In this study, we show that a detailed understanding of beam electron-sample interactions is required to achieve high-resolution structural imaging of two-dimensional materials. We start to derive basic understanding from atomically-resolved, time-dependent in-situ TEM imaging of inorganic two-dimensional (2D) transition metal dichalcogenides using the chromatic- and spherical-aberration-corrected low-voltage SALVE instrument operating in the voltage range between 80kV and 20kV [1-3]. We elucidate the accelerating-voltage-dependent formation of defects and find that elastic and inelastic interactions are strongly connected, resulting in a two-step interaction process. Density functional theory molecular dynamics shows that excitations in the electronic system can form vacancies through ballistic energy transfer at electron energies, which are much lower than the knock-on threshold for the ground state [4]. After the material under electron irradiation lost its ordered structure, the evaluation of the unordered structure is performed by developing a U-net-based neural network (NN).

The knowledge gained for the study of 2D inorganic materials we apply to the study of 2D polymers [5,6] and 2D metal-organic frameworks (MOFs) [7], where however atomically-resolved imaging is hindered due to much lower resilience against electron irradiation. We present key strategies to achieve higher resolution in high-resolution TEM images of imine-based 2D polymer films [8], which include the selection of the appropriate electron accelerating voltage [9]. When comparing the achievable resolution at 300kV, 200kV, 120kV and 80kV, we found that imaging at 120kV offers the highest resolution (1.9A). This resolution allowed even imaging the molecular nature of interstitial defects, which could be identified by means of quantum mechanical calculations [9]. The U-net-based NN developed for the analysis of inorganic amorphous 2D structures was also succsessfully applied on the evaluation of amorphous polymeric networks to understand the pore size distributions [9]. In addition, we show that even sub-Angstrom resolution can be achieved for hydrogen-free 2D BHT-Cu (BHT = benzenehexathiol) MOFs using the Cc/Cs-corrected SALVE microscope operating at 80 kV, resulting in imaging single atoms with high contrast.

Further, we study experimentally and computationally the role of different organometallic bonds and hydrogen content on electron radiation stability, using a group of four structurally similar Cu-based 2D MOFs with well-defined differences to allow for a direct comparison of hydrogen-containing and hydrogen-free MOFs, and of the presence of Cu - N, Cu - O and Cu - S chemical bonds. Trends in e-beam resilience among the 2D-MOFs found experimentally showed good agreement with ab initio molecular dynamics simulations of ejection cross-section.