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. Single‐crystal red phosphors easily grown by solvent exchange exhibit considerably better efficiency and stability. To further improve the chemical stability, a core–shell CTFM@CTF single crystal was prepared through an epitaxial growth route and a unique three‐step PL intensity change has been proposed.