Defect Healing during Single-Walled Carbon Nanotube Growth: A Density-Functional Tight-Binding Molecular Dynamics Investigation

Quantum chemical molecular dynamics have been employed to investigate the healing of single-walled carbon nanotubes (SWNTs) during growth. In trajectories based on self-consistent-charge density-functional tight-binding (SCC-DFTB) energies and gradients, gas-phase carbon atoms were supplied to the carbon−iron boundary of a model C<sub>40</sub>-Fe<sub>38</sub> complex at two different rates (1 C/0.5 ps and 1 C/10 ps). The lower rate of carbon supply was observed to promote SWNT growth, compared to the higher rate, for the same number of carbon atoms supplied. This promotion of growth was ascribed to the suppression of pentagon and heptagon incorporation in the sp<sup>2</sup> carbon network observed at lower carbon supply rates. The most successful example of growth occurred when the respective periods of hexagon and pentagon formation were out of phase and heptagon formation was limited. Higher carbon supply rates tended to result in the encapsulation of the Fe<sub>38</sub> cluster by the extended sp<sup>2</sup> carbon cap, due to a saturation of pentagon and heptagon defects in the latter. The greater tendency toward hexagon formation found using a lower carbon supply rate was attributed to the relative rates of defect removal and addition from the sp<sup>2</sup> carbon cap during the growth process. The defect removal (i.e., healing) process of the sp<sup>2</sup> carbon cap occurred via ring isomerization, which resulted in the removal of 5-7, adatom, and monovacancy defects. These healing mechanisms generally occurred over time scales of several picoseconds and depended largely on the presence of the catalyst surface. The healing mechanisms observed in this work represent a possible pathway by which control over the (<i>n</i>, <i>m</i>) chirality of a nascent SWNT is obtained during the growth process.