therefore, the distortions of the two sites are interdependence and the removal of one can make the other disappear. In the experiment, when the Mo6+ substituted for the W6+ ions at high content, the distortion of B-site cations became severe and the crystal structure was damaged since the Mo6+ had stronger SOJT distortion in octahedral-coordination than the W6+ ions. And thus, the La2MoO6 appeared when the value of m increased to 0.5.
Fig. 1 XRD patterns of typical NaLa0.95Eu0.05Mg(W1-mMom)O6 samples together with the standard data of NaLaMgWO6 and La2MoO6 for comparison.
To further validate that the Mo6+ ions have doped into the host lattice substituting for the W6+ ions, we employed the Rietveld structure refinement by the GSAS (general structure analysis system) program taking NaLa0.95Eu0.05MgW0.6Mo0.4O6 as example. The crystallographic data of NaLaMgWO6 previously reported by Sekiya [25] were employed as the initial structure parameters during the refinement. Figure 2 shows the refined patterns of our prepared sample, while the refined results are listed in Table 1 in detail. All the observed peaks satisfy the reflection conditions, which can be concluded from the reliability factors χ2 = 5.051, Rwp = 9.00% and Rp = 6.73%. The refinement results indicate that the MoO6 are suscessfully exist in the lattice substituting for the WO6, and thus the regulation of LMCT band by this substitution can be expected.
Fig. 2 Observed (crosses) and calculated (red solid line) powder XRD patterns of the NaLa0.95Eu0.05MgW0.6Mo0.4O6 sample. The blue solid line is the difference between experimental and calculated data and the green sticks present the Bragg reflection positions.
Table 1. Final refined structure parameters of NaLa0.95Eu0.05MgW0.6Mo0.4O6 derived from the Rietveld refinement of X-ray diffraction data
3.2 Band structure analysis
Density functional calculation was performed to better understand the effects of Mo6+ substitution on band structure and electronic properties of host. The calculated band structures have been shown in Fig. 3. Energy gap between the lowest energy of the conduction band (CB) and highest energy of valance band (VB) were determined to be 3.82 eV for NaLaMgWO6 and 3.20 eV for NaLaMgW0.6Mo0.4O6 hosts, indicating that the substitution of Mo6+ for W6+ ions had decreased the energy gap obviously. In this case, a lower energy can lead to the ligand-to-metal charge transfer of the Mo-substituted host. When the Eu3+ activator doped into the host, the energy transfer from the charge transfer band to Eu3+ ions can be expected, which can make the Eu3+ doped phosphor efficiently excited by light with low energy. Assignment of the electronic bands can be performed on the basis of the calculated partial and total density of states (DOSs) diagrams as shown in Fig. 3. The VBs of the two samples both start from the Fermi level around 0 eV and decay near −5.7 eV. The VB of NaLaMgWO6 host is mainly derived from W-6s, 5d and O-2p states, while the VB band of Mo-doped sample also consist Mo-4d state. The conductive band (CB) of NaLaMgWO6 stretching from 3.70 eV to 5.28 eV is comprised of La-4f, O-2p and W-5d states. The CB of NaLaMgW0.6Mo0.4O6 is separated into two parts: the lower energy part between 3.12 eV and 3.73 eV is contributed by O-2p and Mo-4d states, and the upper part between 4.29 eV and 5.28 eV is derived from La-4f, O-2p and W-5d states. From the analysis of electronic properties of the two hosts we can conclude that the decrement of energy gap is mainly caused by the Mo-4d state, which can make the LMCT band of Mo4d-O2p have a lower energy than W5d-O2p. Moreover, the crystal structure parameters for NaLaMgWO6 are optimized to be a = b = 5.495 Å, c = 7.8799 Å, α = β = γ = 90°, while the optimized crystal structure parameters for NaLaMgW0.6Mo0.4O6 are determined to be a = b = 5.493 Å, c = 7.8277 Å, α = β = γ = 90°. Generally, the undistorted and fully ordered AA’BB’O6 perovskite has tetragonal symmetry with P4/nmm space group. However, the octahedral tilting would lower its symmetry and degenerate the cell into monoclinic in practice, which can be the reason for the difference between the refined structure parameters and optimized structure parameters [28]. The little shrinkage of the optimized structure parameters for Mo-substituted sample is caused by the smaller ionic radii of Mo6+ than W6+ ion.
Fig. 3 Calculated band structures and density of states (DOSs) of (a) NaLaMgWO6 and (b) NaLaMgW0.6Mo0.4O6 near the Fermi energy (EF) level.
3.3 Photoluminescence properties
Diffuse reflection spectra of typical samples were measured and depicted in Fig. 4. The NaLaMgWO6 host had an absorption band below 375 nm caused by the O2-→W6+ charge transfer band. Compared with the host, the NaLaMgWO6:0.05Eu3+ had a broader absorption resulted from both O2-→W6+ and O2-→Eu3+ charge transfer bands. When substituted Mo6+ for W6+ ions in NaLaMgWO6:0.05Eu3+ samples, the absorption bands shifted gradually to the long wavelength with the increasing content of Mo6+ ions caused by the addition of O2-→Mo6+ charge transfer, which can be further explained by the cation-ligand covalency. As we know that the electronegativity value of Mo (2.16) is lower than that of W (2.36). Correspondingly, the electron-cloud overlap between center cation Mo6+ and its coordination anion O2- will enlarge and the charge transfer from O to Mo can occur easily due to the decrement of energy gap, causing the charge transfer band shift to lower energy. The optical energy gap can also be estimated from the reflectance spectrum using the following equation [38]:
where is the photon energy; Eg is the value of optical band gap; A stands for a constant; F(R∞) is the so called Kubelka−Munk function which can be written asin which S, K, and R represent for of scattering, absorption, and reflection coefficients, respectively. As can be seen from Fig. 5, the band gap energy can be obtained by plotting [F(R∞)hν]2 versus hν. For NaLaMgWO6 and NaLaMgW0.6Mo0.4O6 host, the Eg are determined to be 3.58 eV and 3.26 eV, respectively. The values of Eg obtained from reflectance spectra are similar with the calculated band gap values.Fig. 5 Calculated energy gaps (Eg) of (a) NaLaMgWO6 and (b) NaLaMgW0.6Mo0.4O6 from the diffuse reflectance spectra.
Figure 6 illustrated the PLE and PL spectra of NaLaMgWO6, NaLaMgWO6:0.05Eu3+, and NaLaMgW0.6Mo0.4O6:0.05Eu3+ samples. The NaLaMgWO6 host had a weak excitation band with center at 275 nm due to the electron transition from 2p orbits of the O2- ions to 5d orbits of W6+ ions. Upon 275 nm irradiation, the phosphor showed a broad emission band peaking at 475 nm owning to the recombination of electrons in CB and holes in VB of the host. However, the emission intensity of this combination was pretty low. From Fig. 6(b) we can see that under 320 nm irradiation, the Eu3+ ions doped NaLaMgWO6 phosphor exhibited a strong red emission with peak at 620 nm caused by 5D0→7F2 electric dipole transition of the Eu3+ ions. The neighboring peak at 596 nm is resulted from the 5D0→7F1 magnetic dipole transition. The relative intensity of these two transitions is hypersensitive to the coordination environment of Eu3+ ions. Generally, if Eu3+ ions occupy the sites with inversion symmetry, the 5D0→7F1 transition will dominate the emission. Otherwise, the 5D0→7F2 transition of Eu3+ ions will be stronger than 5D0→7F1 transition. In Fig. 6(b) the PL spectrum of NaLaMgWO6:0.05Eu3+ was dominated by hypersensitive red emission of the 5D0-7F2 electric-dipole transition, manifesting that the Eu3+ ions were located at sites substituting for the La3+ ions without inversion symmetry [39,40]. Monitored at 620 nm, the PLE spectrum consisted of a broad band (225―375 nm) and several peaks. The excitation band at 320 nm and weak shoulder band at 275 nm were caused by the Eu-O and W-O charge transfer, respectively. The excitation peaks at 381, 399, 416 and 467 nm were caused by the 7F0-5L7, 7F0-5L6, 7F1-5D3 and 7F0-5D2 transition of Eu3+ ions, respectively.
Fig. 6 The excitation and emission spectra of (a) NaLaMgWO6, (b) NaLaMgWO6:0.05Eu3+ and (c) NaLaMgW0.6Mo0.4O6:0.05Eu3+ phosphors.
To study the effect of Mo6+ substitution on luminescence properties of Eu3+ doped phosphor, the spectra of samples with Mo6+ substituting for W6+ in different contents had been measured. Figure 6(c) depicted the results of typical NaLaMgW0.6Mo0.4O6:0.05Eu3+ sample. Under 375 nm irradiation, the emission spectrum of sample with Mo6+ substitution is exactly the same with NaLaMgWO6:0.05Eu3+. However, the excitation band of NaLaMgW0.6Mo0.4O6:0.05Eu3+ shift to longer wavelength with strong intensity in the n-UV range, matching with the emission of InGaN-based LEDs successfully. One can also see that the intensity of W-O charge transfer band centered at 275 nm increased in Fig. 6(c) compared with that in Fig. 6(b). It can be explained by the decrease of distance between the WO6 octahedral with the increasing of Mo content. In NaLaMgWO6:0.05Eu3+ phosphor, the distance between the WO6 is short and the absorbed energy of WO6 quenches seriously when transferring to Eu3+ ions along the (MgW)O6 framework [37]. The substitution of MoO6 will make the distance between the WO6 get longer, resulting in more absorbed energy of WO6 transferring to Eu3+ ions. Thus, the excitation spectrum in Fig. 6(c) exhibits not only strong Mo/Eu-O charge transfer band but also W-O chare transfer band. In this way, we had obtained the n-UV light excited red emitting phosphor with good color purity by substituting Mo6+ for W6+ ions.
To study the effect of Eu3+ content on the luminescence property of the phosphor, a series of NaLa1-xEuxMgW0.6Mo0.4O6 samples with x being 0.05, 0.10, 0.15, 0.20, 0.25, 0.30 and 0.5. had been prepred. When x increased to 0.30, impurity phase appeared as can be seen from the XRD patterns of NaLa1-xEuxMgW0.6Mo0.4O6 shown in Fig. 7. In this case, the luminescence properties of the NaLaMgW0.6Mo0.4O6:xEu3+ (x = 0.05, 0.10, 0.15, 0.20 and 0.25) samples with single phase had been characterized. With the Eu3+ content increasing gradually, the emission intensity of the samples had a monotonous increment as shown in Fig. 8(b). Figure 8(a) demonstated the excitation and emisson spectra of NaLaMgW0.6Mo0.4O6:0.25Eu3+ together with its reflection spectrum. The excitation spectrum which agrees well with the reflection specturm has been extended largely. Under irradiation of 375 nm, the phosphor shows intense red emssion with peak at 620 nm. A red LED lamp was fabricated using a 375 nm UV chip combined with NaLaMgW0.6Mo0.4O6:0.25Eu3+ phosphor. Figure 8 depcited the electroluminescence spectrum of the LED device, which is similar with the photoluminescence spectrum of the red phosphor. By integrating the corresponding emission spectrum, the CIE coordinates were determined to be (0.622, 0.343) locating in the red region. The inset of Fig. 9 depicted the CIE chromaticity diagram together with digital photograph of the red LED device. From Fig. 9 we can see that the obtained NaLaMgW0.6Mo0.4O6:0.25Eu3+ phosphor can be efficiently exctied by the UV chip and might have potential value for WLEDs.
Fig. 8 (a) Photoluminescence and reflection spectra of NaLaMgW0.6Mo0.4O6:0.25Eu3+ phosphor. (b) Emission intensity of NaLaMgW0.6Mo0.4O6:xEu3+ samples with different Eu3+ contents.
Fig. 9 Electroluminescent spectrum of the red LED device encapsulated with a 375 nm UV chip and as-prepared NaLaMgW0.6Mo0.4O6:0.25Eu3+ phosphor. The inset shows the CIE coordinate diagram together with a digital image of the LED device.
To study the dynamic luminesce properties of our prepared phosphors, the decay curves of NaLaMgW0.6Mo0.4O6:xEu3+ phosphors had been measured and shown in Fig. 10 on the logarithmic intensity scale. From Fig. 10 we can clearly see that the emission intensity of the phosphors all decrease in single exponential way, which is in good agreement with the structure character that there is one kind of La3+ cation site which can be occupied by Eu3+ ion forming emission center. The decay lifetimes were calculated to be 0.47, 0.47, 0.46, 0.45, and 0.46 ms for NaLaMgW0.6Mo0.4O6:xEu3+ samples with x = 0.05, 0.10, 0.15, 0.20, and 0.25, respectively. Thus, the increment of Eu3+ content neither change the decay process nor the value of lifetime, revealing that the energy migration among the Eu3+ ions hardly take place. For NaLaMgW0.6Mo0.4O6:xEu3+ (x = 0.05, 0.10, 0.15, 0.20, and 0.25) phosphors, the internal quantum efficiencies (IQE) have been determined to be 10.3%, 15.6%, 19.0%, 23.1%, and 26.0%, respectively. The value of IQE increases gradually with the increment of Eu3+ content and may be further improved by changing the experimental conditions as well as the compositions of the phosphors.
Generally, the temperature dependent luminescence property of prepared phosphor is important since it can influence the light output and color rendering index. Phosphors have to sustain a stable luminescence property at high temperature since the working temperature of WLEDs can reach about 150°C [41]. For most phosphors, the emission intensity will decrease due to the temperature quenching caused by the intense electron–phonon interaction in both ground and excited states at high temperature. Figure 11(a) depicted the emission spectra of NaLa0.75Eu0.25MgW0.6Mo0.4O6 in a temperature range from 77 K to 450 K. The emission intensity of phosphor decreased monotonously with the temperature increment. Figure 11(b) showed the dependence of luminescence intensity and CIE coordinates on temperature. At 450 K, the emission intensity decreased to 22.6% and 44.0% of the value at 77 K and 300 K, respectively. The coordinate values of the phosphor almost do not change with the temperature increment, indicating that the phosphor has good thermal stability in color hue.
Fig. 11 (a) Emission spectra at different temperatures and (b) dependence of emission intensity and CIE coordinates on temperature of NaLaMgW0.6Mo0.4O6:0.25Eu3+ phosphor.
4. Conclusions
In summary, we have synthesized a series of Eu3+ doped NaLaMg(W,Mo)O6 red phosphors with different ratio of W/Mo ions by tradition solid state reaction. According to the DFT calculation, the substitution of Mo6+ for W6+ ions had decreased the energy gap from 3.82 eV to 3.20 eV due to the Mo-4f state. The regulated band structure of the host resulted in a broad excitation band matching successfully with the InGaN based LED chips. The Eu3+ doped NaLaMgW0.6Mo0.4O6 phosphors exhibited intense red emission peaking at 620 nm under 375 nm irradiation. The CIE coordinates and emission intensity of NaLaMgW0.6Mo0.4O6:0.25Eu3+ had been investigated in a temperature ranging from 77 K to 450 K. Moreover, a LED device with intense red emission had been obtained by coating the prepared phosphor on a 375 nm UV chip. Our investigation results suggest that the prepared NaLaMgW0.6Mo0.4O6:0.25Eu3+ phosphor might be a potential red phosphor with good colour purity for WLEDs.
Funding
National Natural Science Foundation of China (NSFC) (11374132, 11574125); Natural Science Foundation of Shandong Province (ZR2015JL024); Taishan Scholars project of Shandong Province (ts201511055).
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