Using atomic force microscopy (AFM), we study the adhesive, frictional and elastic properties of supported and suspended graphene exfoliated onto pit-patterned silicon dioxide-on-silicon (SiO2/Si) substrates. In spite of the greater adhesive force between the AFM tip and supported graphene in comparison with that between the AFM tip and bare SiO2 surface, we observe significantly lower friction on supported graphene than that on bare SiO2. This is explained by a greater material compliance and tip-sample contact area combined with lower interfacial shear stress for supported graphene. For suspended graphene in the form of circular membranes, we measured load-dependent topography to quantify prestress and in-plane Youngs modulus, as well as adhesive force and friction. We observed increasing adhesive forces for graphene membranes (suspended structures) with an increasing number of layers, which we explain in terms of van der Waals interactions between the AFM tip and the subsurface layers of the membrane. Interestingly, at low applied normal loads we also observed increasing friction for suspended graphene with increasing number of layers, in contrast to the established opposite trend for supported graphene. Furthermore, we found that the trend can be reversed for membranes by increasing the applied load. We attribute these observations to a competition between van der Waals forces acting between the AFM tip and the subsurface layers and local deformation of the layer directly below the AFM tip.
Citation: Physical Review B
Pub Type: Journals
graphene, friction, atomic force microscopy, simulation, adhesion