Single molecule transistor could revolutionize electronic miniaturization

June 2, 2005

Researchers at the University of Alberta have proven the potential for constructing electronic circuitry on a molecular scale, a breakthrough that could shatter the limitations of conventional transistor technology and pave the way for smaller, faster, cheaper microelectronic devices.

The report by National Research Council National Institute of Nanotechnology's Molecular Scale Development Group, led by U of A physics professor and iCORE Chair in Nanoscale Information and Communication Technologies Dr. Robert Wolkow, has been published in the June 2005 issue of the scientific journal Nature.

Wolkow said his team has proven that a single molecule can be controllably charged while all the surrounding molecules remain neutral, causing it to act as a basic transistor. Transistors control the flow of current in most electronic devices and are combined to form integrated circuits used to make the microprocessors and memory chips that drive everything from computers and cell phones to household appliances.

But where conventional transistors might use a million electrons to switch a current, Wolkow's team was able to control the current through a hydrocarbon molecule using a single atom.

Wolkow emphasized that, while the concept his team tested is a long way from practical application, it undoubtedly fits the definition of a transistor, which has three terminals - an 'in,' and 'out,' and a control outlet.

"To call something a transistor, it needs a control element," Wolkow said. "We have control, but it's very sluggish and slow right now. It takes us on the order of minutes to change conditions that make current go or not, so for any computer technology, this thing is today impractical. But it's not hopeless. There are many hurdles, but there aren't any we see as insurmountable."

In fact, the research team has already cleared what appeared to be insurmountable obstacles in manipulating molecules measuring one one-billionth of a metre in size.

"It's very hard to connect wires to a molecule," Wolkow said. "Imagine trying to bring three watermelons together all to touch something the size of a poppy seed. You couldn't do it - you could make two watermelons touch a poppy seed, and even that would be kind of difficult, holding that poppy seed in place. But then to bring in the third watermelon is impossible - you can't have all three touching such a small object."

To solve this problem, the "transistor" molecule was placed on a silicon surface that had been exposed to hydrogen gas, so that each silicon atom was capped with a hydrogen atom. By removing the hydrogen cap from single silicon atom, that silicon atom could be made to conduct a charge while the surrounding atoms remained neutral. The tip of a powerful scanning tunneling microscope served as the on/off switch.

Practical nanoscale transistors may be decades away, Wolkow said, but the potential to create smaller, faster, more efficient electronic devices with minimal energy and material requirements is a powerful incentive to pursue this line of research. But, he added, the challenges are considerable.

"We need to make such an entity work without the need for a million-dollar scanning tunneling microscope hovering over each molecule. We'd like to get these things down to where they cost pennies. It's an engineering feat to put the right structures in place. We need to make a solid-state structure that provides that other contact which was provided by the probe of our scanning microscope."

Wolkow said the lead author of the study, U of A postdoctoral fellow Dr. Paul Piva, deserves special mention for championing the research and mustering the expertise of the his collaborators to design the concept and test it "in every way imaginable." Funding for the research was provided by iCORE, the National Research Council, Science and Engineering Research Canada (NSERC), the Canada Foundation for Innovation, the U of A and the Canadian Institute for Advanced Research.

Source: University of Alberta

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