Radial-breathing mode frequencies for nanotubes encapsulating fullerenes

Radial-breathing mode (RBM) frequencies of armchair nanotubes encapsulating C60 molecules (peapods) are calculated on the basis of the local density approximation in the density functional theory. The RBM frequencies of peapods exhibit unusual shifts from those of corresponding nanotubes depending on the space between the walls of nanotubes and C60. Encapsulation of C60 molecules is found to cause a higher frequency shift for (10, 10) nanotube but a lower shift for (11, 11) and (12, 12) nanotubes. This diameter dependence of the frequency shift is ascribed to the charge redistribution caused by weak hybridization between π-electron states and nearly free electron states.