Molecular switches that assemble themselves

Saturday, 05 January, 2002


"Smaller means faster and more efficient" is the maxim of the electronics industry as it strives towards the further miniaturisation of components.

Conventional fabrication methods are reaching their limits, with the production of components around 100 nm in size.

The next step is to produce components 10 times smaller than the smallest of today: true nanometre-scale devices.

Making such devices takes manufacturers into strange new worlds. And these micro zones have already been entered by scientists at Liverpool University in Britain who have produced self-assembling working switches and other components fewer than 10 nanometres in size.

The physicist and Nobel laureate Richard Feynman predicted a future of nanotechnology in 1959, in a lecture he entitled 'There's Plenty of Room at the Bottom'.

But to break through into the spacious world at the bottom, the world of molecule-sized components, means inventing new nano technologies.

For a start, nanosize components must be able to build themselves. No tweezers or screwdrivers are small enough to assemble them.

Lithography, the technique used to make today's electronic components, reaches its limits at about 50 nm.

The technique cannot resolve detail much smaller than that and certainly not down to the 10 nm which is envisaged.

Components that size cannot be made. They have to assemble themselves from even smaller components by a process of self-assembly and which is chemical rather than mechanical. Scientists around the world have devised and are now testing electronic components each consisting of a few molecules (or even of a single molecule) that have the electronic properties of the components of today that they will one day replace.

But for such molecular transistors, capacitors and other components to do their job they must be connected to the outside world by wires and switches to control current flow.

A vital step towards practical nanoscale electronics has been taken by the team led by Prof David Schiffrin, director of the Centre for Nanoscale Science at Liverpool University, north-west England.

The team has developed a working nanoscale switch with nanoscale connections ready to link it to nanoscale circuitry but designed eventually be operated from outside the nanoworld. The switch consists of a gold nanoparticle, a cluster of gold atoms, connected to a gold electrode which could form part of a nanoscale computer or other nano device of the future. The gold nanoparticle, about six nanometres in diameter, is linked to the electrode by a bundle of about 60 nano wires, each one a single organic molecule.

In the middle of each molecular wire is the working part of the switch, a chemical grouping called a redox group which readily accepts or loses electrons.

When electrons are fed down to the redox groups, resistance to the flow of current through the 'wires' is greatly reduced.

Resistance increases when electrons are removed from the redox groups. Therefore the assembly acts as a switch - which can be turned on or off from the outside world - and the current measured by using a well-established laboratory tool, a scanning tunnelling microscope (STM) to feed in electrons.

The STM is able to pinpoint a gold nanoparticle with great precision, to make contact. Each nanowire is 'soldered' to the nanoparticle at one end and the gold electrode at the other by sulphur groups which stick naturally to the gold atoms.

These help to make the switch potentially part of self-assembling nanoscale electronic circuitry.

The Liverpool team has shown that gold nanoparticles of two different sizes will spontaneously assembly themselves in regular patterns. This represents another step towards self-assembling nano circuitry.

By using complex organic molecules as substrates and templates - which dictate the structures of the nanocircuitry assembling itself on them - the Liverpool team and others are opening up the vast resources of organic chemistry, including biochemistry and nature's self-assembling structures, for applications in designing and building nanoscale computers and other devices.

Another challenge is to link the components in such devices together and to the outside world with appropriate nanoscale wiring. The Liverpool team has already produced nanowires which are strings of single gold atoms.

Clearly, much progress has already been made towards nanocomputers and other nanoscale electronic devices.

When they appear they will find applications in many areas of human life, including medicine, especially drug delivery, and new catalysts for chemical industry, as well as in information technology of all kinds.

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