van der Waals Epitaxial Formation of Atomic Layered alpha-MoO3 on MoS2 by Oxidation.

The electronic, catalytic, and optical properties of transition metal dichalcogenides (TMDs) are significantly affected by oxidation and using oxidation to tune the properties of TMDs has been actively explored. In particular, since transition metal oxides (TMOs) are promising hole injection layers, a TMD-TMO heterostructure can be potentially applied as a p-type semiconductor. However, the oxidation of TMDs has not been clearly elucidated due to the structural instability and the extremely small quantity of oxides formed. Here, we reveal the phases and morphologies of oxides formed on two-dimensional (2D) molybdenum disulfide (MoS2) using transmission electron microscopy (TEM) analysis. We find that MoS2 starts to oxidize around 400 to form orthorhombic phase molybdenum trioxide (alpha-MoO3) nanosheets. The alpha-MoO3 nanosheets so formed are stacked layer-by-layer on the underlying MoS2 via van der Waals interaction and the nanosheets are aligned epitaxially with six possible orientations. Furthermore, the bandgap of MoS2 is increased from 1.27 to 3.0 eV through oxidation. Our study can be extended to most TMDs to form TMO-TMD heterostructures, which are potentially interesting as p-type transistors, gas sensors, or photocatalysts.