High-resolution electron microscopy imaging of MOFs at optimized electron dose

Local and high-resolution structural investigation of metal-organic frameworks (MOFs) is essential for understanding the role of defects and incorporated elements. In this paper, we characterize the structure of metalated versions of (Hf)PCN-222(H2) and locate the position of the additional metal atoms. Transmission electron microscopy (TEM) is a powerful technique for this purpose, but MOFs are highly sensitive to the electron beam. To avoid structural alterations, it is therefore crucial to establish the maximum electron dose that can be applied. In this study, we apply a systematic workflow to measure the critical electron dose, enabling the identification of the optimal technique for extracting reliable information about the local structure of MOFs. We examined the electron beam stability of benchmarked (Zr)NU-1000, (Hf)PCN-222(H2) and its metalated versions, (Hf)PCN-222(Fe) and (Hf)PCN-222(Pd), and identified factors influencing the stability under the electron beam. After the threshold for electron dose was established, we applied low-dose, four-dimensional scanning transmission electron microscopy (4D-STEM). We then compared annular bright field (ABF), annular dark field (ADF), and real-time integrated center of mass (riCOM) images that could be extracted from the 4D dataset. The riCOM technique successfully revealed the structure of investigated MOFs with minimal beam-induced alterations and provides insights into local features, including organic linkers and additional metalation elements.