Electronic properties of carbon nanotubes by transmission electron energy-loss spectroscopy - art. no. 195404

We present electron energy-loss spectroscopy measurements on isolated singl e-walled carbon nanotubes and their bundles performed in a scanning transmi ssion-electron microscope, Spectra are obtained over a range of impact para meters, from the centers of the samples to several nanometers outside the m aterial. Curve-fitting techniques reveal five peaks in the range similar to 2-30 eV. These include surface and bulk plasmons and direct pi to pi* inter band transitions. The energies, heights, and widths of these peaks are trac ked throughout the data sets in order to determine their dependence on bund le diameter and impact parameter. The heights of the externally excited sur face mode peaks are compared to a model that varies exponentially with impa ct parameter and by a power law with bundle diameter. The power-law exponen t varies from similar to0.4 for the pi to pi* mode to similar to1.3 for the pi+sigma bulk plasmon, with the surface plasmons close to 1. The peak heig ht data suggest a sensitivity to density inhomogeneities in one of the bund les and possibly to random variations in the chiral vectors among the singl e tubes and small bundles. These patterns are further elucidated in the pea k energy and width data.