Surface Reconstruction in Quasi‐2D Perovskite Films Treated with Cesium Halide Nanocrystals: Halide Exchange or Phase Transformation

The formation of heterostructure interfaces from quantum dots (or nanocrystals) and lower‐dimensional (2D or quasi‐2D) materials enables interfacial and optoelectronic property tuning. However, this strategy has not been sufficiently characterized, for example, the application of cesium halide nanocrystals to quasi‐2D perovskite structures is underexplored, and the mechanisms of the resulting structural modifications and specific nanocrystal roles are not fully understood. Herein, the effects of postsynthetically surface‐modifying quasi‐2D perovskite films with CsX ( X = Cl, Br, I) nanocrystals are examined to bridge this gap. The purposeful choice of X enables the selective induction of halide exchange or a structural phase transformation at the nanocrystal–perovskite interface, which leads to optical bandgap and luminescence property modulation over a wide range of the visible spectrum (450–620 nm). Results of in situ spectroscopic analyses and temperature‐dependent kinetic studies reveal that the activation energy for the halide exchange (24–29 kJ mol −1 ) is lower than that for the structural phase transformation to 0D Cs 4 PbX 6 nanocrystals (39 kJ mol −1 ), indicating the kinetic favorability of the former process. The potential of the developed strategy is showcased through the fabrication of efficient color‐tunable light‐emitting diodes with quasi‐2D perovskite films surface modified with CsX as active emission layers.