PITTSBURGH--Steadily increasing the length of a purified conducting polymer
vastly improves its ability to conduct electricity, report researchers at Carnegie
Mellon University, whose work appeared March 22 in the Journal of the American
Chemical Society. Their study of regioregular polythiophenes (RRPs) establishes
benchmark properties for these materials that suggest how to optimize their
use for a new generation of diverse materials, including solar panels, transistors
in radio frequency identification tags, and light-weight, flexible, organic
light-emitting displays.
"We found that by growing very pure, single RRP
chains made of uniform small units, we dramatically
increased the ability of these polymers to conduct
electricity," said Richard D. McCullough, who initially
discovered RRPs in 1992. "This work establishes basic
properties that researchers everywhere need to know
to create new, better conducting plastics. In fact,
designing materials based on these results could
completely revolutionize the printable electronics
industry."
"Our results are very significant, since they cast
new light on the mechanism by which polymers conduct
electricity," said Tomasz Kowalewski, associate professor
of chemistry and senior author on the study.
Unlike plastics that insulate, or prevent, the flow
of electrical charges, conducting plastics actually
facilitate current through their nanostructure. Conducting
plastics are the subject of intense research, given
that they could offer light-weight, flexible, energy-saving
alternatives for materials used in solar panels and
screen displays. And because they can be dissolved
in solution, affixed to a variety of templates like
silicon and manufactured on an industrial scale,
RRPs are considered among the most promising conducting
plastics in nanotech research today, according to
McCullough, dean of the Mellon College of Science
and professor of chemistry.
"Our tests showed that highly uniform RRPs self-assemble
into well-defined elongated aggregates called nanofibrils,
which stack one against the other," Kowalewski said. "About
5,000 of these nanofibrils would fit side by side
in the width of a human hair. The presence of these
well-defined structures allowed us for the first
time to make a connection between the size of polymer
molecules, the type of structure they form and the
ease with which current can move through nanofibril
aggregates." (See image.)
The vast improvement in conductivity is tied to
several key properties that were unambiguously shown
for the first time in this study, according to Kowalewski.
"We made the key discovery that mobility -- how
easily electrons move -- increases exponentially
as the width of a nanofibril increases," Kowalewski
said. Each rope-like nanofibril actually is a stack
of RRP molecules, so the longer these molecules,
the wider the nanofibril and the faster the electrical
conductivity. (See image insert of RRP stacks.) In
this way, electricity moves preferably perpendicular
through the rows of naturally aligned nanofibrils.
"We found that charge carriers encounter fewer hurdles
when jumping between wider nanofibrils," said Kowalewski. "Ultimately
through this study, we found that the nanostructure
of our conducting plastic profoundly enhances its
ability to conduct electricity."
Conductivity increases with the length of an RRP
molecule -- and hence the width of each nanofibril
-- because it takes less time for a charge carrier
to cross through wider nanofibrils than narrower
ones. (Charge carriers are unbound particles that
carry an electric charge through a molecular structure).
All this can be tied to the fact that a charge carrier
that enters a short molecule disrupts its energetic
environment considerably more than if that same charge
carrier enters a long molecule. This energetic hurdle,
called reorganization energy, thus slows the movement
of charge carriers that move from short molecule
to short molecule. The energetic hurdle is lower
for a long molecule, which can absorb changes to
its electrical environment more easily. This phenomenon
could be one of the reasons why charge carriers jump
more quickly from long molecule to long molecule,
according to Kowalewski.
"We hope that these findings will stimulate further
theoretical and experimental work which will significantly
improve the performance of polymer-based electronics
and open the way to a wide range of applications," Kowalewski
said.
To
show that increasing the width of RRP nanofibrils
exponentially increased charge carrier mobility,
the Carnegie Mellon team first created pure RRPs
of uniform size, or molecular weight. Next, they
placed the drops of RRPs dissolved in a solvent
onto silicon chips whose surfaces were specially
prepared for use as nanotransistors. Such "drop casting" allowed
the team to create a series of nanostructures that
varied in accordance with the length of the RRP chains
initially present in solution.
The team ran a current through these different RRP-based
nanotransistors to measure their ability to conduct
electricity. They used atomic force microscopy and
a technique called grazing-incidence small-angle
X-ray scattering to verify that periodic, stacked
structure of different RRPs indeed formed nanofibrils
of corresponding widths. The latter technique was
performed using the High Energy Synchrotron Source
at Cornell University.
The team of investigators included students Rui
Zhang in the Department of Chemistry; Bo Li in the
laboratory of David Lambeth, professor of electrical
and computer engineering; and faculty from the Department
of Physics, who participated in X-ray scattering
studies.
The research is supported by grants and contracts
from the National Science Foundation, the Air Force
Office of Scientific Research, and the National Institute
of Occupational Safety and Health/Centers for Disease
Control.
Contact: Lauren Ward
wardle@andrew.cmu.edu
412-268-7761
Carnegie Mellon University
|
