We co-encapsulated the fullerenes and benzene by sonicating the SWCNTs for 2 hours in a benzene: C 60 solution of 1 m g / m l. Fullerenes were obtained from a commercial source (Hoechst, Super Gold Grade C 60, purity 99.9 %) and we used natural (Sigma) and fully 13 C enriched ( 13 C 6 H 6, Eurisotop, France) benzene. Prior to the filling process, the nanotubes were opened by heating in air at 450 ∘ C for 0.5 h. This diameter is ideal for the growth of inner tubes as it can energetically well encompass the filled-in fullerenes.
![gausssum for raman gausssum for raman](https://i.ytimg.com/vi/fA3odPmkV-U/hqdefault.jpg)
The SWCNT host sample was prepared by the arc-discharge method and was from the same batch as in previous studies 17 with a mean diameter of 1.4 n m. Wall which are separated from other such rings by natural carbon regions. Our calculations show that the Raman spectra can be explained by assuming the clusterization of the 13 C nuclei: the 13 C atoms form benzene-like rings on the inner nanotube This is supported by a semi-empirical method based modeling of the Raman modes. This is clearly incompatible with the random distribution of the different carbon isotopes and it suggests a clustering of 13 C.
![gausssum for raman gausssum for raman](https://www.frontiersin.org/files/Articles/653527/fenrg-09-653527-HTML/image_m/fenrg-09-653527-g006.jpg)
In contrast, we observe a 2D Raman component which is downshifted as if it had an 85 % isotope enrichment. Random distribution of the isotopes would cause a uniform downshift of the Raman mode, with its center corresponding to the ∼ 9 % isotope enrichment. We study the 2D Raman line, which is one of the most energetic phonon modes, and observe an unexpected change when the benzene is fully 13 C enriched. Motivated by the above findings, in this work we study the final inner tube product, which is made of benzene and C 60, which are co-encapsulated inside the host nanotubes.