Figure 4. Electron density color filling diagram. Figures (a) and (b)
are the electron density between the hexagonal carbon atoms at the cap
end, and the electron density between the C adatom and adjacent atoms of
the Cap-(9, 0)-Def [6, 6]. Figures (c) and (d) are the electron
density for Cap-(9, 0)-Def [5, 6] between the hexagonal carbon atoms
at the cap end, and between the C adatom and adjacent atoms. The white
part in the figure means that the electron density in this area exceeds
the upper limit 0.35.
To further understand the mechanism of electronic states formation, we
analyzed molecular orbitals (MOs), as shown in Figure 3. Since the
ground state of the Cap-(9, 0)-Def [6, 6] CNT is sextet, that is,
there are five unpaired electrons. The corresponding five
single-occupied MOs are the highest occupied molecular orbital minus one
for the α-electrons (denoted as HOMO-1α), HOMO-2α, HOMO-4α, HOMO-5α and
HOMO-6α (Figure 3 (a)). Among them, HOMO-1α and HOMO-2α are two
degenerate MOs, and they are mainly contributed by carbon atoms at
zigzag edges. While HOMO-4α, HOMO-5α and HOMO-6α are mainly derived from
the adatom at cap end. The ground state of the Cap-(9, 0)-Def [5, 6]
CNT is quartet with three unpaired electrons, and these three
single-occupied MOs are mainly contributed by carbon atoms at the zigzag
edges and adatom at cap end (as Figure 3 (b)), similar to Cap-(9, 0)-Def
[6, 6]. Therefore, the spin density of Cap-(9, 0)-Def [6, 6] and
Cap-(9, 0)-Def [5, 6] CNTs are mainly contributed by their
single-occupied MOs.
The electron density diagram, as displayed in Figure 4, can clearly
reflect the electron distribution of Cap-(9, 0)-Def [6, 6] and
Cap-(9, 0)-Def [5, 6] systems. Figures 4(a) and (c) show the
electron density of the hexagonal carbon atoms at the cap end. The color
distribution as a direct indication of the bonding strength reveals that
there are strong electron accumulations between C atoms. And the
electron accumulation for Cap-(9, 0)-Def [6, 6] (Figure a) is
stronger than Cap-(9, 0)-Def [5, 6] (Figure b). Figures 4(b) and (d)
are the electron density diagrams between the C adatom and two adjacent
C atoms. It can be seen that there are also strong electron
accumulations between the adatom and two adjacent C atoms, and the
electron accumulation for Cap-(9, 0)-Def [5, 6] CNT (Figure b) is
slightly stronger than Cap-(9, 0)-Def [6, 6] CNT (Figure d). These
results show that the C-C bond at hexagon for Cap-(9, 0)-Def [6, 6]
is stronger than that for Cap-(9, 0)-Def [5, 6]. While the bond
between the C adatom and two adjacent C atoms for Cap-(9, 0)-Def [5,
6] structure is slightly stronger than that for the Cap-(9, 0)-Def
[6, 6] structure. Thus, electron density indicates that different
adsorbed positions show different electron accumulation.