Electronic Structures and Energetics of [5,5] and [9,0] Single-Walled Carbon Nanotubes

Abstract

Standard enthalpies of formation, ionization potentials, electron affinities, and band gaps of finite-length [5,5] armchair and [9,0] zigzag single-walled carbon nanotubes (SWNTs) capped with C(30) hemispheres obtained by halving the C(60) fullerene have been computed at the B3LYP/6-311G* level of theory. Properties of SWNTs are found to depend strongly on the tube length and, in the case of the [9,0] zigzag species, on the relative orientation of the caps. The metallic character of an uncapped infinite-length [5,5] armchair SWNT manifests itself in the oscillatory dependence of the properties of capped finite-length tubes on their size. An infinite-length [9,0] zigzag SWNT is predicted to be a semiconductor rather than a metal irrespective of the presence of caps. The present results underscore the slow convergence of SWNT properties with respect to the tube length and uncover small but significant radial distortions along the long axes of SWNTs.

Semiempirical calculations on the electronic properties of finite-length models of carbon nanotubes based on Clar sextet theory

“…The calculations presented in this work were performed on FLCC models of CNTs ranging from 2 to 6 replica of the Clar unit cell. Figure 1 shows the optimised geometry of FLCC models of (7,7), (8,0), (6,4) and (6,5) CNTs for the case of three Clar cells. Geometries obtained at the AM1 level are in excellent agreement with the geometries evaluated at the DFT level [15], the larger deviations being related to the position of terminal hydrogens.…”

“…As shown in Figure 4 for the examples of the (8,0) and (6,5) CNTs, total dipole moments of FLCC models converge promptly to a finite value, with an accuracy of about 0.02 Debyes for 5 Clar cells. As stated above, semiempirical AM1 calculations were also performed on models of (n,n-2) (n=5, 6,7,8,9) CNTs to assess the effect of the nanotube diameter on the electronic properties for a set of analogous systems. In this case, the HOMO-LUMO energy gap decreases monotonically (see Figure 3), as expected for rolled graphene sheets of decreasing diameters.…”

“…The results and the methodology presented in this work can be extended to studies where the electronic properties of CNTs play a crucial role, as in the case of investigations on the reactivity at the sidewall . 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 (6,5). Figure 2.…”

“…In particular, as representatives of more general cases, models of (7,7), (8,0), (6,4) and (6,5) CNTs were taken into account. Moreover, the dependence of electronic properties with the nanotube diameter for an analogous set of systems was studied for semiconducting chiral (n,n-2) CNTs, with n ranging from 5 to 9.…”

“…On the other hand, calculations on periodic systems are computational demanding, especially when the periodic unit cell is constituted by hundreds of atoms, as in the case of chiral CNTs. An alternative is based on the use of finite-length models, as used in previous work [5][6][7]. However, the representativity of finite-length models in describing the properties of nanosized objects is a non-trivial problem, and this also applies to the case of CNTs [8].…”