New results force scientists to rethink single-molecule wires

Single-molecule switches have the potential to shrink computing circuits dramatically, but new results from the Arizona State University lab that first described how to wire a single molecule between gold contacts now show that laboratory-standard wired molecules have an unavoidable tendency to “blink” randomly.

In the May 30, 2003, Science, Stuart Lindsay and colleagues identify the cause of this blinking behavior as random, temporary breaks in the chemical bond between the wired molecule and the gold contacts, making this particular wired-molecule arrangement unsuitable for electronic circuits. The National Science Foundation, the federal government agency responsible for supporting all areas of science and engineering, supported the research.

“There is a substantial interest in building single-molecule switches for molecular computing,” said Lindsay, a professor of biophysics. “The observation from scanning tunneling microscopes is that these wired molecules ’blink’ on and off. It was assumed that this was due to some property of the molecules, and if that behavior could be controlled, they could be used as molecular switches.” The various molecules examined typically blink once every 30 seconds to four minutes.

The research team includes Arizona State postdoctoral researchers Ganesh Ramachandran and Alex Primak, and researchers Theresa Hopson, Adam Rawlett and Larry Nagahara from Motorola Labs’ Physical Sciences Research Laboratories.

In 2001, Lindsay’s research group was the first to perfect a technique that allowed long, thin molecules capped at both ends with sulfur atoms to be wired individually to a gold electrode. Since then, researchers have studied the behavior and properties of various exotic molecules, almost all wired via sulfur atoms to gold electrodes using the Lindsay group’s procedure.

To isolate the cause of the blinking, Lindsay’s team compared the wired behavior of a more complex molecule to that of extremely simple molecules. While various explanations for the blinking had been proposed for complex molecules, none could possibly apply to the simpler molecules. Yet Lindsay’s team saw the blinking in all cases.

“We were left with two possibilities,” Lindsay said. “Either the molecule itself or the contact was switching. The conclusion we had to reach was that the lower contact was coming apart.”

However, that conclusion runs contrary to some basic chemistry: sulfur-gold bonds simply do not break at room temperature or even at temperatures as high as 60 degrees Celsius (140 degrees Fahrenheit).

Lindsay’s team explains the apparent contradiction with some equally basic chemistry. While the sulfur-gold bonds don’t break permanently, the bonds are unstable and will break temporarily and then re-connect. In fact, a chemical process called “annealing”–commonly used to harden metals and also in the procedure to prepare the gold electrodes to accept wired molecules–relies on the unstable bonds coming apart. The team reports that, as expected, the blinking becomes more frequent when the wired molecules are heated to annealing temperatures.

The blinking, in other words, is not a controllable behavior of the molecules. The on-and-off blinking happens naturally and randomly at the sulfur contact between the molecule and the gold electrodes.

“This doesn’t help anyone build better computing devices, but neither does it mean that single-molecule wires are useless,” Lindsay said. “It’s just not as naïve as it first seemed. The gold surface is not the best electrode material, but there are other options.”