The Mechanism of Single-Walled Carbon Nanotube Growth and Chirality Selection Induced by Carbon Atom and Dimer Addition

On the basis of abounding density function calculations, a mechanism is proposed to explain single-walled carbon nanotube (SWCNT) growth and chirality selection induced by single C atom and C(2) dimer addition under catalyst-free conditions. Two competitive reaction paths, chirality change induced by single C atom and nanotube growth through C(2) dimer addition, are identified. The structures of the intermediates and transition states along the potential energy surfaces during the formation of near-armchair (6,5), (7,5), (8,5), and (9,5) caps initiated from the armchair carbon cap (5,5) are elucidated in detail. The results show that the direct adsorptions of C atom or C(2) dimer on growing carbon caps have no energy barrier. Moreover, the incorporations of adsorbed C atom or C(2) dimer display low energy barriers, indicating SWCNT growth and chirality change are thermodynamically and kinetically feasible under catalyst-free growth conditions. In addition, the results also highlight that the concentrations of C atoms and C(2) dimers in the experimental environment would play a critical role in the chiral-selective SWCNT synthesis. Potential opportunities exist in achieving the (n,m) selective growth by delivering single C atom or C(2) dimers at different ratios during different reaction stages.