  Ab initio calculations indicate that topological-defect networks in graphene
display the full variety of single-particle electronic structures, including
Dirac-fermion null-gap semiconductors, as well as metallic and semiconducting
systems of very low formation energies with respect to a pristine graphene
sheet. Corrugation induced by the topological defects further reduces the
energy and tends to reduce the density of states at the Fermi level, to widen
the gaps, or even to lead to gap opening in some cases where the parent planar
geometry is metallic.
