Low-frequency excitations ofC60chains inserted inside single-walled carbon nanotubes

Abstract

The low-frequency excitations of ${\mathrm{C}}_{60}$ chains inserted inside single-walled carbon nanotubes (SWNTs) have been studied by inelastic neutron scattering on a high-quality sample of peapods. The comparison of the neutron-derived generalized phonon density of states (GDOS) of the peapods sample with that of raw SWNTs allows the vibrational properties of the ${\mathrm{C}}_{60}$ chains encapsulated in the hollow core of the SWNTs to be probed. Lattice dynamical models are used to calculate the GDOS of chains of monomers, dimers, and polymers inserted into SWNTs, which are compared to the experimental data. The presence of strong interactions between ${\mathrm{C}}_{60}$ cages inside the nanotube is clearly demonstrated by an excess of mode density in the frequency range around 10 meV. However, the presence of a quasielastic signal indicates that some of the ${\mathrm{C}}_{60}$’s undergo rotational motion. This suggests that peapods are made from a mixture of ${\mathrm{C}}_{60}$ monomers and ${\mathrm{C}}_{60}$ $n$-mer (dimer, trimer, …, polymer) structures.