Substrate effect on thickness- dependent friction on graphene

Using friction force microscopy, we have investigated the frictional behavior of graphene deposited on various substrates as well as over micro-fabricated wells. Both graphene on SiO2/ Si substrates and graphene freely suspended over the wells showed a trend of increasing frictionwith decreasing number of atomic layers of graphene. However, this trend with thickness was absent for graphene deposited onmica, where the graphene is strongly bonded to the substrate.Measurements together with a mechanics model suggest that mechanical confinement to the substrate plays an important role in the frictional behavior of these atomically thin graphite sheets. Loosely bound or suspended graphene sheets can pucker in the out-of-plane direction due to tip-graphene adhesion. This increases contact area, and also allows further deformation of the graphene when sliding, leading to higher friction. Since thinner samples have lower bending stiffness, the puckering effect and frictional resistance are greater. However, if the graphene is strongly bound to the substrate, the puckering effect will be suppressed and no thickness dependence should be observed. The results can provide potentially useful guidelines in the rational design and use of graphene for nano-mechanical applications, including nano-lubricants and components in microand nanodevices.