Direct identification of monolayer rhenium diselenide by an individual diffraction pattern

In the current extensive studies of layered two-dimensional (2D) materials, compared to hexagonal structures such as graphene, hBN, and MoS₂, low- symmetry 2D materials have shown great potential for applications in anisotropic devices. Rhenium diselenide (ReSe₂) possesses the bulk space group P1 and belongs to the triclinic crystal system with a deformed cadmium-iodide-type structure. Here, we propose an electron diffraction-based method to distinguish the monolayer ReSe₂ membrane from multilayer ReSe₂ and its two different vertical orientations. Our method is also applicable to other low-symmetry crystal systems, including both triclinic and monoclinic lattices, as long as their third unit-cell basis vectors are not perpendicular to the basal plane. Our experimental results are well explained by kinematical electron diffraction theory and the corresponding simulations. Generalization of our method to other 2D materials, such as graphene, is also discussed.