Theoretical study of the molecular and electronic structure of one-dimensional crystals of potassium iodide and composites formed upon intercalation in single-walled carbon nanotubes

We report first principles density functional pseudopotential calculations on the molecular and electronic structures of one-dimensional crystals of KI and composites formed upon the intercalation of these ionic crystals within single wall carbon nanotubes (SWNT). For a series of K24I24@SWNT composites the influence of the diameter and chirality of the SWNT on the structures of the 2x2 KI crystals is discussed. The calculated I-I interplanar spacings along and across the KI crystal in the KI@(10,10) composite (3.40 A and 3.66 A, respectively) were in good agreement with the experimental lattice measurements reported by Sloan [3.46(0.03) A and 3.98(0.31) A] for 2x2 KI encapsulated within a SWNT of similar diameter [Chem. Phys. Lett. 329, 61 (2000)]. The energy of intercalating K24I24 within a SWNT was calculated and a Mulliken population analysis was performed for each of the four composites considered here. These Mulliken analyses indicate that a small amount of charge transfer occurs from the SWNT to the KI crystal and that the extent of this charge transfer is dependent on the diameter of the confining nanotube. The intercalation energies exhibit a similar dependency on the diameter of the confining nanotube, with insertion of the 2x2 crystal of KI becoming increasingly exothermic upon reduction of the nanotube diameter from similar to 1.36 nm to similar to 1.10 nm.