ASU Research E-Magazine: Nature's Smallest Shock

Molecular-scale electronics has been widely touted as “the next step” in electronic miniaturization. Scientists suggest that single molecules may take the place of today’s much larger electronic components. However, manipulating single molecules so that their electronic capabilities can be tested and then applied is a problem.

“Progress in the field has been hampered by two problems,” says Devens Gust, a professor of chemistry at Arizona State University. “The first has been in making robust, reproducible electrical connections to both ends of molecules. After this has been achieved, the next problem is finding out how many molecules there actually are between the electrical contacts.”

“The results of studying the conductivity of molecules attached to wires have been all over the map,” says Stuart Lindsay, an ASU professor of physics. “For example, according to published articles, DNA has been found to be everything from an insulator to a superconductor. The problem has been that no one has been able to reliably connect a single molecule.”

Past attempts to measure the electrical properties of small numbers of molecules have given a wide range of values for their conductivities. Most previous studies have relied on a “mechanical” contact between molecules and a metallic wire, where the two are simply pushed together.

“Any hobbyist knows that the best electrical contacts are made by soldering the components together,” Gust says. “What we’ve needed is a way to ‘solder’ individual molecules on a molecular ‘circuit board,’”

Mission accomplished. The work was recently done by a multidisciplinary team that includes Gust, Lindsay, ASU physicists Xiadong Cui, John Tomfohr, and Otto Sankey, ASU chemists Alex Primak, Xristo Zarate, Ana Moore, and Thomas Moore, and Motorola, Inc. scientist Gari Harris.

Their work is described in a paper published in the October 19, 2001 issue of the journal Science. The team reported a method for creating through-bond electrical contacts with single molecules and the achievement of reproducible measurements of the molecules’ conductivity.

The researchers began with a uniform atomic layer of gold atoms. They then attached long, octanethiol ‘insulator’ molecules to it through chemical bonds, forming a fur-like coating of aligned molecules. They removed a few of the insulators using a solvent and replaced them with molecules of 1,8-octanedithiol, a molecule that is similar, but is capable of bonding with gold at both ends and acting as a molecular “wire”.

Tiny gold particles were then added to the solvent, where they bonded to the free ends of the 1,8-octanedithiol molecules. The result was a bonded metallic “contact” at either end of the conducting molecules. To test conductivity, the scientists used a gold-coated conducting atomic force microscope probe—a conducting probe with an atom-sized tip. They ran the probe across the surface. Conductivity was measured when the probe made contact with the gold particles.

Gust says that when electrical measurements were made on over 4,000 gold particles, virtually all measurements fell into one of five groups (five distinct conductivity curves). The conductivity curves were distinct whole number multiples of a single, ‘fundamental’ curve.

“This answers the basic question of how you know when you are measuring just one molecule,” explains Gust.

The fundamental curve represents conduction by a single molecule of octanedithiol attached to the two gold contacts. When more than a single molecule was bound, each additional molecule increased the current capacity by the single unit amount of current that could be carried by one molecule. When the probe encountered octanethiol “insulator” molecules, which could not bond with a gold particle, a much higher electrical resistance was recorded.

“The experimental results closely agree with theoretical quantum mechanical calculations for the conductivity of these molecules. This gives us confidence that current theories can provide useful guidance for future experiments,” Gust says.

“The molecule becomes a much better conductor when it is ‘soldered’ into the circuit by the bonds to gold at each end,” Gust adds. “This suggests how we can wire single molecule components into a molecular circuit board, and lays some important groundwork for doing practical molecular electronics.”—James Hathaway