First-principles calculations were based on the framework of DFT, as implemented in Quantum ESPRESSO , with the PBE exchange and correlation functional. Ultra-soft pseudo-potentials of the RRKJUS type were used . We employed a plane wave basis set with kinetic energy cutoffs of 40 Ry for the wave functions. The Van der Waals interactions were described using the potential of Grimme . Both supercell and nanoflake models were used. The flakes had 16 carbon atoms per layer, whereas the supercell model contained 32 atoms per layer. For the latter, the Brillouin zone was sampled using a Γ-centered 6×6×1 Monkhorst-Pack (MP) grid . A supercell periodicity of 30-40 Angstrom in the direction perpendicular to the layers was used to avoid spurious interactions between replicas.