A Theoretical Study of Carbon Nanotube Formation: Carriers of the Diffuse Interstellar Bands?

Density functional theory (DFT) calculations have been conducted to probe the formation of carbon nanotubes (CNTs) from graphitic sheets in the presence of a simulated circumstellar shock wave. The sheets serve as a model for graphite layers formed on surfaces of SiC grains, as produced in experiments. The computations show that when the top graphite layer is displaced toward a lower layer under the mechanical stress of a shock, the sheets bond and curl into a CNT. The bonding between sheets always occurs along the zigzag edges, exclusively producing the “armchair” CNT or (n, m) conformer, where n = m, as opposed to the “zigzag” or “chiral” forms (n ≠ m). Multilayered graphite sheets showed the same behavior when the top sheet was mechanically perturbed, also leading to formation of energy-minimum armchair CNT structures. The optical-transition energies from the ground to first excited state for a series of armchair CNTs, E₁₁, were calculated. Within uncertainties associated with DFT calculations, the wavelengths of the E₁₁ transitions of the (5, 5), (8, 8), (9, 9), (10, 10), and (12, 12) CNTs match the pattern of five of the six strongest diffuse interstellar bands (DIBs), including those at 4430, 5780, and 6284 Å. These DIBs are also found in the planetary nebulae Tc-1 and M1-20, where C₆₀ and C₇₀ have been identified. These data further support the hypothesis that CNTs, as well as fullerenes, are formed in shocked circumstellar gas from the destruction of SiC grains and are transported into the diffuse interstellar medium.