Results and discussion
After optimization, the C adatom on the Cap-(9, 0)-Def [6, 6]
structure destroys the [6, 6] bond and bonds with two adjacent C
atoms, forming two heptagons (Figure 1a). So C adatom enlarges the
distance between these two atoms by 0.74 Å (from 1.40 to 2.14 Å).
Similarly, for Cap-(9, 0)-Def [5, 6], the C adatom also destroys the
[5, 6] bond and bonds with adjacent C atoms to form a heptagon and a
hexagon (Figure 1b), which also enlarges the distance between these two
atoms by 0.77 Å (from 1.40 to 2.17 Å). For the Cap-(9, 0)-Def [6, 6]
CNT, its ground state is sextet, while the ground state of Cap-(9,
0)-Def [5, 6] CNT is quartet. And the total energy for Cap-(9,
0)-Def [6, 6] is lower than that for Cap-(9, 0)-Def [7, 6] about
0.19 eV, so the former structure is more stable. To illustrate in
detail, the relative energies are listed in the Table S1 and S2 in
Supporting Information.