  Anisotropic material properties of two-dimensional (2D) crystals are not only
intriguing but also of potential use for many applications. In this work, we
report the anisotropy of bond-breaking reactions is governed by the structural
anisotropy and significantly material-dependent for 2D semiconducting
transition metal dichalcogenides (TMDs). The degree of the anisotropy that led
to trigonal oxidation patterns was much higher in MoS2 and MoSe2 than WS2 and
WSe2. Using optical second-harmonic generation spectroscopy, we establish
crystal-facet-resolved kinetic measurements and show that the reactions proceed
fastest (slowest) for chalcogen (metal)-terminated zigzag edges. Edge-specific
reaction rates fed into kinetic Wulff construction also verified the
material-dependent anisotropy. We also show that the reactions are initiated at
substrate-mediated defects that are located on the bottom and top surfaces of
2D TMDs.
