  In this work, we have examined how the multi-vacancy defects induced in the
horizontal direction change the energetics and the electronic structure of
semiconducting Single-Walled Carbon Nanotubes (SWCNTs). The electronic
structure of SWCNTs is computed for each deformed configuration by means of
real space, Order(N) Tight Binding Molecular Dynamic (O(N) TBMD) simulations.
Energy band gap is obtained in real space through the behavior of electronic
density of states (eDOS) near the Fermi level. Vacancies can effectively change
the energetics and hence the electronic structure of SWCNTs. In this study, we
choose three different kinds of semiconducting zigzag SWCNTs and determine the
band gap modifications. We have selected (12,0), (13,0) and (14,0) zigzag
SWCNTs according to n (mod 3) = 0, n (mod 3) = 1 and n (mod 3) = 2
classification. (12,0) SWCNT is metallic in its pristine state. The application
of vacancies opens the electronic band gap and it goes up to 0.13 eV for a di-
vacancy defected tube. On the other hand (13,0) and (14,0) SWCNTs are
semiconductors with energy band gap values of 0.44 eV and 0.55 eV in their
pristine state, respectively. Their energy band gap values decrease to 0.07 eV
and 0.09 eV when mono-vacancy defects are induced in their horizontal
directions. Then the di-vacancy defects open the band gap again. So in both
cases, the semiconducting-metallic - semiconducting transitions occur. It is
also shown that the band gap modification exhibits irreversible
characteristics, which means that band gap values of the nanotubes do not reach
their pristine values with increasing number of vacancies.
