π plasmons in two-dimensional arrays of aligned carbon nanotubes

The pi-electronic collective excitations (pi plasmons) in a two-dimensional (2D) array of aligned carbon nanotubes are studied within the self-consistent-field approach. The loss function has several peak structures. The most prominent one at omega>5 eV is identified as the pi plasmon. The pi-band structure and the geometric structure of the 2D nanotube array are directly reflected in the electronic excitations. Consequently, the pi plasmons strongly depend on the nanotube geometry (radius and chiral: angle), and the direction (theta) and the magnitude (q) of the transferred momentum. The oscillator strength of collective excitations and the pi plasmon frequency markedly decrease with the polar angle theta. The pi plasmon frequency clearly grows with q except at large theta and q. The pi plasmons in a 2D carbon nanotube array quite differ from those in a 3D carbon nanotube bundle or a 1D carbon nanotube, mainly owing to the very different Coulomb interactions. [S0163-1829(99)00739-0].