By Peg Brickley
Small Times Correspondent
Scientists at Harvard and Cornell universities have created single-atom transistors -- moving a significant step toward making molecular electronic devices a reality.
Working separately, the two groups of scientists pulled off the difficult technological trick of trapping a single atom between two contacts, according to papers to be published in this week's Nature magazine by teams from the two universities, with help from the University of California, Berkeley, on both projects.
This week's achievements from the academic laboratories demonstrate the technology for wiring a short molecule, or even a single atom, as a nanoscale transistor.
"Do these realizations of a single-atom transistor mean that molecular electronics is just around the corner?" asked a group of scientists from Delft University of Technology, in the Netherlands, in an analysis accompanying the publications.
Not yet, according to the Dutch scientists. The day when working logic circuits are built from single-atom or single-molecule transistors is still a way off, according to Silvano De Franceschi and Leo Kouwenhoven of the ERATO Mesoscopic Correlations Project at Delft.
What the news out of Cornell and Harvard shows is that the science is in place to study how electrons move through nanoscale objects, they say. Perhaps just as important, when it comes to chemical processes that can successfully fabricate electronic devices on single molecule, we now have the technology.
"It's a long way from commercial electronic uses," said Daniel C. Ralph, one of the principal scientists on the Cornell-based project, which was funded largely by a National Science Foundation grant. But the basic research provides a clearer view into the process of molecular electronics.
"The point is to see how electrons move on such a small scale," Ralph said. "This opens up the ability to use synthetic chemistry to design molecules that can be adjusted and measured one at a time."
The difficulty of wiring an atom has long been one of the biggest challenges to nanotechnologists attempting to build viable electronics that operate with a single molecule or atom. Silicon transistors do not depend on an ability to control the movement of each individual electron. They count on the average flow behavior of a large number of electrons.
But single-molecule transistors will not work unless they can get electrons to hop on and off, one-by-one, requiring a level of control that macroscale electronics don't contemplate.
Carbon nanotubes have shown themselves useful as molecular conductors, but that was partly due to their relative length: several micrometers. Size limits likewise afflicted techniques like lithography, which were used to assemble electrode structures precise enough to grab a single molecule. Given the 10-nanometer resolution of conventional lithographic assembly, it was not possible to get a single molecule into the gap between electrodes.
By combining electron-beam lithography and electron migration, the Harvard and Cornell researchers managed to get their electrodes 1 nanometer apart, boosting the chances of catching and holding a single atom.
Building a better atom-trap, however, is only the beginning.
"There are lots of molecules out there with interesting properties," Ralph said. "There are molecules that might store information, or lead to higher speed transportation. There are molecules that might allow us to combine them in natural ways so we don't have to hook things up one at a time."
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