The wonders of nanoelectronics

NANOSCALE materials are not only one of the most interesting areas of research but also have potentially far-reaching applications in areas like electronics. The ultimate goal of miniaturisation is to fabricate devices that are made of a single molecule of the material. Unbelievable as it may sound, recent work by two groups has shown that the goal is not as utopian as it may seem.

M Bockrath and his team at Berkeley, California, us, and Sander Tans and co-workers at the Delft University of Technology, the Nether lands, have recently shown that it is pos sible to fabricate and study single molecule devices bas6d on carbon nanotubes (Nature, Vol 386, No 6624). Ordinary transistors, based on semiconducting materials like silicon, are usually about one micrometre in size (one micrometre is one-millionth of a metre). The newly developed nanometre-size (one- thousandth of a micrometre) transistors would increase the device density in integrated circuits many-fold.

But fabricating these molecular devices until now has been a formidable task. The first hurdle has been that almost all long molecules are electrically non-conducting. This makes it impossible to build electronic devices with known molecules. This problem was solved with the advent of carbon nanotubes. These are made of a new form of carbon called c-60. These tubes are about a nanometre in diameter and conduct electricity.

The second problem was more technological in that, until recently, it was difficult to envisage connecting wires to a single molecule. But now, with advances in deposition technology, this is possible.

First, electron beam lithography is used to deposit metallic strips on to a substrate. On top of these, carbon nanotubes are laid in a random fashion. With the help of extremely powerful microscopes called atomic force microscopes, the sample is inspected and those areas picked out where only one nanotube is connecting two metallic strips. Thus, what we have is a transistor made of two metallic strips and a carbon nanotube, the latter consisting of a single molecule with a typical size of a few tenths of a nanometre.

The small size and single-molecule nature of the device also leads to it having characteristics radically different from those of ordinary transistors. The microscopic size of the device means that the behaviour of the electrons in it is completely governed by quantum mechanics, unlike in standard silicon devices. What is more, unlike quantum dots - nanoscale devices that show similar characteristics but need extremely low temperatures to operate - these carbon-based transistors can operate at room temperature.

Although the integration of these devices into nanoelectronic circuits is still a long way off, their unusual characteristics make them the favourites of scientists as a research tool to understand the properties of matter. These technological advances may lead to a revolution in the field of electronics, which may rival the microelectronics revolution.

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