Abstract
The neighboring layers in bi-layer (and few-layer) graphenes of both AA and AB
stacking motifs are known to be separated at a distance corresponding to van der Waals (vdW)
interactions. In this Letter, we present for the first time a new aspect of graphene chemistry in
terms of a special chemical bonding between the giant graphene “molecules”. Through rigorous
theoretical calculations, we demonstrate that the N-doped graphenes (NGPs) with various doping
levels can form an unusual two-dimensional (2D) pi−pi bonding in bi-layer NGPs bringing the
neighboring NGPs to significantly reduced interlayer separations. The interlayer binding energies
can be enhanced by up to 50% compared to the pristine graphene bi-layers that are characterized
by only vdW interactions. Such an unusual chemical bonding arises from the pi−pi overlap across
the vdW gap while the individual layers maintain their in-plane pi-conjugation and are
accordingly planar. The existence of the resulting interlayer covalent-like bonding is corroborated
by electronic structure calculations and crystal orbital overlap population (COOP) analyses. In
NGP-based graphite with the optimal doping level, the NGP layers are uniformly stacked and the
3D bulk exhibits metallic characteristics both in the in-plane and along the stacking directions.
