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 B 22 (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 B 22 (3) and staggered B 22 (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 B 22 (3) and B22 (4),  similar to the situation in monolayer graphene which contains one delocalized 6c-2e π bond over each C 6 hexagon. Furthermore,  these BL-α+ borophenes appear to remain highly stable on Ag(111) substrate, presenting the possibility to form supported BL-α+  borophenes.