Temperature-induced transformations in hydrogenated and fluorinated single-wall carbon nanotubes studied by Raman scattering

Raman spectra of hydrogenated and fluorinated single-wall carbon nanotubes (SWCNTs) are measured at ambient temperature before and after heat treatment. The spectra of the as-prepared hydrogenated SWCNTs show a giant structureless band in the visible region that screens the Raman peaks related to the carbon atom vibrations. The onset of this strong band follows the excitation laser line, which is typical of hot luminescence. The intensity of the luminescence background decreases exponentially with the annealing time, while the dependence of the luminescence decay time constant on the annealing temperature is of the Arrhenius type with the activation energy E _a = 465 ± 44 meV. The luminescence background in the Raman spectra of the fluorinated SWCNTs is comparable with the Raman peak intensity and decreases exponentially with the annealing time. The dependence of the decay time constant on the temperature is again of the Arrhenius type with the activation energy E _a = 90 ± 8 meV. The appearance of hot luminescence is related to the upshift of the fundamental energy gap in functionalized SWCNTs and the structural disorder induced by random binding of hydrogen or fluorine atoms. The luminescence background disappears upon annealing in vacuum or in air after removal of hydrogen (fluorine), while the annealed samples still demonstrate large structural disorder.