ELECTRONIC-PROPERTIES OF CARBON NANOTUBES WITH POLYGONIZED CROSS-SECTIONS

The electronic properties of carbon nanotubes having polygonized cross sections instead of purely circular ones, such as recently observed u sing transmission electron microscopy, are investigated with plane-wav e ab initio pseudopotential local-density-functional calculations and simple light-binding models. Strong sigma-pi* hybridization effects o ccur in zigzag nanotubes due to the: high curvature located near the e dges of the polygonal cross-section prism. These effects, combined wit h a lowering of symmetry, dramatically affect the electronic propertie s of the nanotubes. It is found that modified low-lying conduction-ban d states are introduced either into the bandgap of insulating nanotube s, or below the degenerate states that form the top of the valence ban d of metallic nanotubes, leading the corresponding nanostructures to b e metals, semimetals, or at least very-small-gap semiconductors. The d egree of the polygon representing the cross section of the tube, and t he sharpness of the edge angles, are found to be major factors in the hybridization effect, and consequently govern the electronic behavior at the Fermi level.