Electronic and Optical Properties of 2D Heterostructure Bilayers of Graphene, Borophene and 2D Boron Carbides from First Principles

In the present work the atomic, electronic and optical properties of two-dimensional graphene, borophene, and boron carbide heterojunction bilayer systems (Graphene-BC₃, Graphene-Borophene and Graphene-B₄C₃) as well as their constituent monolayers are investigated on the basis of first-principles calculations using the HSE06 hybrid functional. Our calculations show that while borophene is metallic, both monolayer BC₃ and B₄C₃ are indirect semiconductors, with band-gaps of 1.822 eV and 2.381 eV as obtained using HSE06. The Graphene-BC₃ and Graphene-B₄C₃ bilayer heterojunction systems maintain the Dirac point-like character of graphene at the K-point with the opening of a very small gap (20-50 meV) and are essentially semi-metals, while Graphene-Borophene is metallic. All bilayer heterostructure systems possess absorbance in the visible region where the resonance frequency and resonance absorption peak intensity vary between structures. Remarkably, all heterojunctions support plasmons within the range 16.5-18.5 eV, while Graphene-B₄C₃ and Graphene-Borophene exhibit a π-type plasmon within the region 4-6 eV, with the latter possessing an additional plasmon at the lower energy of 1.5-3 eV. The dielectric tensor for Graphene-B₄C₃ exhibits complex off-diagonal elements due to the lower P3 space group symmetry indicating it has anisotropic dielectric properties and could exhibit optically active (chiral) effects. Our study shows that the two-dimensional heterostructures have desirable optical properties broadening the potential applications of the constituent monolayers.