Low-frequency Raman studies of multiwalled carbon nanotubes: Experiments and theory

In this paper, we investigate the low frequency Raman spectra of multiwalled carbon nanotubes (MWNT's) prepared by the electric arc method. Low frequency Raman modes are unambiguously identified on purified samples thanks to the small internal diameter of the MWNT's. We propose a model to describe these modes. They originate from the radial breathing vibrations of the individual walls coupled through the van der Waals interaction between adjacent concentric walls. The intensity of the modes is described in the framework of bond polarization theory. Using this model and the structural characteristics of the nanotubes obtained from transmission electron microscopy, we simulated the experimental low frequency Raman spectra with excellent agreement. Therefore Raman spectroscopy can be as useful in the characterization of MWNT's as it is for single-wall nanotubes.