By Kenneth Chang
12 November 2003
The recipe for a computer chip of the future may read
something like this: Take some wires. Add DNA. Stir.
In an advance that might provide a practical method for
making molecular-size circuits, the smallest possible,
scientists in Israel used strands of DNA, the computer code
of life, to create tiny transistors that can literally
build themselves.
"What we've done is to bring biology to self-assemble an
electronic device in a test tube," said Dr. Erez Braun, a
professor of physics at the Technion-Israel Institute of
Technology in Haifa, Israel, and a senior author of a paper
describing the research today in the journal Science.
Scientists have in the last few years accomplished feats of
the incredibly small, constructing devices not much larger
than individual molecules, but they also realize that their
current painstaking techniques are too slow and
inefficient.
"In order to construct a circuit," Dr. Braun said, "you
need to invent ways to tell molecules where to go and how
to connect to each other."
To that end, many scientists have turned to the
biologically inspired notion of self-assembly, using
molecules like DNA and proteins that can automatically link
together in the correct configuration.
"It's all of the dynamics on that scale rather than just
making small stuff," Dr. Horst Stormer, a professor of
physics at Columbia, said.
Dr. Stormer, who was not involved in the new research,
described the work as a "good first step" toward
self-assembling electronic devices.
The Technion-Israel scientists constructed transistors out
of carbon nanotubes, cylindrical molecules that are about
one ten-millionth of an inch in diameter and resemble
rolled-up chicken wire.
Current computer chip technology, which fashions
transistors out of silicon, will hit fundamental limits in
about a decade. To continue the progression toward ever
faster computers, many scientists are looking to molecular
electronics like the nanotube transistors to step in.
Other researchers have made similar transistors, which
already perform better than their silicon counterparts. But
making them in quantity is a major unsolved challenge.
In the earliest work, the nanotubes were randomly placed.
By chance, some made the correct electrical connections.
Since then, researchers have looked for a more practical
way to wire together the billions of transistors that would
be needed for a computer chip. Scientists at Duke
University reported in August that they had coated DNA with
silver to produce ultrathin wires.
The Israeli group is the
first to use DNA to build a working electronic device.
"It's a very interesting demonstration of a completely new
concept of assembling devices," said Dr. Cees Dekker, a
professor of physics at the Delft University of Technology
in the Netherlands who research group made the first nanotube transistor in 1998.
The new technique takes advantage of a biological process
known as recombination, where a segment of DNA is swapped
out for an almost identical piece. The cell uses
recombination to repair damaged DNA and to swap genes. A
special protein helps connect the replacement DNA to the
desired location.
By attaching a nanotube to the protein, the nanotube moves
to an exact location along the DNA strand.
"The DNA serves as a scaffold, a template that will
determine where the carbon nanotubes will sit," Dr. Braun
said. "That's the beauty of using biology."
The scientists then coated the DNA with gold, producing a
simple electronic device consisting of the nanotube
connected to gold wires at each end. Current through the nanotube could be switched on or off by applying an
electric field - the definition of a transistor.
In earlier work, the same researchers showed that they
could stretch DNA across a surface to provide a template to
hook together the transistors into a circuit. The next step
would be to build the circuit, Dr. Braun said.
Other groups are looking at alternative ways to build
molecular circuits. Dr. Dekker's group now lays catalyst
that will grow nanotubes at desired places. He is also
exploring using DNA, although using a different approach.
DNA molecules attached at the end of nanotubes would act
like "smart glue."
Each strand would be able to attach to
only one other strand.
"It's programmable Velcro," Dr. Dekker said.
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