Credit: © 2007 AAAS

The drive to miniaturize electronic circuits has spurred investigations into the use of single molecules as components in nanoscale devices such as diodes, transistors and memories. Whereas the switching of some of these systems relies on dramatic changes in the shape or chemistry of a molecule, a team from the IBM Zurich Research Laboratory in Switzerland and the University of Regensburg in Germany have now shown that small structural rearrangements can be equally effective.

Peter Liljeroth and co-workers1 deposited molecules of naphthalocyanine — a flat organic compound — on single crystals of copper coated with insulating films. Locating the tip of a scanning tunnelling microscope above a single molecule and increasing the bias voltage induces a chemical reaction, known as tautomerization, in which two hydrogen atoms move to different positions in the molecule. This reversible and localized process is accompanied by a significant change in tunnelling current and so the system can be considered to be a conductance switch. The two states of a given molecule are easily distinguished from the different STM images of their molecular orbitals taken before and after rearrangement of the hydrogen atoms.

Because the atomic framework of this system is largely unaffected by the switching process, it is suggested that it could serve as a robust building block for molecular electronic devices. And although there is certainly potential along these lines, the experiments are currently limited by temperature, being performed at only 5 K, and so there are practical barriers that must be overcome first.