001102-10 10.1038/nsu001102-10 2 November 2000 technology Turning on the nanoworld PhilipBall Self-soldering molecular wires with built-in switches bring nanotechnologyÕs promise a step nearer. Philip Ball reports. .

Midas touch: gold particles taking us closer to nanocomputing
Nanotechnology -- molecule-sized engineering -- promises wonders: from ultra-dense computer memories to cell-sized robots. Now this promise comes a step closer to being realized, thanks to a team of chemists from Liverpool University in the UK. They have solved the problem of connecting electronic components that are not much larger than molecules.

David Schiffrin and his co-workers have linked a tiny lump of gold -- just six nanometres (or six millionths of a millimetre) across -- to a gold electrode. They used wires consisting of single molecules that can also act as switches, so that the flow of electrical current through them can be turned on and off.

In theory nanoparticles like this could act as memory elements in a 'nanocomputer', storing bits of information at incredibly high density in the form of electrical charge.

Schiffrin and colleagues used a class of organic molecules called thiols that stick to gold. At the end of a chain-like molecule, a thiol group reacts with a gold atom to form a strong link. Molecules with thiols at both ends can link themselves between two gold surfaces. So the team attached gold nanoparticles to a flat gold surface, tethered by two-headed thiol molecules. Each gold particle, they say, is probably linked to the surface by dozens of these molecules.

This process of chemical 'self-assembly' will be crucial to nanotechnology -- the alternative, manipulating and securing molecules 'by hand', is extremely difficult.

Schiffrin's team also report in Nature1 that their linkers can conduct a current. They used a device called a scanning tunnelling microscope (STM) to pull a current from the gold surface, up through the linking molecules and into the gold nanoparticles.

The STM has a very fine metal tip. When voltage is applied to this tip and it is brought very close to the nanoparticles, a current flows from the particles into the tip. But by altering this voltage, the researchers were able to switch the current on and off, because their linker molecules were voltage-sensitive.

For large negative voltages, the linkers existed in a conducting state. But as the voltage became more positive, electrons were stripped from the molecules, and their conductivity plummeted. Then current could no longer find its way to the gold nanoparticles.

If the nanoparticles are connected by, say, 30 or so molecules, then this switch is flipped by shifting just this many electrons -- a tiny 'control' current for an electronic device.

Unfortunately such a switch would be far too slow to be useful in a conventional computer, says Dan Feldheim of North Carolina State University. If computer engineers want to take advantage of the miniaturization possibilities that these devices hold, they will need to completely rethink their circuit designs.

Gittins, D. I. Bethell, D., Schiffrin, D. J. & Nichols, R. J. A nanometre-scale electronic switch consisting of a metal cluster and redox addressable groups. Nature 408, 6769 2000.