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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