The high-energy vibrational modes of metallic carbon nanotubes are believed to be softened compared to the semiconducting ones by a Peierls-like transition. The Raman modes, when excited with a red laser to enhance the metallic tubes, were found to exhibit an exceptionally high sensitivity to electrochemical doping. Our data may be interpreted as controlling the Peierls-like instability in metallic tubes with the applied potential. We also discuss the limits of applicability of the double-layer charging model and show Raman evidence for a hysteretic transition to a chemical doping regime.

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Using Raman spectroscopy we determined the van-der-Waals component of the low-frequency vibration in ropes of single-walled nanotubes at 171 cm À1 . While Raman peaks in this frequency range are commonly believed to correspond to the pure radial breathing mode of a single tube, our pressure and temperature-dependent measurements show that van-der-Waals contribution of the peak observed at 514.5 nm excitation is necessary to explain its large pressure coefficient of 9.7 cm À1 /GPa. Our results are consistent with the small elastic modulus predicted for nanotube ropes.

Resonance excitation profiles of the high-frequency peaks in the Raman spectra of single-walled carbon nanotubes normalized to the scattering intensity of CaF 2 are presented. We find separate resonances of metallic and semiconducting tubes throughout the visible and the near-IR excitation range. The resonance shift of samples with different mean diameters confirm the inverse proportionality of the resonant transition energy to the tube diameter. Smaller diameter tubes are found to have sharper resonances than larger tubes due to a 1/d 2 dependence of the Raman cross section.

We obtained a Raman signal from an individual WS 2 nanotube mounted on an atomic force microscopy cantilever tip. We discuss the implications for simultaneous investigations of the mechanical properties of WS 2 nanotubes by combining different experimental methods. From the orientation dependence of this nanotube's resonant Raman intensity, we estimate the ratio of the perpendicular to parallel polarizabilities ␣ XX / ␣ ZZ Ϸ 0.16. We compare the WS 2 nanotube with single-walled carbon nanotubes and expect a similarly strong depolarization effect for multiwalled carbon nanotubes.