Fabrication And Luminescent Properties Of ZnWO₄:RE³⁺(Sm³⁺,Eu³⁺,Dy³⁺) White Light-Emitting Phosphors For W-LED

White light-emitting diodes（W-LED） due to their excellent properties such as lower energy consumption, high luminous efficiency, long working lifetime and good reliability have been considered as the forth generation solid-state light sources, and widely used in indicator, neon lights, indoor decorative light, LCD monitor, etc. The commercial method to realize white light-emitting was based on the blue LED chips combined with YAG:Ce3+ yellow phosphor, which exhibit poor color rendering index and high correlated color temperature due to lack of red component. In order to achieve white light with high CRI and suitable CCT, the tri-color phosphors excited at near-UV light In Ga N chips were designed. However, these different phosphors exist the re-absorption and regulation problem. Therefore, in realization of white light emission, a single phased phosphor is of great significance. In this paper, through hydrothermal method, the single phased white light-emitting phosphors under UV light excitation were prepared by using ZnWO₄ as host material doping with rare earth ions. The samples were characterized by X-ray diffraction（XRD）, fourier transform infrared（FTIR） spectrum, raman spectra, scanning electron microscope（SEM）, transmission electron microscope（TEM）, diffused reflectance spectra, photoluminescence excitation and emission spectra. The effects of different synthesis condition, the doping concentration of rare earth ions, the surfactant and charge compensation agent on luminescence properties of phosphors were stimultaneously investigated. The main results are as follows:（1） ZnWO₄:Sm³⁺ white light emitting phosphors were successfully prepared via mild hydrothermal process with calcination. The p H, hydrothermal temperature and calcination temperature have a great influence on the phase structure and luminescent properties of the samples. The crystal structure of the as-synthesized ZnWO₄:Sm³⁺ is same as the pure monoclinic ZnWO₄ under the p H value of 6, hydrothermal temperature of 180 oC and calcination temperature of 700 oC. These samples are approximately spherical particles with the particles size from 100 to 220 nm. The characteristic emission peaks of ZnWO₄:Sm³⁺ phosphors reached the highest when the doping concentration of Sm³⁺ was 0.5%. However, the emission intensity decresased when the Sm³⁺ concentration is beyond it, and the concentration quenching occurs. The mechanism can be attributed to the dipole-dipole interaction and cross-relaxation（4G5/2 + 6H5/2 → 6H9/2 + 6H9/2） among Sm³⁺ ions. Under 254 nm excitation, the chromaticity coordinates of ZnWO₄:x%Sm³⁺（x=0.5, 1, 3） are located in cold white light zone and the colour render index are higher than 80.（2） The introduction of PEG into the reaction system has little effect on the phase structure of tha sample, and the as-synthesized ZnWO₄:Sm³⁺ samples still have monoclinic wolframite structure. But the dispersibility of the particles has been improved and the particle size becomes uniform. At the same time, the emission intensitiy of WO42-（blue-greenish emission） and characteristic emission intensities of Sm³⁺ are all improved remarkablely.（3） ZnWO₄:Eu³⁺,Dy³⁺ white light-emitting phosphors were prepared by a hydrothermal method followed with calcination process. The obtained phosphors have monoclinic wolframite structure. The particles of the phosphors were nearly spherical in shape, and the particle size is about 70-100 nm. Upon excitation at UV light, the emission spectra contains the blue-green emission of tungstate group, the red emission of Eu³⁺, and blue and yellow emission of Dy³⁺. With the Dy³⁺ concentration increasing, the chromaticity coordinates of Zn0.98-xWO4:0.02Eu³⁺,x Dy³⁺（x=0, 0.005, 0.015, 0.025, 0.03, 0.04） move toward ideal white light point, especially when the x=0.025, the chromaticity coordinate（x=0.3161,y=0.3304） is very close to the standard white light point. Based on the excitation/emission spectra and decay curves, the energy transfer and photoluminescence mechanism for ZnWO₄:Eu³⁺,Dy³⁺ system were discussed. Meanwhile, external quantum efficiency of ZnWO₄:Eu³⁺,Dy³⁺ was also determined.（4） Co-doped with Li+ compensator has little influence on the crystal structure of ZnWO₄:Eu³⁺,Dy³⁺ phosphors, but has a great influence on the emission intensites of the samples. With the Li+ concentration increasing, the characteristic emission intensity of Dy³⁺ increases, but the emission intensity of WO42- gradually decreases, and the characteristic emission intensity of Eu³⁺ is almost unchanged. The result indicated that the energy transfer efficiency from WO42- to Dy³⁺ has been improved through Li+ doping. When the Li+ concentration is 2.5%, the yellow emission intensity of Dy³⁺（576 nm） reaches the highest. It has been enhanced by 1.26 times compared with the un-doped sample. The emission color of phosphors can be tuned from the blue-white light zone to the yellowish-white zone with the Li+ concentration increasing.