ABSTRACT
High luminescence efficiency is obtained in halide- and chalcogenide-based phosphors, but they are impractical because of their poor chemical durability. Here we report a halide-based nanocomposite phosphor with excellent luminescence efficiency and sufficient durability for practical use. Our approach was to disperse luminescent single nanocrystals of CaI2:Eu2+ in a chemically stable, translucent crystalline SiO2 matrix. Using this approach, we successfully prepared a nanocomposite phosphor by means of self-organization through a simple solid-state reaction. Single nanocrystals of 6H polytype (thr notation) CaI2:Eu2+ with diameters of about 50 nm could be generated not only in a SiO2 amorphous powder but also in a SiO2 glass plate. The nanocomposite phosphor formed upon solidification of molten CaI2 left behind in the crystalline SiO2 that formed from the amorphous SiO2 under the influence of a CaI2 flux effect. The resulting nanocomposite phosphor emitted brilliant blue luminescence with an internal quantum efficiency up to 98% upon 407 nm violet excitation. We used cathodoluminescence microscopy, scanning transmission electron microscopy, and Rietveld refinement of the X-ray diffraction patterns to confirm that the blue luminescence was generated only by the CaI2:Eu2+ single nanocrystals. The phosphor was chemically durable because the luminescence sites were embedded in the crystalline SiO2 matrix. The phosphor is suitable for use in near-ultraviolet light-emitting diodes. The concept for this nanocomposite phosphor can be expected to be effective for improvements in the practicality of poorly durable materials such as halides and chalcogenides.
ABSTRACT
The luminous efficiency of white light-emitting diodes, which are used as light sources for next-generation illumination, is continuously improving. Presently available white light-emitting diodes emit with extremely high luminance because their emission areas are much smaller than those of conventional light sources. Consequently, white light-emitting diodes produce a glare that is uncomfortable to the human eye. Here we report a yellow-emitting phosphor, the Eu(2+)-doped chlorometasilicate (Ca(1-x-y,)Sr(x,)Eu(y))(7)(SiO(3))(6)Cl(2), which can be used to create glareless white light-emitting diodes. The (Ca(1-x-y,)Sr(x,)Eu(y))(7)(SiO(3))(6)Cl(2) exhibits a large Stokes shift, efficiently converting violet excitation light to yellow luminescence, and phosphors based on this host material have much less blue absorption than other phosphors. We used crystal structure analysis to determine the origin of the desired luminescence, and we used (Ca(1-x-y,)Sr(x,)Eu(y))(7)(SiO(3))(6)Cl(2) and a blue-emitting phosphor in combination with a violet chip to fabricate glareless white light-emitting diodes that have large emission areas and are suitable for general illumination.
