Highly Luminous N3–-SubstitutedLi2MSiO4−δN2/3δ:Eu2+ (M = Ca Sr and Ba) for White NUV Light-EmittingDiodes

摘要

The N^3–-substituted Li2MSiO4:Eu²⁺ (M = Ca, Sr, and Ba) phosphors were systematically prepared and analyzed. Secondary-ion mass spectroscopy measurements revealed that the average N^3– contents are 0.003 for Ca, 0.009 for Sr, and 0.032 for Ba. Furthermore, the N^3– incorporation in the host lattices was corroborated by infrared and X-ray photoelectron spectroscopies. From the photoluminescence spectra of Li2MSiO4:Eu²⁺ (M = Ca, Sr, and Ba) phosphors before and after N^3– doping, it was verified that the enhanced emission intensity of the phosphors is most likely due to the N^3– doping. In Li2MSiO4:Eu²⁺ (M = Ca, Sr, and Ba) phosphors, the maximum wavelengths of the emission band were red-shifted in the order Ca < Ba < Sr, which is not consistent with the trend of crystal field splitting: Ba < Sr < Ca. This discrepancy was clearly explained by electron–electron repulsions among polyhedra, LiO4–MOn, SiO4–MOn, and MOn–M’On associated with structural difference in the host lattices. Therefore, the energylevels associated with the 4f⁶5d energy levels of Eu²⁺ are definitely established in the following order: Li2CaSiO4:Eu²⁺ > Li2BaSiO4:Eu²⁺ > Li₂SrSiO₄:Eu²⁺. Furthermore, using the Williamson–Hall (W–H)method, the determined structural strains of Li₂MSiO₄:Eu²⁺ (M = Ca, Sr, and Ba) phosphors revealed thatthe increased compressive strain after N^3– dopinginduces the enhanced emission intensity of these phosphors. Whitelight-emitting diodes made by three N^3–-doped phosphorsand a 365 nm emitting InGaN chip showed the (0.333, 0.373) color coordinateand high color-rendering index (R_a = 83).These phosphor materials may provide a platform for development ofnew efficient phosphors in solid-state lighting field.