Stable structures and electronic properties of the oriented Bi nanowires and nanotubes from first-principle calculations

We report the results of first-principle calculations on Bi nanowires and nanotubes oriented along the [012] direction. The shapes of the cross section have an important effect on the stability of Bi nanowires resulting from different reconstructions. Among the Bi nanowires studied, the most stable ones have the core-shell structures and are indirect gap semiconductors. The band gap decreases with increasing diameter, which explicitly demonstrates strong quantum size effects observed in previous experiments. For the Bi nanotubes, our results show that the nanotube with a square cross-section is more stable energetically than the corresponding circular one, and the electronic properties of Bi nanotubes depend sensitively on the wall thicknesses rather than the diameters of nanotubes. These findings might have important implications for understanding the formation of Bi nanowires and nanotubes and utilizing them as building blocks for nanoscale devices.