The in situ Raman spectroscopy technique was used to investigate the ion transport and to determine the concomitant electrochemical tuning of Fermi level in single-wall carbon nanotubes. The variation of structural bonding in a single-wall carbon nanotube bundle dipped in aqueous alkaline earth halide electrolyte such as CaCl2 with electrochemical biasing was monitored. This is because Raman scattering can detect changes in C–C bond length through radial breathing mode (RBM) at ∼184 cm−1, which varies inversely with the nanotube diameter and the G band at ∼1590 cm−1, varying with the axial bond length. Consistent reversible and substantial variation in Raman intensity of both modes was induced by electrode potential point at the fine and continuous tuning (alternatively, emptying/depleting or filling) of the specific bonding and anti-bonding molecular states. Qualitatively, the results were explained in terms of changes in the energy gap occurring between the one-dimensional van Hove singularities present in the electron density of states, possibly arising due to the alterations in the overlap integral of π bonds between the p orbitals of the adjacent carbon atoms. We estimated the extent of variation of the absolute potential of the Fermi level and overlap integral (γ0) between the nearest-neighbor carbon atoms by modeling the electrochemical potential dependence of Raman intensity. Observations also suggested that the work function of the tube becomes larger for the metallic nanotubes in contrast to the simultaneously present semiconducting nanotubes.
Ion transport and electrochemical tuning of Fermi level in single-wall carbon nanotube probed by in situ Raman scattering
