Helical superstructures of fullerene peapods and empty single-walled carbon nanotubes formed in water

Aqueous dispersions of fullerene C₇₀-filled carbon nanotubes (C₇₀@SWNTs or peapods) and empty single-walled carbon nanotubes (empty SWNTs) were prepared with the aid of trimethyl-(2-oxo-2-pyrene-1-yl- ethyl)-ammonium bromide (1), which is a carbon nanotube solubilizer. This is the first report describing the preparation and characterization of the transparent dispersion/dissolution of the peapods. The UV - vis - near-IR spectra of C ₇₀@SWNTs-1 and empty SWNTs-1 were almost identical. We found by means of transmission electron microscopy and atomic force microscopy that the empty SWNTs and C₇₀-peapods form helical nanostructures in the shapes of rings, irregular rings, lassos, handcuffs, catenanes, pseudorotaxanes, and figure-eight structures. The mechanism of the superstructure formation has been discussed in relation to the unique characteristics of stiff polymer chains with the aid of an off-lattice Monte Carlo simulation.