Possible defect structures can be classified into three groups: topological, (corresponding to the presence of rings other than hexagons, for example pentagon/heptagon pairs), incomplete bonding defects (e.g., vacancies, dislocations etc.) and rehybridization - chemical defects consisting of atoms/groups covalently attached to the carbon lattice of the tube .
The most typical type of defects in crystalline lattices are point vacancies. A graphene vacancy breaks three short, strong C=C bonds. Vacancy defects are produced post-synthesis. For example, hit events involving high energy electron, ion, or neutron radiation can fully remove a carbon atom. This type of vacancy initially results in three dangling bonds that will immediately rehybridize or react with surrounding molecules .
Dangling bonds are somewhat academic when CNTs are surrounded by a typical experimental environment. Unlike vacancies produced deep in a graphite crystal, CNT surfaces interact with adsorbed gases, moisture, supporting substrates, and nearby amorphous carbons, all of which provide spontaneous reaction pathways to saturate dangling bonds. Under all those conditions, most intermediates are susceptible to nucleophilic hit of H2O, making -OH terminated vacancies, one of the most likely, and physically relevant, configurations, see Fig.1.
The metastable chemistry of a vacancy also drives a tendency towards vacancy coalescence. In graphite, a di-vacancy formed by two missing atoms only. The di-vacancy has the ability to reconstruct into a pentagon, octagon, and pentagon (5-8-5) structure that is free of dangling bonds, see Fig.1. With the additional strain of curvature, di-vacancies in SWNTs are believed to have smaller formation energies than mono-vacancies by nearly 1.5 eV.
The root of versatility of carbon is its ability to ryhebridize between sp sp2 and sp3. Diamond and graphite are examples of pure sp3 and sp2 hybridized states of carbon, however many intermediate degrees of hybridization are possible . CNT rehybridization defects are atomic carbons covalently bonded between two carbon shells and, rarely, to just one. Covalent attachments by other atoms or molecules (adsorbates)are termed adducts .
Geometrical size other than hexagons, such as pentagons and heptagons, in the graphene sheet can be treated as local defects . While vacancies and rehybridization defects are highly disfavored, bond rotation is the most prevalent type of defect in high quality graphites during synthesis. A single bond rotation can be incorporated into graphene at approximately 3.5 eV without disturbing the sheet's topology or sp2 conjugation. The rotation only affects four adjacent hexagons, converting two into pentagons and two into seven-sided heptagons. This particular 5-7-7-5 configuration (see Fig. 2) has been studied extensively and is known in the literature as a Stone-Wales (SW) defect (, ). SW defects are difficult to observe experimentally. In SWCNTs, the SW defect is presumed to be as predominant as it is in graphite, even though CNT synthesis, especially chemical vapor deposition (CVD) synthesis, proceeds at lower temperatures than typical graphitization. As noted in , SWNTs synthesized at 3000 K will contain 1 SW defect per μm, on average. Furthermore, these SW defects are long-lived after the initial synthesis, being trapped in the lattice by the high dissolution barrier.
A slightly simpler defect than the SW configuration is a single 5-7 pair (see Fig. 2), in which apentagon adjacent to a heptagon replaces two hexagons. CNT growth appears to have no preferred defect configuration: 5-7 defects can appear as singles and they are just as likely to be incorporated as pairs into a SW defect.
In the cylindrical CNT geometry, the dislocation introduced by a 5-7 defect is manifested as a change in helicity. A SCWNT with indices (n,m) will seamlessly change to (n ±1,m-+1); the incorporation of d 5-7 defects around a SWNT circumference can change its indices to (n ± d,m-+d), (see ).
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