Thermal fluorination and annealing of single-wall carbon nanotubes

Single-wall carbon nanotubes (SWNTs) were fluorinated with F₂ at 250°C, and then some were heated under He to temperatures ranging from 100 to 400°C to desorb the fluorine. The resulting samples were studied with X-ray photoelectron spectroscopy, Raman spectroscopy, IR spectroscopy, and electrical resistance measurements. The goals were to identify, as a function of fluorination and defluorination, the nature of the fluorine bonding, the damage to the tubes, the changes to the average tube diameter and proportions of metallic and semiconducting tubes, and the effects on the electrical conductivity and its mechanism. The stoichiometry of the as-fluorinated tubes was CF_0.43. Most of the time, F atoms bonded covalently to one carbon atom. Smaller amounts of CF₂ and CF₃ were probably bonded primarily at various defect sites. The fluorinated single-wall nanotubes were highly insulating. Upon heating, the largest fluorine loss occurred between 200 and 300°C, and defluorination was virtually complete at 400°C, but the largest resistance change occurred at 150-200°C. Raman showed that both fluorination and thermal defluorination damaged the tubes by creating defects and/or amorphous carbon phases. Upon heating, most of the fluorinated SWNTs apparently reverted to SWNTs, rather than being etched. Thermal defluorination slightly increased the average tube diameter, but there was no Raman evidence of a change in the metallic/semiconducting tube ratio. However, the average length of tube bundles was substantially reduced. The resistance of the defluorinated SWNTs returned to that of the starting material, but a different conduction mechanism such as an amorphous carbon percolation network cannot be excluded.