Multishell Fullerenes Research Articles

Structural transformations, reactions, and electronic properties of fullerenes, onions, and buckytubes

We describe the results of extensive quantum molecular dynamics calculations of the properties of fullerenes and microtubules. The topics to be discussed include: (i) stability of C 60 isomers and barriers to isomerization; (ii) reactivity of C 60 and C 58 with C 2 and C 3, and its implications on the formation and growth of fullerenes; and (iii) atomic and electronic structure and doping of semiconducting microtubules. We also discuss the structures, stabilities and atomic transformations of large multishell fullerenes and offer an explanation for the formation of spheroidal “onions” under high fluence electron irradiation conditions. The last results, which involved calculations for up to ∼ 15 000 atoms, were obtained using classical three-body potentials.

Electronic structure of single- and multiple-shell carbon fullerenes.

We study the electronic states of giant single-shell and the recently discovered nested multi-shell carbon fullerenes within the tight-binding approximation. We use two different approaches, one based on iterations and the other on symmetry, to obtain the $\pi$-state energy spectra of large fullerene cages: $C_{240}$, $C_{540}$, $C_{960}$, $C_{1500}$, $C_{2160}$ and $C_{2940}$. Our iteration technique reduces the dimensionality of the problem by more than one order of magnitude (factors of $\sim 12$ and $20$), while the symmetry-based approach reduces it by a factor of $10$. We also find formulae for the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) energies of $C_{60{\cdot}n^{2}}$ fullerenes as a function of $n$, demonstrating a tendency towards metallic regime for increasing $n$. For multi-shell fullerenes, we analytically obtain the eigenvalues of the intershell interaction.

Computation of the ultraviolet absorption and electron inelastic scattering cross section of multishell fullerenes.

We develop a model in which the ultraviolet dielectric tensor of planar graphite is transported to the spherical geometry of a nanoscale multishell fullerene with a central cavity. This is accomplished by assigning to every point of the multishell fullerene a local dielectric tensor identical to that of graphite with its c axis aligned along the local radial direction. The dynamic, multipolar polarizabilities of the model fullerene are obtained from the exact solutions of the nonretarded Maxwell equations. The ultraviolet absorption spectrum of the hollow fullerene is calculated as a function of the ratio of the inner and outer radii. Comparisons of the theoretical absorption spectra with the 2175-\AA{} interstellar extinction hump and with recent absorption measurements for synthetic multishell fullerenes indicate that the dielectric properties of graphite are qualitatively adequate for understanding the optical data. However, difficulties persist with both the astrophysical and laboratory absorption peaks which lead us to consider the possible role of multishell fullerene aggregation into small or large clusters. It is found that the effect of clustering is important and reduces but does not remove completely the quantitative difficulties of the graphitic multishell model. Finally theoretical electron-energy-loss spectra (EELS) of these structures with an empty or filled cavity are calculated from the multipolar polarizabilities of the model. The results indicate that spatially resolved EELS measurements should be ideally suited to study the dielectric properties of individual multishell fullerenes and to ascertain to what extent they differ from those of planar graphite.

High yield synthesis and HREM study of fullerene tubules and fullerene onions

The fullerene tubules and fullerene onions are synthesized in macroscopic quantity with high yield by modified Kratschmer et. al. method. Besides ordinary concentric fullerene tubules, also are observed abnormal nonconcentric multilayer glaphitic tubules with varying layer spacings. The curved graphitic tubules are observed, showing the tendency to form torus. Under appropriate helium pressure, about half of the synthesized product is the multi-shell fullerene onions, polyhedral in shape in accord with the theoretical calculation. [11]

Stability of multishell fullerenes.

We investigate the equilibrium geometry of very large carbon clusters with special emphasis on nested multishell fullerene derivatives of spherical, cylindrical, and conic shape. For cluster sizes above 20 atoms, we find spherical shapes to be most stable. For very large sizes, a transition from single-shell fullerenes to multishell structures, locally similar to graphite, is energetically favored by the weakly attractive interaction between the shells.