Single-Crystal Red Phosphors and Their Core–Shell Structure for Improved Water-Resistance for Laser Diodes Applications

A solvent-vapor transport route produces centimeter-sized single-crystal red phosphors. The epitaxial growth route to yield its core–shell structure at ambient temperature was adopted. These red phosphors could be applied in all-inorganic WLED devices. Cs2TiF6:Mn4+ (CTFM) single crystal provides enhancement of quantum efficiency, moisture resistance, and thermal stability compared to polycrystalline powders. The internal quantum efficiency can reach as high as 98.7 %. To further improve waterproof stability, the Cs2TiF6 (CTF) shell with tunable thickness has been epitaxially grown on the CTFM single crystal surface and a unique three-step photoluminescence intensity evolution mechanism has been proposed. By combining as-prepared CTFM@CTF core–shell structured single crystal, YAG:Ce single crystal and blue-chip, warm WLEDs with excellent color rendition (Ra=90, R9=94), low correlated color temperature (CCT=3155 K), and high luminous efficacy were fabricated without any organic resins.