C-57 carbon: A two-dimensional metallic carbon allotrope with pentagonal and heptagonal rings

By means of the first-principles calculations, we have theoretically investigated the structural stability and electronic properties of a two-dimensional planar metallic carbon allotrope named C-57 carbon which possesses the P (6) over bar 2m (D-3h(3)) symmetry. This carbon allotrope is an all-sp(2) hybridized bonding network consisting of 5-7 rings of carbon atoms. The stability of C-57 carbon is confirmed through phonon-mode analysis, total energy and elastic constants calculations, as well as first-principles molecular dynamics simulations. We conceived that the metallicity of C-57 carbon is attributed to the large states across Fermi-level contributed by p(y) orbital due to the bond distortion, which is much different from that of graphite. This new carbon sheet can also serve as a precursor for stable one-dimensional nanotubes with metallic character. These results broaden our understanding of two-dimensional carbon allotropes and will attract more researchers to focus the research on the field of two-dimensional carbon materials. Besides, the C-57 carbon may be useful for designing of nano-electronic devices.