Properties of Carbon Nanostructures

Carbon nanostructures have attract considerable attention due to their special properties. The most important properties of carbon nanotubes and fullerenes are stated below. Carbon nanotubes have excellent molecular, electronic, optical and mechanical properties due to their nearly one-dimensional structure.

11.8.1 Molecular Properties

A perfect SWCNT has no functional groups. Therefore these quasi-1D cylindrical aromatic macromolecules are chemically inert. However, curvature-induced pyramidalization and misalignment of the H-orbitals of the carbon atoms induces a local strain, and carbon nanotubes are expected to be more reactive than a flat graphene sheet (Niyogi et al. 2002). From the standpoint of chemistry, it is conceptually useful to divide the carbon nanotubes into two regions: the end caps and the sidewall. The end caps will always be quite reactive, irrespective of the diameter of the carbon nanotubes.

11.8.2 Electronic Properties

CNTs are quasi-one-dimensional materials made of sp2-hybridized carbon networks and have been the subject of extensive research and discussion. In particular, the electronic structure of a single CNT has been studied theoretically, which predicts that CNT becomes either metallic or semiconducting depending on its chiral vector, i.e., boundary conditions in the circumference direction. An electron in a nanotube is a massless neutrino on a cylinder surface with a fictitious Aharonov- Bohm flux determined by its structure. A nanotube becomes a metal or a semiconductor, depending on whether the amount of the flux vanishes or not.

The electron accepting ability of C60 has been explored extensively. Apparently, oxidative electrochemistry of fullerenes is not as rich as reductive electrochemistry. Scientists have also discovered that the electronic properties are not the same among different types of fullerenes. For example, chirality in fullerene molecules could alter significantly the electronic properties of a cagelike system. Regarding quasi-spherical giant fullerenes, it is demonstrated that heptagons and additional pentagons produce dramatic changes in the charge distribution.

11.8.3 Optical Properties

A newly EMA method indicates that the effect of the larger internal radii of the nanotubes cannot be neglected (Lu et al. 2001). The optical activity of chiral nanotubes disappears if the nanotubes become larger (Damnjanovic et al. 1999). Pristine C60 is a molecule of high symmetry, with p-electrons delocalized along the whole 3-D structure. These properties make C60an interesting material in the nonlinear optical field (Brusatin et al. 2002). It also shows a wide variety of uncommon physical properties ranging from optical limiting to superconductivity and photoconductivity. It has been clearly observed that the final linear and nonlinear optical properties of fullerenes are strictly related to the matrix used for inclusion, to the possibility of functionalizing the pristine C60, and to the processing protocol of the solid nanocomposite. Another work got a similar result that the lowering of symmetry and the presence of a crystal field in fullerenes affect the selection rules and the energies of the intermolecular excitations (shift and splitting of degenerate electronic levels). The optical properties of fullerenes depend equally on intra- and intermolecular processes (Makarova 2001).

11.8.4 Mechanical Properties

The mechanical properties of CNTs have attracted much attention since they were discovered in 1991. CNT exhibits extraordinary mechanical properties: The Young's modulus is over 1 Tera Pascal. It is stiff as diamond. The estimated tensile strength is about 200 Giga Pascal. Weak region of mechanical strength of CNT was the center of the tube, not the connection region, which meant that the connection region was stronger than the tube itself (Abe et al. 2004). The mechanical strength at the connection region must be significantly improved for the real application and one plausible candidate might be the particle method.

11.8.5 Periodic Properties

Despite their complexity in structure, fullerenes still have their own periodic properties (Torrens 2004). First, the properties of the fullerenes are not repeated; only, and perhaps, their chemical character. Second, the relationships that any fullerene p has with its neighbor p + 1 are approximately repeated for each period (p stands for the number of edges common to two pentagons). Nanoballs are electric materials. Due to easily adsorptions of amorphous carbon and impurities to the surface of nanoballs, their electric capability may decrease (Liu et al. 2001). Another specific property is the polymerization. Two bucky onions in similar dimension may open the chemical bond of the atoms in out wall and connect together to form a stable dimer. Nanofibers have many different types including exotic properties. For example, vapor-grown carbon nanofibers (VGNFs) have electric properties (Wei et al. 2004). Their electric resistance of composite prepared from the polyester-grafted carbon nanofiber and poly(ethylene glycol) suddenly increased in methanol vapor over 1000 times, and returned to initial resistance when it was transferred into dry air. Graphic carbon nanofibers (GCNFs) have high dispersion and electrocatalytic properties. Researches still underway are exploring the novel properties of nanofibers.

In summary, some interesting properties of several types of carbon nanostructures have been discussed. Properties of nanotubes and fullerenes attract scientists so deeply that their characters are discovered and concluded systematically. The study of nanoballs and nanofibers is still in its infancy and requires substantial, prolonged development, many new exotic properties are waiting for exploiting.

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