Structure and stability of bilayer borophene: The roles of hexagonal holes and interlayer bonding

Two-dimensional (2D) boron monolayers with diversity of structures and extraordinary physical properties have been extensively investigated using first-principles calculations. A series of boron bilayer sheets with pillars and hexagonal holes have been constructed. Many of them have lower formation energy than α-sheet boron monolayer. The structural stability and chemical bonding character of these boron bilayers are analyzed by charge density, electron localization function and Bader charge, indicating that the optimal proportions of pillars and hexagonal holes can be obtained by balancing the surplus electrons. Meanwhile, the distribution and arrangement of hexagonal holes can cause ignorable effect on the stability of structures. These findings pave the way for the structural diversity of 2D boron materials.