Moving Dislocations: The mechanical properties of crystalline materials are limited by the presence and motion of defects caused by extra or missing atoms in the crystal lattice. Plastic deformation of a material causes these defects, known as dislocations, to move and multiply. Much is known about the motion of dislocations in three dimensions but less so in two. Warner et al. (p. 209 ; see the Perspective by Bonilla and Carpio ) used graphene as a model material to track dislocation dynamics in real time. The strain fields in the graphene sheet were mapped, which suggests that the dislocation motion is connected to the stretching, rotating, and breaking of individual carbon bonds.

Layered transition metal dichalcogenides (TMDs) offer monolayer 2D systems with diverse properties that extend beyond what graphene alone can achieve. The properties of TMDs are heavily influenced by the atomic structure and in particular imperfects in the crystallinity in the form of vacancy defects, grain boundaries, cracks, impurity dopants, ripples and edge terminations. This review will cover the current knowledge of the detailed structural forms of some of the most intensively studied 2D TMDs, such as MoS₂, WSe₂, MoTe₂, WTe₂, NbSe₂, PtSe₂, and also covers MXenes. The review will utilize results achieved using state-of-the-art aberration corrected transmission electron microscopy, including annular dark-field scanning transmission electron microscopy (ADF-STEM) and electron energy loss spectroscopy (EELS), showing how elemental discrimination can be achieved to understand structure at a deep level. The review will also cover the impact of single atom substitutional dopants, such as Cr, V and Mn, and electron energy loss spectroscopy used to understand the local bonding configuration. It is expected that this review will provide an atomic level understanding of 2D TMDs with a connection to imperfections that can arise from chemical vapour deposition synthesis, intentional doping, rips and tears, dislocations, strain, polycrystallinity and confinement to nanoribbons.