MoS₂ edges and heterophase interfaces: energy, structure and phase engineering

The transition metal dichalcogenides exhibit polymorphism; i.e. both 2H and 1T′ crystal structures, each with unique electronic properties. These two phases can coexist within the same monolayer microstructure, producing 2H/1T′ interfaces. Here we report a systematic investigation of the energetics of the experimentally most important MoS₂ heterophase interfaces and edges. The stable interface and edge structures change with chemical potential (these edges/interfaces are usually non-stoichiometric). Stable edges tend to be those of highest atomic density and the stable interfaces correspond to those with local atomic structure very similar to the 2H crystal. The interfacial energies are lower than those of the edges, and the 1T′ edges have lower energy than the 2H edges. Because the 1T′ edges have much lower energy than the 2H edges, a sufficiently narrow 1T′ ribbon will be more stable than the corresponding 2H ribbon (this critical width is much larger in MoTe₂ than in MoS₂). Similarly, a large 2H flake have an equilibrium strip of 1T′ along its edge (again this effect is much larger in MoTe₂ than in MoS₂). Application of tensile strains can increase the width of the stable 1T′ strip or the critical thickness below which a ribbon favors the 1T′ structure. These effects provide a means to phase engineer transition metal dichalcogenide microstructures.