The new inorganic semiconductor - carbon nanotubes

Carbon nanotubes, also known as "bucky tubes" could be an alternative to organic semiconductors. Carbon nanotubes can be considered as extended carbon "bucky balls" - hollow cylinders of structured carbon atoms (tubular fullerene) which can be capped at the two ends with a hemisphere of a bucky ball. Multi wall and single wall nanotubes are available, with single wall nanotubes exhibiting a regular, near perfect structure when carefully produced. These have a diameter of 0.7-2nm with length being thousands of times the diameter.

 

Developed first by the Japanese and now globally, nanotubes exhibit unique thermal and electrical conductive capabilities. They are even being billed as a wire strong enough to connect a satellite to the Earth.

 

The electrical conductivity of single walled nanotubes is similar to that of metals. This is because its unique structure means electrons can freely pass along the material. This is currently the first and only known molecule that can electronically act as a true metallic conductor. By spacing and controlling the size of the tubes they can behave as semiconductors, similar to silicon.

 

Single walled nanotubes are also suitable additives for conductive plastics. For applications such as antistatic and EMI shielding coatings, carbon black is currently the most common filler because it is relatively inexpensive. However, a lot of it is required to provide the necessary conductivity. At 30-50%, it also weakens the bulk properties of the material. The cylindrical shape of the nanotube means the aspect ratio (length-width) enables less amount of nanotubes to be used to achieve the same desired conductivity - less than 1% in some cases.

 

However, being a relatively new technology it is expensive. Some of the purest forms of carbon nanotubes cost $500 per gram and some have likened the cost of nanotube production plants to that of silicon foundries. However, if cheap enough in future it could outperform some organic semiconductors and enable faster circuits.