Causes of energy destabilization in carbon nanotubes with topological defects

The relative stability of a family of carbon nanotubes (CNT) with defects has been investigated theoretically with first-principles density functional theory (DFT) calculations, B3LYP/6-31G*. A set of (12,0)-(8,0) CNT heterojunctions with an increasing number (n = 1-4) of pentagon/heptagon defects were studied systematically in different arrangements, and the results were compared with a set of small defective graphene fragments. In addition, tubular structures with two pairs of defects distributed variedly (along and around the CNT) with increasing distances were considered. Within the defective structures, those containing the well-known Stone-Wales defect proved to be the most stable. However, when more than two pairs of defects coexisted, situations where the defects appeared together seemed to be preferred, in sharp contrast to the isolated pentagon rule (IPR) for fullerenes, although this agrees with some previous works on this topic. The junctions studied here constitute different arrangements that help us to identify which effects (geometry and energy) arise from the particular positions and orientations of the defects in nanotubes. Moreover, a close correlation was found between the energy stability and the geometric deformation, measured with the average pyramidalization angle (POAV) and the average trigonal deformation (D₁₂₀). For this purpose, the different contributions to molecular strain were analysed with the TubeAnalyzer software.