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 PbBr64- 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.