  Based on ab initio calculations, we study the effect of intercalating twisted
bilayer graphene with carbon. Surprisingly, we find that the intercalant pulls
the atoms in the two layers closer together locally when placed in certain
regions in between the layers, and the process is energetically favorable as
well. This arises because in these regions of the supercell, the local
environment allows the intercalant to form tetrahedral bonding with nearest
atoms in the layers. Intercalating AB- or AA-bilayer graphene with carbon does
not produce this effect; therefore, the nontrivial effect owes its origin to
both using carbon as an intercalant and using twisted bilayer graphene as the
host. This opens new routes to manipulating bilayer and multilayer van der
Waals heterostructures and tuning their properties in an unconventional way.
