Vapor‐Phase Deposition of CsPbBr ₃ Shells on Iodide‐Rich Perovskite Cores in SiO ₂ for Efficient and Robust Red Emission

While high‐temperature solid‐phase confined synthesis has established itself as a viable approach for producing ultra‐stable halide perovskite nanocrystals (NCs), the photoluminescence quantum yield (PLQY) of the resulting red‐emitting NCs, whether pure‐iodide or mixed‐halide, remains unsatisfactory. In this work, we introduce a high‐temperature vapor‐phase deposition for the controlled synthesis of CsPbI 3 /CsPbBr 3 and CsPb(Br/I) 3 /CsPbBr 3 heterostructure NCs encapsulated within a robust SiO 2 matrix. This method spatially separates the CsPbBr 3 shell precursor from the preformed iodine‐rich core in the vapor phase, effectively suppressing undesirable halide diffusion during high‐temperature processing at 700°C. By leveraging the strong Br─Cs/Pb interactions, the CsPbBr 3 vapor promotes preferential interfacial deposition on CsPbI 3 or CsPb(Br/I) 3 cores. The resulting NCs display highly efficient red emission, achieving a PLQY as high as 85%. Furthermore, benefiting from the dual protection of the SiO 2 matrix and the heterostructure, these red‐emitting materials exhibit outstanding environmental stability while retaining their high performance.