H(D)-lattice interactions in single wall carbon nanotubes

A thermally activated relaxation process has been found in the anelastic spectrum of HiPco carbon nanotubes at 25 K. This peak is depressed in the hydrogenated samples, but grows after subsequent thermal treatments which partially remove hydrogen. The activation energy obtained by the peak shift with frequency is Ea = 54.7 meV, and the pre-exponential factor of the Arrhenius relaxation time is τ0 = 10-14 s, which is typical of point defect relaxation. The observation of an isotope peak shift confirms that the relaxation is due to hydrogen. The height of the peak firstly increases with decrease of cH as more free sites are made available for the H jumps. On further reducing the hydrogen content, the peak height decreases, as a small number of relaxing species is left in the sample. The high mobility of the species giving rise to this peak indicates that it can hardly be ascribed to a hydrogen molecule. Moreover, classical models cannot reproduce the peak, suggesting a dynamics of atomic hydrogen governed by quantum tunneling.