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 Cs₄PbX₆ 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.