The electronic (band) structure of carbon nanotubes (CNT) can be roughly understood from that of graphene by imposing periodic boundary conditions along one axis. We immediately run into a problem – how did the two edges of the graphene join? Is the longitudinal axis oriented directly parallel with para- positions of the hexagons, or is it at a slight angle? Another way to put the same question, does a curve around the nanotube’s periphery and joining unit cell centers describe a circle or a spiral?
It turns out that the band structure critically depends on just this geometry. CNT’s can be metallic, semi-conductive or semi-metallic according to how they zip up. (The nanotube’s diameter plays a lesser role.) Experimentally, there is no good way to synthesize only one type.
A review posted this week explores the electronic structure of CNT’s in some detail. Among the many implications are that electrons in CNT’s display surprising spin-orbit coupling, which affects the lifetime of prepared spin states, among other things.
Futhermore, electrons can display very strong correlation. In many CNT’s, a valence electron has an effective Coulomb radius in the same order of magnitude as the radius of the CNT. Therefore, electrons “line up” along the nanotube axis, unable to get out of each other’s way. (This is, of course, a classical picture. Quantally, the electrons are indistinguishable.)
Reference: Laird et al, “Quantum transport in carbon nanotubes”, arXiv:1403.6113.