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.