We have carried out MD simulations for coating of carbon nanotubes (CNTs) using a beam of energetic carbon atoms. At low energies Ebeam < 10 eV the sp2 network remains nearly intact and CNTs grow in diameter. The growth is not uniform but the deposited atoms tend to cluster (see figure). At intermediate energies (Ebeam = 10 and 20 eV) knock-out events start to occur and carbon chains are observed. At the highest energy (40 eV) top and bottom surfaces of CNTs with respect to the beam are damaged (but not the sides) leading to graphitization and amorphization.
The deposited atoms tend to cluster. This is because addition of an atom is energetically easier near an existing defect and because desorption probability is lower near existing defects.