Chemistry Reference
In-Depth Information
Fig. 1 Chemical structure of C
60
, a single wall carbon nanotube and graphene
1
Introduction
From the 118 elements which have been identified according to the periodic table,
only 94 elements are naturally occurring on earth while the remainder are consid-
ered artificial elements. Among the natural elements, carbon is the only one
providing the basic requirements for life. Its ability of hybridization of atomic
orbitals to produce sp
3
,sp
2
, and sp hybrid orbitals confers upon it the singular
property of having a variety of allotropic forms. Surprisingly, it is less than three
decades since only two allotropes of carbon, namely diamond - constituted by sp
3
carbon atoms - and graphite - formed by sp
2
carbon atoms - were known by the
scientific community. Both allotropes show a reticular structure with carbon atoms
spreading infinitely through the three space directions.
A new scenario emerged in 1985 with the advent of fullerenes, the third and only
molecular allotropic form of carbon, formed by highly symmetric closed cages of a
well-defined number of carbon atoms [
1
]. Interestingly, fullerenes have been
present on our planet from its very beginning as well as in outer space (the presence
of fullerenes C
60
and C
70
has recently been detected by IR in huge amounts in a
young planetary nebula (Tc 1). The fullerene content is around 1.5% of the carbon
present in the nebula, roughly corresponding to the mass of three moons [
2
]).
Soon after the discovery of fullerenes, other important different forms of carbon
were found, namely, in chronological order, multiwall [
3
] and singlewall [
4
,
5
] carbon
nanotubes, and most recently graphenes [
6
] which have provoked great excitement
and expectation in the scientific community (Fig.
1
). In the meantime, a wide variety
of other less common nanoforms of carbon also emerged such as nanohorns,
nanoonions, nanotori, nanobuds, peapods, etc., whose properties and chemical reac-
tivity are less well known to date [
7
]. Furthermore, fullerenes have been skillfully
combined with other elements allocated to their inner empty space, affording a large,
singular, and promising family of
so-called
endohedral fullerenes - those containing
an atom, molecule, or complex in their inner cavity - whose properties and chemical
reactivity are strongly influenced by the elements inside the ball [
8
].
