Single pot synthesis of indirect band gap 2D CsPb2Br5 nanosheets from direct band gap 3D CsPbBr3 nanocrystals and the origin of their luminescence properties

Abstract

Two-dimensional (2D) perovskites recently attracted significant interest due to their unique and novel optoelectronic properties. CsPb₂Br₅, a 2D inorganic perovskite halide, is an indirect band gap semiconductor, and hence it is not supposed to be luminescent. However, a fundamental understanding of the origin of its luminescence properties is still lacking as there are contradictory literature reports present concerning its luminescence properties. Here, we demonstrate a single pot solution based transformation of 2D CsPb₂Br₅ nanosheets from the nanocrystals of 3D CsPbBr₃ and investigate the origin of its luminescence properties by detailed experiments and density functional theory (DFT) calculations. The photoluminescence of CsPb₂Br₅ originates from the different amorphous lead bromide ammonium complexes which are present at the surface of the nanosheets. We have also highlighted the formation mechanism of 2D nanosheets from 3D CsPbBr₃ nanocrystals. These combined theoretical and experimental studies offer significant insights into the optical properties and formation mechanism of 2D CsPb₂Br₅ perovskites.