Carbon nanotube plectonemes: Loops of twisted helices

The relaxation of twist in elastic filaments often drives conformational changes. We explore this paradigm using all-atom computations and report the formation of novel supercoiled shapes in individual carbon nanotubes (CNTs). Decreasing the end distance of torsionally constrained CNTs leads to spontaneous nucleation and growth of a nanotube plectoneme. We develop a stability diagram and comparisons with theoretical frameworks reveal the importance of non-local van Der Waals interactions. In some cases, they stabilize the supercoiling to an extent that its tip locally kinks and then irreversibly reconstructs into a disordered yet strengthened structure that involves $sp^3$ bonding. The ability to engineer supercoiled conformations of CNTs and related nanoscale filaments opens the possibility of a unique set of tunable functional properties at the nanoscale.