  Due to the drastically different intralayer versus interlayer bonding
strengths, the mechanical, thermal, and electrical properties of
two-dimensional (2D) materials are highly anisotropic between the in-plane and
out-of-plane directions. The structural anisotropy may also play a role in
chemical reactions, such as oxidation, reduction, and etching. Here, the
composition, structure, and electrical properties of mechanically exfoliated
WSe2 nano- sheets on SiO2/Si substrates were studied as a function of the
extent of thermal oxidation. A major component of the oxidation, as indicated
from optical and Raman data, starts from the nano-sheet edges and propagates
laterally towards the center. Partial oxidation also occurs in certain areas at
the surface of the flakes, which are shown to be highly conductive by microwave
impedance microscopy. Using secondary ion mass spectroscopy, we also observed
extensive oxidation at the WSe2/SiO2 interface. The combination of multiple
microcopy methods can thus provide vital information on the spatial evolution
of chemical reactions on 2D materials and the nanoscale electrical properties
of the reaction products.
