As a high energy efficiency, ecofriendly, long-lifetime, and compact alternative to traditional lighting sources, solid-state lighting (SSL) in form of white light-emitting diodes (white LEDs) has received considerable attention. Nowadays, the phosphor-converted white LEDs (pc-white-LEDs) fabricated using a blue GaN chip or near-UV LED (370–420 nm) coupled with red, green, and blue phosphors are becoming the most attractive SSL devices because of its continuing improvement in the efficiency.Reference Schubert and Kim^1–Reference Hashimoto, Wu, Speck and Nakamura⁵ The eventual performance of pc-white-LEDs devices strongly depends on the luminescence properties of the phosphors used. Therefore, the state-of-the-art SSL devices would be notably advanced by the discovery of phosphor materials that are optimized for pc-white-LEDs. The most frequently used red-emitting phosphor for near-UV LEDs is Y₂O₂S:Eu³⁺. However, Y₂O₂S:Eu³⁺ has poor absorption in near-UV region where LED chips emission occurs. A good strategy to overcome this problem is to search for a host lattice that can absorb energy efficiently in the near-UV region and transfer the energy to the activator, resulting in emission in the red region (host-sensitized emission).Reference Neeraj, Kijima and Cheetham^6, Reference Neeraj, Kijima and Cheetham⁷ In view of practical applications, improvement of luminescence of phosphors is of great importance, especially for pc-white-LEDs while retaining the high quantum yield and high luminescence quenching temperature. Currently, modification of phosphor composition by chemical substitution is a highly effective strategy for the designing of new materials and improving luminescent properties of already known compounds.Reference Nyman, Rodriguez, Shea-Rohwer, Martin and Provencio^8–Reference Xie, Yuan, Hai, Wang, Wang and Li¹²

Lanthanum molybdate (La₂Mo₂O₉) has ever been studied as catalyst, fast oxide-ion conductor, and ferroelectric material.Reference Lacorre, Goutenoire, Bohnke, Retoux and Laligant^13–Reference Kim and Kang¹⁵ Marrero-López et al. reported for the first time the photoluminescence properties of Eu³⁺-doped La₂Mo₂O₉ phosphor.Reference Marrero-López, Núñez, Abril, Lavín, Rodríguez-Mendoza and Rodríguez¹⁶ Recently, the development of SSL has lead to a rebirth of interest in this material because it can be used as a host for optically active lanthanide ions.Reference Li, Yang, Guan, Liu and Li^17, Reference Gong, Shi and Zhou¹⁸ The effects of flux on structure, morphology, and luminescent properties of La₂Mo₂O₉:Eu³⁺ were investigated by Li et al.Reference Li, Yang, Guan, Liu and Li¹⁷ Gong et al. found that the emission intensity of La₂Mo₂O₉:Eu³⁺ can be strengthened by doping small amounts of rare earth (Gd, Dy, Y, and Yb) ions.Reference Gong, Shi and Zhou¹⁸ However, there is no report on the luminescent enhancement of Eu³⁺-activated La₂Mo₂O₉ red-emitting phosphor through chemical substitution. The aim of the present study is to explore the varying composition and attempt to improve luminescence intensity of Eu³⁺-activated La₂Mo₂O₉ red-emitting phosphor. Herein, a series of novel red-emitting phosphors with nominal compositions La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) with strong near-UV absorption were prepared by solid-state process. The enhanced luminescence of La₂Mo₂O₉:0.05Eu³⁺ by substituting Mo⁶⁺ with Si⁴⁺ was investigated.

La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) sample phosphors were prepared by a typical solid-state reaction. The starting materials La₂O₃ (99.99%), MoO₃ (99.5%), SiO₂ (99.90%), and Eu₂O₃ (99.99%) were purchased from Shanghai Chemical Reagent Ltd. Co., China, and used without any further purification. The required stoichiometric amounts of these materials were weighed and then thoroughly wet mixed in an agate mortar with acetone as the wetting medium. Prior to calcinations, the mixing samples were dried in an air oven. The obtained mixture was heated at 550 °C for 4 h in a muffle furnace, followed by the calcination at 900 °C for 8 h. Finally, the samples are ground into powder for characterizations. For obtaining the samples with almost the same size and narrow particle distribution, we separated powders by particle sizes using the multilevel sieves after above route,Reference Nazarov¹⁹ with the aim to avoid the differences in luminescence intensities due to the size effects.

The samples were characterized by x-ray powder diffraction (XRD), UV–vis diffuse reflectivity, and photoluminescence spectrum. XRD was carried out with a Japan Rigaku D/max 2500 diffractometer, using Ni-filtered Cu K_α radiation. A scan rate of 0.02°/s was applied to record the patterns in the 2θ range 10–90°. UV–vis diffuse reflectivity spectra of phosphors were measured with Cary 500 UV–vis–NIR spectrophotometer using a polytetrafluoroethylene standard as reference. The excitation and emission spectra of powders were recorded using fluorescence spectrofluorometer (Varain Cary-Eclipse). For the comparison of excitation and emission intensity, the quantity of the phosphor samples has been normalized and measurement conditions (i.e., the packing density of samples, width of slit of the excitation and emission monochromators, the photomultiplier tube detector voltage, and sensitivity) were kept consistent from sample to sample in the measurements. All the measurements were performed at room temperature.

The body colors of as-obtained phosphor powders are white. La₂Mo₂O₉ has two different structures: α- La₂Mo₂O₉ and β- La₂Mo₂O₉, which belong to the monoclinic phase and the cubic phase, respectively. XRD patterns of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) series phosphors are presented in Fig. 1. As the Si⁴⁺ contents (i.e., z value) increase, XRD profiles are similar without showing discernable shifting. No impurity phases related to the starting materials and intermediates were found, clearly indicating that the compounds formed are of single phase. This also illuminates that the effective incorporation of Si⁴⁺ into host lattice have been achieved. Si⁴⁺ was supposed to occupy the Mo⁶⁺ sites in this phosphor because of the almost identical ionic radii between Mo⁶⁺ (0.041 nm) and Si⁴⁺ (0.040 nm). All diffraction peaks can be indexed to the cubic phase La₂Mo₂O₉, which is in agreement with JCPDS card no. 28-0509. The strong and sharp diffraction peaks suggest that these samples are highly crystallized which is advantageous to luminescence.

FIG. 1.

X-ray powder diffraction patterns of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) phosphors.

UV–vis diffuse reflectivity spectra of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) phosphors are represented in Fig. 2. All samples show a broad absorption band in the near-UV region with the steep absorption edges in the range of 350–430 nm. With increasing of Si⁴⁺ doping content, the absorption band tends to shift toward longer wave length, i.e., the optical band gap energy (E _g) decreased, which also indicates the successful incorporation of Si⁴⁺ into host lattice and is in agreement with the XRD results. It is well established that there is a direct dependence between the E _g values and the presence of intermediary energy levels within the band gap.Reference Hank, Barnes, Benjamin and Patrick²⁰ Generally, the origin of these energy states is caused by the formation of oxygen vacancies into the structure. For La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ phosphors, the increasing of Si⁴⁺ content (i.e., z value) induces the increase of oxygen vacancies and consequently increasing the concentration of intermediary electronic states within band gap and decreasing the E _g value. Furthermore, the shoulder band beside the absorption edge for the Si⁴⁺-containing phosphors was then observed in the long wave length side, especially in the case of z = 0.03. However, the shoulder band was not observed in the phosphor without Si⁴⁺-doping. These results indicated that this band is related to introduction of Si⁴⁺ into La₂Mo₂O₉:Eu³⁺ lattice. Based on the above results, it can be concluded that substitution of Si⁴⁺ for Mo⁶⁺ into host lattice can modulate the band structure of La₂Mo₂O₉:Eu³⁺ phosphors by generating oxygen vacancies.

FIG. 2.

UV–vis diffuse reflectivity spectra of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) phosphors.

It was reported that the red emission of CaMoO₄:Eu³⁺ phosphor can be enhanced by substituting Mo⁶⁺ with Si⁴⁺.Reference Ci, Wang, Zhang and Sun^11, Reference Xie, Yuan, Hai, Wang, Wang and Li¹² A similar improvement in luminescence of LiEuMo₂O₈ phosphor by introduction of Si⁴⁺ ion was observed by Dai et al.Reference Dai, Zhang, Li, Wang, Zhao and Liu²¹ To improve luminescence of La₂Mo₂O₉:0.05Eu³⁺ phosphor, Si⁴⁺ was introduced into host lattice as partial substitution for Mo⁶⁺. Figure 3 depicts that the excitation spectra of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) phosphors monitored the ⁵D₀ → ⁷F₂ transition of Eu³⁺ at 615 nm. The spectrum is comprised of a strong broadband from 250 to 430 nm and several sharp peaks at 395, 465, and 533 nm. The strong broad band with peak at 350 nm is assigned to charge transfer (CT) transitions of O²⁻–Eu³⁺ and O²–Mo⁶⁺. Sharp lines at 395, 465, and 533 nm can be ascribed to ⁷F₀ → ⁵L₆, ⁷F₀ → ⁵D₂, and ⁷F₀ → ⁵D₁ transitions of Eu³⁺ ions, respectively. The intensity of CT band was comparable to that of 4f–4f transition of Eu³⁺ ions. The presence of the strong broadband in the excitation spectrum indicates that there exists an energy transfer from host to Eu³⁺ ions. With increasing of Si⁴⁺ content (i.e., z value), both of the CT band and sharp peaks gradually increase and reach a maximum when the doping concentration of Si⁴⁺ is 0.03 mol. In addition, the CT band is considerably broadened and red-shifted, which are consistent with those observed in the above UV–vis spectra (shown in Fig. 2). The enhancement of CT band indicated that the Si⁴⁺ incorporation improved the host-sensitized emission of Eu³⁺. When the z value is bigger than 0.03, the excitation intensities of CT band and sharp peaks reduce, which may be due to excess defects formed by the introduction of superfluous Si⁴⁺ ions.Reference Ci, Wang, Zhang and Sun¹¹

FIG. 3.

Excitation spectra of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) phosphors (λ_em = 615 nm).

The emission spectra of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) phosphors upon excitation with CT band are depicted in Fig. 4. The emission spectra contain some sharp peaks at 535, 554, 587, 615, and 700 nm, which are assigned to ⁵D₁ → ⁷F₁, ⁵D₁ → ⁷F₂, ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F_2, and ⁵D₀ → ⁷F₄ transition of Eu³⁺, respectively. However, no emission corresponding to host is observed under excitation of CT band. The phosphors exhibit characteristic emission of Eu³⁺, which clearly indicates the energy transfer from host to Eu³⁺ ions. With increasing of Si⁴⁺ content (i.e., z value), the emission intensity increases and reaches a maximum when doping concentration of Si⁴⁺ is 0.03 mol. However, when the Si⁴⁺ concentration is higher than 0.03 mol, the luminescent intensity decreases.

FIG. 4.

Emission spectra of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) phosphors under excitation of CT band (λ_ex = 350 nm).

The reasons for the enhanced luminescence under excitation of CT band by Si⁴⁺ doping may be attributed to the following aspects. As indicated above, Si⁴⁺ was supposed to occupy the Mo⁶⁺ site in host because of the almost identical ionic radii between Mo⁶⁺ and Si⁴⁺. Si⁴⁺ ions with low valence occupying Mo⁶⁺ sites in the lattices generate extra negative charges, (Si_M0)^··, which are supposed to generate oxygen vacancies, (V_O)^·· for charge neutrality. The presence of the oxygen vacancies generated by the introduction of Si⁴⁺ can promote the energy transfer from excited carriers in host lattice to Eu³⁺.Reference Ryu, Singh, Bartwal, Brik and Kityk^22–Reference Wang, Jiang, Shen, Fang, Zheng, Wu and Shen²⁴ However, when the Si⁴⁺ content exceeds a certain amount, excessive oxygen vacancies will be created, which leads to the luminescence quenching.Reference Ci, Wang, Zhang and Sun¹¹

Figure 5 shows the emission spectra of Si⁴⁺-doped series samples upon excitation with direct 4f–4f transition of Eu³⁺ ions. The emission spectra were also dominated by 615-nm peak due to ⁵D₀ → ⁷F₂ transition of Eu³⁺, which clearly implies that Eu³⁺ occupies a noncentrosymmetric site. With increasing of Si⁴⁺ content, the luminescence enhancement was accompanied by the further splitting of ⁵D₀ → ⁷F₂ transition. While z value reaches 0.10, two peaks located at 615 and 620 nm due to ⁵D₀ → ⁷F₂ transition of Eu³⁺ can be observed. This hints that the local symmetry around Eu³⁺ ions was to some extent lowered by the incorporation of Si⁴⁺.²⁵ Hence, it is deduced that more odd rank crystal field components are induced, and more opposite parity components (4f⁵5d states) are mixed into 4f⁶ transitional levels of Eu³⁺, which leads to the enhancement of ⁵D₀ → ⁷F₂ transition.Reference Gong, Shi and Zhou¹⁸ Consequently, the partial substitution of Mo⁶⁺ by Si⁴⁺ increases the fluorescence of the as-obtained phosphors.

FIG. 5.

Emission spectra of La₂(Mo_1−zSi_z)₂O₉:0.05Eu³⁺ (0 ≤ z ≤ 0.10) phosphors upon excitation with direct 4f–4f transition of Eu³⁺ ions (λ_ex = 465 nm).

Eu³⁺-activated La₂(Mo_1−zSi_z)₂O₉ (0 ≤ z ≤ 0.10) phosphors were prepared by high-temperature solid-state reaction. A significant enhancement of red photoluminescence has been demonstrated for La₂Mo₂O₉:0.05Eu³⁺ phosphor by chemical substitution. XRD results show that Si⁴⁺ was effectively incorporated into host lattice and the obtained samples are of single phase. The excitation spectrum included a strong broadband ranging from 250 to 430 nm due to the CT transitions of O²⁻–Eu³⁺ and O²–Mo⁶⁺, and sharp peaks at 395, 465, and 533 nm corresponding to intra-4f transitions of Eu³⁺. Upon excitation with near-UV and blue light, the as-obtained phosphors exhibited a satisfactory red emission peaked at 615 nm. Partially replacing Mo⁶⁺ with Si⁴⁺ can enhance the red emission under excitation of CT band as well as direct 4f–4f excitation-mediated luminescence. The former is mainly attributed to the oxygen vacancies as sensitizer for the effective energy transfer from host to Eu³⁺, while the latter may be result from the relatively lower local symmetry around Eu³⁺ ions. Our results show that the chemical substitution is of great significance for the development of highly efficient phosphors used in solid-state lighting equipment.