Environmental Engineering Reference
In-Depth Information
Axis of tube
Zig Zag tube C
(7,0)
Chiral tube
C
(5,2)
a
1
a
2
na
1
ma
2
Basic Vector system
Armchair Tube C
(4,4)
Axis of
tube
2D Graphene sheet
Figure 2.18
The labelling of different forms of carbon nanotubes using the vector system.
or either end or open at one end and blocked with a metal nanoparticle at the
other. The carbon nanotubes themselves may take a range of forms and are usually
termed as C
(n,m)
tubes. The numbers in the name refer to the helicity of the tube
(Figure 2.18), two special cases being armchair [4,4] and zigzag [7,0] tubes. This
results in a range of properties ranging from conductive to semiconducting proper-
ties. There is still a signifi cant challenge in preparing any single form of nanotube
by design and therefore most samples contain many different forms.
Carbon nanotubes can be prepared by similar methods to those used to prepare
C
60
by simply varying the pressure of argon in the reaction atmosphere. However,
control of carbon nanotube growth had been a particular challenge, along with the
drive to fi nd a cheap and scaleable method for their preparation. Recently the use
of chemical vapour deposition (CVD) based methods has allowed the preparation
of carbon nanotubes of both multi and single walled types by careful deposition of
metal particles on a surface. The vapour- liquid -solid (VLS) method allows the
preparation of long carbon nanotubes attached to a surface. Typically, a surface is
coated with nanoparticles of a suitable metal, such as gold, and a vapour of argon
containing a suitable carbon source, such as carbon monoxide or ethylene, is then
passed over the substrate whilst it is heated to high temperatures. The metal
nanoparticles melt and act as nucleation sites for the formation of the carbon
nanotubes. The initial carbon is dissolved in the metal droplets and begins to form
a graphite - like fi lm. The fi lm becomes insoluble in the metal droplet and partitions
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