Methods
Based on the asymmetric structure of cap-(9,0) CNTs previously
studied8, we adsorbed C atoms on it to form asymmetric
defect structure. Similar to C60,28C30 also has [6, 6] bond (connecting two hexagons)
and [5, 6] bond (connecting a pentagon and a hexagon), which means
that only two adatom defects can be formed on the surface of
C30. Therefore, in this paper, we adsorbed a C atom on
the [6, 6] and [5, 6] bond at cap end to form cap-(9, 0)-Def
[6, 6] and cap-(9, 0)-Def [5, 6] structures, respectively, as
shown in Figure 1 (a) and (b).
For carbon-based materials, the DFT methods deal with spin polarizations
caused by various defects and edge unsaturation.29-31We used the screened exchange hybrid density functional HSE06 method
proposed by Heyd, Scuseria and Ernzerhof.32, 33Because the maximum Milliken spin density values calculated by it are
more reliable than those obtained using the local spin density
approximation (LSDA) and semi-local gradient corrected functional of
Perdew, Burke and Ernzerhof (PBE).15 Moreover, all
calculations were performed using the Gaussian 09
program34 with the 6–31G** basis set35.