Fe³⁺ Assisted Synthesis of Stable 3D‐in‐2D CsPbBr₃/CsPb₂Br₅ Nanocomposites for Optical Gain Media

Abstract Metal halide perovskite nanocrystals are sought after for many optical and optoelectronic applications, such as light‐emitting diode and solar cells, due to their outstanding optical properties. However, their ionic nature makes them susceptible to ambient conditions. One rational solution to this challenge is the passivation or encapsulation of perovskite nanocrystals to isolate them from their environments. Thus, there is an urgent need to develop efficient methods for encapsulating emissive perovskite nanocrystals. A facile post‐synthesis method is proposed to treat Cs_xFA_(1−x)PbBr₃ nanocrystals, in the presence of Fe³⁺ cations, to create a robust and water‐stable nanocomposite structure, where 3D CsPbBr₃ nanocrystals are embedded in and thus protected by the 2D CsPb₂Br₅ nanosheets (named as CsPbBr₃/CsPb₂Br₅ hereafter). These Fe³⁺ cations facilitate the formation of the CsPbBr₃/CsPb₂Br₅ composite and regulate the growth of 2D CsPb₂Br₅ sheets. By performing controlled experiments, the possible mechanism of 2D nanosheet growth is proposed and discussed in detail. More importantly, the composite can remain stable in water for three months and exhibits amplified spontaneous emission under femtosecond laser irradiation. This work presents a synthesis pathway for producing durable perovskite composites that are promising for future lasing applications.