Strong Repulsive Lifshitz-van der Waals Forces on Suspended Graphene
Understanding surface forces of two-dimensional (2D) materials is of fundamental importance as they govern molecular dynamics and atomic deposition in nanoscale proximity. Despite recent observations in wetting transparency and remote epitaxy on substrate-supported graphene, very little is known about the many-body effects on their van der Waals (vdW) interactions, such as the role of surrounding vacuum in wettability of suspended 2D monolayers. Here we report on a stark repulsive Lifshitz-van der Waals (vdW) force generated at surfaces of suspended 2D materials, arising from quantum fluctuation coupled with the atomic thickness and birefringence of 2D monolayer. In combination with our theoretical framework taking into account the many-body Lifshitz formulism, we present direct measurement of Lifshitz-vdW repulsion on suspended graphene using atomic force microscopy. We report a repulsive force of up to 1.4 kN/m$^2$ at a separation of 8.8 nm between a gold-coated AFM tip and a sheet of suspended graphene, more than two orders of magnitude greater than the Casimir-Lifshitz repulsion demonstrated in fluids. Our findings suggest that suspended 2D materials are intrinsically repulsive surfaces with substantially lowered wettability. The amplified Lifshitz-vdW repulsion could offer technological opportunities such as molecular actuation and controlled atomic assembly.