  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 (AFM). 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.
