We present electron energy-loss spectroscopy measurements on isolated single-walled carbon nanotubes and their bundles performed in a scanning transmission-electron microscope. Spectra are obtained over a range of impact parameters, from the centers of the samples to several nanometers outside the material. Curve-fitting techniques reveal five peaks in the range ∼2–30 eV. These include surface and bulk plasmons and direct π to ${\pi }^{*}$ interband transitions. The energies, heights, and widths of these peaks are tracked throughout the data sets in order to determine their dependence on bundle diameter and impact parameter. The heights of the externally excited surface mode peaks are compared to a model that varies exponentially with impact parameter and by a power law with bundle diameter. The power-law exponent varies from ∼0.4 for the π to ${\pi }^{*}$ mode to ∼1.3 for the $\pi +\sigma$ bulk plasmon, with the surface plasmons close to 1. The peak height data suggest a sensitivity to density inhomogeneities in one of the bundles and possibly to random variations in the chiral vectors among the single tubes and small bundles. These patterns are further elucidated in the peak energy and width data.

• Received 3 April 2001

DOI:https://doi.org/10.1103/PhysRevB.64.195404