Since their discovery, fullerenes and their derivatives have received great attentions because they opened many new fields in science and technology. Among them, endohedral fullerenes, fullerenes encapsulating one or more atoms in their hollow carbon cages, have been predicted to have novel properties which are unexpected for empty fullerenes. However, the difficulty in producing pure samples of endohedral fullerenes in large quantities have hindered their experimental exploration. The motivation of this research is to understand their properties....[ Read more ]

Since their discovery, fullerenes and their derivatives have received great attentions because they opened many new fields in science and technology. Among them, endohedral fullerenes, fullerenes encapsulating one or more atoms in their hollow carbon cages, have been predicted to have novel properties which are unexpected for empty fullerenes. However, the difficulty in producing pure samples of endohedral fullerenes in large quantities have hindered their experimental exploration. The motivation of this research is to understand their properties.

In this research, endohedral monometallofullerenes M@C82 (M=Nd, Dy) have been investigated by in-situ ultra high vacuum scanning tunneling microscopy and tunneling spectroscopy.

As a comparison, the well-understood fullerene C60 has been studied first by the same technique. On the basal plane of cleaved highly oriented pyrolytic graphite (HOPG) (0001) surface, arriving C60 molecules assemble and grow into highly ordered, extended overlayers in hexagonal-close-packing. Starting from the second layer, the growth mode of C60 transforms from 2-dimensional dendrite islanding to 3- dimensional island formation upon the variation of growth rate and substrate temperature. A band gap of 1.5⁺_-0.2 eV is observed in the tunneling spectroscopy of the C60 films.

On the contrary, endohedral metallofullerenes do not form ordered structures on HOPG surface under the same conditions of growing C60 films, instead, they gather into clumped clusters. These molecules may induce a symmetric distributed ([square root]3X[square root]3)R30° superstructure of the HOPG substrate around them. The superstructure can be realized theoretically by a computer simulation. The HOMO-LUMO gap of a single molecule is measured to be 0.6±0.2 eV by tunneling spectroscopy, consistent with the UV-VIS-Near IR absorption spectrum of M@C82

On the top of C60 films, Nd@C82 molecules form close-packed 2-D structures of dimers, trimers, . . . ., and crystalline islands, while Dy@C82 molecules still aggregate as 3-D clusters. The intermolecular interaction is believed to be dominated by a dipole-dipole interaction. After experiencing a strong electric field applied by an STM tip, the Nd@C82 molecules will undergo a structural transformation from the close-packing to ring like configurations. The rings are well-shaped (as a hexagonal polygon), well-numbered (consisting of six or multiples of six molecules) and stable. The mechanism of the tip-driven transformation are discussed. We suggest that molecules within these rings are bonded covalently.

As a byproduct, we found a new path to generate carbon nanotubes. After the overlaying mixture of C60 and M@C82 on an HOPG surface has been irradiated with KeV electrons, carbon nanotubes are observed laying on the surface. They are straight, clean and have diameters in the range of 2-50 nm and length up to 3 μm. The detailed atomic images of the tube walls show helical arrangements of the honeycomb structure of a graphite(OOO1) surface. The formation mechanism of these tubes also is discussed.

Future research directions are identified. The systematic study of Lanthanide's ehdohedral fullerenes would uncover the roles of the metal atoms. More detailed understanding of the formation of the rings and the nanotubes from endohedral metallofullerenes could lead to finding methods for their mass production and novel applications.