Size Dependence in the Stabilities and Electronic Properties of α-Graphyne and Its Boron Nitride Analogue

We predict the stabilities of alpha-graphynes and their boron nitride analogues (alpha-BNyne), which are considered as competitors of graphene and two-dimensional hexagonal BN. On the basis of the first-principles plane wave method, we investigated the stability and structural transformations of these materials at different sizes using phonon dispersion calculations and ab initio finite temperature, molecular dynamics simulations. Depending on the number of additional atoms in the edges between the corner atoms of the hexagons, n, both alpha-graphyne(n) and alpha-BNyne(n) are stable for even n but unstable for odd n. alpha-Graphyne(3) undergoes a structural transformation, where the symmetry of hexagons is broken. We present the structure-optimized cohesive energies and electronic, magnetic, and mechanical properties of stable structures. Our calculations reveal the existence of Dirac cones in the electronic structures of alpha-graphynes of all sizes, where the Fermi velocities decrease with increasing n. The electronic and magnetic properties of these structures are modified by hydrogenation. A single hydrogen vacancy renders a magnetic moment of one Bohr magneton. We finally present the properties of the bilayer alpha-graphyne and alpha-BNyne structures. We expect that these layered materials can function as frameworks in various chemical and electronic applications.