Stable double helical iodine chains inside single-walled carbon nanotubes

• The stable double helical iodine chain inside single-walled carbon nanotubes is proposed.
• The influence of tube's diameter and chirality on the stability of encapsulated iodine chains is studied.
• The metastable double helical structures may be co-existence with the stable structure but not in the same tubes.

The helicity of stable double helical iodine chains inside single-walled carbon nanotubes (SWCNTs) is studied by calculating the systematic interaction energy. Our results present clear images of stable double helical structures inside SWCNTs. The optimum helical radius and helical angle increase and decrease with increasing diameter, respectively. The tube's diameter plays a leading role in the helicity of encapsulated structures, while the tube's chirality may induce different metastable structures. This study indicates that the observed double helical iodine chains in experiments are not necessarily the optimum structures, but may also be metastable structures.