Twist-to-Untwist Evolution and Cation Polarization Behavior of Hybrid Halide Perovskite Nanoplatelets Revealed by Cryogenic Transmission Electron Microscopy

Hybrid methylammonium lead iodide (MAPbI₃) perovskite nanoplatelets (NPLs) have emerged as promising optoelectronic materials because of their remarkable properties in defect tolerance, band gap tunability, and light emission. However, the detailed formation mechanism, in particular the atomic structure information in the initial nucleation stage, stands as a mystery because of the intrinsic vulnerability toward moisture, electron beams, etc. By virtue of the imaging technique under the extremely low electron dose of the cryogenic TEM, atomic structures of MAPbI₃ NPLs are imaged, and a twist-to-untwist structural evolution is captured. According to theoretical calculation results, the twist-to-untwist evolution is a spontaneous process, and the band gap will be reduced, which is further verified by the red shift of photoluminescence peaks with aging time. Moreover, MA cation polarization is observed by quantitative analysis of the atomic-resolution image of single-crystalline MAPbI₃ NPLs, which demonstrates the high ion mobility in the lattice of the hybrid halide perovskites.