  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.
