Abstract Submitted to the NANOTUBE'02 Workshop:

Spatially resolved EELS (Electron Energy-Loss Spectroscopy) in a STEM (Scanning Transmission Electron Microscope) is perfectly suited to investigate the electron excitation spectra in individual nanotubes, which can simultaneously be visualized by standard TEM imaging techniques. Consequently, it can be applied to nanotubes with variable morphologies (single-walled, multi-walled, bundles, peapods...) and compositions (simple or hybrid, made of C, BN, WS₂..). When scanning a 0.5nm incident electron probe across selected nanotubes, one can discriminate bulk plasmons in the intersecting configuration from surface plasmons visible when the probe travels outside of the tubes at a distance which can be controlled typically between 1 and 10 nm from the external surface (i.e. in the "near-field" geometry). This contribution will emphasize, among others, several aspects of the dielectric response of the nanotubes over a wide energy domain in the far UV range : (i) the experimental evidence of surface plasmon coupling in anisotropic hollow nanoparticles. It splits the mode into two contributions of longitudinal and transversal characters, which are discriminated as a consequence of the local anisotropic character of the object; (ii) the variation of the response when considering tubes of ultra small widths down to the single wall case. The major parameter is then r/R (with r and R repectively the inner and outer diameters of the tube). In the r/R=1 limit, only one contribution remains, the longitudinal one. The agreement of these experimental results with those of a theoretical modelling relying on a dielectric description, with the tensor of dielectric coefficients valid locally at any coordinate in the object (see accompanying contribution by D. Taverna et al.), will be presented.