Introduction

By virtue of admirable advantages of long working lifetime, energy saving, low cost, high luminous efficiency and environmental compatibility, the phosphor-converted white light-emitting diodes (WLEDs) which are considered as the next-generation illumination sources to supersede the conventional fluorescent lamps have been extensively used in indoor lighting, automobile displays and flashlights1,2,3,4,5. Presently, the commercial WLEDs which are made up of a blue-emitting InGaN LED chip and Y3Al5O12:Ce3+ yellow-emitting phosphors suffer from poor color rending index (CRI ~ 70–80) and high correlated color temperature (CCT ~ 7000 K) as a result of inefficient red emission component6,7,8. To circumvent these drawbacks, a new strategy utilizing the near-ultraviolet (NUV) LED chip to pump the hybrid tricolor (blue, green and red) phosphors is performed to emit warm white light9,10,11. From the aforementioned combinations, one knows that the eventual behaviors of WLED devices can be significantly affected by the phosphors and they are expected to be efficiently excited by NUV or blue light. In comparison with commercial blue-emitting and green-emitting phosphors, the current red-emitting phosphors, by taking Y2O2S:Eu3+ for example, still exhibit some unsatisfied characteristics such as weak absorption in the NUV/blue region, low luminous efficiency and poor stability12,13. In order to improve the performance of red-emitting phosphors, the nitride- and germanide-based red-emitting phosphors, such as Ca2Si5N8:Eu2+, Sr[LiAl3N4]:Eu2+, Sr3Y2Ge3O12:Eu2+ and Sr2GeO4:Eu2+, were developed14,15,16,17. Unfortunately, to synthesis these compounds, a reduced atmosphere and a high sintering temperature are required, leading to high investment as well as environmental issues. Therefore, the development of novel red-emitting phosphors that can be excited by NUV or blue light is highly desirable.

Nowadays, tremendous interests have been attracted in rare-earth (RE) ions-based luminescent materials because of their potential feasibility in many fields of solar cells, thermometry, field emission displays, WLEDs and biomedicine18,19,20,21,22. In comparison, the Eu3+ ion, as an obbligato member of RE ions, is most frequently used as a red-emitting activator since its narrow red emission originating from 5D0 → 7F2 transition23,24. Up to date, some Eu3+-activated red-emitting phosphors, such as Li3Ba2Y3(WO4)8:Eu3+, Ca2Ga2SiO7:Eu3+ and Y2Mo4O15:Eu3+, were successfully synthesized25,26,27. However, these red-emitting phosphors suffer from narrow excitation bands and do not match well with the emitting band of NUV or blue LED chip which limits their promising applications in WLEDs. To solve this problem, an appropriate luminescent host material should be selected. According to previous literatures, one obtains that the tungstates and molybdates are promising candidates for luminescent host materials because of their outstanding metrics of high stability, low phonon energy, relatively low synthetic temperature and admirable intrinsic luminescent performance28,29,30. Meanwhile, both the tungstates and molybdates exhibit a broad absorption band in the UV region arising from the charge transfer (CT) transitions of O2− → W6+ and O2− → Mo6+, respectively31,32. Furthermore, owing to the small difference in the ionic radii between W6+ and Mo6+ ions, they can be easily substituted by each other, leading to the formation of molybdates-tungstates compounds12,32. It was revealed that the absorption bands of the Eu3+-activated tungstates-molybdates compounds were shifted from UV region to longer wavelength compared with the pure tungstates and molybdates. Thus, the developed products can be perfectly excited by NUV or blue light. And some impressive achievements have been obtained in these strategies, such as Sr2ZnW1−x Mo x O6:Eu3+,Li+, La3BW1−x Mo x O9:Eu3+, NaLa(MoO4)2−x (WO4) x :Eu3+ and (Sr x Ba1−x )2CaMo1−yWyO6:Eu3+  32,33,34,35. Obviously, most of the current researches mainly focus on the effect of transition metal (Mo6+ and W6+) ions on the luminescent properties of Eu3+-activated molybdates-tungstates compounds, while the influence of Eu3+ ion concentration on their luminescent performance is barely investigated.

Recently, the La2MoO6 has been intensively studied as the luminescent host material on account of its intrinsic luminescent properties and high stability36,37. Furthermore, it was also found that the La2WO6 had broad absorption band in the UV region38. However, these RE ions activated La2MoO6 or La2WO6 phosphors can only be efficiently excited by the deep UV light which makes them insufficient for NUV chip-based WLEDs. To figure out this shortage and improve their optical performance, the W6+ ions were introduced into the La2MoO6 host lattice and the La2Mo0.6W0.4O6 (La2MoO6-La2WO6) was produced. In present work, a facile citrate-assisted sol-gel route was applied to prepare the Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors. The phase structure, morphology, lifetime, luminescent behaviors and thermal stability of the final products were detailedly studied. In addition, a theoretical calculation based on the Judd-Ofelt theory was also carried out to analyze the local crystal environment surrounding the Eu3+ ions. Finally, to clarify the applicability of resultant compounds for indoor lighting, a WLED device was implemented by utilizing an NUV LED chip and a mixture of synthesized red-emitting phosphors, commercial blue-emitting and green-emitting phosphors.

Results and Discussion

The phase compositions of the final products were identified by X-ray diffraction (XRD). From the XRD patterns (see Fig. 1(a)), it is evident that all the samples exhibit similar diffraction patterns. According to previous literatures28,39,40, one knows that the resultant compounds consist of the mixed phases of La2MoO6 (ICSD#25611) and β-La2WO6 (ICSD#246256), revealing that the Eu3+ ions are incorporated into the host lattices and Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors are successfully prepared. Moreover, with the increase of Eu3+ ion concentration, the diffraction peaks shift to larger angle which is attributed to inconsistent ionic radii between the Eu3+ and La3+ ions, as displayed in Fig. 1(b). The unit cell crystal structures of La2MoO6 and La2WO6 are presented in Fig. 1(c). As disclosed, in La2MoO6, the La3+ ions are surrounded by six oxygen atoms and the Mo6+ ions are coordinated with four oxygen atoms. In comparison, the La3+ ions in the La2WO6 are surrounded by eight oxygen atoms and the W6+ ions are coordinated with four oxygen atoms. Figure 1(d) shows the FTIR spectrum of La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors in the range of 450–4000 cm−1. The absorption peaks centered at around 3301 and 1671 cm−1 are associated to the O-H symmetric stretching vibration16,41. The absorption band located at around 1431 cm−1 is ascribed to the H-O-H blending vibration42. Furthermore, the intense absorption bands at about 837, 762, 652 and 512 cm−1 are related to the Mo-O-Mo, W-O-W, W-O and Mo-O stretching vibration modes, respectively41,43,44.

Figure 1
figure 1

(a) Representative XRD patterns of La2MoO6-La2WO6:2xEu3+ (x = 0.02, 0.06, 0.08, 0.12 and 0.16) red-emitting phosphors sintered at 850 °C. (b) Magnified XRD patterns in the 2θ range of 25–30.5°. (c) Crystal structures of La2MoO6 and La2WO6. (d) FTIR spectrum and (e) diffuse reflectance spectrum of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors. Inset depicts the calculation of band gap of the resultant samples utilizing Kubellka-Munk function.

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The diffuse reflectance spectrum of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors was recorded as shown in Fig. 1(e). As demonstrated, the studied samples possess strong absorption in the NUV region corresponding to the absorption of the host material which coincides well with the excitation spectrum. Meanwhile, two narrow bands located at approximately 466 and 532 nm originating from the characteristic absorption of Eu3+ ions are also observed44. As is known, the diffuse reflectance spectrum can be converted to the Kubelka-Munk function (F(R ∞)) with the help of following formula45:

$$F({R}_{\infty })=\frac{{(1-R)}^{2}}{2R}=\frac{k}{s},$$
(1)

where R is the sample reflectivity, k denotes the molar absorption constant of the compound and s stands for the scattering coefficient. Moreover, the relationship between the optical band gap and absorption coefficient of luminescent materials can be roughly expressed as44,45:

$$\alpha hv\approx A{(hv-{E}_{g})}^{1/2}.$$
(2)

In this equation, α, hv, E g and A present the optical absorption coefficient, phonon energy, band gap and proportionality coefficient, respectively. Combined with Eqs (1) and (2), the following expression is achieved:

$${[hvF({R}_{\infty })]}^{2}=B(hv-{E}_{g}).$$
(3)

To estimate the optical band gap of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors, the plot of [hvF(R ∞)]2 versus hv is drawn, as presented in the inset of Fig. 1(e). As demonstrated (see the inset of Fig. 1(e)), the optical band gap is determined to be about 2.79 eV by deducing the linear fitted region to [hvF(R ∞)]2 = 0.

The microstructure and morphological properties of the prepared samples are characterized by FE-SEM. From the FE-SEM images, as described in Fig. 2(a) and (b), the synthesized compounds are made up of two different sized particles including small nanoparticles with the size around 70 nm and large nanoparticles with the average size about 600 nm, further indicating that the resultant samples consist of the hybrid phases of La2MoO6 and La2WO6. The EDX spectrum shown in Fig. 2(c) reveals the presence of La, Mo, W, O and Eu in the prepared samples. Furthermore, the occurrence of Pt peak in the EDX spectrum is assigned to the platinum electrode for measuring the FE-SEM image. In addition, the elemental mapping result suggests that the elements presented in the studied samples are homogeneously distributed (see Fig. 2(d–i)). These characteristics further verify the successful formation of Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors, which coincides well with the deduction obtained from the XRD pattern.

Figure 2
figure 2

(a and b) FE-SEM images, (c) EDX spectrum and (d–i) elemental mapping of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors.

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The photoluminescence (PL) excitation spectrum of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors monitored at the emission wavelength of Eu3+ ions (612 nm) is described in Fig. 3(a). Obviously, the excitation spectrum consists of two ultrabroad absorption bands in the UV/NUV region and a narrow peak in the blue region. The first broad band, which is marked as CT(I), ranging from 220 to 320 nm centered at around 283 nm is assigned to the charger transfer from the oxygen ligands to tungsten ions12,46. Moreover, the second broad band, named as CT(II), with a central wavelength of 379 nm, which matches well with the commercial NUV LED chip, is related to the overlapped transitions of O2− → Mo6+ and O2− → Eu3+  28,32. The sharp absorption band at 462 nm is ascribed to the 7F0 → 5D2 transition of Eu3+ ions33. Furthermore, the excitation spectral profiles are slightly varied with the increase of Eu3+ ion concentration, as depicted in Fig. S1. Note that, in comparison with other peaks, the excitation band located at 379 nm exhibits the strongest intensity, demonstrating that the studied samples can be efficiently excited by NUV light which is helpful for its application in solid-state lighting. Upon the irradiation of 379 nm light, the PL emission spectrum presented in Fig. 3(a) is dominated by an intense red emission at about 612 nm arising from the 5D0 → 7F2 transition of Eu3+ ions5,42. Apart from the strong red emission, four weak emission peaks situated at approximately 578, 594, 650 and 708 nm which are attributed to the 5D0 → 7F0, 5D0 → 7F1, 5D0 → 7F3 and 5D0 → 7F4 intra-configurational transitions of Eu3+ ions, respectively are also detected in the emission spectrum12,21. It is widely accepted that the yellow (5D0 → 7F1) and red (5D0 → 7F2) emissions are two featured emissions of Eu3+ ions. In particular, the 5D0 → 7F1 transition is regarded as the magnetic dipole transition (∆J = 0, ±1) which is insensitive to the crystal environment surrounding the Eu3+ ions, whereas the red emission corresponding to the 5D0 → 7F2 transition pertains to the hypersensitive electric dipole transition (∆J ≤ 6, when J or J′ = 0, ∆J = 2, 4, 6) and its intensity is largely dependent on the crystal field around the Eu3+ ions. In general, the 5D0 → 7F2 transition prevails in the luminescent spectrum when the Eu3+ ions occupy the sites with non-inversion symmetry, while the yellow emission (5D0 → 7F2) becomes the strongest one when the Eu3+ ions are located at symmetric cation circumstance28,47. As described in Fig. 3(a), it is evident that the emission intensity of the 5D0 → 7F2 transition is much higher than that of the 5D0 → 7F1 transition, demonstrating that the Eu3+ ions occupy the positions with low symmetry and non-inversion center in the host lattices. In addition, with the help of Judd-Ofelt theory, the optical transition intensity parameters, Ω2 and Ω4, were calculated to better comprehend the local structure environment surrounding the Eu3+ ions. The values of Ω2 and Ω4 are determined to be about 6.8 × 10−20 and 1.2 × 10−20 cm2, respectively, (see Supplementary Information), further verifying that the Eu3+ ions take up the low symmetry sites.

Figure 3
figure 3

(a) PL excitation (λem = 612 nm) and emission (λex = 379 nm) spectra of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors. (b) Simplified energy level diagram as well as the luminescent processes in the Eu3+-activated La2MoO6-La2WO6 compounds. (c) PL emission spectra of the La2MoO6-La2WO6:2xEu3+ red-emitting phosphors as a function of Eu3+ ion concentration. (d) CIE chromaticity diagram of La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors. Inset shows the luminescent image excited at 365 nm of LED lamp.

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The three-dimensional (3D) PL emission spectra and contour lines of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors were measured in the excitation wavelength range of 220–420 nm, as depicted in Fig. S2(a) and (b), respectively. As presented in Fig. S2(b), the contour lines exhibit the characteristic emissions of Eu3+ ions originating from the 5D0 excited level to the 7F0, 7F1, 7F2, 7F3 and 7F4 ground states. Furthermore, both the 3D emission spectra and contour lines reveal the intense emission intensities between 270 and 400 nm excitation wavelengths, suggesting that the resultant products can be pumped by UV/NUV light which is in good agreement with the deduction achieved from the excitation spectrum. The result also confirms that the NUV LED chips are the efficient pumping sources for the La2MoO6-La2WO6:2xEu3+ red-emitting phosphors which make them suitable for solid-state lighting. In order to expound the involved luminescent mechanism in the La2MoO6-La2WO6:2xEu3+ system, the simplified energy level diagram as well as the proposed luminescent processes is displayed in Fig. 3(b). In brief, upon UV/NUV light excitation, the incident photons are absorbed by the host lattices. Subsequently, the energy is transferred to the adjacent Eu3+ ions and the 5L6 level is populated. Then, the electrons located at the 5L6 level decay to the 5D0 excited level by means of nonradiative (NR) transition. Ultimately, the emissions of Eu3+ ions are generated through the radiative transitions of 5D0 → 7F J (J = 0, 1, 2, 3, 4), as described in Fig. 3(b).

As we know, the luminescent performance of the RE ions activated materials is greatly dependent on the dopant concentration. For the purpose of exploring the optimal doping concentration of Eu3+ ions in the La2MoO6-La2WO6 host lattices, a series of Eu3+-activated La2MoO6-La2WO6 compounds were prepared and their luminescent behaviors were studied in detail. Figure 3(c) describes the PL emission spectra of the La2MoO6-La2WO6:2xEu3+ samples as a function of Eu3+ ion concentration. Obviously, all of the compounds emit the specific emissions of Eu3+ ions and the spectral emission profiles are barely changed with raising the doping concentration except the emission intensity. From the doping concentration-dependent PL emission intensity curve, as demonstrated in Fig. S3(a), it is clear that the emission intensity increases sharply with the increment of Eu3+ ion concentration and the optimum doping concentration is found to be 12 mol%. However, with further addition of the Eu3+ ions, the concentration quenching phenomenon occurs, which is associated with NR energy transfer among the dopants. Generally, the NR energy transfer among the dopants can be realized by means of radiation reabsorption and electric multipolar interaction. As presented in Fig. 3(a), there are no any overlaps between the excitation and emission spectra, revealing that the NR energy transfer among the Eu3+ ions is not caused by the radiation reabsorption. Herein, the NR energy transfer among the Eu3+ ions should be controlled by electric multipolar interaction. On the basis of Dexter theory, the following expression is given48:

$$\frac{I}{x}=k{[1+\beta {(x)}^{\theta /3}]}^{-1},$$
(4)

where I is the emission intensity, x denotes the doping concentration, k and β are constants, and θ = 6, 8 and 10 are related to dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interactions, respectively. The plot of log(I/x) versus log(x) is shown in Fig. S3(b). As displayed, the experimental data can be linearly fitted with a slope of −1.85 and thus, the θ value is calculated to be 5.55 which approaches to 6, confirming that the concentration quenching of Eu3+ ions in the La2MoO6-La2WO6:2xEu3+ compounds is dominated by dipole-dipole interaction.

The Commission International de I’Eclairage (CIE) chromaticity coordinate of Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors with optimal doping concentration was calculated, as described in Fig. 3(d). The estimated CIE chromaticity coordinate (0.657, 0.342) is located in the edge of red region which is very close to the ideal red light (0.670, 0.330), and outclasses that of the commercial Y2O2S:Eu3+ (0.622, 0.351) red-emitting phosphors. Meanwhile, under the irradiation of NUV light, the synthesized samples emit dazzling red emissions (see the inset of Fig. 3(d)). Apart from the color coordinate, the color purity is another essential parameter to evaluate the chromatic properties of the resultant phosphors which can be defined as12,42:

$$Color\,\,purity=\frac{\sqrt{{(x-{x}_{i})}^{2}+{(y-{y}_{i})}^{2}}}{\sqrt{{({x}_{d}-{x}_{i})}^{2}+{({y}_{d}-{y}_{i})}^{2}}}\times 100 \% ,$$
(5)

where (x, y), (x i, y i) and (x d , y d ) denote the color coordinates of the studied samples, white illumination and dominate wavelength, respectively. Herein, (x, y) = (0.657, 0.342), (x i, y i) = (0.310, 0.316) and (x d , y d ) = (0.672, 0.328). As a consequence, the color purity of red emission is determined to be as high as 96.1% which is much superior to previous reports, such as CaMo0.6W0.4O4:Eu3+ (93.8%) and SrMoO4:Eu3+ (85.8%)12,49. These results demonstrate that the Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors with high color purity, good color coordinate as well as superior luminescent properties may have promising applications in solid-state lighting.

For the sake of understanding the luminescence dynamics, the room-temperature decay curves of the final products were recorded. Figure 4(a) shows the representative decay curves of the La2MoO6-La2WO6:2xEu3+ (x = 0.02, 0.08, 0.12, 0.16 and 0.18) red-emitting phosphors (λex = 379 nm, λem = 612 nm). As presented, the recorded curves can be perfectly fitted with a single-exponential decay model, as defined below:

$$I(t)={I}_{0}+A\,\exp (-t/\tau ),$$
(6)

where I(t) and I 0 refer to the emission intensities at times t and t = 0, A is constant and τ denotes the lifetime. According to the fitting results, the measured lifetime is found to be 469, 444, 395, 381 and 362 μs, respectively when the Eu3+ ion concentration is 2, 8, 12, 16 and 18 mol%. Clearly, the decay time exhibits a tendency of decrease with the increase of doping concentration, suggesting the existence of NR energy transfer and concentration quenching in Eu3+ ions-based compounds. As is known, with the introduction of Eu3+ ions, the distance between the dopants will be decreased, leading to the enhanced NR energy transfer possibility between the Eu3+ ions as well as the decreased lifetime.

Figure 4
figure 4

(a) Representative decay curves of the La2MoO6-La2WO6:2xEu3+ (x = 0.02, 0.08, 0.12, 0.16 and 0.18) red-emitting phosphors. (b) PL emission intensity as a function of temperature. (c) Plot of ln(I 0/I−1) versus 1/kT. Inset shows the possible channels for the thermal quenching behavior of the studied samples.

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For verifying the feasibility of the synthesized phosphors for solid-state lighting application, their thermal stability should be evaluated since it can vastly affect the light output, working lifetime and CRI of the LED device. Under 379 nm light excitation, the temperature-dependent PL emission spectra of La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors were measured as presented in Fig. S4. It can be seen that the emission peaks scarcely changed with raising the temperature from 303 to 483 K which is beneficial to achieve stable emission color at high temperature. In comparison, the emission intensity decreases rapidly with the increment of temperature owing to the NR phonon relaxation from the high populated energy level via crossover process and the possible channels for the thermal quenching are shown in the inset of Fig. 4(b) 25. Furthermore, the relatively small optical band gap may also be reasonable for the sharply deceased emission intensity with the elevated temperature. The thermal quenching temperature (T0.5), defined as the temperature at which the emission intensity decreases to half of its initial value, is determined to be about 338 K (see Fig. 4(b)). In order to better understand the thermal quenching phenomenon, the following expression is employed to evaluate the activation energy50,51:

$$\mathrm{ln}(\frac{{I}_{0}}{I}-1)=\,\mathrm{ln}\,A-\frac{{\rm{\Delta }}E}{kT}.$$
(7)

In this expression, I 0 is the initial emission intensity, I refers to the emission intensity at different temperatures, A is constant, k is Boltzmann constant and ∆E is the activation energy from 5D0 level to the CT band. According to the plot of ln(I 0/I−1) versus 1/kT, as presented in Fig. 4(c), the recorded data are linearly fitted with a slope of −0.472. Therefore, the activation energy for the thermal quenching is determined to be 0.472 eV.

Apart from the chromatic properties, color purity and thermal stability, the quantum efficiency of the resultant phosphors is another vital factor to identify their suitability for solid-state lighting application. As is known, the quantum efficiency of the Eu3+ ions activated phosphors can be estimated by means of the emission spectrum and lifetime, which was proposed by Kodaira et al. and subsequently applied by other researchers52,53,54. According to the previous report by Kodaira et al., the following expressions can be achieved52:

$${A}_{0-J}={A}_{0-1}\frac{{I}_{0-J}}{{I}_{0-1}}\frac{h{v}_{0-1}}{h{v}_{0-J}},$$
(8)
$${A}_{rad}=\sum _{J=0,1,2,3,4}{A}_{0-J},$$
(9)
$$\frac{1}{\tau }={A}_{rad}+{A}_{nrad},$$
(10)
$$\eta =\frac{{A}_{rad}}{{A}_{rad}+{A}_{nrad}}.$$
(11)

In these expressions, A 0-J stands for the Einstein coefficient of spontaneous emission corresponding to the 5D0 → 7F J transitions. A rad and A nrad show the radiative and non-radiative transition rates, respectively, I 0-J is the integrated emission intensities of 5D0 → 7F J transitions, hv 0-J exhibits the energy of the 5D0 → 7F J transitions and τ is the decay time. Since the 5D0 → 7F1 transition belongs to the magnetic transition and it is independent of the crystals field55, the value of A 0-1 is determined to be approximately 50 s−1. From the recorded decay curve (see Fig. 4(a)), one knows that the lifetime of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors is 395 μs. As a result, with the help of Eqs (8–11), the quantum efficiency of the Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors with the optimal doping concentration is calculated to be 27.7%. It is evident that the achieved value is comparable with other reported red-emitting phosphors, in which their quantum efficiencies were also estimated by utilizing the above method, such as (Ca,Sr)(Mo,W)O4:Eu3+ (28.6%), Ca3Sn3Nb2O14:Eu3+ (30.29%), La2W1.6Mo0.4O9:Eu3+ (22.38%) and SrNb2O6:Eu3+ (17.84%)56,57,58,59, further implying that the resultant red-emitting phosphors are promising for WLEDs. Additionally, to further understand the optical performance of the resultant phosphors, the quantum efficiency of the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors was measured. Under 379 nm of excitation, the quantum efficiency of the studied phosphors is determined to be about 8.9% which is smaller than the theoretical value (27.7%). Similar phenomenon was reported in Eu3+-activated KYP2O7 red-emitting phosphors54. As is known, the quantum efficiency is measured for a special excitation wavelength, while the theoretically estimated quantum efficiency is greatly dependent on the A 0–1 parameter which has to be fixed at a reasonable value of the characteristics of the Eu3+ ions in similar compounds54, resulting in the difference between the measured quantum efficiency and theoretically calculated quantum efficiency.

To better confirm the suitability of the resultant phosphors for solid-state lighting application, a red-emitting LED device was fabricated by coating the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors onto the surface of a NUV LED chip and its EL emission spectrum is shown in Fig. 5S(a). As disclosed, the EL emission spectrum consists of an intense peak located at around 412 nm originating from the NUV LED chip and several sharp emission bands in the wavelength range of 575–725 nm corresponding to the featured emissions of Eu3+ ions. Meanwhile, under a forward bias current of 50 mA, the fabricated LED device emits glaring red emission that can be seen by naked eyes (see Fig. S5(b) and (c)), suggesting that the Eu3+-activated La2MoO6-La2WO6 phosphors are promising candidates for red-emitting phosphors for indoor lighting. As a proof of the above deduction, a WLED device was prepared by integrating an NUV LED chip and a mixture of BaMgAl10O17:Eu2+ (BAM:Eu2+) blue-emitting phosphors, (Ba,Sr)2SiO4:Eu2+ (BaSrSi:Eu2+) green-emitting phosphors and La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors. The EL emission spectrum of the fabricated WLED device, which was driven by a forward bias current of 50 mA, was recorded, as shown in Fig. 5(a). Clearly, the EL emission spectrum can be divided into four parts, that is, an emission peak situated at 412 nm arising from the NUV LED chip, two broad emission bands centered at 461 and 521 nm originating from the commercial blue-emitting and green-emitting phosphors, respectively, and a series of narrow emission peaks are attributed to the characteristic emissions of Eu3+ ions in the La2MoO6-La2WO6:0.24Eu3+ red-emitting phosphors. Furthermore, the CCT and CRI values of the designed WLEDs device are 6492 K and 84.6, respectively. Figure 5(b) shows the fully packaged WLED device. Under different forward bias currents, the packaged WLEDs device emits dazzling white light and the emitting color barely changes with increasing the forward bias current, as displayed in Fig. 5(c–e). These characteristics make the Eu3+-activated La2MoO6-La2WO6 phosphors suitable for WLEDs as red-emitting phosphors.

Figure 5
figure 5

(a) EL emission spectrum of the fabricated WLED device under different operating currents. (b) Fabricated WLED device. (c–e) Digital luminescent images of the WLED device at different input currents of 30, 40 and 50 mA, respectively. Inset shows the schematic diagrams of the packaged WLED device.

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Conclusion

In summary, a series of novel Eu3+-activated La2MoO6-La2WO6 red-emitting phosphors were synthesized via a facile citrate-assisted sol-gel route. The resultant compounds possessed an ultrabroad intense excitation band from 220 to 450 nm and emitted glaring red emission with high color purity of 96.1% under 379 nm light excitation. The Eu3+ doping concentration in La2MoO6-La2WO6 host lattices was optimized as 12 mol% and the dipole-dipole interaction is revealed to contribute to the concentration quenching. Both the emission spectrum and theoretical calculation confirm that the Eu3+ ions occupy the low symmetry sites in the host lattices. The thermal stability of the prepared phosphors was characterized by temperature-dependent emission spectra and the activation energy is found to be 0.472 eV. The red-emitting LED device, which is made only with La2MoO6-La2MoO6:0.24Eu3+ phosphors, demonstrates that the synthesized compounds are suitable for indoor lighting applications. Additionally, the WLEDs packaged by a NUV LED chip, La2MoO6-La2MoO6:0.24Eu3+ red-emitting phosphors, commercial blue-emitting and green-emitting phosphors emitted dazzling white light with high CRI (84.6) and proper CCT (6492 K), further indicating that the Eu3+-activated La2MoO6-La2WO6 compounds are promising red-emitting phosphors for solid-state lighting.

Experimental Section

Materials and Synthesis

The citrate-assisted sol-gel technique was employed to synthesize the Eu3+-activated La2MoO6-La2WO6 (La2MoO6-La2WO6:2xEu3+; x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16 and 0.18) red-emitting phosphors. Stoichiometric amounts of La(NO3)3∙6H2O, (NH4)6Mo7O24∙4H2O, Na2WO4∙2H2O and Eu(NO3)3∙5H2O were weighted and dissolved into 200 ml of de-ionized water to form a transparent homogenous solution. Subsequently, citric acid acting as a complexing agent was added into the above mixture. Then, the solution was covered with a polyethylene lid and heated at 80 °C for 30 min with drastic mechanical stirring. Afterwards, the lid was removed from the beaker and the solution was gradually evaporated to generate a yellow wet-gel. Later, the wet-gel was dried in oven at 120 °C for 12 h, and the xerogel was obtained. Ultimately, the xerogel was put into an alumina crucible and calcined at 850 °C for 6 h to form the Eu3+-activated La2MoO6-La2WO6 phosphors.

Material Characterization

The phase compositions of the studied samples were examined on a Bruker D8 Advance diffractometer. The morphological behaviors and elemental composition of the synthesized compounds were characterized by using a field-emission scanning electron microscope (FE-SEM) (LEO SUPRA 55, Carl Zeiss) equipped with an energy-dispersive X-ray (EDX) spectrometer. The luminescent spectra of the phosphors were recorded by utilizing a fluorescence spectrometer (Scinco FluroMate FS-2). The spectrofluorometer (Edinburgh FS5) attached with an integrating sphere coated with barium sulfate was employed to measure the quantum efficiency. The temperature ranging from 303 to 483 K was controlled by a temperature controlled stage (NOVA ST540). The Fourier transform infrared (FTIR) and diffuse reflectance spectra were recorded by using a Thermo Nicolet-570 FTIR spectrophotometer and V-670 (JASCO) UV-vis spectrophotometer, respectively. Under a forward bias current of 50 mA, the multi-channel spectroradiometer (OL 770) was used to monitor the electroluminescence (EL) spectrum.