Researchers demonstrate how to join single molecules to nanowires using a scanning tunnelling microscope

Scientists in Japan and Switzerland have demonstrated how to wire up single molecules with conductive nanowires. The technique, called chemical soldering, is a big step towards single molecule electronics.

Molecules have long been proposed as alternative circuit components. In 1974, IBM researchers Mark Ratner and Arieh Aviram described how a single molecule could pass current in just one direction, thereby acting as a diode, and since then various other components, such as switches and transistors, have been put forward. In theory, single molecule electronics would be smaller than their silicon counterparts, and may require less power.

A problem in realising single molecule electronics, however, is how to wire up the molecules in the first place. Some scientists have tried connecting metal electrodes to molecules, but it has proved difficult to reduce their width sufficiently. An alternative is to connect molecules with conductive polymers, but in the past it has only been possible to connect several molecules at a time.


Source: © J. Am. Chem. Soc.

Single molecule electronics could use less power than their silicon counterparts

Now, Yuji Okawa of the National Institute for Materials Science in Tsukuba, Japan, and colleagues have developed a method to wire and bond single molecules. Okawa’s group starts with a monomolecular film of a diacetylene on a graphite substrate. The researchers then deposit a small amount of phthalocyanine, which forms nanoclusters on the surface. Finally, the researchers bring the tip of a scanning tunnelling microscope to one of the phthalocyanine molecules and, applying a pulsed voltage across the tip and surface, initiate chain polymerisation of the diacetylene. This forms a polymer nanowire that bonds to the phthalocyanine molecule. 

Phthalocyanine molecules are used as dyes, and in organic semiconductors and solar cells. Okawa’s group believe their structure will behave like a diode. 

Robert Stadler, an expert in molecular electronics at the University of Vienna, Austria, considers chemical soldering a ’breakthrough’. Yet he notes that it won’t bring single molecule electronics to the market anytime soon. ’While the authors are right that they address one of the problems on the way towards integrated molecular circuits, it is certainly not the only - or even most important - one,’ he says. 

Okawa says his group’s next task is to test phthalocyanine molecules as diodes, and to try wiring up other molecules. ’The final goal of our research is to fabricate and demonstrate a single molecule electronic circuit,’ he says. 

Jon Cartwright