Thermal Oxidation of WSe<sub>2</sub> Nanosheets Adhered
on SiO<sub>2</sub>/Si Substrates
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Abstract
Because
of 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 WSe<sub>2</sub> nanosheets on SiO<sub>2</sub>/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 nanosheet edges and propagates laterally toward 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 WSe<sub>2</sub>–SiO<sub>2</sub> 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