Nanocomposite Phosphor Consisting of CaI₂:Eu²⁺ Single Nanocrystals Embedded in Crystalline SiO₂

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.