Metastable defects in monolayer and few-layer films of MoS₂

We report on structural and electronic properties of defects in chemical vapor-deposited monolayer and few-layer MoS₂ films. We use scanning tunneling microscopy and Kelvin probe force microscopy in order to obtain measurements of the local density of states, work function and nature of defects in MoS₂ films. We track the evolution of defects that are formed under annealing in ultra-high vacuum conditions. We observe formation of metastable domains with different work function values after annealing the material in ultra-high vacuum to moderate temperatures. We attribute these metastable values of the work function to evolution of crystal defects forming during the annealing. The experiments show that sulfur vacancies formed after exposure to elevated temperatures diffuse, coalesce, and migrate bringing the system from a metastable to equilibrium ground state. The process could be thermally activated with estimated energy barrier for sulfur vacancy migration of 0.6 eV in single unit cell MoS₂. The results provide estimates of the thermal budgets available for reliable fabrication of MoS₂-based integrated circuit electronics and indicate the importance of defect control and layer passivation.