Reconstruction of Mn4+-free shell achieving highly stable red-emitting fluoride phosphors for light-emitting diodes

Poor humidity resistance is a bottleneck problem to be solved urgently for Mn4+ activated fluoride red phosphors. At present, some strategies such as outer coating, cation exchange (CE), and surface passivation (SP) have been developed, but the effect is limited. The key reason is that the easily hydrolyzed Mn4+ ions on the surface are difficult to be completely removed. With the goal of achieving a significant improvement in the moisture resistance of fluoride, this work proposes a new idea of using the reduction-assisted surface recrystallization (RSRC) strategy to construct a real core-shell structure without Mn4+ in the shell. The quantum efficiency (QE) of the K2SiF6:Mn4+ treated by the RSRC method is 99.95%. After boiling in water, the QE can even be maintained at 96.7%, which is significantly higher than that of the fluorides treated by CE (15.53%) or SP (62.64%) methods. The Mn-free shell is further determined by a combination of in-situ lift-out and STEM-EDX techniques. Finally, the aging results of LED devices in high temperature (85 °C) and high humidity (85%) environments further confirmed that the color stability of the K2SiF6:Mn4+ was significantly improved.