  Using molecular dynamics simulations, we find an in-plane negative Poisson's
ratio intrinsically existing in the graphene-based three-dimensional (3D)
carbon foams (CFs) when they are compressed uniaxially. Our study shows that
the negative Poisson's ratio in the present CFs is attributed to their unique
molecular structures and triggered by the buckling of the CF structures. This
novel mechanism makes the negative Poisson's ratio of CFs strongly depend on
their cell length, which offers us an efficient means to tune the negative
Poisson's ratio in nanomaterials. Moreover, as the buckling modes of CFs are
topographically different when they are compressed in different directions,
their negative Poisson's ratio is found to be strongly anisotropic, which is in
contrast to the isotropic positive Poisson's ratio observed in CFs prior to the
buckling. The discovery of the intrinsic negative Poisson's ratio in 3D CFs
will significantly expand the family of auxetic nanomaterials. Meanwhile, the
novel mechanism of nano-auxetics proposed here may open a door to manufacture
new auxetic materials at the nanoscale.
