Computational searching for new stable graphyne structures and their electronic properties

We devised a useful computational strategy to search for new types of stable two-dimensional carbon allotropes. As an illustration, we considered all the isomers of C18H6 and found three stable building units for graphynes. Then, we investigated the stability of each unit {hexagonal/triangular/parallelogrammatic C18H6 (C18H6h/C18H6t/C18H6p)} in that π-conjugation effects improve the stability and conductivity of the system. The resonance energies of the above C18H6 are smaller than that of benzene, but still large; among the three types, C18H6t is the most stable, followed by C18H6p. This supports why the graphyne composed of C18H6t was already synthesized, while it is likely that the graphynes composed of C18H6h and C18H6p could also be synthesized. The graphynes composed of C18H6h and C18H6p were predicted to have zero band gaps with Dirac cones, whereas that composed of C18H6t turned out to be a semiconductor with the band gap of 0.49 eV. In particular, C18H6p has not only direction-dependent Dirac cones but also their unconventional locations in the first Brillouin zone.