Vacancy-induced intramolecular junctions and quantum transport in metallic carbon nanotubes

First-principles calculations are carried out to study atomic reconstruction and the electronic structures of metallic armchair and zigzag single-walled carbon nanotubes (SWCNT) in the presence of hexa-vacancy defects. The introduction of hexa-vacancy defects can effectively give rise to the formation of intramolecular junctions in both armchair- and zigzag-type SWCNTs. Two kinds of vacancy distribution, six atoms removed along the zigzag chain and a pristine hexagonal lattice, lead to asymmetric and symmetric intramolecular junctions with respect to the axis of the SWCNT after reconstruction, respectively. It is found that the (7,7) symmetric intramolecular junction exhibits more favorable electronic transport compared to the corresponding asymmetric intramolecular junction, whereas the (12,0) symmetric exhibits smaller current in contrast to the asymmetric configuration. This unexpected behavior is attributed to a competition between a widening of the transmission windows that opens additional transmission channels and transport quenching due to orbital misalignment induced by the applied bias.