Boron Nitride vs. Carbon Nanotubes

Theory of Graphitic Boron Nitride Nanotubes

Angel Rubio, Jennifer L. Corkill, and Marvin L. Cohen

PRB 49, 5081--5084 (1994)



URL: http://link.aps.org/abstract/PRB/v49/p5081

This paper came out just a year after the discovery of carbon nanotubes. It really was a recent discovery when this paper was written. The authors discuss what to expect from boron nitride nanotubes.

I didn't realize how different boron nitride was until I read this paper. It's definitely not graphene with a sublattice asymmetry! The lattice spacing is similar: 1.45 angstroms in BN, 1.42 in graphene. Apparently, the average on-site potential is similar as well. The authors say this can be inferred from the band withs of the parent crystals. However, the similarity ends there.

Boron nitride is a semiconductor with an INDIRECT gap on the order of 5 electron volts. All boron nitride nanotubes are semiconducting as well.

The most surprising result is the scaling of bandgap with tube radius. In carbon nanotubes, the well-known result is that the band gap is inversely proportional to the nanotube radius. In boron nitride, the band gap INCREASES with increasing radius until it approaches the free BN sheet band gap. The effect of curvature reduces the band gap of boron nitride.

This made no sense to me, with my background in nanotubes. The authors point out that the band gap of hexagonal boron nitride decreases with increasing pressure. Equating the strain of a curved tube with pressure, I can at least make sense of the effect.

It is interesting that only some of the boron nitride nanotubes are indirect gap semiconductors even though the parent BN sheet is. The (n,0) tubes are direct gap. All carbon nanotubes are direct gap semiconductors.

The authors use a tight-binding model with first and second nearest neighbor interactions. I'd like to know how many parameters are in their model. Surely more than the two parameters of the corresonding graphene model.

I'll have to keep these differences in mind as I continue my research. I would not have expected such disparity between these two similar structures.