HOMO-LUMO GAPS IN ZIGZAG CARBON NANOTUBES WITH STONE-WALES DEFECT
Presented by Dr. Noriyuki MIZOGUCHI
Type: Oral presentation
1. Introduction An interesting aspect of finite-length carbon nanotubes (CNTs) is the quantum finite-size effects of the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). It has been shown that the HOMO-LUMO gaps in the zigzag CNTs show the oscillation depending on the number of hexagons around the peripheral circuits. We have shown that this HOMO-LUMO gap oscillation arises from the existence of 4k-membered conjugated circuits in (n,0)m zigzag CNTs. Various kinds of defects are known to be present in carbon nanotubes, which have significant influence on electronic, mechanical, and transport properties of the tubes. The Stone-Wales defect plays a key role, enabling large-scale structural rearrangements in graphitic networks. It creates two pentagons and two heptagons. In this paper we evaluate the effects of SW defect on the HOMO-LUMO gaps in zigzag CNTs by using semiempirical PM3 method. 2. Formation Energies There are two types of SW defect with different configurations. We call the two types of SW defect, H-type and V-type, respectively. The results of PM3 calculations showed that (n,0)m zigzag CNTs with SW defect have larger energies than pristine zigzag CNTs, regardless of types of SW defect. This implies that the formation energies of SW defect are positive, regardless of types of SW defect. The formation energies of SW defect of V-type are larger than those of H-type. 3. HOMO-LUMO gaps oscillation The V-type SW defects increase the magnitudes of HOMO-LUMO gaps in pristine zigzag CNTs at each radius n, implying that zigzag CNTs with V-type SW defects are chemically more stable than pristine zigzag CNTs. On the other hand H-type SW defects do not significantly affect HOMO-LUMO gaps. The HOMO-LUMO gaps in the zigzag CNTs with SW defects of H-type and V-type exhibit similar oscillation with period two depending on the number of hexagons around the peripheral circuits to those in the pristine zigzag CNTs.