Reversible oxidation effect in Raman scattering from metallic single-wall carbon nanotubes

Raman scattering from individual single-wall carbon nanotube (SWNT) bundles was measured using confocal optical microscopy with 632 nm laser excitatio n. The: Raman scattering from metallic SWNTs was found to depend sensitivel y on adsorbed oxidizing molecules. Tangential mode Raman lines of HNO3-trea ted SWNTs exhibited different line shapes from those of H2SO4/H2O2-treated SWNTs. However, the line shapes became identical after high power laser irr adiation, which heats the bundle to similar to 750 K based on measurement o f the breathing mode Stokes to anti-Stokes Raman intensity ratio. Thermal a nnealing of both samples at 873 K caused the same change in Raman spectra a s with laser irradiation, which is attributed to degassing of doping adsorb ates. A mechanism of charge transfer between carbon nanotubes and adsorbate molecules is proposed to explain the large change in Raman scattering from metallic SWNTs upon chemical doping and degassing. Raman scattering from d egassed SWNT bundles was found to show a linear dependence on excitation in tensity, suggesting that Raman cross sections do not change with temperatur e in the range 300-750 K.