Preferred orientations of encapsulated C₆₀ molecules inside single wall carbon nanotubes

A systematical study of the orientational behavior of C₆₀ molecules in single wall carbon nanotubes (SWCNTs) with different chirality and diameter has been performed by using a model of an infinite long nanotube filled with two C₆₀ (denoted as C₆₀-1 and C₆₀-2) molecules. We studied the preferred orientation of the C₆₀-1 molecule when the neighboring C₆₀-2 molecule was fixed at the pentagon, double-bond, and hexagon orientations respectively. Our results showed that the C₆₀-1 molecule prefers the pentagon (hexagon) orientation when the tube diameter is smaller (larger) than 1.31 nm (1.36 nm). For the tube diameter in between, the preferred molecular orientation of C₆₀-1 changes from pentagon to hexagon with the increasing tube diameter when the neighboring C₆₀-2 molecule is fixed at the pentagon or double-bond orientation. A novel vertex orientation for the C₆₀-1 molecule has been found when the C₆₀-2 molecule is fixed at the hexagon orientation.