Molecular Dynamics and Phase Transition in One-Dimensional Crystal of C-60 Encapsulated Inside Single Wall Carbon Nanotubes
Résumé
One-dimensional crystals of 25% C-13-enriched C-60 encapsulated inside highly magnetically purified SWNTs were investigated by following the temperature dependence of the C-13 NMR line shapes and the relaxation rates from 300 K down to 5 K. High-resolution MAS techniques reveal that 32% of the encapsulated molecules, so-called the C-60(alpha), are blocked at room temperature and 68%, labeled C-60(beta), are shown to reversly undergo molecular reorlentational dynamics. Contrary to previous NMR studies, spin - lattice relaxation time reveals a phase transition at 100 K associated with the changes in the nature of the C-60(beta) dynamics. Above the transition, the C-60(beta) exhibits continuous rotational diffusion; below the transition, C-60(beta) executes uniaxial hindered rotations most likely along the nanotubes axis and freeze out below 25 K. The associated activation energies of these two dynamical regimes are measured to be 6 times lower than in fcc-C-60, suggesting a quiet smooth orientational dependence of the interaction between C-60(beta) molecules and the inner surface of the nanotubes.