Growth of chiral single-walled carbon nanotube caps in the presence of a cobaltcluster

Abstract

Density functional theory is used to simulate nanotube growth by addition of a pair ofcarbon atoms to a composite chiral nanotube cap/cobalt cluster system, with capscorresponding to near-armchair (6, 5), (7, 5) and near-zigzag (9, 1) nanotubes. Two differentcarbon addition processes are evaluated: in the first, the new carbon atoms are located inthe vicinity of the armchair site of the cap rim, and thus this process provides insight intothe root-growth mechanism; in the second the carbon atoms are initially located under thecobalt cluster, and thus this process helps one to evaluate the dissolution of carbon insidethe metal cluster. The geometric evolution and energetics of the system are used toexplain features of the mechanism of nanotube growth. The root-growth reaction isshown to occur by displacement of a cobalt atom initially interacting with thearmchair site while the added carbon atoms bond to each other forming a newhexagonal ring, whereas the carbon dissolution process shows formation of dimersinside the cluster only for the (6, 5) system. The energetics for both steps revealsthat the dissolution stage is probably controlling the overall nanotube growthrate.