Synthesis and Properties of Rare-earth Doped Phosphors for White Light Emitting Diodes (WLEDs) Application
|Keywords||White LED Doping Rare earth phosphor Li2SrSiO4 ZnMoO4|
White light-emitting diodes (WLEDs) have been attracted more attention because they have advantages of high luminescent efficiency, long lifetime, reliability, energy savings and small packaging. As for general lighting applications, the luminous efficiency, colorimetric parameters and price of LEDs are of great importance and depend on LED chips used and the phosphors coated on the chips. In our present study, we paid attention to the synthesis of novel phosphors for WLEDs application via blue or near-UV LEDs.Therefore, Li2SrSiO4:Eu2+and ZnMoO4:Eu3+ phosphors were selected as research objects for enhancing luminescence intensity and tuning excitation and emission wavelenghth by doping.Li2Sr0.995SiO4:0.005Eu2+is a yellow phosphor with intense emission peak at 580 nm under excitation of blue light (400-480 nm). Our results showed that co-doping PO43- or SO42- into Li2Sr0.995SiO4:0.005Eu2+enhanced the fluorescence intensity of as-synthesized phosphors. The optimum compositions were determined to be Li2Sr0.995(SiO4)0.9775(P04)0.03:0.005Eu2+and Li2Sr0.995(SiO4)0.945(SO4)0.11:0.005Eu2+ Their luminescent intensity under excitation of 460 nm blue light showed 1.8 or 2.2 times higher than that of Li2Sr0.995SiO4:0.005Eu2+. In addition, the incorporation of SO42- ions also tuned the emission peak from 580 nm to 610 nm and excitation peak from 410nm to 460nm, indicating a novel route to synthesize red phosphors in LED application via blue LED chips. On the other hand, red phosphors under blue light excitation have been synthesized by co-doping Pr3+ into Li2Sr0.995SiO4:0.005Eu2+. The intense emission at about 610 nm is attributed to 3P0-3H6 and 1D2-3H4 transition of Pr3+. Furthermore, we synthesized Pr3+ doped Li2SrSiO4 red phosphors for the first time and demonstrated that these phosphors can be effectively excited by a blue LED chip and emit strong red light, promising that they are important candidates as blue-convertible phosphors for white light-emitting diodes.ZnMoO4:Eu3+ phosphors can emit intense red light at 615 nm when excited by near-UV (about 395 nm) and blue (about 466 nm) light. In order to enhance their luminescent intensity, we doped them with different amount of Li+as charge compensator. The luminescent intensity of as-synthesized phosphors showed that the doping of Li+can obviously improve the emission intensity of Eu3+doped ZnMoO4, and the optimum composition was found to be Zn0.5MoO4:Eu3+0.25Li+0.25·Furthermore, we co-doped Zn0.5MoO4:Eu3+0.25Li+0.25 with different amount of Bi3+, Sm3+or La3+, Y3+, Gd3+for the purpose of of enhancing luminescent intensity and widening excitation band. The luminescence property of as-synthesized phosphors proved that the doping of Bi3+, Sm3+or La3+, Y3+, Gd3+is beneficial for the enhencement of luminescent intensity. Under 395 nm or 466 nm light excitation, the luminescent intensities of as-synthesized phosphors with optimum composition, Zn0.454MoO4:Eu3+0.25Li+0.25Bi3+0.04Sm3+0.006 and Zn0.44MoO4:Eu3+0.25Li+0.25Gd3+0.06 are all over two times higher than that of Zn0.75MoO4:Eu3+0.25. At the same time, the excitation peak at around 395 nm was widened by the doping of Sm3+. These facts indicate that the as-synthesized phosphors promise a potential for the application of light converting to both near-UV and blue LED chips in the fabrications of white light-emitting diodes.