In present-day electronic devices, different materials play different
roles, just as a football team would have separate specialists playing
offensive and defensive positions. Copper, stretched into wires, conveys
electrical power. Silicon, the semiconductor, is responsible for the calculating
prowess of computer chips.
Plastic -- the stuff of foam cups, food wrap and polyester suits --
has remained mostly on the sidelines, wrapped around wires to prevent short
circuits, shaped into computer boxes and not much else.
That is changing. Plastic and other carbon-based molecules -- what chemists
call organic chemicals, even though most bear little resemblance to anything
found in living organisms -- are evolving into versatile electronic do-it-alls,
scientists say.
Over the next few years, organic materials will start appearing in thin,
bright displays on cell phones and other hand-held devices, the scientists
predict. Meanwhile, researchers have also fashioned the materials into
lasers, transistors and magnets.
This year's Nobel Prize in Chemistry recognized
one landmark achievement in the rise of organic electronics. Plastics,
in which thousands to millions of identical carbon-based molecules link
together into long chains known as polymers, are generally insulators,
blocking the passage of electrical currents. The prize was awarded to Dr.
Alan G. MacDiarmid of the University of Pennsylvania, Dr. Alan J. Heeger
of the University of California at Santa Barbara and Dr. Hideki Shirakawa
of the University of Tsukuba in Japan for becoming the first to transform
a plastic into an electrical conductor.
That discovery in 1977, along with parallel research by others into
the electrical properties of smaller, unlinked carbon-based molecules,
spurred scientists into imagining new possibilities.
''I think it opens up the opportunity of new kind of electronics for
the 21st century based on organics,'' said Dr. Arthur J. Epstein, a professor
of chemistry and physics at Ohio State University.
''It's really fun to see all the things that have now come out of it
and where it might be going.''
In some aspects, the organic molecules are a step backward compared
to conventional materials. Copper is a better conductor of electricity,
and silicon makes for faster computer chips.
''We're not going to replace silicon with organic materials,'' said
Dr. Stephen R. Forrest, a professor of electrical engineering at Princeton
University.
But organic molecules offer advantages. They are light, flexible, cheaper
to make and easier to shape.
Organic chemists can also draw from a library of millions of molecules
and custom build new variations by swapping one piece for another, almost
the way a child plays with Tinker Toys. ''Your materials palette is infinite,''
Dr. Forrest said. ''You can vary function with composition in a much more
facile way than, for example, with an inorganic semiconductor where you're
stuck with a couple of compounds in the middle of the periodic table.''
The conducting polymers are already added as an antistatic layer in
some photographic film to prevent electrical discharges that might ruin
the images.
A much larger market, video displays, is beginning to open up, and the
unlinked carbon-based molecules currently have the lead over polymer plastics.
Researchers discovered decades ago that some of the unlinked molecules
emit light if electric currents are run through them, but they consumed
too much energy to be fashioned into practical light-emitting diodes, or
L.E.D.'s. In 1980, Dr. Ching W. Tang, a senior research associate at the
Eastman Kodak Company in Rochester, produced the first low-energy organic
L.E.D. More impressively, the L.E.D. emitted blue light, and at the time,
there were no blue L.E.D.'s made out of any materials.
''That kind of spurred us further,'' Dr. Tang said. ''We kept on it
and improving the efficiency.''
The first products using the Kodak organic L.E.D.'s appeared earlier
this year as displays on some Pioneer car stereos and Motorola mobile phones.
Dr. Forrest and his colleague, Dr. Mark Thompson, a chemistry professor
at the University of Southern California, have made L.E.D.'s out of different
unlinked organic molecules, and Universal Display Corporation of Ewing,
N.J., is similarly turning that research into video displays and hopes
to have products on the market by the end of next year.
Similar, more recent advances have also come out of the polymer chains.
At Cambridge University in 1988, Dr. Richard Friend, a professor of
physics, and Dr. Jeremy Burroughes, then a graduate student, constructed
a transistor out of plastics with siliconlike semiconductor properties.
Later, while measuring the electrical properties of a plastic semiconductor,
they discovered a new property.
''We saw light coming out of it,'' Dr. Friend said. ''And having seen
the light, it was fairly easy to work backward figuring out why it was
putting out the light.''
The polymers have a potential advantage over their unlinked counterparts:
dissolved in a chemical solvent, they can be printed into circuits with
an inkjet printer. ''Quite a lot of it is done with an inkjet head that's
off a $100 printer, because $100 printers do a very good job,'' Dr. Friend
said. ''In principle, it can be printed onto almost anything. You put the
materials where you want it and not where you don't want it.''
Most of the unlinked carbon-based molecules, including Kodak's organic
L.E.D.'s, need to be deposited on a circuit board in a vacuum.
The company Cambridge Display Technologies, of Cambridge, England, where
Dr. Friend is chief scientist and Dr. Burroughes is technical director,
is turning the light-emitting polymers to commercial products. The first
uses will probably appear in 12 to 18 months in the video displays of cell
phones and other hand-held devices, said Dr. Daniel McCaughan, president
of the company.
Video displays are not the only electronic devices that might come out
of an inkjet printer. Lucent Technologies Inc.'s Bell Labs in Murray Hill,
N.J., is the one of the companies working on printable semiconductor circuits.
The researchers at Bell Labs started with a carbon-based molecule with
two good qualities: it was a known semiconductor and it was easy to manipulate.
''We envisioned we could attach a lot of side groups to it easily,'' Dr.
Katz said. The molecule also had two definite drawbacks: the molecules
didn't link together very well, and air and water caused the material to
break apart. ''It only works if you do the entire experiment under vacuum,''
said Dr. Howard Katz, a chemist at Bell Labs.
Dr. Katz solved both problems by adding a small side branch consisting
of a carbon and two fluorine atoms. ''It's the same kind of chain you would
find in Teflon,'' he said.
The original molecule did not stack very well, Dr. Katz said. The side
chains of the modified version, which Bell Labs named F15, fill in the
empty space. ''The side chain makes for a much better film,'' Dr. Katz
said. ''And also those chains seem to protect the molecules against the
atmosphere.''
The side branch also made the molecule dissolvable, making it amenable
to printing.
Designing molecules is part skill, part trial and error. The researchers
at Bell Labs figured out the correct side chain after about half a dozen
tries. ''We have analogs of F15 that are very similar looking, but then
they don't work nearly the same way,'' Dr. Katz said. ''You really do have
to hit it head on for it to work. There's a certain amount of learning
from your mistakes here.''
The low cost of printable circuits makes inexpensive, disposable electronics
possible. One idea is to print the radio equivalent of bar codes on packaging
that would absorb the energy of a radio signal and send a response. That
would allow companies to inventory warehouses by simply sending out a querying
radio signal and tabulating the responses from the packages.
The researchers at Bell Labs have also made the first electrically driven
laser out of a carbon-based material. The same molecule also turns into
a superconductor at extremely low temperatures.
At Ohio State University, Dr. Epstein is turning carbon-based molecules
into magnets. Many molecules act as tiny magnets, but molecules tend to
line up in an alternating up-and-down pattern that cancels out the magnetic
field. Through various chemical manipulations, Dr. Epstein and his colleagues
have maneuvered the molecular magnets of both polymers and unlinked molecules
into the same direction.
Electric motors and transformers are heavy, ''because they have a big
piece of iron in them, which acts to guide the magnetic fields,'' Dr. Epstein
said. With lighter organic magnets, ''that'd be great for energy conservation,''
he said.
He envisions the materials may also be used for computer memory and
hard disks and in thin films to shield electronic components. ''You don't
want someone to come and erase everything on your computer,'' he said.
Eventually, carbon-based molecules may push out metals and silicon even
in high-performance applications. Over the next couple of decades, conventional
computer chip technology, where performance has been speeded by shrinking
the size of the circuits, will hit a fundamental roadblock. Metal wires,
if only a few atoms wide, are not good conductors, and at the scale of
atoms and molecules, silicon circuits would no longer operate the same
way.
''A grander vision is to have electronic circuits which are at the quantum
scale,'' Dr. Forrest said. Today's work with carbon-based materials could
lead to a wire made of a single polymer strand and calculations performed
by interconnected molecules.
''We don't have a clue how to do that today,'' Dr. Forrest said, ''but
that's the vision.''
Organizations mentioned in this article:
Related Terms:
Electronics; Chemistry; Plastics; Research
