Physicist Writes the Rules for Conducting Electricity in Molecules
In 2001, at the dawn of the nanoscale era, the newly formed Columbia Nanoscale Science and Engineering Center received a 10-year grant from the National Science Foundation large enough to support 20 professors with one or two graduate students apiece. One of its first projects was to decipher the fundamentals of how electrical charges move in circuits that are built with a single molecule wired up to electrodes.
That was a basic mystery of this brand new science. “We knew the rules about how charges move in macroscopic objects, we knew the rules about how charges move in atoms, but we didn’t know the rules about how charges move in various nanoscale systems,” says Jim Yardley, the center’s director.
Experiment after experiment wasn’t working. No one had yet succeeded in inventing a reliable way to create the most basic ingredient of electric circuits out of a single molecule—the essential switch at the heart of the modern computer known as the transistor.
The idea that transistors, which are embedded in silicon chips, could be created with a single molecule was extremely appealing. But in order for such electronic components to work efficiently, they would need to share similar, predictable electronic characteristics. How much voltage it took to turn a switch on and off would have to be the same in all circuits built with the same molecule. If this varied widely, there would be no way to predict how much voltage would be needed to turn any molecular circuits on or off.
“Everyone was publishing papers where they were making claims about how much current they could drive through their single molecule circuits,” recalls Latha Venkataraman, a professor of applied physics and applied mathematics. “But each lab had different results and nothing was reproducible; the numbers spanned a huge range. It was extremely messy.”
Venkataraman—who was not trained in chemistry—eventually came to suspect that the problem lay in the chemical attachments used to fabricate the nanoscale circuits—and that a whole new approach was needed.
To turn a specific molecule into a circuit, it is necessary to connect each end of the molecule to a metal capable of conducting electricity. Most scientists were using gold atoms as the conductive metal. To form the terminals—the end points of each circuit—researchers were using groups of sulfur atoms, which bind very strongly and easily with gold.
Like the other researchers, Venkataraman had been working with these materials for two and a half years—and she, like everyone else, had been hitting brick walls. After consulting with Chemistry Professor Colin Nuckolls, she had confirmation that her suspicions about the chemical links were correct. The sulfur-gold attachments were varying the rate at which electricity passed across each molecular device.
What was needed, she decided, was a more reliable attachment that would achieve consistent measurements at the nanoscale. After speaking with chemists about different systems that might work, Venkataraman went online and began searching for different compounds from commercial chemical vendors. She succeeded with the second new compound she tried—using nitrogen-gold attachments. By then, Venkataraman had become so accustomed to inconsistent measurements that she thought her suddenly consistent results must be a mistake.
“We were all about ready to throw that whole experiment out the window, and then one day she tried a different molecule in the system and all of a sudden it worked,” said Yardley. Her innovation “has created a whole new world of research.”
Indeed, it allows scientists to dig far deeper into the unique electrical properties of some nanoscale compounds. Venkataraman has built a strong research collaboration with Nuckolls and University Professor Ronald Breslow.
“It also shows how critical it is to have chemists working together with—in Latha’s case—the physicist to get the ideas of what happens on a nanoscale working together,” said Yardley.
These breakthroughs in understanding and controlling current flow between very different systems, such as metals and organic molecules, have potentially broad nanoscale applications in the areas of electronics and energy. “It put in place a whole set of research programs really laying out all of what we would call the chemical guide rules, or the chemical rules, for how electrical conduction happens in molecules,” Yardley said.