Nanotubes as -
transistors in Computer chips
Semi-conducting (n and p) nanotubes and conducting nanotubes can be combined to produce miniature transistors that can be used inside computer chips.
Theoretically, individual nanotubes can be laid on a bed of insulating silicon
oxide, producing a nano-sized printed circuit board. This is possible due
to the low resistance of the tubes, and because they are very close together,
and are capable of carrying very high current densities. It is therefore theoretically
possible to improve chip speeds dramatically in the future, possibly even
continuing Moore 's exponential growth law.
Problems
One problem is that when nanotubes are created using conventional means, no one type of nanotube is produced exclusively [12]. For example, the debris from Sumio Iijima's experiment contained a great number of variations of nanotubes. There were many conducting, as well as semi-conducting tubes, making the finding of individual semi-conducting tubes painstakingly slow. Bundles of tubes are useless, as the conductivity of the conducting nanotubes far outweighs that of the semi-conducting tubes. One method of separating and manipulating the tubes is with an Atomic Force Microscope, which can be used to manipulate individual tubes. Unfortunately, using this method it is far too difficult to separate the tubes.
Another method of separation is called “Constructive Destruction” [12]. This involves passing a large current through the nanotube bundle, vaporising the conducting nanotubes. The semi-conducting nanotubes, on the other hand, are ‘switched off' using a silicon wafer either side of the bundle, and so become insulators, and are not vaporised.
A second problem is that of the remaining nanotubes, there are some “single-walled” versions (i.e. like those already discussed), but also “multi-walled” versions (one nanotube inside another, inside another, etc.). The multi-walled tubes do not have the same conductive properties and are not as efficient at conducting/semi-conducting, so also must be removed. In the US, DuPont is working on a solution, involving specifically designed DNA [1], which bonds to the chosen type of nanotube, making it easy to identify and remove. This process is still in the early stages of development though and has not been tested.