Chemistry Reference
In-Depth Information
dropwise to a stirred solution of diisopropylamine in dry THF at low temperature
(about 195 K) under N
2
. Then
-4T in dry THF is added dropwise, the mixture is
stirred and anhydrous CuCl
2
is added. After further stirring and addition of HCl,
solid
α
-6T is formed. Synthesis and crystallization, discussed in the next chapter,
go hand in hand when the obtained materials form solids.
α
2.3 Hexagons and pentagons
Molecules made exclusively of hexagons necessarily form open structures, un-
less a large torus-shaped molecule is obtained (genus
=
1), but can become closed
simply by adding pentagons (genus
0). The buckminsterfullerene, C
60
,isavery
special and rather unique molecule and perhaps the best example of combinations
of hexagons and pentagons (see Table 1.1). It exhibits the truncated icosahedral
structure and was named after the American architect and engineer R. Buckminster
Fuller (1895-1983) because of his celebrated geodesic domes. C
60
is prepared by
physical
synthesis instead of following a chemical route (Kroto
et al.
, 1985). It is
obtained by laser evaporation of graphite using a focused pulsed laser and a high
density helium flow (see Section 3.2 for details of the laser ablation technique). A
large number of carbon clusters are obtained and under certain conditions, for a
range of helium pressure, C
60
dominates. A higher mass cluster, C
70
, is also gener-
ated. The most effective way of producing C
60
is the arc discharge technique with
graphite electrodes in a helium atmosphere (Kratschmer
et al.
, 1990). The resulting
black soot is scraped from the collecting surfaces inside the evaporation chamber
and dispersed in C
6
H
6
. The solution is separated from the remaining soot and dried.
C
60
is stable at temperatures well above RT, is chemically active but stable in air.
In addition to C
60
the soot consists of other materials such as amorphous carbon,
C
36
, a range of other fullerenes (e.g., C
70
), multilayered carbon onions, single and
MWNTs and nanotube bundles. Purification of the soot is thus a major part of the
preparation.
Nanotubes can be thought of as arising from rolling graphene layers and capping
with two halves of C
60
molecules. Multiwalled nanotubes (MWNTs) are made of
concentric cylinders of rolled-up graphene sheets (Iijima, 1991). The length of
a tube is in the range of a few micrometres and the diameter of 10-20 nm. A
major breakthrough was achieved when single-walled nanotubes (SWNTs) were
synthesized usingmetallic catalyst particles (Bethune
et al.
, 1993). These structures
can show1Dmetallic or semiconducting properties, depending on how the graphene
sheets are rolled up. Carbon onions were first observed in an electron microscope
after electron irradiation of graphitic materials (Ugarte, 1992). They consist of
concentric spherical layers stacked one inside the other, like Russian dolls.
=
