Bilayer Borophene Allotropes: Structural Stabilities and Electronic Properties

Borophene with rich polymorphs has largely enriched the family of two-dimensional materials and attracted great research interest. Recently, the evolution of bilayer borophene (BLB) structures has aroused curiosity about the exploration of more members of BLB and their stability mechanism. Herein, we predicted a number of stable BLBs built by borophene monolayers using density functional theory calculations. Interestingly, four BLB polymorphs, such as α 5 -BLB, β 8 -BLB, α 4 -BLB, and β 9 -BLB, are identified as semiconductors with band gaps of 0.81, 0.34, 0.04, and 0.02 eV, respectively. In addition, four other types of BLBs, i.e., α 3 -BLB, β 1 -BLB, β 6 -BLB, and β 12 -BLB, are found to be stable metals. It is revealed that the interlayer B–B pillars play a crucial role in balancing the distribution of surplus electronic density in BLBs and are responsible for their super stabilities than those of borophene monolayers. Besides, the strong interlayer covalent interaction plays an important role in the semiconducting properties of these BLBs. Our results are helpful to understand the stability mechanism of BLB structures and expand the family of bilayer borophenes.