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UA physicists (l to r) Sumit Mazumdar, David
Cardamone and Charles Stafford have created molecules that are
working transistors. (Photo: Lori Stiles)
Artist's conception of a Quantum Interference
Effect Transistor (QuIET). The colored spheres represent
individual carbon (green), hydrogen (purple), and sulfur (yellow)
atoms, while the three gold structures represent the metallic
contacts. A voltage applied to the leftmost contact regulates the
flow of current between the other two. (IMAGE: ACS Nano Letters)
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"All transistors in current technology, and
almost all proposed transistors, regulate current flow by raising and
lowering an energy barrier," University of Arizona physicist Charles
A. Stafford said. "Using electricity to raise and lower energy
barriers has worked for a century of switches, but that approach is
about to hit the wall."
Transistors can't shrink much smaller than 25 nanometers, or 1/40,000
the width of a pinhead, because scaling down further creates
intractable energy problems, Stafford said. Even if it were possible
to build an ultra-advanced laptop computer with molecule-sized
transistors using current transistor technology, it would take a
city's worth of electricity to run the laptop, and the thing would get
so hot it would probably vaporize.
Stafford, UA physicist Sumit Mazumdar and David Cardamone, who
received his doctorate from UA in 2005, began thinking about the
problem of next-generation transistor technology three years ago. They
realized that quantum mechanics can solve the problem of how to
regulate current flow in a single-molecule transistor that would work
at room temperature.
"Our approach is a little more finesse than brute force," Cardamone
said. "We don't put up a wall to stop current. It's just that we can
regulate how electron waves combine to turn the transistor on or off."
The simplest molecule they propose for a transistor is benzene, a
ring-like molecule. They propose attaching two electrical leads to the
ring to create two alternate paths through which current can flow.
They also propose attaching a third lead opposite one of the
electrical leads. Other researchers have succeeded in attaching two
contacts to a molecule this small, but attaching the third is the
trick -- and the point. The third lead is what turns the device on and
off, the "valve."
"In classical physics, the two currents through each arm of the ring
would just add," Stafford said. "But quantum mechanically, the two
electron waves interfere with each other destructively, so no current
gets through. That's the 'off' state of the transistor."
The transistor is turned on by changing the phase of the waves so they
don't destructively interfere with each other, opening up addiitonal
paths through the third lead.
"It took a while to go from the idea of how this could work to
developing realistic calculations of this kind of system," Stafford
said. "We were able to do the simplest kind of quantum chemical
calculations which neglect interactions between different electrons
within a few weeks. But it took some time to put in all the electron
interactions that demonstrate this really is a very robust device."
According to the Semiconductor Research Corp. it typically takes a
dozen years for a new idea to go from initial scientific publication
to commercial technological application, Stafford noted.
"That means if the computer industry is to continue its recent pace in
making smaller-scale computers, we should have had this idea yesterday,
" Cardamone said.
Why all this effort to make incomprehensibly small computers? Why
expend so much brainpower on nanocomputing?
More computing power will result in more realistic simulations,
whether you're a scientist modeling global warming or supernovae
explosions, or an entertainment industry animator creating believable
emotion in a simulated human face, Stafford said.
Nanocomputers could have a major impact in medicine, Cardamone said.
"These machines could operate in solution, in vivo. There already are
clinical trials of nanoparticles to deliver medicinal drugs. Imagine
how much more powerful those little nanoparticles or nanorobots would
be if they could count, or do simple computation. With our transistors
packed at maximum density, you could put a microprocessor as powerful
as the top-of-the-line workstation on the back of an E. coli."
"Have you seen the movie, Fantastic Voyage?" Stafford asked. A
nano-sized surgical team journeyed through a human body in the 1966
sci-fi flick. That's a different story, but with a similar theme.
"We're not futurists at all and can't predict it, but imagine that you
could make an artificial intelligence, that you could have this little
submarine that goes inside somebody's arteries and capillaries to
repair them," Stafford said.
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