Effect of Metal Cluster-Cap Interactions on the Catalyzed Growth of Single-Wall Carbon Nanotubes

Density functional theory is employed to investigate the interactions between a nine-atom cobalt cluster and single-wall carbon nanotube caps with chiral angles ranging from near zigzag to armchair. With the aim of analyzing the initial stages of nanotube growth, the caps used in our study are the minimal units for which chirality can be defined. Electrostatic potential maps, atomic charges, and electronic density of states are analyzed for the individual caps and cluster and for the cap/cluster coupled system to detect changes triggered by the cap/cluster interactions. It is observed that the metal cluster becomes oxidized, and significant changes are detected in its geometry; such changes are dependent on the cap chirality. Strong electrostatic interactions are found between the cap rim atoms and the cobalt atoms in contact with them. However, the bonding orbitals involved in such strong interactions are dominated by inner rather than by frontier orbitals, which are the ones that most likely would tie able to participate in nanotube growth reactions. Among other findings, our results suggest that armchair and near-armchair systems should be the most favored when the growth mechanism is dominated by reactions such as C-2 addition to the cap rim atoms.