Coil Formation in Multishell Carbon Nanotubes: Competition between Curvature Elasticity and Interlayer Adhesion

To study the shape formation process of carbon nanotubes, a string equation describing the possible existing shapes of the axis curve of multishell carbon tubes (MCTs) is obtained in the continuum limit by minimizing the shape energy. It is shown that there exists a threshold relation of the outmost and inmost radii that gives a parameter regime in which a straight MCT will be bent or twisted. Among the deformed shapes, the regular coiled MCTs are shown to be one of the solutions of the string equation. In particular, the optimal ratio of pitch $p$ and radius $r_0$ for such a coil is found to be equal to $2\pi$, which is in good agreement with recent observation of coil formation in MCTs by Zhang et al.