Preparation,Photoluminescence and Properties Regulation of Broad Band Green Phosphors(Lu0.99-xLaxBi0.01)2WO6
Phosphor conversion LEDs(pc-LEDs)have many advantages and have been widely used as green lighting sources.At present,the most common strategies to realize white lighting is to coat YAG∶Ce3+yellow phosphor on blue LED chip,or combine tricol-or(RGB)phosphor with near ultraviolet chip(nUV-LED).The chip production process is relatively mature,and the phosphor plays a key role in pc-LED,which largely determines the performance of the device.Due to the lack of a cyan(470～510 nm)spectral compo-nent between the PL spectra of blue and green phosphors,the obtained pc-WLEDs have a low color rendering index(CRI,70～80),which limits their wide applications in general lighting.Thus,developing ultraviolet or blue light excitable cyan-emitting phosphors be-comes a logical-led solution,and the color rendering index of a pc-LED device incorporating a cyan phosphor can indeed be improved in this way.However,using multiple phosphors will lead to fabrication complexity and reabsorption.Broadband green phosphors have important applications in overcoming the lack of blue-green(cyan)470～510 nm spectra range in phosphor-converted white LED(pc-WLED)and achieving high color rendering index white lighting.Bismuth is an element with multiple valence states(Bi0,Bi1+,Bi2+,Bi3+,Bi5+).The electronic configuration of Bi3+is[Xe]4f145d106s2,which is relatively stable.The 6s electrons of Bi3+are easily affected by the surrounding environment,so the emission and excitation positions of Bi3+doped phosphors strongly depended on the covalence of the surrounding crystal field and the matrix lattice.Therefore,the tunable emission of Bi3+can be precisely realized by adjusting the matrix composition and crystal structure.In this work,a broadband green phosphor(Lu0.99Bi0.01)2WO6 was synthesized by a high-tem-perature solid-state method,and co-doping La3+was adopted to regulate the phase,morphology,and luminescence performance of(Lu0.99-xLaxBi0.01)2WO6 phosphors.The phase,composition,and microstructure of the samples were analyzed by X-ray diffraction(XRD),field emission scanning electron microscopy(FE-SEM)and X-ray photoelectron spectroscopy(XPS),and the luminescence properties of the products were tested by fluorescence spectrophotometer.The warm white pc-LED with high color rendering index was prepared using the best performance phosphor.The results showed that La3+co-doping could significantly improve the crystallinity and luminescence intensity of(Lu0.99-xLaxBi0.01)2WO6(x≤0.10)samples,and phase transition happened when x＞0.20.XRD patterns of(Lu0.99-xLaxBi0.01)2WO6 samples were refined by Rietveld structure refinement using Total Pattern Solution(TOPAS),which further proved the purity of the samples.La3+doping had a significant effect on the sample's microstructure,through which a micron rod mor-phology with uniform size and good dispersion was obtained.The surface chemical composition and the valence state of Bi in the pre-pared samples were also studied via XPS.All XPS spectral characteristics were attributed to the constituent elements in(Lu0.99-xLax-Bi0.01)2WO6 samples,and it was observed that the two peaks for the two lattices Bi 4f7/2 and Bi 4f5/2 were asymmetric due to the fact that Bi3+was located in multiple polyhedra and the binding energies of Bi in different polyhedra were slightly different.Under the excitation of 346 nm,(Lu0.99-xLaxBi0.01)2WO6 samples showed broadband green emission in the range of 400～800 nm.The strongest emission was at 515 nm,which corresponded to the 3P1→1S0 transition of Bi3+,and the full width half maximum(FWHM)could reach 135 nm,well covering the cyan gap.At the same time,La3+co-doping significantly improved the emission intensity of Bi3+.The enhanced lumines-cence of La3+codoped(Lu0.99-xLaxBi0.01)2WO6 samples was due to the enhanced crystallinity of the samples.Because higher crystallinity would produce better luminescence intensity.The decrease of luminescence observed by further increasing La3+doping concentration might be due to the generation of phase transition and the lattice expansion caused by the incorporation of large ion radius La3+,and the decrease of stiffness,thus enhancing phonon vibration and phonon-photon coupling to a certain extent.Through multiple evidences,it was proved that the three luminescent centers of Bi3+are the cause of broadband green emission.The phosphor showed broadband exci-tation in the range of 200～400 nm,which corresponded well to commercial UV LED.A warm white pc-LED device was fabricated by using the obtained optimal emitting green phosphor((Lu0.94La0.05Bi0.01)2WO6),commercial CaAlSiN3∶Eu2+red phosphor,BaMgAl10O17∶Eu2+blue phosphor and 365 nm commercial chip.The performance of pc-WLED device was tested by OHSP-350M LED spectrometer.The results showed that the obtained white pc-LED device had high color rendering index(Ra=93,R1～R15＞80)and low color temper-ature(correlated color temperature(CCT)=3928 K),which showed that(Lu0.99-xLaxBi0.01)2WO6 phosphor had great potential applica-tion in high-quality warm white LED lighting.
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