Chemistry Reference
In-Depth Information
junctions could act as multiterminal electronic station or building blocks for three-
dimensional nanotube networks.
4 Chemistry of Carbon Nanotubes
The outstanding properties exhibited by CNTs make them exceptional candidates
for materials and biomedical applications. However, there is a big difference
between the promising potential of a new material and its practical application on
a widespread basis. As-produced CNTs are insoluble in most organic or aqueous
solvents and tend to aggregate as a result of strong van der Waals interactions
between individuals tubes, rendering difficult their manipulation and limiting their
processability. Thus, for example, in biomedical application a valid solution is the
modification of the nanotube surface in order to prepare aqueous solutions of
nontoxic and biocompatible systems.
Another key problem is that, according to the procedure used to synthesize
CNTs, mixtures of metallic and semiconducting CNTs coexist with their corre-
spondingly different electronic structures. The presence of these mixtures limits the
applications of CNTs as versatile building blocks for molecular electronics.
Impurities also interfere with most of the desired properties as well as with
biocompatibility. For these reasons, purification and separation of the tubes has
been a matter of intensive study.
Hence, given all these challenges, the use of chemistry has emerged as one of the
most effective means for manipulating CNTs and even separating metallic from
semiconducting species. A large number of chemical treatments that include both
covalent and noncovalent approaches have been described, giving scientists the
ability to combine these structures with other types of materials.
In the next two sections we will explore the advances in the formation of
nanotube derivatives and the combination of techniques used for their characteri-
zation. Because of the huge amount of work in this area, this will not be a
comprehensive review but will at least be representative of the progress in this field.
4.1 The Covalent Approach
The hexagonal network of sp
2
-hybridized carbons of CNTs can be covalently
functionalized by taking advantage of the curvature of the tubular walls. Similar
to fullerenes, there is indeed a local strain relief enhancing carbon nanotubes
reactivity, which is induced by the change of pyramidalization and
-orbitals
misalignment of conjugated carbon atoms [
47
,
48
]. Although fullerenes are much
more reactive than CNTs as a consequence of the spherical geometry and the
smaller diameter, a big set of reactions is allowed for both allotropes [
49
-
51
].
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