Expansion of nanotube cap due to migration of sp atoms from lateral surface
Possible mechanism of cap expansion of preliminarily prepared carbon nanotube is proposed and confirmed by DFT calculations. According to this mechanism single carbon atoms adsorbed on lateral surface of the nanotube (sp atoms) migrate along the nanotube and are trapped at its cap in the potential well. When pair of sp atoms meet at the cap they annihilate with significant energy gain so that expansion of pure sp2 structure occurs. According to performed calculations on example of (9,0) nanotube the energy gain (or depth of the potential well) with displacement of the first and second sp atom from the lateral surface of nanotube to its cap is of about 1.5 eV. The calculated barriers along these sp-atom displacement paths toward the cap are low and lay in the range 0.25–1.5 eV. The calculated energy gain at the sp-atom pair annihilation is 5–6 eV. The analysis of topological changes at the proposed nanotube cap expansion shows that it leads to the change of the cap structure and therefore to change of the related properties that can be used in applications.