  Atomic-scale investigation on mechanical behaviors is highly necessary to
fully understand the fracture mechanics especially of brittle materials, which
are determined by atomic-scale phenomena (e.g., lattice trapping). Here,
exfoliated anisotropic rhenium disulfide (ReS2) flakes are used to investigate
atomic-scale crack propagation depending on the propagation directions. While
the conventional strain-stress curves exhibit a strong anisotropy depending on
the cleavage direction of ReS2, but our experimental results show a reduced
cleavage anisotropy due to the lattice reconstruction in [100] cracking with
high resistance to fracture. In other words, [010] and [110] cracks with low
barriers to cleavage exhibit the ultimate sharpness of the crack tip without
plastic deformation, whereas [100] cracks drive lattice rotation on one side of
the crack, leading to a non-flat grain boundary formation. Finally,
crystallographic reconstruction associated with the high lattice randomness of
two-dimensional materials drives to a modified cleavage tendency, further
indicating the importance of atomic-scale studies for a complete understanding
of the mechanics.
