Electronic structure of multiwall boron nitride nanotubes

We report on electron energy-loss spectroscopy studies of the electronic properties of multiwall boron nitride nanotubes, in terms of the B1s and N1s excitation edges and of the q-dependent energy-loss function. The q-dependent dielectric function shows a strong dispersion in momentum of the pi and sigma+pi plasmons indicating that they are spatially delocalized along the tube axis as expected according to their characteristic two-dimensional graphiticlike structure, and nondispersing excitations at 11 and 12 eV. The dielectric function epsilon of the boron nitride nanotubes reveals an intense pi-pi* interband transition at 5.4 eV, which is shifted to lower energies by 0.6 eV when compared to hexagonal BN in good agreement with recent theoretical band-structure calculations of boron nitride nanotubes.