Unveiling Twist Domains in Monolayer MoS 2 through 4D‐STEM and Unsupervised Machine Learning

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Abstract
Dichalcogenides, such as molybdenum disulfide (MoS 2 ), are being studied extensively due to their 2D feature and various material properties. Although crystal structures are critical for applications, conventional atomic structure analyses have a limited field of view. In this study, the crystal domains of monolayer MoS 2 synthesized by metal–organic chemical vapor deposition (MOCVD) are analyzed using 4D scanning transmission electron microscopy (STEM) and unsupervised machine learning. Twist domains (±11°) are identified through the nonnegative matrix factorization (NMF) and hierarchical clustering of numerous (>22k) diffraction patterns from a wide field of view. Preprocessing for detecting noncentrosymmetry effectively visualizes the polarities of distinct MoS 2 domains by highlighting the violation of Friedel's law in diffraction physics. Analyses reveal that the specimen deposited on Al 2 O 3 (0001) at 850 °C consists of domains measuring ≈100 nm in size and featuring many mirror‐twin boundaries. The findings provide valuable insights into optimizing the MOCVD process and elucidating crystal growth mechanisms.

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