Direct Measurement of Nonadditive van der Waals Forces From Graphene Nanostacks

Abstract Theoretical evidence suggests strong nonadditivity of van der Waals (vdW) forces at the nanoscale(1,2), but direct experimental confirmation is missing(3). By aligning the underlying surface sample with a macroscopic tipless Si cantilever, we present essentially noise-free vdW force measurements up to micrometer separations from multilayered graphene (MLG) nanostacks of variable thickness (1-21 layers) adsorbed on SiO2 as well as from macroscopically-thick highly ordered pyrolytic graphite (HOPG). At contact cantilever-surface separation, the vdW force increases non-additively but monotonically from graphene to HOPG. Conversely, at separations D>100 nm the vdW forces from MLG exhibit tunable power laws and the magnitude of the force on top of thick MLG nanostacks substantially exceeds the force measured on top of HOPG, despite the latter showing a slower decaying power law (D^-4 for MLG vs D^-3 for HOPG). Such a strong nonadditivity and tunable power laws of vdW forces at the nanoscale are rationalized by many-body calculations that include the nonlocal polarization response in graphene nanostacks.