Understanding the Independent and Interdependent Role of Water and Oxidation on the Tribology of Ultrathin Molybdenum Disulfide (MoS2)

In this work, the tribological behavior of ultrathin MoS2 is investigated to understand the independent roles of water and oxidation. Water adsorption is identified as the primary interfacial mechanism for both SiO2/pristine-MoS2 and SiO2/graphene interfaces, however, tribological behavior of pristine-MoS2 is observed to be more sensitive to presence of water due to stronger MoS2-water interaction. Comparison of pristine-MoS2 and oxidized-MoS2 reveals that the oxidation of MoS2 significantly increases its friction and sensitivity to water by playing a more detrimental role. The specific effect of oxygen on friction via chemical interactions is studied in isolation through density functional theory simulations of a tip sliding on MoS2 basal planes and over edges before and after oxidation. The maximum change in energy, or energy barrier correlating with friction, as the tip moves across the surface, increases after oxidation by up to 66% for the basal plane and by 25% at the edge. Charge density analysis suggests that the more localized and nonuniform interfacial charge distribution on oxygen-rich surfaces, as compared to pristine surfaces, leads to higher resistance to sliding. This confirms that oxygen presence alone increases friction and when coupled with the presence of water, both effects are additive in increasing friction.