  In the current extensive studies of transition metal dichalcogenides (TMDCs),
compared to hexagonal layered materials, like graphene, hBN and MoS2, low
symmetry layered 2D crystals showed great potential for applications in
anisotropic devices. Rhenium diselenide (ReSe2) has the bulk space group P1 and
belongs to triclinic crystal system with a deformed cadmium iodide type
structure. Here we unambiguously determined monolayer and its vertical
orientation of rhenium diselenide membrane with an individual electron
diffraction pattern, which could be applicable to low symmetry crystal systems,
including both triclinic and monoclinic lattices, as long as their third
unit-cell basis vector is not perpendicular to basal plane. Atomically resolved
image from probe corrected annular dark field scanning transmission electron
microscope (ADF-STEM) was employed to validate layer number. Finally,
experimental results were well explained by kinematical electron diffraction
theory and corresponding simulations.
