The diffusion behaviors of small hydrocarbon radicals (-CH, -CH2 and -CH3) on the outer wall of a (9,0) carbon nanotube were investigated by performing tight-binding potential calculations. It was found that either -CH or - CH2 preferentially performed zig-zags on the wall of the tube, whereas -CH3 hopped from the top of a carbon atom to its nearest one, by overcoming energy barriers of less than 1.3eV. If 5-1 dangling bond defect, a type of monovacancies, existed in the tube, it seems a trap to capture the radicals diffusing nearby. Two typical pathways of the radicals migrating into a 5-1 dangling bond defect were revealed here. In one of the pathways, -CH and -CH2 diffused from the defect-free region to the dangling bond atom in the 5-1 dangling bond defect by crossing the hexagon directly. In the other one, the radicals diffused to the pentagon, where either -CH or -CH2 converted the initial 5-1 dangling bond defect to be an overturned one (by breaking and re-bonding C-C bonds), with -CH or -CH2 attached on the new dangling bond atom. Furthermore, it was found that -CH or -CH2 bonding at the dangling bond atom could ultimately convert the 5-1 dangling bond defect to be a three-fold coordinated or bridge-like structure only at an energy cost of 0.5eV, improving the local structure of the monovacancy. However, a -CH3 radical could not improve the structure of the 5-1 dangling bond defect anyway.

Diffusion of Small Hydrocarbon Radicals on the Outer Wall of a (9, 0) Carbon Nanotube. He, H.Y., Pan, B.C.: Physica E, 2008, 40[3], 542-9