http://www.physorg.com/news102081690.html

Chemists advance organic semiconductor processing...

Any machinist will tell you that a little grease goes a long way toward
making a tool work better. And that may soon hold true for plastic
electronics as well.
Carnegie Mellon University chemists have found that grease can make some
innovative plastics vastly better electrical conductors. This discovery,
published June 25 in Advanced Materials, outlines a chemical process that
could become widely adopted to produce the next generation of tiny switches
for transistors in radio frequency identification tags, flexible screen
displays, and debit or key cards.

"This research brings us closer to developing organic semiconductors with
electrical and physical properties far superior to those that exist today,"
said principal investigator Richard D. McCullough, professor of chemistry
and dean of the Mellon College of Science at Carnegie Mellon. "We were
surprised and amazed with our findings." The new process involves adding a
little grease in two ways, say the investigators. The first step involves
chemically combining an inherently conducting polymer (ICP) with a
grease-like chemical. The second step involves depositing this hybrid
material - called a block copolymer - onto a greased platform.

On the surface layer of a transistor, ICPs make good electrical conductors
that provide the switch element for a transistor to turn on and off. But
ICPs are by nature brittle. To counter this brittleness, scientists
chemically link ICPs with grease-like, elastic polymers to make block
copolymers.

"These block copolymers are very promising for creating future materials,
such as lightweight, thin composite films for ebook readers that you could
roll up like today's newspapers," said Genevieve Sauvé, a research associate
who conducted the latest research under conditions similar to a commercial
production setting.

While they provide much-needed flexibility, elastic polymers insulate rather
than conduct electricity. Block copolymers that contain grease-like polymers
are less effective electrical conductors than pure ICPs. Yet in the right
processing setting, the opposite can hold true, the Carnegie Mellon
scientists now report. It just depends how you treat a transistor's silicon
dioxide base layer.

As part of the current study, the Carnegie Mellon team tested four block
copolymers, each with a different ratio of insulating elastic polymer to
conducting polymer. When they applied thin films of these different polymers
to untreated silicon dioxide, they found the greater the overall amount of
insulating polymer in the final film, the worse that film performed in
conducting an electric charge. The result is a flexible switch layer that
doesn't work very well.

But when the scientists pretreated the transistor's silicon dioxide platform
with OTS-8 - a chemical that creates a grease-like coating - they found that
transistors incorporating any of the four block copolymers conducted an
electric charge with remarkable ease, even when the insulating polymer
constituted more than half of the applied block copolymer.

"Something amazing is happening at the molecular interface between our block
copolymer and the OTS-8-treated surface so the block copolymers
self-assemble with great precision," Sauvé said. "In fact, we think that the
grease-like, insulating polymer in the material and the grease-coated
surface both somehow exert important effects in driving this self-assembly."

Block copolymers with up to 57 percent insulating polymer performed 10 times
better on OTS-8-treated surfaces than they did on untreated surfaces,
according to the investigators. More importantly, the block copolymers were
nearly equal in their performance to ICPs alone on treated surfaces,
according to McCullough.

"This is the first report that copolymers are good organic semiconductors,"
McCullough said. "These results mean that we could soon design devices that
are both flexible and highly functional."

OTS-8 appears to help the block copolymers assemble into nanowires that are
much more highly organized than those that self-assemble on untreated
silicon dioxide, according to Sauvé. (See available images)

The Carnegie Mellon team used block copolymers containing ICPs called
regioregular polythiophenes (rr-P3HTs), which were initially described by
McCullough in 1992. In subsequent research, McCullough's laboratory has
developed cost-efficient methods to produce rr-P3HTs so they can be put into
solution and sprayed onto surfaces using ink-jet printing. McCullough has
also shown that rr-3PHTs can be modified to attach to different surfaces. By
chemically linking rr-P3HTs with other elastic polymers, McCullough's group
has also produced conductive plastics with a range of physical properties
that could suit different device applications.

The insulating, elastic polymer used in this latest work is
poly(methylacrylate), or PMA. Sauvé is using this system to evaluate
nanowire assembly and conductive properties of block copolymers made with
polymers other than PMA. These additional polymers are being developed by
research scientist Mihaela Iovu in McCullough's lab.

Eventually, Sauvé says, polymer chemists could replace a silicon dioxide
base with a flexible plastic so consumers could roll up plastic displays.

Link: http://www3.interscience.wiley.com/cgi-bin/fulltext/114282726/PDFSTART

-- 

Ken

"Buddhism elucidates why we are sentient."
"Buddhism follows thought throughout the Universe."
"Karma means that you don't get away with anything."