Single-molecule Conductivity

MOLECULAR ELECTRONICS

Researchers in Canada have discovered a new way to control the flow of electrical current through individual molecules. The study broadens understanding of fundamental molecular processes and may hasten development of single-molecule-based detectors and other types of molecular electronic devices.

Electronic circuits based on just a few molecules hold the promise of advancing today's miniaturized silicon-based electronics to even smaller, faster, and more densely packed electronic components. But taking the big step toward tiny electronic devices requires detailed understanding and unprecedented control of the flow of current through individual molecules.

A new method for exerting that type of control over single-molecule conductivity has just been demonstrated by a team of researchers at the University of Alberta and the Canadian National Research Council's Institute for Nanotechnology in Edmonton. Postdoctoral associate Paul G. Piva, physics professor Robert A. Wolkow, and their colleagues have shown that a single point charge, such as a surface-bound ion, generates an electrostatic field that can be exploited to regulate electrical conductivity in nearby surface-attached molecules. By controlling the charge state of the ion or its spatial relationship to a nearby molecule, the molecule's conductivity can be switched on and off (Nature 2005, 435, 658).

Unlike other studies in which scientists have tried to examine individual molecules trapped at a junction between two metal leads, the Edmonton team uses a scanning tunneling microscope (STM) tip to probe molecules covalently bound to a silicon (semiconductor) electrode. That setup avoids the influence of metal electrodes on the transport process, which is, "in many ways, the most vexing problem in dealing with such junctions," according to a commentary by Mark A. Ratner, a chemistry professor at Northwestern University.

Using methods they developed a few years ago, Wolkow and coworkers prepared silicon surfaces adorned with rows of styrene-derived molecules that terminate near silicon dangling bonds (unsaturated valencies). Then they varied the silicon doping level, which alters the charge state of the dangling bond, and measured conductivity through the molecules.

The team points out that when a large bias (about 2.5 V) is applied to the electrode, all of the molecules in the row conduct current. But when a much smaller bias is applied, usually the molecules cannot transport charge. Surprisingly, Wolkow and colleagues found that under conditions that ordinarily do not support electrical conductivity, the molecules closest to charged dangling bonds conduct current anyhow. In fact, the effect is so pronounced, Wolkow stresses, that it is readily observed at room temperature--not just at the cryogenic temperatures typically required for these types of investigations.

On the basis of STM analysis and quantum mechanical calculations, the group attributes the observations to changes in the molecules' energy levels that are induced by the dangling bonds' electric fields.

Ratner remarks that the study "constitutes direct evidence that localized charges profoundly affect charge transport in single-molecule structures on silicon surfaces at room temperature."