Synthesis And Luminescence Characteristics Of Red-emitting Molybdate Phosphors For White LED

Recently, the white LED preparation technology has rapidly being developing. With the widen applications and increased demands, the studies on the syntheses and luminescent properties of the phosphors for white LEDs have become a hot issue of research. Now white LEDs are gradually occupying the lighting industry because of their excellent properties, such as energy saving, no mercury pollution, high efficiency, short response time and miniaturization. Because they can preferably meet the present requirements to the lighting sources, white LEDs are expected to become the fourth generation “green” lighting source.Because of their excellent physical and chemical performance, tungstates and molybdates have caused extensive concern of scholars and frequently been used as the matrixes of luminescent materials. Molybdates phosphors can be prepared at lower temperature and have good chemical and thermal stability. The structures of molybdates generally are tetrahedral, in which Mo(VI) atoms coordinate with four O atoms, and matrix cations and rare earth ions are in the eight coordinated positions. Molybdates as the matrix can effectively absorb the energies of the light in near UV region and transfer the energies to the activator ions. The activator Eu3 + ion can emit very pure red light by using its 4f-4f transition.The red phosphors KR(Mo O4)2: Eu3+(R = La, Y),(Sr0.75Ba0.25)Mo O4 and K2R(Mo O4)(PO4): Eu3+(R = La, Lu, Gd) were synthesized by using the high temperature solid phase reaction. For the synthesized phosphors, the crystal structures were characterized, the luminescence properties studied, and the luminescent concentration quenching mechanisms and chromaticity analyzed. The results showed that the synthesized red phosphors have different crystal structures due to the differences between their respective matrixes. The doping of Eu3+ ions and the concentration changes of the doped Eu3 + ions did not influence the crystal structures of the samples. However, because there are the difference between the radius of the Eu3 + ion and that of the substituted ion, the diffraction peaks in XRD patterns of the synthesized phosphors shifted compared to those in the matrixes. With the increasing concentration of doped ions, the fluorescence intensity increased, but when the doping ion reach a certain concentration the concentration quenching occurs. The concentration quenching mechanisms of the phosphors were studied according to Dexter ’s concentration quenching theory. The synthesized red phosphors can be effectively excited by the light in near UV region and emit highly pure red light.