November 9, 2001
Nanowires May Lead to Superfast Computer Chips
By KENNETH CHANG
n the race to make computer chips that are ever faster and ever smaller, scientists at Harvard University
have
grown tiny crystal rods of silicon and other semiconductors, then sluiced them onto chips to form
rudimentary
circuits that perform basic logic operations.
Compared with competing techniques, the semiconductor rods, or nanowires, are easier to make and
manipulate,
and they may be easier to miniaturize to the sizes needed for superfast computer chips. "They have a lot of
advantages in that we can control their properties quite well," said Dr. Charles M. Lieber, a professor of
chemistry
at Harvard who led the research.
Dr. Lieber said the nanowires might also make "unbelievably good sensors" for proteins, DNA and other
biological
molecules. Among other things, that could aid the development of devices to detect pathogens like anthrax.
The findings are reported today in the journal Science.
For several decades, the technology of carving transistor circuits into silicon has been improved to pack
more
transistors onto chips. In 10 to 15 years it is expected to hit fundamental physical barriers that will halt that
progress.
Scientists at many companies and universities have explored possible successors to silicon, including
custom-designed molecules and rolled-up cylinders of carbon known as nanotubes.
Dr. Lieber and his colleagues are sticking with silicon. But instead of carving it, they build it up from
individual
atoms. Out of a droplet of solvent saturated with silicon or another semiconductor like gallium nitride, they
grow
perfect, rod-shaped crystals less than a millionth of an inch wide and several thousandths of an inch long.
A solution containing the nanowires is squirted onto a silicon oxide wafer. A chemical on the wafer guides
the
wires to the right place.
Each intersection where one nanowire crosses another acts like a transistor, not much different from the
tens of
millions of transistors in current computer chips, just much smaller. Transistors are essentially voltagecontrolled
switches.
The researchers have shown that the nanowire transistors can be wired together to perform all of the basic
logic
operations needed for computer computations. To build dense circuitry, the researchers would move the
nanowires
closer together. "Voilà," Dr. Liber said. "You have a billion devices."
Practical computer chips using nanowires are probably a decade away. Dr. Lieber said that in a year or two
the
nanowire transistors could be used as biological sensors by adding sites for specific molecules — say a
piece of
anthrax — to bind to the nanowires. Because DNA and proteins carry electrical charges, they would switch
the
transistors on, setting off an alert.
Other molecular electronics researchers are reporting important advances in today's Science. Scientists at
Lucent
Technologies (news/quote)' Bell Labs — who reported last month that they had built a transistor whose
active
switching layer is one molecule thick but consists of several hundred thousand molecules — now report
that they
have constructed a transistor where a single molecule acts as the switch.
And researchers at the Delft University of Technology in the Netherlands report that they have constructed
logic
circuits out of nanotubes.
In April, researchers at I.B.M.'s Thomas Watson Research Center in Yorktown Heights, N.Y., reported that
they
had built arrays of transistors out of nanotubes and had made the simplest possible logic circuit. The Dutch
researchers, like Dr. Lieber, have built logic circuits to perform each of the fundamental operations.
"This is the sort of toolbox that typical electronics people would do," Dr. Cees Dekker, the lead researcher,
said.
Until the advances this year, molecular electronics researchers have made switches that can turn electric
current on
and off, but cannot amplify signals as transistors do. To be used for computation, such switches would
require a
radically different architecture.
The new transistors suggest that post-silicon computers may be much like today's, only smaller and faster.
"We really have a period of momentous development in molecular- scale electronics," said Dr. James C.
Ellenbogen, a pioneer in molecular electronics. Advances that had seemed impossible a short while ago
"suddenly
are made to look almost obvious," Dr. Ellenbogen said.
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