Alignment of single-wall carbon nanotubes by inclusion of dipolar wires

When a semiconducting single wall nanotube is filled by a dipolar, one-dimensional, molecular wire, its polarization is strongly enhanced leading to the occurrence of a large induced dipole. Upon exposure to an external electric field on the order of 10(-2) V A(-1), we show that the orientational energy of a tube with length equal to 30 A filled by a set of 11 aligned dipolar molecules forming a wire can be 10 times (5 times) smaller than for a similar empty tube at 0 K (300 K). This factor increases to 60 (20) at 0 K (300 K) for a tube length that becomes infinite with an external field equal to 10(-4) V A(-1). The tube orientation is therefore largely stabilized by the wire inclusion. This energy gain is mainly due to the induced dipole which fully dominates the permanent dipole of the wire due to the screening of the nanotube. Extension of these results to two identical nearest neighbor tubes indicates that an assembly of filled nanotubes should be more stable in terms of orientational energy than a set of empty tubes.