The filling of flexible carbon nanotubes by molten salts

The direct filling of flexible single-walled carbon nanotubes by model molten salts are described. The simulations advance on previous work, in which the carbon nanotubes were treated as fixed atomistic entities, by considering the carbon nanotubes as fully-flexible. The molten salts are described using two potential models, which vary in the relative energetic stability of two key bulk crystal structures. Small diameter carbon nanotubes are found to favour relatively low symmetry "ladder'' and "hinged'' structures, which can both be considered as formed from sections of square net sheet, in contrast to cylindrical geometries favoured when filling fixed carbon tubes. The small diameter tubes are also found to favour structures based on folding square, rather than hexagonal, nets. Additional energy minimisation calculations are performed in order to understand both the formation of the less symmetric structures and the favouring of square net-based structures. The role of the flexible tubes in controlling the filling mechanisms and kinetics is discussed. Larger diameter carbon tubes are found to favour relatively disordered internal structures.