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Incoming links: Stone-Wales defect
This is a multi-walled carbon nanotube with a hollow interior and approximately concentric graphitic shells running along its axis. The walls show many resolved (002) fringes: a relatively straight section supports an approximate count of 16 shells, with some uncertainty from contrast and overlapping/terminated fringes. The tube is not a uniform straight cylinder: the inner channel narrows and changes direction, walls kink and curve, and some inner fringes terminate or join a partly closed internal region. The rough outer boundary contains disordered carbon or incomplete outer layers. The high-contrast material below the tube should not simply be counted as additional concentric shells.
Bending, incomplete walls and changes of atomic bonding scatter charge carriers and can mix shells, reducing the mean free path and causing extra resistance or localization. Covalent intershell links can also increase transfer between shells. Nonhexagonal rings or vacancies are plausible atomic causes, but this image does not resolve a unique ring inventory or prove a specific Stone-Wales defect. Different shell chiralities can intrinsically give different metallic or semiconducting behaviour; a (002) image alone does not assign those chiralities.
Three point-defect structures are a Stone-Wales defect, a reconstructed graphene monovacancy and a reconstructed graphene divacancy. The original sketch gives ring inventories rather than fitted atomic coordinates.
A Stone-Wales defect rotates one C–C bond, converting four hexagons into a 5–7–7–5 group without changing the number of atoms. A graphene monovacancy removes one carbon; a common reconstruction gives a 5–9 group with residual under-coordination. A graphene divacancy removes two adjacent carbons; a 5–8–5 reconstruction can eliminate dangling bonds. Other divacancy reconstructions exist, so atom loss does not uniquely select one topology.
Irradiation, energetic processing, etching and incomplete growth can create vacancies; thermally or beam-activated bond rearrangement can change their structures. Electron microscopy itself can create defects, so compare exposures before attributing a time sequence to unperturbed material.
Pentagons favor positive curvature and heptagons negative curvature. Such defects can bend tubes, help form caps or junctions, or change a local wrapping relationship. They scatter carriers and alter the local density of states; vacancies can add localized states and chemical reactivity. Whether a clean band gap appears depends on defect identity, concentration and arrangement, not merely on the presence of a defect.
The important distinctions are changes of ring topology, missing or added atoms, and changes of local bonding:
- A Stone-Wales defect rotates one carbon-carbon bond by approximately , converting four hexagons into a --- ring arrangement without adding or removing atoms. It changes strain and electronic scattering.
- A carbon nanotube vacancy defect removes an atom. Reconstruction commonly gives a pentagon and a nine-membered ring, with an undercoordinated site; a reconstructed divacancy can give a -- arrangement. Larger vacancies make holes or broken walls.
- An adjacent pentagon-heptagon pair acts as a dislocation in the hexagonal network. It can connect regions with different wrapping indices, producing a bend or an electronic junction. Isolated nonhexagonal rings impose curvature: pentagons favor positive curvature and heptagons negative curvature.
- An interstitial defect, an adsorbed atom, or chemical functionalization can distort the network and locally change bonding toward . These defects can scatter carriers and create chemically reactive sites.
Closed end caps require nonhexagonal rings, typically six pentagons per hemispherical cap; these are part of the cap topology rather than necessarily damage. Kinks, buckling, branches and incomplete closure are larger-scale structural imperfections.
A multi-walled carbon nanotube can contain any of these defects in individual shells, plus interrupted or terminated shells, extra incomplete graphitic layers, irregular interwall spacing, and covalent interwall bridges formed by displaced atoms. Such bridges change intershell coupling and sliding. Different shells having different wrapping indices is normal and is not, by itself, a defect.