An Anomalous Enhancement of the A_g(2) Mode in the Resonance Raman Spectra of C₆₀ Embedded in Single-Walled Carbon Nanotubes during Anodic Charging

The effect of electrochemical charging on C 60 fullerenes embedded in single-walled carbon nanotubes (peapods) has been studied. The Raman A g (2) mode of C 60 fullerene has been used as a probe. The spectra were measured using seven different laser excitation energies. For cathodic charging we observed bleaching of the A g (2) mode independent of laser excitation energy. On the other hand for anodic charging the behavior of the A g (2) mode was dependent on laser excitation energy. The intensity of the A g (2) mode decreased only for the 1.16 eV laser excitation energy. In the case of other laser excitation energies (2.70, 2.60, 2.54, 2.41, 2.33, 2.18 eV) an increase of intensity has been observed. The dependence of the slope of the intensity on the electrode potential has been found to be a function of laser excitation energy. Observation of the upshift of the A g (2) frequency supported by first principles calculations suggests that only a small amount of charge penetrates the nanotube, resulting in partial charge on the encapsulated fullerenes.