Collective excitations of multishell carbon microstructures: Multishell fullerenes and coaxial nanotubes.

s and p plasmons in coaxial carbon nanotubes and multishell fullerenes are modeled in analogy with coupled collective excitations in finite, layered, two-dimensional-electron-gas, planar semiconductor superlattices. The curvature of the surface of these complex carbon clusters plays an important role in shaping the dimensionality ~one dimensional, two dimensional, or three dimensional! of the plasmons. Direct crossover from a one-dimensional to a three-dimensional regime is found under readily fulfilled conditions for carbon nanotubes in the case of small finite longitudinal momentum transfer \q , while for q50 bulk graphitic plasmons fail to develop. For large q , a two-dimensional behavior is found. The case of multishell fullerenes resembles in all instances the q50 behavior of carbon nanotubes. Such behavior correlates with the observed systematic redshift of the strong interstellar absorption band as compared to the p plasmon of bulk oriented graphite ~i.e., the 5.7 eV position of the former compared to the 6.2 eV energy of the latter!. Furthermore, in the case of p plasmons in carbon nanotubes, a special surface mode can develop for large q , due to the difference in the values of the dielectric constants between the graphitic structures and the surrounding medium.