4P142
Density-functional tight-binding molecular dynamics simulationsof the self-capping process in open-ended (n,n) SWCNTs (n=3 to 10)(1Graduate school of science, 2Graduate school of information science, Nagoya University)
1Hironori Hara, 2Genki Ichinose, 1Stephan Irle
Introduction
Although many important and fundamental studies of CNTs have been conducted so far,there are still many unknows about these wondrous molecules. Especially, their formationand transformation mechanism at high temperatures such as the here investigated self-capping
mechanism has not been completely revealed yet today. Since CNTs consist of graphitic
material, !-conjugation effects are important. Previously used calculation methods such as
semiclassical reactive empirical bond-order (REBO) interatomic carbon-carbon molecularmechanics (MM) force fields are computationally cheap but do not include !-conjugation or
aromaticity at all. Accurate quantum chemical methods such as Moller-Plesset or Density
Functional Theory (DFT) approaches are computationally too expensive. The
density-functional tight-binding molecular dynamics (DFTB/MD) method, which allows thestudy of the time evolution of molecules using potentials and gradients from quantum
mechanical electronic structure calculations, has proven to be a very useful approach toinvestigate the formation and transformation processes of carbon nanomaterials over thecourse of ~100 ps. Using this method, we previously elucidated the mechanism of fullerene
formation starting from short open-ended SWCNT models [1,2]. According to theseprevious studies, a suitable temperature range for self-capping is 3000 K to 3500 K, and it
was discovered that small-diameter armchair-type SWCNTs are able to anneal relatively
faster than zigzag-type SWCNTs. In the present study, we tried to answer whether and how
larger-diameter tubes are also able to anneal, namely how they form half-hemisphere-likecaps, and to find a correlation between the tube diameter and the self-capping time attemperatures of 3000 K and 3500 K.
Simulation Details
Using canonical (constant temperature, NVT) DFTB/MD simulations, we studied theself-capping processes of 35  -long, open-ended armchair (n,n) single-walled carbon
nanotubes (SWCNTs) (n=3 to 10, the diameter ranges from 4  for n=3 to 13.5  for n=10)
during annealing at temperatures of 3000 K and 3500 K, using 3 trajectories for each (n,n)
SWCNT type and temperature. These systems are far more large than the ones studied in[1,2] and have diameters closer to real SWCNTs. The open ends were modeled by simply
cutting the tube perpendicular to the tube direction and leaving dangling bonds in one set ofmodel systems (CUT), and by saturating the resulting dangling bonds with carboxyl groups
and atomic hydrogen in a 1:1 ratio in another set of model systems (OX).
Results and Discussion
We find that the open ends tend to lose mostly C2 units in case of CUT models, and inaddition CO2, CO, Cn, CnH, OH and so forth in case of OX models, in dependence ontemperature. With the exception of the (3,3) SWCNT the all-carbon ends of the open-ended
armchair SWCNTs are able to self-cap during simulation times on the order of 100 ps. The
time required for self-capping increases steeply with the tube diameter beyond an “optimal”
diameter which depends on the temperature and corresponds to roughly n=5 to 6 at 3000 K
and 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 and their therefore
more difficult creation. The very small (3,3) tube is highly strained and typically falls apartat the present temperatures. Ring statistics show that the initial hexagon-only SWCNT
structures begin to incorporate between 10 to <20 pentagons (associated with positive
curvature) during the self-capping process as a natural consequence of the buildup of capcurvature. Heptagons (associated with negative curvature) are rarely to never seen during thedynamics, in agreement with expectation and previous fullerene self-assembly studies.

Fig.1 The initial structures and both-sides self-capped structures andare shown.

simulation time length to obtain themTimes are the average of 3 trajectories for the same n and temperature.

Fig.2 Average time length of self-capping.

 is the time of closing of the first end, and 2 is that of the other end.

Reference[1] Irle, S.; Zheng, G.; Elstner, M.; Morokuma, K., Nano Lett. (2003) ,3 ,465-470.
[2] Zheng, G.; Irle, S.; Elstner, M.; Morokuma, K., J.Phys. Chem. A (2004), 108, 3182-3194.