Intercalated Fullerenes and Carbon Nanotubes

Since the discovery of soccer-ball-shaped C60 (Kroto et al., 1985), fullerenes have been added to the family of allotropes of carbon element. During the fullerene formation by arc-discharge of graphite electrodes, carbon nanotubes were simultaneously grown as a deposit on the electrode (Iijima, 1991). The carbon nanotubes consist of single or multiple graphene sheets rolled in the form of a seamless cylinder, with the diameter of the hollow core being almost 10 Å (similar to that of fullerenes) or even as small as 4 Å. For these new forms of nanometer-sized carbon, so-called nanocarbons, basically similar concepts as GICs have been applied from the aspects of structures, electronic properties, and functionalities that can be controlled by doping or intercalation process. That is, the bonding force between nanoballs or nanotubes is governed by weak van der Waals forces, so that foreign species such as atoms or molecules can be intercalated (or doped) in the van der Waals gaps, similar to graphite. So, from applications and the basic science of these new carbon families, intercalation as well as doping to these hosts has been studied intensively in the last ten years. There are three kinds of doping reactions of guest species into these host materials, which are reflected in their specific structure. Guest atoms can be introduced by substituting the carbon atoms of the hosts. This process is generally called “doping,” as there is a similarity with the doping process in semiconductors, where, in general, there is no long-range periodicity in the guest arrangement against the host crystal. Guests can locate in the hollow cores of fullerenes or carbon nanotubes as well as on their outer surfaces. GICs establish the super-lattice structure between the host of the graphite lattice and the inserted guest species, where the long-range periodicity along the c-axis as well as on the a-b plane is formed. According to the original meaning of “intercalation,” periodic doping to the host materials is defined as intercalation. So, the three kinds of doping are: (1) endohedral doping into the hollow cage; (2) substitutional doping by replacing the carbon atoms on the cages; and (3) exohedral doping where the dopants are sited in the gaps between the cage molecules of a fullerene crystal or between carbon nanotubes in the array of the bundle form.