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Posted: September 21, 2006

Quantum effect offers molecular transistors

(Nanowerk News) A molecular switch that operates in a fundamentally new way could usher in a generation of microprocessors that work on the molecular scale.
The switch exploits the strange laws of quantum mechanics to tackle one of the biggest barriers facing researchers developing molecular electronics, say scientists.
With transistors becoming ever smaller, researchers have long known that they would eventually have to tackle the problems of building circuits on the molecular scale. A number of groups have proposed molecules that should work as transistors and have even used them to build rudimentary circuits.
But these molecular transistors work in essentially the same way as larger, silicon ones. Current flows into the molecule from one electrode and out through another. A third electrode switches the current on and off by raising and lowering an electrical potential barrier that prevents the flow of electrons.
The trouble with this type of switching is that it requires relatively large amounts of power. And building more complex microprocessors featuring molecular transistors would increase power requirements dramatically.
Powering up
A conventional laptop with transistors that measure roughly 100 nanometres across consumes something in the region of 100 watts. Molecular scale transistors each about 1 nm across would require substantially more, if packed into the same physical space.
"Power dissipation goes up to a million watts," says Charles Stafford, a physicist at the University of Arizona in Tucson, US. "Not every Starbucks is going to be happy for you to plug [your computer] in."
Now, Stafford and colleagues have come up with an entirely different type of molecular switch which could solve this power problem, based on a quantum process called "interference".
Quantum interference
On the molecular scale, electrons behave like waves and can interfere with themselves and each other. When the peaks of two waves coincide they combine to form a larger peak, a process called "constructive interference". But when the peaks of one wave coincide with the troughs of another they cancel out through "destructive interference".
Stafford realised that current could be switched off in a circuit if it could be constructed so that the electrons travelling through it naturally cancel each other out through destructive interference.
This quantum behaviour of electrons is fragile and can easily be obliterated by disturbances, such as the movement of nearby atoms. This movement destroys quantum interference, allowing electrons to flow again switching the current back on.
The result could be to produce an entirely new type of transistor that regulates the flow of current by switching quantum interference on and off. "This is a very robust effect," Stafford says.
Beautiful symmetries
Stafford and colleagues say that various well-known molecules have exactly the structure needed to work as so-called "quantum interference effect transistors".
For example, they have calculated that the ring-like molecule benzene could work well as electrons exactly cancel each other out as they flow in opposite directions around its ring. "We simply exploit the beautiful symmetries that occur naturally in these molecules," he says.
George Kirczenow at Simon Fraser University in Burnaby, Canada is impressed with idea. "It's a very innovative approach that could in principle require much less power than conventional switching," he says.
However, the next step is to connect a suitable molecule to three electrodes so that it can operate as a transistor, with the electrodes taking the form of long chains of conducting hydrocarbons. Attaching them to a single molecule will be difficult, Kirczenow says: "People have done it with two electrodes but not with three. The problem of creating interconnects is at least as challenging as making the transistors themselves."
Stafford agrees but is philosophical about the challenge: "I'm a theoretical physicist I've done my bit."
The paper is titled in ("Controlling Quantum Transport through a Single Molecule")
Source: newscientist.com news service (Justin Mullins)
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