Nanoparticles, particles with dimensions less than 100
nm, have been studied and produced for many different applications
including polishing, fluid thermal conductivity enhancement, impingement
cooling enhancement, and thermal interface material enhancement. Thus,
many types of nanoparticles have emerged including carbon nanotubes and
fullerene. Various techniques have been developed to produce these
particles.
Fullerene/Buckminsterfullerene
Fullerene occurs naturally and can even be found in
everyday candle soot. However, to create a purified fullerene a
laboratory environment is necessary. Buckminsterfullerene (Buckyball) is
a form of fullerene in a soccer ball pattern about 0.7 nm in diameter with
60 carbon atoms bonded into hexagonal and pentagonal shapes like with a
traditional soccer ball. The most well known method used to create
Buckminsterfullerene is the carbon-arc method and was the method used at
Sussex University during the initial investigations and discovery. In
this method a large amount of current is sent through two barely touching
carbon rods for about 10 seconds creating a plasma arc between them. The
rods are contained in a chamber filled with an inert gas (such as helium
or argon). During this period, a sooty material is formed within the
container. At the ideal temperature and gas pressure around 10 percent of
this soot is C60 and another one percent is C70.
To test for the Buckminsterfullerene an infrared
absorption spectrum is measured, due to Buckminsterfullerene’s unique
icosohedral symmetry, it has only 4 IR absorptions at approximately 7.00
μm, 8.45 μm, 17.33 μm, and 18.94 μm [1] making it easy to
identify. First the fullerenes need to be purified by the use of
chromatography, which is a separation method used to separate components
in a mixture.
Fullerenes have also been produced in other ways. In
an effort to reduce the production cost, solar production has been studied
to utilize sunlight to vaporize the graphite. Another method studied to
produce a specific type of fullerene that does not need to be separated is
the pyrolysis of polycyclic aromatic hydrocarbons, which are believed to
be precursors of fullerenes.
Carbon Nanotubes
Nanotubes have also been synthesized in other
materials including boron nitride and silicon, however, carbon nanotubes
are by far the most popular due to the wide variety of possible
applications being researched. Like fullerene, carbon nanotubes occur
naturally and can be found in soot when methane, ethylene, or benzene is
burned. However, to produce more consistent sized and shaped nanotubes a
laboratory environment is necessary. Multiple methods exist for growing
carbon nanotubes and the use of various methods depends on the application
of the nanotubes being grown.
Multiple applications of nanotubes require a good
thermal or electrical bond to the surface, for example, to measure the
thermal or electrical properties of a nanotube. For these applications,
chemical vapor deposition utilizing a catalyst is the primary technique
since the nanotubes are grown on the surface of interest. Chemical vapor
deposition also allows for excellent alignment, position and size control
of the nanotubes.
Chemical vapor deposition begins with a substrate that
is coated with a layer of metal catalyst particles typically by sputtering
a transition metal and either etching or thermally annealing to create
particles for the nanotubes to grow from or from a patterned deposition of
the metal. The sizes of these particles are directly related to the
diameter of the nanotubes grown. The substrate is then heated to between
650 and 900 oC and immersed in two gases, one that facilitates
the process like hydrogen or nitrogen and another that contains carbon
like methane, acetylene, or carbon monoxide. The gas with carbon is
broken down at the catalyst particles allowing the nanotubes to grow at
these sights. Through the application of an electric field the nanotubes
can be aligned into an array of parallel nanotubes.
Chemical vapor deposition can also be used for bulk
growth of nanotubes by adding more catalyst. These can then be removed
from the surface to be free particles. This has proven to be one of the
more efficient methods of creating bulk amounts of consistently sized
nanotubes. However, other methods including arc discharge similar to that
used to create fullerene and laser ablation of a graphite target in at
high temperature in an environment similar to the arc discharge method are
also used to produce the carbon nanotubes.
The best technique to view the carbon nanotubes
currently is an electron microscope. Scanning electron microscope images
of failed carbon nanotube production and actual carbon nanotubes produced
are shown in Figure 1a and 1b respectively.
Fullerene in Carbon Nanotubes
Recently, new studies have been done on the synthesis
of buckminsterfullerene spheres inside carbon nanotubes, which have been
nicknamed peapods. These peapods are of particular interest for
electronics and thermal application and their properties are currently
being studied. Various methods have been developed to produce the
peapods, which are all very similar.
 |
| Figure 2: TEM image of peapod fullerene in nanotube [2]. |
One particular method grows single walled nanotubes on a silicon substrate using a chemical vapor deposition method similar to the one described above and the cap at the end of the nanotube was opened. The end caps of nanotubes are often opened using a H2O2 reflux followed by HCl oxidation or by annealing in a mixture of N2 and O2. To insert the fullerenes into the nanotubes by what is called a vapor phase method [2]. This consists of immersing the substrate with the nanotubes in a sealed glass chamber filled with vapor containing the fullerene molecules heated to 500 oC [3]. This allows the fullerene to move around through Brownian motion within the chamber and after a given period of time enter in the nanotube. Once in the nanotube, the fullerene is restricted in its motion and cannot escape. After a period of time the fullerene fills up the nanotubes yielding a peapod as shown in the TEM image in Figure 2 [2].
© 2007 P. E. Gharagozloo. The author
grants permission to copy, distribute and display this work in unaltered
form, with attribution to the author, for noncommercial purposes only.
All other rights, including commercial rights, are reserved to the
author.
References
[1] W. Kratschmer, K. Fostiropoulos, D. R. Huffman,
"The infrared and ultraviolet absorption spectra of laboratory-produced
carbon dust: evidence for the presence of the C60 molecule,"
Chemical Physics Letters 170, 167 (1990).
[2] Y. Ohno et al., "Synthesis of Carbon Nanotube
Peapods Directly on Si Substrates," Appl. Phys. Lett. 86, 023109-1
(2005).
[3] K. Hirahara et al., "One-dimensional
metallofullerene crystal generated inside single-walled carbon nanotubes,"
Phys. Rev. Lett. 85, 5384 (2000).
