Understanding the Bonding Mechanism in Hexagonal B–S Monolayers: As Examples of B ₆ S ₂ and B ₇ S ₂

The unique properties of boron have driven advancements in the development of various boron-based two-dimensional (2D) materials. In most stable 2D B–S compounds, each S atom bonds with three of its surrounding B atoms instead of two. Here, we proposed two novel hexagonal B–S monolayers, B6S2 and B7S2, by using the first-principles calculations. Both monolayers have relatively large cohesive energies (more than 5.3 eV for each atom), suggesting their stability and experimental viability. In both monolayers, each S atom takes some electrons from B atoms and forms three covalent bonds with its three nearest B atoms. Interestingly, the three lone electron pairs of the S atom occupy the sp2 bonding orbitals of the B–S covalent bonds, and the remaining electrons occupy the pz orbitals of both B and S atoms. The B atoms form sp2 covalent bonds with each other and form hexagonal boron rings in the B6S2 monolayer, while the central boron atoms within the boron rings exhibit partially delocalized metallic bonding states in the B7S2 monolayer. Due to the partially occupied pz bonding orbitals of B atoms, both B6S2 and B7S2 monolayers are metallic. Our calculations are helpful for understanding the bonding mechanism in 2D B–S compounds and provide pathways to explore novel boron-based 2D materials.