Computers Made of Molecule-Size Parts Could Build Themselves Source: U.S.News & World Report May 1, 2000
When James Tour thinks about computers, the Rice
University scientist pictures big, big numbers-but minuscule machines.
That's because he's not a computer designer in the traditional sense
at all. He's a chemist, and his notion of computer parts makes today's
technology seem as unwieldy as
the radio tubes of days past. Indeed, if Tour has his way, the silicon
components of today's machines will ultimately go the way of those
electronic dinosaurs.
"Consider this," he writes. "There are [a billion
trillion] water molecules in just one drop of water." That's hundreds
of times the sum total of all the transistors in every computer ever
made, he reckons. If just a fraction of the molecules in a speck of
matter could be made to act as electronic
switches, able to control the electrical currents that are the basic
language of computers, present-day computers with their paltry
billions of transistors could quickly become obsolete. So would
to-day's giant and expensive chip fabrication plants: A molecular
computer might build itself in a set of chemical steps.
If this sounds like rank speculation, it's not. Those
molecular switches are real. Over the past year, Tour and other
researchers have made and tested complex, artful molecules that work
like switches you might buy at RadioShack, flipping from "off" to "on"
and back when tripped by pulses of voltage. Other groups have made
wires that are only atoms across,
equipped with the mo-lecular connectors that might soon hook up the
tiny switches into computer circuits.
A droplet-size supercomputer will become a reality only
after scientists learn how to connect astronomical numbers of
molecules into a working computer architecture and link the minuscule
computer to the larger-scale world we live in. That could take
decades. But simpler devices that use dense patches of molecular
switches to soup up conventional chips are already in the works.
Computer switches need to do two jobs: open or close in
a flash to process information, and stay open or closed for long
enough to act as short-term memory. Last November, Tour, Mark Reed of
Yale University, and their colleagues reported that they had met the
first requirement by tailoring an
electrically conductive molecule. When they positioned a few hundred
of the molecules between tiny metal electrodes and increased the
voltage across them, they found that at about 2 volts, the molecular
switches turned on, allowing current to flow. Above or below that
voltage, the switches were off. Now they have modified their switch
molecule to create memory: After
being flipped on or off, it stays that way for a few minutes. "You can
read, write, erase it," Tour says.
At the University of California-Los Angeles, Fraser
Stoddart, James Heath, and their colleagues have built similar
switches from molecules that actually change shape when they are
zapped with a charge. "Think of a dumbbell with a ring that slides
back and forth" along a shaft, turning the switch on or off, says
Stoddart.
The wiring problem. Connecting these tiny switches with
wires as coarse as those on an ordinary microchip would squander much
of the molecules' size advantage. So the UCLA group's collaborators at
Hewlett-Packard are learning how to make metal or silicon wires just
10 or so atoms across. As a
bonus, these wires don't need to be laid down one by one, like the
connectors on current silicon chips; instead, they assemble themselves
en masse, like long, thin crystals. At Pennsylvania State University,
another group is making tiny wires that are predesigned to snap
together into larger structures. The researchers coax platinum and
gold atoms into the narrow pores in a membrane-then dissolve away the
membrane to free breathtakingly thin wires that are striped with bands
of the different metals, like a roll of LifeSavers. Molecular switches
should stick readily to some of the bands but not others, as do other
molecules, including DNA, that serve as glue for linking the wires to
one another.
Stanley Williams of Hewlett-Packard says that putting
together the first molecule-based devices should take 15 months. By
then, he says, "we hope to be able to deliver an entire 16-bit memory
unit that could fit on top of the smallest wire in today's integrated
circuits, with room to spare." Other
groups may also be unveiling prototype molecular processors and
memories in two or three years, and one device, from a company founded
by Reed, Tour, and others, may even be taking its first steps to
market.
by Tim Appenzeller
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