Electron energy-loss spectroscopy study of the electronic structure of boron nitride nanotubes

Electron energy-loss spectra were obtained from single boron-nitride nanotubes (BNTs), which were synthesized by the are-discharge method. The pi: and pi + sigma plasmon energies of the BNTs were smaller than those of hexagonal boron-nitride (h-BN). The pi + sigma plasmon energy is explained by the surface plasmon excitation. Dielectric functions of the BNTs were derived from the loss functions by Kramers-Kronig analysis. The bandgap energy was obtained to be similar to 5 eV, which is smaller than that of h-BN of 5.8 eV, from the imaginary part of the dielectric function. Full width at half maximum (FWHM) value of the 1s --> pi* transition peak in boron It-shell excitation spectra was nearly the same as that of h-BN, whereas the FWHM value of the peak of carbon nanotubes was greater than that of graphite. No additional broadening of the peaks of the BNTs may be attributed to a weak interaction between the p(z) orbitals of adjacent boron and nitrogen atoms.