Carbon nanotube supramolecular assemblies: Discovery, characterization, and high-yield synthesis of carbon-60 SWNT

Using primarily high resolution transmission electron microscopy (HRTEM), we have discovered C₆₀@SWNT—a novel supramolecular assembly comprising a linear chain of C₆₀ molecules spaced ∼1 nm center-to-center inside a 1.4 nm diameter single wall carbon nanotube (SWNT)—and solved its structure and synthesis route. C₆₀@SWNT is a van der Waals interacting heterostructure that exhibits a unique response to electron irradiation involving both beam damage and mass transport. Metastable, higher fullerene isomers are sometimes encapsulated as well. In situ materials processing studies show that these assemblies are created during the purification steps usually applied to as-synthesized nanotubes. In particular, the synthesis mechanism involves the vapor phase transport of C₆₀ molecules to openings in the SWNTs' walls and is therefore driven by annealing, which serves to vaporize the fullerenes. Subsequent encapsulation is exothermic, and the interior C₆₀ molecules are retained due to the stabilizing van der Waals coordination. C₆₀@SWNT yields were maximized by exacting control over the many synthesis parameters and were reliably measured by a calibrated weight uptake method. For isochronal experiments, the most important parameters affecting yield under the tested conditions were SWNT purity and annealing temperature. Yields of ∼90% have been achieved. Our high-yield method affords characterization by electron diffraction and Raman spectroscopy. The former indicates a simple 1-D lattice with a parameter of 0.9982 nm, in agreement with simulation and real-space measurement. The latter suggests that the Raman spectrum of a filled SWNT might be different than that of an empty SWNT due to vibrational and/or resonance effects. Filling experiments were duplicated with La₂@C₈₀, incontrovertibly proving that extrinsic molecules can be intentionally inserted into SWNTs. Images of the individual lanthanum atoms in these structures suggest that the properties of both molecules may be perturbed in the assembled state. Our methods afford the possibility of creating novel supramolecular engineering materials using molecules that have optical, electronic, or biological activity.