Single-layer MoS₂ roughness and sliding friction quenching by interaction with atomically flat substrates

We experimentally study the surface roughness and the lateral friction force in single-layer MoS₂ crystals deposited on different substrates: SiO₂, mica, and hexagonal boron nitride (h-BN). Roughness and sliding friction measurements are performed by atomic force microscopy. We find a strong dependence of the MoS₂ roughness on the underlying substrate material, being h-BN the substrate which better preserves the flatness of the MoS₂ crystal. The lateral friction also lowers as the roughness decreases, and attains its lowest value for MoS₂ flakes on h-BN substrates. However, it is still higher than for the surface of a bulk MoS₂ crystal, which we attribute to the deformation of the flake due to competing tip-to-flake and flake-to-substrate interactions.