Onion-shell nuclei on monolayer MoS₂ facilitates friction reduction

Monolayer MoS₂ has garnered significant interest due to its exceptional optoelectronic and tribological properties and potential application as a lubrication layer in micro- and nano-electromechanical systems. Although the nanotribological performance of CVD-grown MoS₂ and the characteristics associated with CVD growth have been extensively studied, challenges remain in designing specific regions on the monolayer MoS₂ surface with reduced friction. Here, we develop nuclei with an onion-shell structure on CVD-grown monolayer MoS₂ to achieve remarkable friction and adhesion reduction. These nuclei, dispersed on high-quality and crystalline MoS₂, consist of an oxi-sulfide core surrounded by a multilayer MoS₂ shell. Lateral force microscopy results indicate that onion-shell nuclei create an ensemble effect that decreases friction and adhesion by up to 45% and 20%, respectively, compared to those of MoS₂ because of the multilayer structure and in-plane tensile strain, both of which minimize out-of-plane deformation. DMT model calculations and step-down load-friction correlations illustrate that the work of adhesion, shear strength, and coefficient of friction on the nucleus decrease by more than 22%, 19%, and 34%, respectively, compared with those on MoS₂. The onion-shell nucleus presents a novel lubrication strategy to mitigate friction and adhesion in CVD-grown 2D materials, with potential applications in lubricating nanoscale friction pairs.