An improved soft-thresholding exit wave reconstruction for imaging beam-sensitive materials.

High-resolution transmission electron microscopy (HRTEM) is a widely-used technique for atomic-scale characterization. However, the conventional dose of HRTEM can destroy beam-sensitive materials such as organic-inorganic halide perovskites. CHNHPbI (MAPbI), a typical perovskite, will be easily damaged after irradiated with the dose of ∼10 e-/Å. Low-dose imaging techniques can protect the specimen but it is difficult to achieve an image which is both directly interpreted and atomically clear. Exit wave reconstruction (EWR), as one of phase retrieval methods, can recover an interpretable phase image at the atomic scale but its signal-to-noise ratio (SNR) is limited by low electron doses. Here, we improve the iterative wave function reconstruction (IWFR) method and present a soft-thresholding L-IWFR. Results from both simulated and experimental focal-series dataset at the extremely low dose show that L-IWFR improves the SNR effectively and has better performance on low-dose datasets than IWFR. Combined with low-dose imaging techniques and various alignment strategies, an atomically clear image of CHNHPbI (MAPbI) is successfully achieved at the total dose of ∼45 e/Å.