Fine conductors

despite tremendous advances in the field of microelectronics, the material used in almost all electronic devices is still silicon. This is because no other material has the electrical properties of silicon, combined with the ease with which it can be fabricated into useful circuits. It is also true that the complexity of connections between integrated circuits on a microchip is nowhere near the connections in a biological system like the brain. The trouble is with making connections which are conducting and yet minature enough to fit large numbers on the chip.

Now, researchers at Stanford University, usa , have reported growing ultra thin carbon wires between metal electrodes. These conducting wires may be the first step towards creating circuits. It may also rival the brain in its complexity of connections.

The material used by H Song and his team at Stanford is pure carbon. Carbon-60, the wonder material of the early 1990s, is easily fabricated into nanometer (a billionth of a meter) scale structures like nanotubes. The tubes are very thin -- 20,000 of them stacked up side-by-side are thinner than a human hair! They offer, perhaps, the most revolutionary form of wiring for miniature circuits. Their electrical properties and size offer tremendous possibilities for circuit designers, who are constantly in search of techniques to pack in more and more circuits into microchips.

The problem with nanotubes is that it is often difficult to grow and place them perfectly between two devices on a chip. One has to also ensure that the electrical contacts between the tubes is good. However, the Stanford team found a novel way to get rid of the problem. They discovered that one could grow the nanotubes from methane gas. Using a catalyst, made primarily of aluminium oxide and iron nitrate, they could grow carbon whiskers in methane gas. The whiskers can grow between two nearby particles of the catalyst as long as the particles are close enough. Using this basic idea, the Stanford team has been able to connect a series of devices on a single microchip.

The electrodes of the device were placed on top of the catalyst particles a few micrometers apart. The nanotubes grew out between them. Most of the time there was only one tube connecting the neighbouring electrodes, but sometimes there was more than one. The team then used an atomic force microscope, a device which allows one to manipulate materials on a very small scale, to cut off the extra connections. The "wires" were tested for their electrical conductivity and they were found to provide excellent contacts.

The use of carbon as a material of choice for fabrication of electronic circuits has been explored by several other groups for the last couple of years. Alex Zetl and his group at the University of California, Berkeley, usa , have fabricated a metal-semiconductor junction from carbon nanotubes. They found that the tubes, behaved like a metal at some points while at certain other points, it mimicked a semiconductor. Within a space of some 10 atoms, there is a perfect metal-semiconductor junction. This can be used to make extremely small semiconductor diodes or nanodiodes. An added advantage of these devices is that they do not heat up in use because the carbon is an excellent conductor of heat. Now that a functioning nanodevice has been demonstrated, it remains to be seen whether the whole process can be easily and economically replicated on a commercial scale ( Applied Physics Letters , Vol 75, No 5).

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