Energy landscape of carbon nanotube caps: lack of intrinsic chirality bias and consequences for selective growth

In the initial stages of carbon nanotube (CNT) growth, a fixed pattern of six pentagons encodes what unique $(n,m)$ chirality a nascent CNT would inherit and can be viewed as its ``inorganic gene''. We shall present the results from a large-scale computational effort designed to establish a quantitative structure--property (intrinsic elastic energy) relation for a set of more than 4500~caps, including all isomers that obey the isolated pentagon rule, corresponding to tube diameters $d \leq 1$~nm. Our analysis shows that the energy scale associated with the CNT caps is small, compared to that of the CNT/catalyst interface. Such a flat energy landscape is irrelevant to chiral selectivity and lends further credibility to interface-controlled scenarios for selective growth of single-walled CNTs.