Spectroelectrochemistry of nanotubes

Spectroelectrochemistry of nanotubes

The in-situ spectroelectrochemistry is a convenient method for the study of the electronic structure of Single Walled Carbon Nanotubes (SWCNT). The main components of the Raman spectra of SWCNT are radial breathing mode (RBM), tangential displacement mode (TG), disorder induced mode (D) and high frequency two-phonon mode (G’). The tangential displacement mode is observed in the region of 1450-1600 cm^-1 and in the case of metallic tubes it exhibits pronounced Breit-Wigner-Fano (BWF) broadening. The RBM frequency of isolated (non-bundled) SWCNT is inversely proportional to the tube diameter:

(1)

where the constant C is reported to be in the range of 224 to 251 nm/cm^-1. The D and G’ modes are observed in all kinds of polycrystalline sp² carbon materials, however their physical origin has been explained only recently in terms of the double resonance theory. The D and G’ modes are observed in the spectral regions of 1250-1450 cm^-1 and  2500-2900 cm^-1, respectively. The one-phonon second order Raman D-band appears only if there is a breakdown in translational crystal symmetry, which can be caused by defects in the structure. On the other hand, the two-phonon second order Raman G’ mode occurs independently of the structural defects.

Fig. 1 shows the development of Raman spectra during the charging of SWCNT film deposited on platinum electrode. The spectra are excited by a 488 nm laser line. The main effect of the electrochemical doping is a subsequent bleaching of all main SWCNT features. The electrochemical doping leads to a shift of the Fermi level. Thus the van Hove singularities of SWCNT are filled/depleted upon cathodic/anodic charging. This induces the loss of the resonance Raman effect due to the loss of visible absorptions between the van Hove singularities and finally leads to the subsequent bleaching of all SWCNT features. Furthermore there are small shifts in the band positions during the doping. The most obvious is the upshift  of the TG mode at high anodic potentials. This effect is usually explained by hardening of carbon-carbon bond by the electrochemical doping.

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Prof. Lothar Dunsch