Design of Two-Dimensional Graphene-like Dirac Materials β12-XBeB5 (X = H, F, Cl) from Non-graphene-like β12-Borophene.

Two-dimensional (2D) Dirac materials and boron sheets have attracted intensive interest recently. However, 2D Dirac materials remain rare and difficult to be realized experimentally, and 2D boron sheets generally have high dynamical instability. Stimulated by the experimental observation of Dirac cones in nongraphene-like β12 boron sheets and based on the understanding of boron sheet electronic organization, we theoretically design new 2D Dirac materials β12-XBeB5 (X = H, F, Cl) with high stability. We confirm β12-HBeB5 as the global energy minimum among its 2D allotropes based on global structure search methods, a strong indication of its experimental feasibility. Our designed β12-HBeB5 has not only a high Fermi velocity, but also a Dirac state very robust against extraordinary large tensile strains, an advantage for flexible electronics applications. Our work opens a new avenue to designing feasible 2D Dirac materials and stabilizing borophene sheets.