challenging part of many experiments is the growth of carbon nanotubes.
At present there is no possibility to control the structure of nanotubes
and all fabrication methods yield mixtures of nanotubes with different
lengths, helicities and diameters. In addition, nanotubes can be further
modified during post-growth treatments such as purification or annealing.
arc discharge was the first available method for the production of both
MWNTs and SWNTs
. It is worth noting that this technique has been in use for a
long time for the production of carbon fibers and that it is very
probable that nanotubes were observed before 1991 but not recognized as
such (follow, e.g.,
can be produced in a carbon arc apparatus similar to the one depicted
below using the method described by Ebbesen and Ajayan. An arc is struck
between two graphite electrodes in a gas atmosphere. MWNTs produced by
arc discharge are long and straight tubes closed at both ends with
graphitic walls running parallel to the tube axis.
et al. and
Bethune et al. reported in 1993 that an arc discharge with a cathode
containing metal catalysts (such as cobalt, iron or nickel) mixed to
graphite powder results in a deposit containing SWNTs. SWNTs are usually
assembled in ropes but some single tubes can also be found in the
Schematics of an arc-discharge
apparatus, along with electron microscopy pictures of the products
obtained with doped and pure anodes.
method to grow SWNTs using laser ablation was demonstrated in 1996 by
Smalley's group and has prompted a lot of interest. Thess et al. showed that the
synthesis could be carried out in a
horizontal flow tube under a flow of inert gas at controlled pressure.
In this set-up the flow tube is heated to ~1200°C by a tube furnace.
Laser pulses enter the tube and strike a target consisting of a mixture
of graphite and a metal catalyst such as Co or Ni. SWNTs condense from
the laser vaporization plume and are deposited on a collector outside
the furnace zone.
Schematics of a laser ablation
set-up, reproduced from
I. Yakobson and R.E. Smalley, American Scientist 85, 324 (1997).
alternative to the arc discharge and laser ablation methods is the
catalytic growth of nanotubes. This method is based on the decomposition
of a hydrocarbon gas over a transition metal to grow nanotubes in a
chemical vapor deposition (CVD) reactor such as the one shown below.
Schematics of a CVD deposition oven.
filaments and fibers have been produced by thermal decomposition of
hydrocarbons since the 1960s. Usually, a catalyst is necessary to promote
similar approach was used for the first time in 1993 by Yacaman et al. to grow MWNTs from
the decomposition of acetylene over iron particles.
the production of MWNTs acetylene is usually used as source of carbon
atoms at temperatures typically between 600 – 800°C. To
grow SWNTs the temperature has to be significantly higher (900 – 1200°C)
due to the fact that they have a higher energy of formation. In this
case carbon monoxide or methane must be used because of their increased
stability at higher temperatures as compared to acetylene.