Chemistry Reference
In-Depth Information
3.3 Defects
The term defect refers to any interruption of the perfect crystallographic structure
periodicity in a regularly-patterned material. Defects in CNTs occur in several
forms, affecting unambiguously their chemical and physical properties. Frequently
the presence of defects is deliberately induced to tune chemical reactivity, charge
injection, metallic behavior, etc.
There are many ways to break the regular hexagonal pattern of the nanotubes,
which are mainly classified into four groups:
topological
, when non-hexagonal
rings are introduced,
rehybridization
, if the carbon atom rehybridizes from sp
2
to
sp
3
configuration,
incomplete bonding defects
, when vacancies or dislocation are
found, and finally
doping
, when other elements are introduced.
For instance, the presence of the pentagon-heptagon pair changes dramatically
the electron densities of states, generating helicity and yielding a metallic tube from
a semiconducting one [
44
].
Carbon nanotubes have also been found to reconstruct spontaneously as a reflex
to uniform atom loss on the surface [
45
]. When controlled electron irradiation is
applied to SWCNTs, atom removal occurs at a slow rate, leading to the formation of
holes that are mended mainly by dangling bond saturation, forming a highly
defective layer cylinder of smaller diameter. Experiment has shown the formation
of rings such as squares, pentagons, heptagons, and unstable high-membered rings
that have been observed to disappear leaving five- and seven-membered rings
(Fig.
12
).
The insertion of the heptagon-pentagon pair into the sp
2
network has been
exploited to design a new class of nanotubes with intrinsic metallic behavior,
independent of chirality and tube diameter. The controlled combination of five-,
six-, and seven-membered rings leads to defective structures called Haeckelite
CNTs (Fig.
13
), in virtue of the similarity to the radiolaria protozoa drawings by
Haeckel, that exhibit unusual high-intensity peaks at the Fermi level indicating
possible superconductivity properties.
Other nanostructures have been developed covalently connecting crossed
single-wall carbon nanotubes by means of electron beam welding at high
temperatures [
46
]. Stable junctions between nanotubes have been created in situ
with “Y”, “X”, and “T” geometries, finding that heptagons played a key role in the
topology (Fig.
14
).
Also in this case the formation of junctions involving seven- or eight-membered
rings occurs as a consequence of the cross-linking of dangling bonds after vacancies
promotion.
Most of these observations have been supported by molecular dynamics studies,
which suggest a strong metallic character in the crossing points and localized donor
states caused by the presence of the heptagons.
The possibility of handling molecular connections between nanotubes opens the
door to the design of new prototypes for nanoelectronics, where different tubular
