Frustration Effects and Microscopic Growth Mechanisms for BN Nanotubes

Using ab initio molecular dynamics simulations, we have studied the growth mechanisms of boron-nitride (BN) nanotubes. In the experimental conditions of temperature, the behavior of single-wall BN nanotubes strongly depends on their helicity. Open-ended ``zigzag'' tubes close into an amorphouslike tip, preventing further growth. In the ``armchair'' case, the formation of squares traps the tip into a flat cap able to revert to a growing hexagonal network by incorporating incoming atoms. These findings are related to the greater stability of B-N bonds as compared to B-B or N-N bonds.