Spectral properties of single-walled carbon nanotubes encapsulating fullerenes

Single-walled carbon nanotubes (SWCNTs) with diameter ranged from 1.22 to 1.6 nm filled with C60, C70 and C60H28 molecules (peapods), as well as double-walled carbon nanotubes (DWCNTs) derived from peapods, were studied by HRTEM, UV–vis–NIR and Raman spectroscopy. Suspensions with accurate concentration were used for spectroscopic studies to enable quantitative comparison of different substances. Filling of the SWCNTs with C70 molecules resulted in a reduced van der Waals interaction between the tubes in a bundle. The DWCNTs have lower intensity of the van Hove bands and weaker photoluminescence. Raman spectra at 633 and 1064 nm excitation wavelengths reveal that RBM frequencies of C60 and C70 peapods are equally downshifted compared to empty tubes. It was found that filling of the nanotubes with C60 and C70 caused spectral shifts of absorption bands: thin tubes display red shifts, while thick ones show blue shifts. DWCNTs and C60H28@SWCNTs do not show any shifts. All the results suggest that the filling of nanotubes with fullerenes alters the average diameter of the electron cloud around SWCNT framework; namely, it increases for thin SWCNTs, and decreases for thick ones. Our attempts to structurally assign thick nanotubes using reported extrapolations from data for thin tubes were unsuccessful.