Pristine edge structures of T′′-phase transition metal dichalcogenides (ReSe2, ReS2) atomic layers

Abstract

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.