Monolayer transition metal dichalcogenides (TMDs) stand out in two-dimensional (2D) materials due to their potential applications in future microelectronic and optoelectronic devices. In experiments, field effect transistors (FET) based on the MoS₂ monolayer are sensitive to environmental gases, especially O₂. Thus, the oxidation of monolayer TMDs is a critical concern. By first-principles calculations, we reveal that a perfect single-layer sheet of TMDs stays intact when exposed in O₂ due to the weak physical adsorption of O₂. However, O₂ can be chemically adsorbed onto the monolayer of TMDs (including MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, and WTe₂) with single vacancies of chalcogen, which are the most common defects in realistic TMD materials. The adsorption configurations and dissociation behavior of the O₂ molecule at vacancy sites, as well as the possible diffusion behavior of oxygen adatoms on the TMD monolayer surface were explored. Oxidation significantly influenced the electronic properties of a defective MoS₂ monolayer, while other defective TMD monolayers (especially MoTe₂ and WTe₂) suffered less from oxidation. Our theoretical results provide valuable atomistic insight into the oxidation of TMD monolayers and are useful for the future design of TMD-based 2D devices.

Band alignments of lateral heterostructures of group-IV monochalcogenides.

Van der Waals (vdW) stacking of two-dimensional (2D) materials to create artificial structures has enabled remarkable discoveries and novel properties in fundamental physics. Here, we report that vdW stacking of centrosymmetric 2D materials, e.g., bilayer MoS ₂ (2LM) and monolayer graphene (1LG), could support remarkable second-harmonic generation (SHG). The required centrosymmetry breaking for second-order hyperpolarizability arises from the interlayer charge transfer between 2LM and 1LG and the imbalanced charge distribution in 2LM, which are verified by first-principles calculations, Raman spectroscopy, and polarization-resolved SHG. The strength of SHG from 2LM/1LG is of the same order of magnitude as that from the monolayer MoS ₂ , which is well recognized with strong second-order nonlinearity. The emergent SHG reveals that the interlayer charge transfer can effectively modify the symmetry and nonlinear optical properties of 2D heterostructures. It also indicates the great opportunity of SHG spectroscopy for characterizing interlayer coupling in vdW heterostructures.