CARBON NANOTUBE DEPOSITION AND GROWTH TECHNIQUE

Lance Delzeit

Carbon nanotubes (NTs) possess electrical, mechanical and physical properties that make them ideal for applications in nanotechnology. A major constraint to the realization of many of these applications is the ability to produce nanotubes in an industrially viable method with the characteristics desired for the given application. A few of these characteristics include quantity, chirality, size, density, distribution and purity of the nanotubes produced. The research described here focuses on the production of NTs with the desired density, distribution, and purity for the application to industrially viable products.

A catalyst deposition and growth technique has been developed that allows for the controlled growth of either single- or multi-walled carbon nanotubes. This technique uses ion-beam sputtering to deposit the catalyst. By changing the catalyst formula and the growth conditions, either single- or multi-walled carbon nanotubes can be grown. Furthermore, by adjusting the conditions used to produce single-walled nanotubes, the density of the nanotubes grown can be controlled from a sparse distribution of individual single-walled nanotubes to dense mats of single-walled nanotube "ropes". "Ropes" are an association of individual nanotubes that forms a larger structure just as individual fibers make up a normal rope. The conditions for the growth of multi-walled nanotubes have been optimized for the growth of "towers". A "tower" is a structure in which the nanotubes grow in the vertical direction because of the high density of the nanotubes in that region. These different structures each have applications to a variety of devices.

A further advantage of this technique is the ability to pattern the catalyst onto the surface. If the application requires the nanotubes to be grown in a confined area, then the ability to restrict the deposition of the catalyst to those areas is critical. This process, with the use of standard shadow masking and lithography techniques, has the ability to create such patterned catalyst deposits for the development of applications.

Finally, for most applications, the nanotubes need to be produced free of impurities and contamination. The two major sources of contamination in the growth of carbon nanotubes are: 1) the build-up of amorphous carbon from the extraneous decomposition of carbon feed gas and 2) contamination by extraneous metal catalyst. The elimination of the extraneous metal catalyst is currently being accomplished by optimizing the catalyst formula, thus reducing the quantity of "inactive" catalyst. The removal of the amorphous carbon is being realized by the use of etching gases that preferentially removes the amorphous carbon while not damaging the carbon nanotubes.