Pentagonal Monolayer Crystals of Carbon, Boron Nitride, and Silver Azide

In this study we present a theoretical investigation of structural, electronic and mechanical properties of pentagonal monolayers of carbon (p-graphene), boron nitride (p-B${}_2$N${}_4$ and p-B${}_4$N${}_2$) and silver azide (p-AgN${}_3$) by performing state-of-the-art first principles calculations. Our total energy calculations suggest feasible formation of monolayer crystal structures composed entirely of pentagons. In addition, electronic band dispersion calculations indicate that while p-graphene and p-AgN${}_3$ are semiconductors with indirect bandgaps, p-BN structures display metallic behavior. We also investigate the mechanical properties (in-plane stiffness and the Poisson’s ratio) of four different pentagonal structures under uniaxial strain. p-graphene is found to have the highest stiffness value and the corresponding Poisson’s ratio is found to be negative. Similarly, p-B${}_2$N${}_4$ and p-B${}_4$N${}_2$ have negative Poisson’s ratio values. On the other hand, the p-AgN${}_3$ has a large and positive Poisson’s ratio. In dynamical stability tests based on calculated phonon spectra of these pentagonal monolayers, we find that only p-graphene and p-B${}_2$N${}_4$ are stable, but p-AgN${}_3$ and p-B${}_4$N${}_2$ are vulnerable against vibrational excitations.