Fig. 4: Synthesis of millimeter-sized mono- to few-layer perovskites.

a Schematic illustration of the PbI₂ to perovskite conversion. b Optical image, (c) atomic force microscope image, (d) transmission electron microscopy image, and (e) selected area electron diffraction of the millimeter-sized monolayer MAPbI₃ nanosheets. f Photoluminescence spectra of 2D perovskites PEA₂PbI₄ and NMA₂PbI₄, as well as 3D perovskites MAPbI₃ and FAPbI₃. The blue area represents 2D perovskite and the red area represents 3D perovskite. g Comparison of the lateral size of monolayer and bilayer halide perovskites prepared using our method and other methods^{17,55,55,56,57,58,59,60,61,63}. The terms of “vapor”, “mechanical”, and “liquid” stand for vapor-phase, mechanical exfoliation, and liquid-phase methods, respectively.