Honeycomb Boron Allotropes with Dirac Cones: A True Analogue to Graphene

Graphene has received much attention for its novel properties, especially Dirac points. Boron, the neighbor of carbon in the periodic table, has also been focused on its Dirac structures. Here, we propose a series of planar boron allotropes with honeycomb topology and demonstrate that their band structures exhibit Dirac cones at the K point, the same as graphene. In particular, the Dirac point of one honeycomb boron sheet locates precisely on the Fermi level, rendering it as a topologically equivalent material to graphene. Its Fermi velocity (v_f) is 6.05 × 10⁵ m/s, close to that of graphene. Although the freestanding honeycomb boron allotropes are higher in energy than α-sheet, our calculations show that a metal substrate can greatly stabilize these new allotropes. They are actually more stable than α-sheet on the Ag(111) surface. Furthermore, we find that the honeycomb boron form low-energy nanoribbons that may open gaps or exhibit strong ferromagnetism at the two edges in contrast to the antiferromagnetic coupling of the graphene nanoribbon edges.