Using Molecules as Transistors

Speaking of quantum investing...

Scientists at the University of Arizona have discovered how to use quantum mechanics to turn molecules into working transistors in the lab, a breakthrough that might one day lead to high-powered computers the size of a postage stamp.

Results of the as-yet-unpublished study came together just weeks before Canadian researchers performed a similar feat using chemical means. That experiment appeared in the journal Nature last week. Together, the two studies could bring the final frontier in nanocomputing -- a single-molecule transistor -- considerably closer to reality.
The transistor -- the essential building block of computers -- is a circuit component that amplifies or halts an electrical signal using three leads: The first two leads are like two ends of a garden hose; the third is like a valve that regulates the flow of water through the hose.

When first developed in the 1940s and '50s, individual transistors were fractions of an inch in size.

The smallest transistors in consumer electronics devices today measure 50 nanometers across -- a million times tinier than their postwar progenitors. (This shrinkage would be equivalent to reducing the continental United States to the size of a hot tub.) Taking transistors down another one or two orders of magnitude, to the realm of individual atoms and molecules, requires a generational leap in technology.

Three years ago, scientists at the University of California at Berkeley and Harvard and Cornell universities announced the fabrication of a transistor from a single organic molecule. But these delicate circuits only operated at single-digit temperatures above absolute zero.

Both the Nature paper and the Arizona study propose transistors able to handle room-temperature environments -- although scaling such designs as these up to mass-production levels still will require years of research and development.

The Arizona paper, soon to be submitted to the journal Physical Review Letters, uses the laws of quantum mechanics as the traffic cop that starts or stops current from flowing.

The Arizona team's proposed transistor is a ring-shaped molecule such as benzene. Attaching the two electrical leads to non-opposite sides of the ring -- at, say, the 12 o'clock and 4 o'clock positions -- allows the electrons to flow through the molecular ring and not destructively interfere with one another. (Due to the quantum wavelike laws of nature that electrons follow, attaching electrical leads at the 12 o'clock and 6 o'clock positions causes the current to cancel itself out.)

However, attaching the third lead (the "valve") opposite one of the two electrical leads enables one to turn this wave interference effect on and off -- and thus turn the flow of electricity through the transistor on and off.

"This is the only proposal that I'm aware of ... to use quantum interference effects in a device at room temperature," said Arizona physicist Charles Stafford.

George Kirczenow of Simon Fraser University in Vancouver, British Columbia, finds the Arizona transistor design a promising development in regulating current flow at the nanometer scale.

"It's an interesting and imaginative thing," said Kirczenow. "These guys have done something quite new."

Perhaps the greatest problem with this design -- as with any single-molecule transistor design -- is the assembly of the components. The Arizona transistor, in fact, only exists on the drawing board, although a team of chemists from the University of Madrid will soon begin the lab work necessary to translate these blueprints into working electronics.

Nature paper co-author Gino DiLabio (.pdf) of Canada's National Institute for Nanotechnology likens his team's transistor molecule, styrene, to a poppy seed.

"When you try to take microscopic leads and converge them on a very small object, you can't fit them into that space," DiLabio said. "(Think of) holding a poppy seed between your thumb and your forefinger. And then try to touch it with another finger. You just can't quite get that other finger in there."

DiLabio's group, led by physicist Robert Wolkow of the University of Alberta, gets around the "third finger" problem by finding a system that doesn't need to be physically touched in three places. The third finger is actually the electric field of a nearby atom. The styrene is attached to a silicon surface, with the head of a scanning tunneling microscope, or STM, hovering just overhead.

In the garden hose analogy, the silicon surface and the STM head are the two ends of the hose. Wolkow et al. found that in this environment, an external electric field acted like the valve. The field in this design comes from one or more nearby silicon atoms on the surface. If the neighboring silicon atoms are all electrically neutral, no current flows between the surface and the head of the tunneling microscope. The transistor is shut off. But if one of the silicon atoms carries a net electric charge, the floodgates open and current flows through the circuit. (See accompanying figure.)

"It's a rather big advance, because I don't think anybody has done anything quite that well-controlled with a single molecule," Kirczenow said of the Wolkow transistor.

In conventional microchips today, DiLabio said, many thousands or even millions of electrons are needed to turn the transistor's valve on and off. "But in this case we have the ultimate efficiency," he added. "A single electron."