Single-walled carbon nanotubes (SWCNT) attract attention as a promising material with exciting chemical and electronic properties. Electron localization at the zigzag edges led to a radical character and increased chemical reactivity at the SWCNT edges. Prior to the study of SWCNTs, geometry optimization of [13,m]-peri-acenes (m = 0–5) series using GFN2-xTB led to directly linked homodimers only at the zigzag edges. The formation of covalently linked homodimers from the corresponding monomers was highly exothermic, while the energy of homodimers from the corresponding Van der Waals dimer is much lower due to the energetic penalty of sp3 hybridization. With the help of the DFT and GFN2-xTB electronic structure calculations, we demonstrate for the first time that the zigzag edges of carbon nanotubes are capable of forming direct interlinking covalent bonds between the SWCNTs. SWCNTs show more prominent stabilization than acenes because of the smaller contact area and weaker Van der Waals interactions between the neighboring SWCNTs. The optimized structures of the SWCNT homodimers with different chirality conclude that the nanotube chirality, diameter, or curvature cannot be used as a sole predictor of the covalent linking between the monomers. Even though initially, the HOMO-LUMO gap shows a good relationship with the bond formation, regardless of small bandgap in longer SWCNTs, homodimers form Van der Waals dimers. Interestingly, Clar’s aromatic sextet theory demonstrated that the zigzag edges are under-stabilized comparing to their armchair counterparts because of the lack of aromatic stabilization. Moreover, the number of zigzag edges in SWCNTs influence the covalent bond formation. Hence, the covalent linking of SWCNTs might enhance the mechanical properties and tensile strength of the multiwall nanotubes, nanotube-containing materials, as well as graphene multilayers.

Stability of single-walled carbon nanotubes toward covalent bonding between the zigzag edges

ACS Spring 2022

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