Abstract Submitted to the    CCTN09 Symposium:

Zigzag Grows Faster Than Armchair: Comparisons of SWNT Growth From Self-Consistent-Charge Density-Functional Tight-Binding Molecular Dynamics Simulations

Alister J Page ¹, Yasuhito Ohta ², Yoshiko Okamoto ¹, Stephan Irle ³, Keiji Morokuma ¹

¹ Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan
² Department of Chemistry, Nara Women’s University, Nara 630-8605, Japan
³ Institute for Advanced Research and Department of Chemistry, Nagoya University, Nagoya 464-8602, Japan

alisterpage@fukui.kyoto-u.ac.jp

XXI. General Studies of Carbon Nanostructures

The growth of single-walled carbon nanotubes (SWNTs) from Fe₃₈ clusters has been simulated using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method, in conjunction with non-equilibrium molecular dynamics. Growth of the nanotube sidewall was induced by the supply of gas-phase atomic carbon at two different rates to randomly chosen sites in the Fe-C boundary region of both (5,5)-SWNT-Fe₃₈ and (8,0)-SWNT-Fe₃₈.^1,2 The dependence of SWNT growth dynamics on both the chiral angle of the SWNT (θ) and the rate of carbon supply were therefore elucidated. SWNTs grown in this manner exhibited a large number of defects. In addition, SWNT healing was driven by the relative rates of defect removal and addition in the nanotube sidewall near the SWNT-Fe₃₈ boundary.³ The mechanisms by which adatom, vacancy and 5-77-5 type defects were removed from the SWNT generally took place over timescales of 1-25 ps. These mechanisms were therefore enhanced using lower rates of carbon supply. Using a rapid supply of carbon, an inverse relationship between θ and the SWNT growth rate was established during the early stages of growth. In particular, (8,0)-SWNTs exhibited a mean growth rate ca. 20-30% greater than that of (5,5)-SWNTs during the first 40 ps of growth. A recently proposed 'steady-state' model of SWNT growth⁴ suggests that the converse is the case when growth is induced by the diffusion of carbon over the catalyst surface to the SWNT-catalyst boundary. However, average SWNT growth and ring-addition rates of (5,5)- and (8,0)-SWNTs can be explained in terms of the direction in which incident carbon extends the sp²-hybridized network with respect to the SWNT axis. It is concluded therefore that the energetic and kinetic properties of SWNT growth mechanisms, and therefore SWNT growth rates, depend on the manner in which carbon density is supplied to the nanotube itself (i.e. direct addition at the SWNT-catalyst interface, as opposed to addition following diffusion over the catalyst surface). It is also postulated that a dependence of SWNT growth rate on the form of carbon feedstock supplied to the SWNT (i.e. C, as opposed to C_n) would be observed.



1. Y. Ohta, Y. Okamoto, S. Irle, K. Morokuma, J. Phys. Chem. C, 113,159, (2009).
2. Y. Ohta, Y. Okamoto, S. Irle, K. Morokuma, K. Phys. Rev. B, (Submitted).
3. A. J. Page, Y. Ohta, S. Irle, Y. Okamoto, K. Morokuma, J. Phys. Chem. C, (Submitted).
4. F. Ding, A. R. Harutyunyan, B. I. Yakobson, PNAS, 106, 2506, (2009).

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