In the quest to miniaturize the computer chip, engineers ultimately run up against the molecule. This is as small as conventional electronic components can get -- anything below that and you're knee-deep in quantum computing.
The single-molecule transistor promises to be a singularly transfiguring piece of nanoscale electronics. And while single-molecule electronic components are still at least a decade away, two teams of American scientists have taken the transistor down into this final realm.
Both teams' work appears in Thursday's edition of the journal Nature.
Technically, the scientists arrived eight months too late for the trophy. Last October, Hendrik Schön of Bell Labs announced his team had broken the single-molecule barrier in transistor fabrication.
However, recently doubts began to emerge about the veracity of Schön's claim when it was discovered that his team published nearly identical results in three different experiments related to this breakthrough.
Schön says he accidentally ran graphs of similar data in different papers. An independent investigation, led by Stanford University's Malcolm Beasley, has been empanelled to consider the controversy.
Meanwhile, Hongkun Park of Harvard says his team has shown that they can work with single molecules.
"We now have a technique to wire up individual molecules into electronic circuits -- and we can do it in a reproducible and reliable fashion," said Park.
Perhaps the greatest challenge to date in wiring a single molecule into a circuit has been making electrodes small enough to attach to either end of the would-be transistor.
Today, conventional lithography can make gaps that are as small as 10 nanometers apart -- roughly 25 diameters of a silicon atom. But this resolution is at least five times too crude to be useful for single-molecule transistors.
Both Park's Harvard/University of California at Berkeley team and the Cornell/Berkeley group solved the dilemma by stealing a page from director David Lean: They built a bridge and then they blew it up.
"What you cannot do at the moment using lithography is to design a molecule-sized gap between two electrodes," Park said. "However, if you just start with a wire and then break it, nature makes that molecule-sized junction for you."
Like blowing a tiny fuse, they ran current through a gold wire at a level that was just barely above its capacity. This produces the nanoscale gaps needed.
Paul McEuen, of the Cornell/Berkeley group, stressed that this work is only a first step. For starters, their single-atom transistor is only a switch -- passing current through if a gate electrode is turned on. It cannot, however, amplify a signal.
"'Transistor' means different things to different people,'" he said. "This is a controllable valve."
Leo Kouwenhoven of Delft University of Technology, author of a review article about the two discoveries in the same issue of Nature, says it's important how wide the transistor's gate swings: When the gate is closed, precious few electrons get through; when it's opened all the way, the transistor conducts current with nearly ideal throughput.
"It is very significant that they have been able to make a low resistance between molecule and metal (electrode)," he said. "They've made it a well-connected molecule."
However, single-molecule electronic components are still a decade or more away.
"It's not a good scheme for making commercial electronics," Kouwenhoven said of the current technology. "But without these kinds of experiments, you can never make a circuit."
