Photoinduced quasi-2D to 3D phase transformation in hybrid halide perovskite nanoplatelets

We present a photo-induced quasi-2D to 3D phase transition of MAPbBr₃ (MA = CH₃NH₃) perovskite nanoplatelets (NPLs). To begin with, we synthesized quasi-2D MAPbBr₃ NPLs (two octahedral layers thick, n = 2). A systematic increase in the thickness of the perovskite platelets is observed as a result of continuous photon irradiation leading to a 78 nm red shift in the emission spectra through different stages. Moreover, the bandgap of the compound decreases from 2.72 eV to 2.2 eV as we move from a quasi-2D to 3D phase. The excitonic Bohr radius of the MAPbBr₃ NPLs is found to be 1.8 nm, whereas the thickness of a single layer of PbBr₆^4- octahedra is 5.9 Å. As the layer thickness increases (>4-6 layers), MAPbBr₃ NPLs move out of the quantum confinement regime, governed by the red shift in the emission spectra. To complement the experimental results, density functional theory calculations were performed on MAPbBr₃ of various layer thicknesses. The van der Waals interaction and a more accurate Heyd-Scuseria-Ernzerhof functional were used to calculate the optical bandgap for MAPbBr₃ platelets of different layer thicknesses, which matches exceptionally well with the experimental results. Our findings disclose an interesting and meaningful phenomenon in the emerging hybrid perovskite NPLs and are beneficial for any future development of perovskite-based devices.