Effect of Ultraviolet-Ozone Treatment on MoS(2) Monolayers: Comparison of Chemical-Vapor-Deposited Polycrystalline Thin Films and Mechanically Exfoliated Single Crystal Flakes.

We report the different oxidation behavior between polycrystalline chemical-vapor-deposited and mechanically exfoliated single crystal MoS(2) monolayers by ultraviolet-ozone treatment. As ultraviolet-ozone treatment time increased from 0 to 5 min, photoluminescence emission and Raman modes of both MoS(2) disappeared, suggesting structural degradation by oxidation. Analysis with optical absorbance and X-ray photoelectron spectroscopy suggested the formation of MoO(3) in both MoS(2) after ultraviolet-ozone treatment. In addition, ultraviolet-ozone treatment possibly led to the formation of oxygen vacancies, molybdenum oxysulfide, or molybdenum sulfates in chemical-vapor-deposited MoS(2). The measurement of electrical resistance after ultraviolet-ozone treatment suggested the transformation of chemical-vapor-deposited MoS(2) into doped MoO(3) and of mechanically exfoliated MoS(2) into negligibly doped MoO(3). These results demonstrate that the crystallinity of monolayer MoS(2) can strongly influence the effect of ultraviolet-ozone treatment, providing important implications on the device integration of MoS(2) and other two-dimensional semiconductors.