Prediction of freestanding semiconducting bilayer borophenes

Supported bilayer α-borophene (BL-α borophene) on Ag(111) substrate has been synthesized in recent experiments. Based on the experimentally observed quasi-planar C6v B36 (1), its monolayer assembly α+-borophene B11 (P6/mmm) (2), and extensive global minimum searches augmented with density functional theory calculations, we predict herein freestanding BL-α+ borophenes B22 (Cmmm) (3) and B22 (C2/m) (4) which, as the most stable BL borophenes reported to date, are composed of interwoven boron triple chains as boron analogs of monolayer graphene (5) consisting of interwoven carbon single chains. The nearly degenerate eclipsed B22 (3) and staggered B22 (4) with the hexagonal hole density of η = 1/12 and interlayer bonding density of u = 1/4 appear to be two-dimensional semiconductors with the indirect band gaps of 0.952 and 1.144 eV, respectively. Detailed bonding analyses reveal one delocalized 12c-2e π bond over each hexagonal hole in both the B22 (3) and B22 (4), similar to the situation in monolayer graphene which contains one delocalized 6c-2e π bond over each C6 hexagon. Furthermore, these BL-α+ borophenes appear to remain highly stable on Ag(111) substrate, presenting the possibility to form supported BL-α+ borophenes.