Boron-mediated growth of long helicity-selected carbon nanotubes

We investigate the growth of B-doped carbon nanotubes combining experimenta l and theoretical techniques. Electron microscopy observations and electron diffraction patterns reveal that B doping considerably increases the lengt h of carbon tubes and leads to a remarkable preferred zigzag chirality. The se findings are corroborated by first-principles static add dynamical simul ations which indicate that, in the zigzag geometry, B atoms act as a surfac tant during growth, preventing tube closure. This mechanism does not extend to armchair tubes, suggesting a helicity selection during growth.