Synthesis And Properties Of Highly Efficient Phosphor-converting Materials For Near Ultra Violet Light-emitting Diodes (NUV-LEDs)

The fabrication of white LED needs highly efficient green phosphor under the radiation of blue or NUV. In this paper, M₂SiO₄:Eu²⁺ and ZnO phosphors were synthesized and some methods for the enhancing of luminescent intensity were investigated.[1] M₂SiO₄:Eu²⁺ phosphors were prepared by solid-state reaction in reductive atmosphere. Effects of partial substitution of Ba by rare earth codopant and fluxes on the luminecenct intensity were investigated. The results showed that Ba₂SiO₄:Eu²⁺ phosphor emits visible light at around 509nm with a half-width of 60nm under excitation by 393nm. The broad intense emission of Ba₂SiO₄:Eu²⁺ is originated from 5d-4f transition of Eu²⁺ ions due to the strong coupling of the 5d electron with host lattice. NH₄F was found to be the best flux, and the co-dopant of La³⁺ or Y³⁺ enhanced the fluorescence intensity, the optimum doping concentration was determined to be of 0.05 (relative molar fraction to Ba²⁺). The excitation spectra of Sr₂SiO₄:Eu²⁺ phosphors displayed a wide peak at around 398nm, and the emission spectra were composed of two adjacent peaks at around 490nm and 560nm. The latter emission peak was enhanced with doping of Y³⁺, the optimum doping concentration is 0.1 (relative molar fraction to Sr²⁺).[2] ZnO green phosphors with highly efficient luminescent emission have been prepared by calcining ZnS in air atmosphere at 800℃with NH₄Br as additive. The X-ray photoelectron spectroscopy and photoluminescence measurement were carried out for explanation of the enhanced luminescence of as-prepared ZnO phosphors. It was suggested that the origin of green emission was ascribed to a transition of a photo-generated electron from the localized defect centers (V_O^*) to a deeply trapped hole (V′_Zn) within the band gap. The improvement of luminescent emission of ZnO was attributed to the increase of oxygen vacancy V_O^* and V′_Zn, which caused by the existence of NH₄Br when calcining ZnS at air atmosphere.