Pristine edge structures of T''-phase transition metal dichalcogenides (ReSe₂, ReS₂) atomic layers

Rhenium dichalcogenides (ReX2, X = S, Se), as a representative type of T''-phase transition metal dichalcogenides (TMDs), have a distinct anisotropic crystal structure as compared to the well-known H- and T-phases and show excellent optical, electronic and catalytic properties. While edges are known to have a profound influence on the physical and chemical properties of two-dimensional materials, they have not been systematically investigated in T''-phase TMDs. We investigated the pristine edge configurations of ReX2 atomic layers using atomic-resolution scanning transmission electron microscopy (STEM) low-dose imaging and density functional theory (DFT) calculations. The pristine edges in monolayer and bilayer ReX2 can be atomically flat with a length up to several tens of nanometers, and are preferentially oriented along either the a axis or b axis. The characteristic 4Re diamond clusters are well preserved along the edges, and ordered structures of the outermost dangling Se atoms were observed, with the Se atoms fully retained, 50% retained or all lost. The edges oriented along the a axis with 100% Se coverage show a ferromagnetic ground state, while their counterparts parallel to b present mid-gap states without appreciable spin-polarization. The anisotropic T'' structure also dictates the cracking direction in ReX2, with cracks propagating mainly along the a and b axes. The strain at the crack edges often causes re-orientation of the lattice, which would change the anisotropic behavior of ReX2. Our work provides new insights into the edge configuration in T'' TMD atomic layers, and offers new opportunities to tailor the performance of ReX2 by edge engineering.