Aberration-Corrected Scanning TEM for 2D Materials at Ultrahigh Resolution

The physical and chemical properties of 2D materials are strongly determined by atomic-scale defects and dopants [1]. The process of characterizing this class of materials therefore often necessitates a tool that can visualize and characterize these materials at the atomic scale. The highest-resolution tool we have available for this is aberration-corrected electron microscopy. The new class of electron lenses called ‘aberration-correctors’ revolutionized electron microscopy, both transmission electron microscopy (TEM) and scanning TEM (STEM). For the first time, it has become possible to correct focus distortions that were previously considered inevitable. Without these distortions, the details that can be resolved are now several times finer than what used to be the case. Suddenly, it has become possible to visualize the individual atoms in 2D films in real space, making this new generation of microscopes an ideal tool for this rapidly-growing class of materials.<br/>This presentation will introduce recent developments in aberration corrected electron microscopy that are relevant to 2D materials. In particular, aberration-corrected STEM is an experimental platform with an enormous potential for new 2D materials applications. It delivers a resolution well below 0.1 nm, with a high beam brightness and monochromaticity for various types of local spectroscopy.<br/>The local distribution of defects and dopants in 2D films can now be quantified by atomic-resolution, real-space imaging with aberration-corrected STEM. It makes this an invaluable tool for gaining insight into the structure-property relationship of these materials. Advances are currently being made in new and optimized imaging methods. These include differential phase-contrast [2] and 4D-STEM mapping for visualizing the atomic structure as well as local electric, magnetic, and strain fields [3]. Moreover, further improvements in resolution, quantification and spatial accuracy are being developed with electron ptychography in the STEM [4,5]. New scan techniques and fast detectors are also under development for very low-dose measurements. These tools can be combined with a gentle beam energy and dedicated sample holders for in-situ experiments, making it possible to observe sample dynamics at the atomic scale, as they happen.<br/>The enormous versatility, sensitivity, and resolution makes aberration-corrected STEM a critical tool for the study of two-dimensional materials for energy applications. The ongoing developments in observing ongoing chemical and physical changes at the atomic scale, and ongoing rapid improvements in hardware and data processing are also making aberration-corrected STEM an indispensable experimental platform for future research on new materials.<br/> <br/> <br/> <br/>The presented work would not have been possible without the invaluable contributions from Hue Thi Bich Do (National University of Singapore, NUS), Loh Leyi (NUS), Zhang Xinyue (NUS), Jagadesh Rangaraj (NUS), Dr. Ning Shoucong (NUS), Prof. Goki Eda (NUS), Daria Kieczka (IMRE, A*STAR), Dr. Zhao Meng (IMRE, A*STAR) and Prof. Wu Lin (SUTD). Funding from the Ministry of Education (MOE) Singapore, under AcRF Tier 2 (MOE2019-T2-1-179), and AcRF Tier 1 (R-284-000-179-133) is kindly acknowledged.<br/> [1] L Loh et al., Substitutional doping in 2D transition metal dichalcogenides. Nano Research 14 (6), 1668-1681 (2021).<br/>[2] N. Shibata et al., Differential phase-contrast microscopy at atomic resolution. Nature Physics 8, 611–615 (2012).<br/>[3] C. Ophus, Four-Dimensional Scanning Transmission Electron Microscopy. Microscopy and Microanalysis 25, 563–582 (2019).<br/>[4] Z. Chen et al. Electron ptychography achieves atomic-resolution limits set by lattice vibrations. Science 372, 826–831 (2021).<br/>[5] S. Ning et al. Accurate and Robust Calibration of the Uniform Affine Transformation Between Scan-Camera Coordinates for Atom-Resolved In-Focus 4D-STEM Datasets. Microscopy and Microanalysis, 28, 622-632 (2022).