Carbon Nanotubes, Rice University

Introduction:
Professor Smalley of Rice University in America was one of the international collaboration of scientists who, in 1985, discovered a new form of carbon. In this new form of carbon the atoms are bound together so as to form a sealed ball. These balls, which can come in various sizes (C60, C70, C80), are now called fullerenes.

In 1991 the Japanese electron microscopist, Sumio Iijima, discovered that these fullerenes can also take on tubular structures. The carbon atoms are bound together in a hexagonal pattern to form long cylinders (see image right). These structures, which are just 1.2nm in diameter, are now known as "carbon nanotubes". Nanotubes can also be formed with multiple layers of walls. Although there is still a lot more to learn about these structures, they hold good prospects for the development of extremely strong materials. Their tensile strength is 1000x greater than steel. They also have good thermal and electrical conductivity. There is now a great deal of ongoing research to investigate the use of these nanotubes in the building of nanostructures and nanocomputing elements.

Nanotube Manufacture:
In Professor Smalley's laboratory nanotubes are manufactured through the pulsed laser vaporization of a metal/carbon target in a furnace at 1100 °C. This produces a sooty material which is about 50% single-walled nanotubes. The rest of the material is amorphous carbon and residual catalyst particles. The material is then purified and rinsed using nitric acid, water and toluene. The end result is single walled nanotubes at a purity of greater than 90% (see micrograph right). The main impurity is leftover Ni/Co catalyst particles. Most tubes are between 0.2 and 2 microns in length.

Although this process is good for in terms of mass production, there is very little control over the resulting length and orientation of the nanotubes. The exertion of more control is now the target of the Rice research group. In October 1999 they reported success in the formation of "Buckyribbons", in-plane-aligned assemblies of nanotubes.

Semiconducting Nanotubes:
The conductivity of carbon single-walled nanotubes can be metallic or semiconducting. Their conductivity is dependent on their diameter and helicity. Kinks and other defects in the tube, as well as impurities can also change their conductivity. This is an extremely important property because it opens up the possibility for building nanotube based transistors. Such transistors could be used in integrated circuits thousands of times smaller than today's silicon base microprocessors.

In 1998 physicists at the IBM Watson Research Center in New York reported having used individual nanotubes as the channel of a field-effect transistor (FET). Although the device acted as an FET at room temperature, at 4K it behaved like a single-electron transistor (SET).

Then in November 1999 physicists at Delf University in Holland and Lucent Technologies in America reported the construction of a nanotube diode. They had produced a kinked nanotube which was made up of a conducting tube connected to a semiconducting tube. Electricity was conducted across the kink in one direction only.

Individual carbon have also recently been shown to act superconductors when cooled to around 1K.

Nanotube Structures:
Scientists at Zyvex, together with Washington University, are currently investigating the nanomanipulation of nanotubes. Their latest piece of equipment, a Transmission Electron Microscope (TEM), is capable of nanotube manipulation whilst being able to simultaneously image the structures as they are being built. Using this equipment they have managed to wrap a nanotube rope around two posts and to build a simple 3-dimensional structure.

Links:
Prof. Smalley: http://cnst.rice.edu/reshome.html
IBM Research: http://www.research.ibm.com/nanoscience
Zyvex Nanotech: http://www.zyvex.com/nano