Edge Defects Promoted Oxidation of Monolayer WS₂ Synthesized on Epitaxial Graphene

Two-dimensional semiconductors are gaining increasing interest for their potential application in several fields. In particular, when combined with graphene into vertical van der Waals heterostructures, they have demonstrated unique properties, such as large spin–orbit coupling at the valence band maximum and ultrafast charge transfer. An understanding of the WS₂ stability on epitaxial graphene in ambient conditions is crucial for the development of potential applications. In this work, we study the environmental aging related degradation of WS₂ monolayers directly synthesized on top of epitaxial graphene. We experimentally demonstrate that the oxidation of the WS₂ monolayers on epitaxial graphene starts from the flake edges, being attributed to a local high concentration of defects. The oxidation leads to the complete deterioration of the semiconducting material in less than 120 days. In addition, we demonstrate that the oxidation mechanism is accelerated on epitaxial graphene in comparison to that on insulating sapphire substrate. First-principles theoretical analysis reveals that the oxidation mechanism is strongly promoted by the presence of sulfur vacancies and is further accelerated by transfer of electron density from the substrate, whereas it is slowed down by depletion of electron density from the support. Our exhaustive approach sheds light on the oxidative process promoted by defects and not self-limited to the oxidation of the monolayer edges.