Reversible Oxidation Effect in Raman Scattering from Metallic Single-Wall Carbon Nanotubes

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Abstract

Raman scattering from individual single-wall carbon nanotube (SWNT) bundles was measured using confocal optical microscopy with 632 nm laser excitation. The Raman scattering from metallic SWNTs was found to depend sensitively on adsorbed oxidizing molecules. Tangential mode Raman lines of HNO3-treated SWNTs exhibited different line shapes from those of H2SO4/H2O2-treated SWNTs. However, the line shapes became identical after high power laser irradiation, which heats the bundle to ∼750 K based on measurement of the breathing mode Stokes to anti-Stokes Raman intensity ratio. Thermal annealing of both samples at 873 K caused the same change in Raman spectra as with laser irradiation, which is attributed to degassing of doping adsorbates. 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 degassed SWNT bundles was found to show a linear dependence on excitation intensity, suggesting that Raman cross sections do not change with temperature in the range 300−750 K.

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