宽带绿色荧光粉(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6)的制备、光致发光及性能调控

Preparation,Photoluminescence and Properties Regulation of Broad Band Green Phosphors(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6)

宽带绿光荧光粉在克服荧光粉转换白光LED(pc-WLED)蓝绿(青色)470~510 nm波段的缺失问题,实现高显色指数白光照明中有重要应用。采用普适的高温固相法合成一种以Bi^(3+)为激活剂的发射宽带绿光荧光粉(Lu_(0.99)Bi_(0.01))_(2)WO_(6)。采用共掺杂La^(3+)的策略对(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6)(x=0~0.25)系列荧光粉的物相、形貌及发光性能进行调控,并采用X射线衍射(XRD)、Rietveld精修、场发射扫描电子显微镜(FE-SEM)、X射线光电子能谱(XPS)及光致发光等检测手段对样品进行了研究。结果表明,La^(3+)掺杂可显著提高(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6)样品的结晶性(x≤0.10)及发光强度,并在x>0.20时开始发生相变。La^(3+)掺杂对样品的微观形貌有显著影响,得益于La^(3+)掺杂获得了尺寸均匀分散性较好的微米棒形貌。在346 nm波长激发下,(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6)样品在400~800 nm范围内呈现出宽带绿色发射,半峰宽(FWHM)可达135 nm,最强峰位于515 nm处,对应于Bi3+的3P1→1S0跃迁。荧光粉呈现宽带激发,激发范围覆盖了200~400 nm的波长区域,与商用紫外LED匹配良好。采用所得的宽带绿光荧光粉(Lu_(0.94)La_(0.05)Bi_(0.01))_(2)WO_(6)与商用CaAlSiN_(3):Eu^(2+)红色荧光粉、BaMgAl10O17:Eu2+蓝色荧光粉及365 nm商用芯片制作出具有较高的显色指数(R,Ra=93,R1~R15均大于80),较低色温(相关色温(CCT)=3928 K)的暖白光pc-LED器件,表明(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6)荧光粉在高质量的暖白光LED照明方向有较大的应用潜力。

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:Ce^(3+)yellow phosphor on blue LED chip,or combine tricolor(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 component 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 becomes 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(pcWLED)and achieving high color rendering index white lighting.Bismuth is an element with multiple valence states(Bi^(0),Bi^(1+),Bi^(2+),Bi^(3+),Bi^(5+)).The electronic configuration of Bi^(3+)is[Xe]4f^(14)5d^(10)6s~2,which is relatively stable.The 6s electrons of Bi^(3+)are easily affected by the surrounding environment,so the emission and excitation positions of Bi^(3+)doped phosphors strongly depended on the covalence of the surrounding crystal field and the matrix lattice.Therefore,the tunable emission of Bi^(3+)can be precisely realized by adjusting the matrix composition and crystal structure.In this work,a broadband green phosphor(Lu_(0.99)Bi_(0.01))_(2)WO_(6) was synthesized by a high-temperature solid-state method,and co-doping La^(3+)was adopted to regulate the phase,morphology,and luminescence performance of(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6) 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 La^(3+)co-doping could significantly improve the crystallinity and luminescence intensity of(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6)(x≤0.10)samples,and phase transition happened when x>0.20.XRD patterns of(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6) samples were refined by Rietveld structure refinement using Total Pattern Solution(TOPAS),which further proved the purity of the samples.La^(3+)doping had a significant effect on the sample's microstructure,through which a micron rod morphology with uniform size and good dispersion was obtained.The surface chemical composition and the valence state of Bi in the prepared samples were also studied via XPS.All XPS spectral characteristics were attributed to the constituent elements in(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6) samples,and it was observed that the two peaks for the two lattices Bi _(4)f_(7/2)and Bi^(4)f_(5/2)were asymmetric due to the fact that Bi^(3+)was located in multiple polyhedra and the binding energies of Bi in different polyhedra were slightly different.Under the excitation of 346 nm,(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6) samples showed broadband green emission in the range of 400~800 nm.The strongest emission was at 515 nm,which corresponded to the ^(3)P_(1)→^(1)S_(0) transition of Bi^(3+),and the full width half maximum(FWHM)could reach 135 nm,well covering the cyan gap.At the same time,La^(3+)co-doping significantly improved the emission intensity of Bi^(3+).The enhanced luminescence of La^(3+)codoped(Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6) 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 La^(3+)doping concentration might be due to the generation of phase transition and the lattice expansion caused by the incorporation of large ion radius La^(3+),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 Bi^(3+)are the cause of broadband green emission.The phosphor showed broadband excitation 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((Lu_(0.99-x)La_(x)Bi_(0.01))_(2)WO_(6)),commercial CaAlSiN_(3):Eu^(2+)red phosphor,BaMgAl_(10)O_(17):Eu^(2+)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(R_a=93,R1~R15>80)and low color temperature(correlated color temperature(CCT)=3928 K),which showed that(Lu_(0.99-x)La_(x) Bi_(0.01))_(2)WO_(6) phosphor had great potential application in high-quality warm white LED lighting.