QM/MD simulations of high-temperature SWCNT self-capping

Using high-temperature molecular dynamics (MD) simulations based on the density-functional tight-binding (DFTB) quantum chemical method, we studied the self-capping process of open-ended single-walled carbon nanotubes (SWCNTs) at annealing at temperatures of 3000 K and 3500 K. The open ends were either modeled by simply cutting the tube perpendicular to the tube direction CUT, creating edge dangling bonds in one set of model systems, and by saturating edge dangling bonds by carboxyl groups and atomic hydrogen in a 1:1 ratio in another set of model systems. We find that the open ends tend to lose pure carbon fragments C with n=1,2,3 in case of CUT models, while OX models additionally lose CO₂, CO, C n C_nH, OH etc. in particular at early stages, depending on temperature. With the exception of the smallest (3,3) SWCNT, all CUT models accomplished rapid self-capping during simulation times on the order of 100 ps. The time required for self-capping naturally increases steeply with the tube diameter beyond an empirically determined optimalself-capping. The value of this optimal diameter depends somewhat on the temperature: (6,6) SWCNTs cap fastest at 3000 K while (5,5) tubes cap fastest at 3500 K. The self-capping process for smaller diameter tubes is less efficient, presumably due to the higher ring strain associated with the resulting caps. Ring statistics show that the initial all hexagon SWCNT structures begin to incorporate between 10 to <20 pentagons (typically associated with positive curvature) during the self-capping process as a natural consequence of the buildup of cap curvature.