Molecule connects contacts

By Kimberly Patch, Technology Research News

One of the challenges of making machines out of small numbers of molecules is figuring out how to connect them individually in order to form electrical circuits.

The trouble is, soldering isn't an option on the molecular scale. Instead, researchers from Arizona State University and Motorola have found a way to chemically bond each end of a molecule to a metal conductor.

They began with a flat gold surface and covered it with a single layer of electrically insulating octanethiol molecules, which are a string of hydrogen and carbon atoms with a sulfur atom on one end. The sulfur bound chemically to the gold surface.

The researchers removed a few of the molecules, leaving gaps, then filled the gaps with related octanedithiol molecules, which have sulfur atoms on both ends. One end of these molecules chemically bonded to the bottom layer of gold. Then the researchers sprinkled gold nanoparticles on the surface, and the opposite ends of the octanedithiol molecules bonded to the nanoparticles.

When the researchers touched a single nanoparticle with the electrified gold tip of an atomic force microscope, it completed a circuit through the molecule to the gold surface. "In essence, we have a single octanedithiol molecule chemically bonded to gold contacts at each end and surrounded by an insulator. This is like a wire soldered into a circuit," said Devens Gust, a professor of chemistry at Arizona State University.

The researchers took 4,000 separate measurements of molecules this way. The connected molecules conducted current more quickly than ordinary molecules, offering four times less resistance, according to Gust.

The length of each molecular wire is a little over one nanometer, which is 1,000 times smaller than the circumference of an E. coli bacterium. A nanometer is one millionth of a millimeter.

There were two main hurdles to connecting single molecules, said Gust.

The first difficulty was designing the chemical layer so that one or only a few molecules were connected to each gold nanoparticle, said Gust. Then they had to figure out how to measure the results, he said. "The second [challenge] was designing and building an atomic force microscope capable of making the... precise current voltage measurements," he said.

The key to attaching a wire to a molecule in a usable way is making a chemical rather than a mechanical bond, said Gust. "We found that when chemical bonds are used at both ends, the conductivity of the molecule increases by a factor of at least 10,000" over methods that mechanically attach a molecule to an electrode, he said. The chemical bond is also not as sensitive to force as a mechanical contact would be, making it a sturdier connection, he said.

Bonds like these can eventually be used to form single-molecule wires, transistors and logic elements that can be incorporated into tiny electronic circuits. It will be at least a few years before even simple circuits that use single molecules become possible, said Gust.

The work is one more step in the progression of molecular-scale electronics, said Vincent Crespi, an associate professor of physics at Pennsylvania State University. The important contribution is the use of bonds to gold on both sides of the molecule, he said.

The work also allows researchers to measure the behavior of single molecules under the influence of electrical current, said Gust. It "shows unambiguously that we are measuring only one molecule, rather than an assembly of some unknown number of molecules."

This is important because one of the puzzles in studying how electricity flows through individual molecules has been untangling the influence of the contact from the influence of the molecule, said Crespi. "In something this small the contact is just as big as a molecule itself, so an understanding of the electron transport depends critically on understanding of the molecule/metal contact," he said.

Gust's research colleagues were Xiaodong Cui, Xristo Zarate, John Tomfohr, Otto Sankey, Ana Moore, Thomas Moore and Stuart Lindsay of Arizona State, and Gari Harris and Alex Primak of Motorola. They published the research in the October 19, 2001 issue of the journal Science. The research was funded by the National Science Foundation (NSF).

Timeline:   > 3 years
Funding:   Government
TRN Categories:   Nanotechnology
Story Type:   News
Related Elements:  Technical paper, "Soldering Molecules for Nano-electronics," Science, October 19, 2001.


October 24, 2001

Page One

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Molecule connects contacts

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