Through first-principlss calculations, we investigate atomic, electronic properties of the carbon nanotubes with vacancy defects and their dynamics. We also study the atomic, electronic and magnetic properties of the transition metal-doped graphene.
First, we focus on the vacancies in the carbon nanotubes. Using first-principels and tight binding calculation, we study the atomic structure of the carbon vacancy clusters Vn, wher n is the number of missing carbon atoms. We find that missing atoms are tend to aligned along the tube axis, rather than form a large vacancy hole. In the vacancy region, the diameter of the tube shrinks, and two pentagon-heptagon pairs are formed at the junction boundary of the pristine tube and the vacancy region. As n increases, pentagon-heptagon pair-kink moves. For the serial network of missing atoms, the parallel alignment along the axis is more stable than the spiral alignment, which is in consistent with the kink motion of the superplastic deformation. We show that the preference of the longitudinal motion of the pentagon-heptagon defect is more prominent in armchair tubes compared with other chiral tubes.
Then, I examine how the monovacancies diffuse and coalesce to the larger vacancy clusters via the action-derived molecular dynamics simulations. Through the minimum-energy transition pathway search, I investigate the monovacancy diffusion modes and vacancy-vacancy coalescence mode in graphene and (5,5), (7,3), (9,0) nanotubes.
Since monovacancies have the much smaller diffusion barrier than the larger vacancy clusters, I focus on the diffusion of the monovacancies and their coalescence. In the (5,5) nanotube, the diffusion barrier along the circumference is the lowest. As the chirality goes to zigzag, however, this energy barrier becomes larger. In the zigzag tube, it is hard to escape the circimference. So generally the coalescence of two single vacancies can be done easily in armchair tube, where two single vacancies move...