Anharmonicity in single-wall carbon nanotubes as evidenced by means of extended energy loss fine structure spectroscopy analysis

A comparative study of the structure of free-standing parallel bundles of single-wall carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs), and highly oriented pyrolytic graphite (HOPG) was achieved by means of transmission electron microscopy and electron energy loss spectroscopy analyses. In particular, the carbon K (1s) extended fine structure of SWCNTs is found to be characterized by an apparent contraction of the nearest neighbors distance. This contraction is interpreted here to originate from an asymmetric pair distribution function, mostly due to the high out-of-plane vibrational motion of the C atoms, as for the case of chemisorbed atoms on clean surfaces. In contrast, the MWCNTs did not exhibit any signature of such an anharmonic effect because of their more rigid structure. This indicates that the SWCNTs pair potential is significantly broader and its effect is much weaker than that experienced by the same C-C pair embedded in a multiwall nanotube.