Structural stabilities and electronic properties of planar C4 carbon sheet and nanoribbons

By means of first-principles calculations, we theoretically studied the structural stabilities and electronic properties of a pure-carbon 2D covalent metal named planar C-4 in P4/mmm (D-4h(1)) symmetry. Planar C-4 is confirmed to be dynamically stable in the ground state based on phonon-mode analysis, and it is more stable than graphyne and the recently prepared graphdiyne. Moreover, it has a higher density of states (DOS) at the Fermi level than any plausible metallic carbon nanotube. Of particular interest, there exist two distinct types of planar C-4 nanoribbons (NRs): type I is predicted to be uniformly metallic regardless of the width change, while type II exhibits remarkable odd-even metal-semiconductor oscillating behavior depending on the width. The edge structure of type II NRs is revealed to be energetically more favored since its formation energy is about 0.45 eV per edge atom lower than that of type I NRs. Our work shows that planar C-4 carbon sheet and its NRs could serve as potential materials for future functional nanodevices.