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

Abstract

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.

Cryogenic transmission electron microscopy on beam-sensitive materials and quantum science

“…This technique even holds promise for quantifying the polarization offset of the methylammonium (MA) cations based on the ideal position of the tetrahedral center, akin to methods employed for inorganic ferroelectric perovskite materials. [79] Our collaborators, led by Loh, synthesized three Mn-based hybrid perovskite single crystals with varying dimensionality by modifying the organic component within the organic-inorganic hybrid perovskite structure. [80] These materials consisted of a 2D material, (4-fluorobenzylamine (FB)) 2 MnCl 4 , a one-dimensional (1D) ((R)-3-fluoropyrrolidinium (FP))MnCl 3 , and a zerodimensional (0D) (pyridine (PD)) 2 MnCl 4 .…”

“…Electron diffraction and HRTEM have been instrumental in elucidating structural complexities within inorganic perovskites and organic-inorganic hybrid perovskites, including ion polarization, structural distortions, vacancies, and grain boundary defects. [47,76,79,81,82] However, research on the crystal structures of all-organic perovskites, which lack metal cations, remains comparatively limited due to the challenges associated with both synthesis and characterization. The primary challenge in synthesizing all-organic perovskites stems from the substantial size difference between organic molecules and inorganic ions within the traditional ABX 3 perovskite structure.…”

Recent progress on advanced transmission electron microscopy characterization for halide perovskite semiconductors

“…Alternatively, Zhang et al employed a direct-detection electron-counting camera to obtain the first atomic resolution image of MAPbBr 3 at a total dose of only 11 e/Å 2 , revealing nanodomain structure where the MA + cations were ordered locally in the single crystal, as shown in Fig. 4(b) [39,81] . Chen et al also used direct-detection electron-counting camera to investigate the decomposition pathway of MAPbI 3 at atomic scale, where the intermediate phase of MA 0.5 PbI 3 with locally ordered vacancies was successfully imaged [82] .…”

“…Rothmann et al were one of the first research groups to report twinning structure in the as-grown MAPbI 3 single crystal sample using electron diffraction [38] . The atomic-resolution structure of MAPbI 3 was further revealed using direct electron detector and cryo-TEM, respectively [39,40] . Yu et al reported a nanodomain structure in CsPbBr 3 2D nanocrystals where tetragonal and cubic phases coexist [41] .…”

“…(Color online) (a) HRTEM of CsPbBr 3 nanocrystals (i)where the coexistence of cubic and orthorhombic phases were demonstrated by FFT patterns (ii, iii), simulated diffraction patterns (iv, v), and illustrated structure (vi, vii)[41] . (b) CTF-corrected denoised HRTEM image (i) of CH 3 NH 3 Pb-Br 3 with different CH 3 NH 3 orientations, where (ii, iii) the structural model (left) and the simulated projected potential map (right) corresponding to region 1 and 2 in (i), respectively[39] . (c) Ptychography reconstructed image of CsPbBr 3 , with the scale bar of 5Å[84] .…”