Figure 1. The geometric structures for cap-(9, 0)-Def [6, 6] and
cap-(9, 0)-Def [5, 6] CNTs.
Figure 2 reflects the net spin distribution of ground states of Cap-(9,
0)-Def [6, 6] and Cap-(9, 0)-Def [5, 6] CNTs. From this figure,
we can clearly see the source of the spin density. The spin density of
open end is mainly derived from the pz electrons of the
carbon atoms at the zigzag edge, which is consistent with previous
reports.8, 16 And our previous study on cap-(9,
0)/(10, 0) CNTs indicated that the spin density of carbon atoms at the
cap is 0, as shown in Figure 2c. And the ground state of perfect
C60 is the closed-shell singlet state, without any net
spin electron.36 For the two defect structures, there
exist spin density at the cap end, and the net spin electrons are mainly
originated from the C adatoms. And the number of net spin electrons
distributed on the C adatoms of Cap-(9, 0)-Def [6, 6] and Cap-(9,
0)-Def [5, 6] are about 1.6 and -0.3, respectively. For
Cap-(9,0)-Def [6, 6] CNT, the net spin orientation of the adatoms at
cap end and the carbon atoms at zigzag edge are both spin up, showing
ferromagnetic coupling phenomenon. While for Cap-(9, 0)-Def [5, 6]
CNT, the net spin orientation of the adatom at the cap is spin down, and
the spin orientation of the carbon atoms at zigzag edges is spin up,
appearing antiferromagnetic coupling. This also shows that different
types of adsorbed atoms can regulate the magnetic properties of
cap-CNTs.