Anomalous twin boundaries in two dimensional materials

Twin boundary defects form in virtually all crystalline materials as part of their response to applied deformation or thermal stress. For nearly six decades, graphite has been used as a textbook example of twinning with illustrations showing atomically sharp interfaces between parent and twin. Using state-of-the-art high-resolution annular dark-field scanning transmission electron microscopy, we have captured atomic resolution images of graphitic twin boundaries and find that these interfaces are far more complex than previously supposed. Density functional theory calculations confirm that the presence of van der Waals bonding eliminates the requirement for an atomically sharp interface, resulting in long-range bending across multiple unit cells. We show these remarkable structures are common to other van der Waals materials, leading to extraordinary microstructures, Raman-active stacking faults, and sub-surface exfoliation within bulk crystals.