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Twisted two-dimensional (2D) material heterostructures provide an exciting platform for investigating fundamental physical phenomena. Many of the most interesting behaviors emerge at small twist angles, where the materials reconstruct to form areas of perfectly stacked crystals separated by partial dislocations. However, understanding the properties of these systems is often impossible without correlative imaging of their local reconstructed domain configuration, which exhibits random variations due to disorder and contamination. In particular, visualization of the local domain configuration allows determination of the local twist angle and, hence, the local lattice strain. Here, we demonstrate a simple and widely accessible route to visualize domains in the as-produced twisted transition metal dichalcogenide (TMD) heterostructures using electron channeling contrast imaging (ECCI) in scanning electron microscopy (SEM). This nondestructive approach is compatible with conventional substrates and allows domains to be visualized even when sealed beneath an encapsulation layer. Complementary theoretical calculations reveal how a combination of elastic and inelastic scattering leads to contrast inversions at the specified detector scattering angles and sample tilts. We demonstrate that optimal domain contrast is therefore achieved by maximizing signal collection while avoiding contrast inversion conditions.

Liquid- phase exfoliation (LPE) is the principal method of producing two-dimensional (2D) materials such as graphene in large quantities with a good balance between quality and cost and is now widely adopted by both the academic and industrial sectors. The fragmentation and exfoliation mechanisms involved have usually been simply attributed to the force induced by ultrasound and the interaction with the solvent molecules. Nonetheless, little is known about how they actually occur, i.e., how thick and large graphite crystals can be exfoliated into thin and small graphene flakes. Here, we demonstrate that during ultrasonic LPE the transition from graphite flakes to graphene takes place in three distinct stages. First, sonication leads to the rupture of large flakes and the formation of kink band striations on the flake surfaces, primarily along zigzag directions. Second, cracks form along these striations, and together with intercalation of solvent, lead to the unzipping and peeling off of thin graphite strips that in the final stage are exfoliated into graphene. The findings will be of great value in the quest to optimize the lateral dimensions, thickness, and yield of graphene and other 2D materials in large-scale LPE for various applications.