Spectroscopic properties of Eu-doped oxynitride glass-ceramics for white light LEDs

Novel oxynitride glass-ceramics with tremendously improved quantum yield and adjustable color coordinates were prepared with the conventional melt-and-quench method and subsequent annealing heat treatment. X-ray diffraction (XRD) and transmission electron microscopy (TEM) images revealed CaAl2Si2O8 and Y20N4Si12O48 crystalline phases were homogenously precipitated in the glass matrix. The optical absorption and photoluminescence (PL) spectrum measurements showed that a large portion of Eu3+ ions were reduced into Eu2+ which were crystallized into CaAl2Si2O8 phase during the heat treatment by substituting Ca2+ ions in them. Emission spectra evidently show that Eu2+ and Eu3+ ions were preferentially incorporated in CaAl2Si2O8 and Y28N4Si12O48 crystalline phases, respectively. This assignment was further supported by cation ionic radii and local charge balances, as well as defect chemistry considerations. The total emission intensity including both Eu2+ and Eu3+ ions was enhanced with increasing the annealing temperature, due to the conversion of Eu3+ to Eu2+ and the enhanced quantum efficiency of Eu3+ in the crystalline phase as compared to the glass matrix. As a result, the quantum yields (QYs) of the glass-ceramics were also enhanced significantly from 2.07% (the precursor glass) up to 43.33% (the maximum of the glass-ceramics). The emission ratio of Eu2+/Eu3+ changed from <1.0 to >4.0, which led this novel glass-ceramics with adjustable color coordinates ranging from red to blue controlled by varying the annealing temperature. (C) 2014 Elsevier B.V. All rights reserved.