Origin of Nanoscale Friction Contrast between Supported Graphene, MoS, and a Graphene/MoS Heterostructure.

Ultralow friction can be achieved with 2D materials, particularly graphene and MoS. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS, and a graphene/MoS heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces.