Zhao Deng, Hua Xu, Nikolai Klimov, Rachel J. Cannara


One of the more elusive elements of nanotribology is the relationship between friction and adhesion. Graphene is a model material for investigating the impact of adhesion on nanoscale frictional energy dissipation with respect to nano- and microscale device structure. Using atomic force microscopy (AFM) with very high spatial resolution and sub-nN force sensitivity, we studied the adhesional and frictional properties of both suspended and supported graphene exfoliated onto pre-fabricated silicon dioxide microstructures. The atomic lattice structure of supported graphene, acquired from stick-slip friction images, showed lattice distortion due to the nanoscale roughness of the underlying substrate. A thirty-degree rotation of the unit cell was observed for double-layer with respect to single-layer suspended graphene. For all of the cases investigated, suspended single-layer graphene exhibited the lowest adhesive forces, as measured by various AFM probe materials. Surprisingly, supported graphene produced the greatest adhesive forces, even in comparison with the silicon dioxide surface. Frictional measurements at low applied normal loads indicated that the suspension of single- and multilayer graphene reverses the friction contrast, which has been observed for supported structures and at high loads both here and elsewhere1: For the low load regime studied here, friction was greater for suspended multilayers than for single-layers, whereas, at high loads and for supported graphene, friction typically decreases with the addition of layers. Similar to the low-load friction, adhesive forces were also greater for the suspended multilayers than for the single-layer membranes. This is unexpected if one considers only the bulk mechanical properties of graphite. With increasing load, the frictional contrast between suspended single-layer and multilayer graphene faded or reversed, as a result of interlayer interactions that can occur for multilayers. The correlation of adhesive forces and friction for suspended graphene provides insight into the effects of elastic properties on frictional energy dissipation.

1 Lee, C. et al., Science 328, 76 (2010).