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.