  The microscopic process of oxidative etching of two-dimensional molybdenum
disulfide (2D MoS2) at an atomic scale is investigated using a correlative
TEM-etching study. MoS2 flakes on graphene TEM grids are precisely tracked and
characterized by TEM before and after the oxidative etching. This allows us to
determine the structural change with an atomic resolution on the edges of the
domains, of well-oriented triangular pits and along the grain boundaries. We
observe that the etching mostly starts from the open edges, grain boundaries
and pre-existing atomic defects. A zigzag Mo edge is assigned as the dominant
termination of the triangular pits, and profound terraces and grooves are
observed on the etched edges. Based on the statistical TEM analysis, we reveal
possible routes for the kinetics of the oxidative etching in 2D MoS2, which
should also be applicable for other 2D transition metal dichalcogenide
materials like MoSe2 and WS2.
