We describe two points using a combination of experimental and computational efforts: an atomistic view of out-of-plane strained graphene and how effects of strain can be used to alter chemical reactivity at the surface; and the adsorption of water and oxygen on MoS2 and their effects on the tribological properties.
Molybdenum Disulfide (MoS2) and graphene are lamellar solids with many diverse applications, including semiconductor devices, catalysis and lubrication, due to their unique mechanical, electrical and optical properties. The layers of these materials interact weakly through van der Waals forces, enabling them to slide with a low coefficient of friction. Their low friction coefficient and flexibility makes MoS2 and graphene appealing solid lubricants for use in satellites and other aerospace applications where liquid lubricants are infeasible. However, exposure to environmental species, e.g. oxygen and water, along with increased reactivity due to out-of-plane strain, has been found to degrade these 2-D materials' lubricating properties, thus limiting usefulness for many potential applications. The precise mechanisms of these interactions and how they degrade tribological performance, however, remain poorly understood. Thus, we used atomistic simulation to construct models that accurately predict results from tribological experiments. By using surface science experiments to monitor the reaction of graphene with 4-nitrobenzenediazonium tetrafluoroborate (4-NBD) on flat silica and on a particle film consisting of 6-nm silica nanoparticles, it has been concluded that graphene conformed to curved surfaces can undergo a greater extent of reaction with its local environment than flat graphene. DFT calculations support this outcome, with the calculated energy of formation for the diazonium and curved graphene greater than with flat graphene. Using a combination of density functional theory and molecular dynamics simulations, we also show that the presence of defects, notably sulfur vacancies, significantly increases the affinity of MoS2 for adsorption of environmental species.
