Ab initioprediction of stable boron sheets and boron nanotubes: Structure, stability, and electronic properties

Using first-principles calculations, we predict a novel stable boron sheet and boron nanotubes which show various electronic properties. The boron sheet is flat and has the structure that the two centers of each three hexagons in the hexagonal lattice are filled with additional atoms, which preserves the symmetry of the triangular lattice. The boron sheet is metal, and there are bands similar to the $\ensuremath{\pi}$ bands in the graphene near the Fermi level. Rolled from the sheet, the nanotubes with diameter larger than $17\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ are metals. Smaller nanotubes are semiconductors with the gap decreasing as the diameter and chiral angle increase.