Thermal Oxidation of WSe2 Nano-sheets Adhered on SiO2/Si Substrates

TL;DR

This study investigates how thermal oxidation affects the composition, structure, and electrical properties of WSe2 nano-sheets on SiO2/Si substrates, revealing edge-initiated oxidation and changes in conductivity.

Contribution

It provides detailed insights into the spatial progression of oxidation and electrical property changes in WSe2 nano-sheets using multiple microscopy techniques.

Findings

Oxidation starts at nano-sheet edges and propagates inward.

Partial oxidation increases surface conductivity.

Extensive oxidation occurs at the WSe2/SiO2 interface.

Abstract

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.