The Mobile and Pinned Grain Boundaries in 2D Monoclinic Rhenium Disulfide.

In bulk crystals, the kinetics of dislocations is usually hindered by the twining boundaries (TB) or grain boundaries (GB), rendering the well-known grain boundary strengthening effects. Nevertheless, here it is found that in 2D rhenium disulfide (ReS), twinning is much easier than dislocation slip. Consequently, the highly mobile TBs or GBs are inversely pinned by the relatively immobile dislocations. Due to the strong in-plane covalent bonding, the GBs in high-symmetry 2D materials such as graphene which consists of defects are immobile at room temperature. In contrast, in monoclinic 2D ReS several types of GBs (including TBs) can be readily generated and driven by mechanical loading. A complete library of the GBs in 2D ReS is established by the (in situ) atomic-scale transmission electron microscopy (TEM) characterizations and density functional theory (DFT) calculations. The twinning (shear) stresses for 2D ReS are estimated as low as 4-30 MPa, one or two orders of magnitude lower than the traditional bulk materials. Full elucidation on the GB structures and especially the intriguing GB kinetics in such anisotropic 2D materials are of fundamental importance to understand the structure-property relationships and develop strain-tunable applications for 2D materials in future.