Optical Transitions in Single-Wall Boron Nitride Nanotubes

Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride ($h\mathrm{\text{−}}\mathrm{B}\mathrm{N}$) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the $h\mathrm{\text{−}}\mathrm{B}\mathrm{N}$ sheet cannot explain a gap width of the boron nitride nanotubes below the $h\mathrm{\text{−}}\mathrm{B}\mathrm{N}$ gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.

Figure 1

HRTEM image of the raw material of boron nitride nanotubes. Single-wall boron nitride nanotubes are observed. $h\mathrm{\text{−}}\mathrm{B}\mathrm{N}$ particles and boron particles encapsulated in a shell of a few $h\mathrm{\text{−}}\mathrm{B}\mathrm{N}$ layers are also observed.

Figure 2

Optical absorption spectrum of hexagonal boron nitride microcrystals deposited on a quartz substrate.

Figure 3

Optical density of an assembly of single-wall boron nitride nanotubes deposited on a quartz substrate. Inset: Optical absorption spectrum of an assembly of single-wall carbon nanotubes deposited on a glass substrate.