Nanotube Development Paves The Way For Nanotube Electronics

IBM researchers reveal improved method of synthesizing carbon nanotubes

Yorktown Heights, NY, USA - 30 Sep 2002: IBM researchers have discovered a new process for synthesizing carbon nanotubes (CNTs) that overcomes important challenges to the feasibility of CNTs as a replacement for silicon transistors.

There are two types of carbon nanotubes: single-walled and multi-walled. Multi-walled CNTs, however, have switching properties that are not ideal for manufacturing transistors. Single-walled nanotubes are ideal for transistors but their growth previously required the use of metal catalysts, which contaminate the nanotubes. While it is usually difficult to control the final attributes of carbon nanotubes, IBM researchers have discovered a process that forms single-walled nanotubes without the use of metals.

Furthermore, IBM researchers have found a way to control the alignment of these single-walled nanotubes so that transistor arrays can be fabricated.For now, nanotubes can only be manufactured in small quantities in the lab. Affordable, mass-production techniques remain years away.

As described in the October issue of Nano Letters, the new method of single-walled nanotube synthesis does not require the use of metal catalysts.

With the original metal catalyst method, scientists heat at high temperatures metal particles, usually nickel, iron or cobalt, with molecules that can act as a source of carbon atoms. The single-walled nanotubes formed are heavily contaminated with the metals used as catalysts and need to be purified before use in electronics. This purification process involves a harsh treatment with oxidizing acids. According to Phaedon Avouris, manager of nanoscale science this treatment damages and dopes the nanotubes.

In the new technique, the nanotubes remain undamaged. Researchers take a crystal of silicon carbide and heat it to about 1600 degrees centigrade in a vacuum. The heat causes the silicon to evaporate and the silicon's absence frees the carbon atoms to bond with other atoms. They bond to neighboring carbon atoms forming fragments of graphite which curl up to form nanotube seeds that grow with the addition of new carbon atoms.

In electronics applications it is very desirable to have arrays of nanotubes. The current work provides a clue on how this can be achieved. At the high temperature of the experiment it was found that the nanotubes become mobile and align themselves with the crystallographic structure of the substrate. Use of pre-patterned substrates could be used to produce a desired alignment.

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