Is Single Layer MoS2 Stable in the Air?

Molybdenum disulfide (MoS2 ) is extensively studied because of its potential applications in catalysis, electronic and optoelectronic devices, and composite nanostructures. However, a recent experimental study indicated that, contrary to current beliefs, MoS2 monolayers lack long-term stability in air. Here, a study is presented on the oxidation of MoS2 monolayers based on density functional theory (DFT) calculations. The results suggest that single-layer MoS2 samples with exposed edge sites are indeed unstable to oxidation, which occurs because of the low energetic barrier to dissociation of oxygen molecules at the Mo-edges of MoS2 . After an oxygen molecule dissociates, oxygen atoms replace sulfur atoms, and further oxidation causes the formation of a one-dimensional chain-like structure resembling that of bulk MoO3 . This MoO3 structure facilitates the spread of oxidation onto the surface, and the stress associated with the misfit between the MoS2 and MoO3 lattices may cause the experimentally observed cracking of MoS2 flakes.