Long-Term Exposure of MoS₂ to Oxygen and Water Promoted Armchair-to-Zigzag-Directional Line Unzippings

Understanding the long-term stability of MoS₂ is important for various optoelectronic applications. Herein, we show that the long-term exposure to an oxygen atmosphere for up to a few months results in zigzag (zz)-directional line unzipping of the MoS₂ basal plane. In contrast to exposure to dry or humid N₂ atmospheres, dry O₂ treatment promotes the initial formation of line defects, mainly along the armchair (ac) direction, and humid O₂ treatment further promotes ac line unzipping near edges. Further incubation of MoS₂ for a few months in an O₂ atmosphere results in massive zz-directional line unzipping. The photoluminescence and the strain-doping plot based on two prominent bands in the Raman spectrum show that, in contrast to dry-N₂-treated MoS₂, the O₂-treated MoS₂ primarily exhibits hole doping, whereas humid-O₂-treated MoS₂ mainly exists in a neutral charge state with tension. This study provides a guideline for MoS₂ preservation and a further method for generating controlled defects.