Boron nitride nanotubes

Pure boron nitride (BN) nanotubes have been synthesised by arc discharge between HfB₂ electrodes in a nitrogen atmosphere. The high resolution electron microscopy (HREM) observations reveal that this route leads to the formation of highly crystalline tubes with reduced numbers of layers, including tubes with only one or two layers. These nanotubes are found to be chiral or non-chiral, however, a preference towards the armchair and zig zag configurations is suggested. Electron energy loss spectroscopy yields a B:N ratio of approximately one and a perfect chemical homogeneity. Tubes are empty and closed at their ends by flat layers perpendicular to the tube axis. A tip model consisting of a triangular facet based on three 120° disclinations and preserving BN bonds in the honeycomb network accounts for the observations. Finally, preliminary electron irradiation experiments on these tubes reveal a specific behaviour. BN tubes transform into aggregates of small cages with diameter between 0.5 and 0.8 nm. These diameters suggest that these shells might be B₁₂N₁₂, B₁₆N₁₆ and B₂₈N₂₈ fullerenes which were predicted to be magic clusters.