General sum rule for chiral index of coalescing ultrathin nanotubes.

We investigated the coalescence of ultrathin carbon nanotubes (UTCNTs) using the tight-binding molecular dynamics simulation technique. We have found that two UTCNTs having the same or different chirality can coalesce without initially introducing atomic defects to enhance the reaction. The chiral index of the coalesced nanotube was found to be given as a vector sum of the indices of the original nanotubes, which can be explained geometrically in terms of the developments of the nanotubes. The results clearly show that the chirality can be changed through chemical reaction, which might suggest a possibility of chirality control.