WEST LAFAYETTE, Ind. -- Move over silicon -- here comes gold.
A group of Purdue University researchers has for the first time created an ultrathin film -- made from tiny clusters of gold atoms -- that conducts electricity by allowing electrons to "hop" one at a time from cluster to cluster.
Electrical current passing through a device in this "jumping" fashion would largely eliminate the problem of heat buildup in electronic devices, which currently rely on a continuous flow of current through silicon-based circuits.
Synthesis of this new material, called a linked cluster network, also is an important step in the development of ultrasmall components, or "nanotechnology." Such elements could be used to build more powerful computers and miniaturized electrical devices, including those that could be inserted in the body.
Current computer chips are made by photolithography, which uses light to produce tiny circuit patterns on the chips.
"Attempts to build nanoscale electronics have been hindered because manufacturing processes such as photolithography can't create structures that small," says Ronald Andres, professor of chemical engineering and part of the interdisciplinary team at Purdue. "Even if they could, such tiny structures are very fragile because they get hot when current flows through them.
"The linked cluster network, or LCN, is a big step toward eliminating both barriers. Single-electron hopping largely eliminates the heat problem. Plus, though our prototype is not much smaller than the smallest silicon devices, those devices can't get much smaller. Ours can, because we can assemble it cluster by cluster, and the clusters are molecule-size."
Details of the research are reported in the Sept. 20 issue of the journal Science.
"Making devices using LCNs is in many ways faster and simpler than the techniques used to make silicon devices, because the cluster network quickly 'self-assembles' and this can be done in air at room temperature, rather than in an ultra-high vacuum at high temperatures," Andres says. "Devices built using LCNs could be low cost, although it will be several years before we see these structures in consumer products."
Other researchers previously have observed the single-electron "hopping" effect, but only at temperatures near absolute zero, which is near -450 degrees Fahrenheit. Earlier this year, Purdue researchers were the first to demonstrate the effect at room temperature, showing that electrons move one at a time through an isolated gold cluster rather than flowing in a continuous stream. The technical term for these single-electron jumps is "tunneling."
"When electrons jump onto a molecule-sized metal cluster, there's a large resistance to the next one coming in until the first one jumps someplace else," Andres explains. "They take their turn, and as a result, there is much less energy dissipated as heat."
The key to achieving such behavior at room temperature is to make the clusters extremely small, Andres says. Roughly 250,000 clusters, lined up end to end in a single line, could fit across the head of a straight pin, says Clifford Kubiak, professor of chemistry and a member of the Purdue research team. Each gold cluster in the LCN is 3.7 nanometers in diameter. A nanometer is one-billionth of a meter.
In their latest achievement, the Purdue researchers used a self-assembly process to build a single-layer film of electronically linked gold clusters that conducts current by single-electron tunneling, again operating at room temperature. When imaged in a transmission electron microscope, the LCN looks like a honeycomb.
Here's how the Purdue researchers built their structure:
Gold clusters, each a perfect crystal made up of about 2,400 gold atoms, are first coated by an organic molecule called an alkyl thiol, which keeps the clusters from "sticking" to each other. "Nanosize metal clusters are so reactive that you have to protect them from each other," Andres says.
The coated clusters are then added to a liquid. A researcher places a drop of the solution onto a flat insulating substrate in the gap between two very small gold electrodes, which will become part of the finished device. As the liquid evaporates, the clusters automatically move together to form a uniform single-layer array, or monolayer, in the gap between the electrodes.
"These clusters want to be close to each other, and like bubbles in a bathtub, they come together and make a uniform array," Andres says. "That's the first step in the self-assembly process."
The researchers start with a known concentration of gold clusters in the solution, so they can control the size of the area that the array covers.
The entire substrate then is immersed in a second solution that contains organic molecules called aryl dithiol molecules.
"These organic molecules are like tiny rigid wires," Andres explains. "They displace the alkyl thiol molecules and link adjacent gold clusters to each other to form a rigid structure, like something built from Tinker Toys."
When a voltage is applied across the gold electrodes, electrons move from cluster to cluster one at a time, instead of streaming continuously through the array.
The organic molecules linking the clusters not only make the structure very stable, Andres says, they also are key to achieving the structure's unique electrical properties.
"Without these organic connectors, electron transmission between the clusters is difficult," Andres says. "Because the aryl molecules are what chemists call conjugated, it is easier for electrons to tunnel from one cluster to the next."
Researchers at Northwestern University recently reported in the journal Nature [382, 607 (1996)] that they had made a self-assembled cluster array similar to the Purdue structure, but the gold clusters in the Northwestern array were joined by DNA, rather than aryl dithiol molecules.
"Joining the clusters with aryl dithiol molecules produces a rigid array that is more robust and one in which we can readily control the electronic interaction between adjacent clusters, a key factor for potential electronic applications," Andres says. "Our linked cluster network also is fabricated using smaller gold clusters, which is critical for achieving single-electron tunneling behavior."
Andres says the next challenge will be to make the gold electrodes smaller.
"The size of any device made using an LCN is limited by our ability to make the electrodes smaller," he says. "Eventually these electrodes also will probably be made from gold clusters."
The interdisciplinary team at Purdue involved in research includes Andres; Kubiak; Supriyo Datta, professor of electrical engineering; Ronald Reifenberger, professor of physics; David Janes, research associate in electrical and computer engineering; and several graduate students from chemistry, chemical engineering, electrical engineering and physics.
The research was funded in part by the Army Research Office and the National Science Foundation.
Source: Ronald Andres, (765) 494-4047; e-mail, firstname.lastname@example.org
Writer: Amanda Siegfried, (765) 494-4709; home, (765) 497-1245; e-mail, email@example.com
NOTE TO JOURNALISTS: A color photo of researchers in the lab and a color photo of a linked cluster network are available from Purdue News Service. Ask for the photo called Andres/Nanotech (download here) or Andres/Nanotech2 (download here), respectively. Copies of the journal article also are available from Amanda Siegfried, (765) 494-4709.