Two-dimensional boron monolayer sheets

Boron, a nearest-neighbor of carbon, is possibly the second element that can possess free-standing flat monolayer structures, evidenced by recent successful synthesis of single-walled and multiwalled boron nanotubes (MWBNTs). From an extensive structural search using the first-principles particle-swarm optimization (PSO) global algorithm, two boron monolayers (α ₁- and β ₁-sheet) are predicted to be the most stable α- and β-types of boron sheets, respectively. Both boron sheets possess greater cohesive energies than the state-of-the-art two-dimensional boron structures (by more than 60 meV/atom based on density functional theory calculation using PBE0 hybrid functional), that is, the α-sheet previously predicted by Tang and Ismail-Beigi and the g _1/8- and g _2/15-sheets (both belonging to the β-type) recently reported by Yakobson and co-workers. Moreover, the PBE0 calculation predicts that the α-sheet is a semiconductor, while the α ₁-, β ₁-, g _1/8-, and g _2/15-sheets are all metals. When two α ₁ monolayers are stacked on top each other, the bilayer α ₁-sheet remains flat with an optimal interlayer distance of ∼3.62 Å, which is close to the measured interlayer distance (∼3.2 Å) in MWBNTs.