Home > Press > Increasing Charge Mobility in Single Molecular Organic Crystals
Studies may help identify best materials for variety of future
Increasing Charge Mobility in Single Molecular Organic Crystals
Los Angeles, CA | March 22, 2005
Flexible displays that can be folded up in your
pocket? More accurate biological and chemical sensors? Biocompatible
electronics? In research that may help determine the best materials
for a wide range of future electronics applications, a scientist from
the U.S. Department of Energy's Brookhaven National Laboratory will
report on the intrinsic electronic properties of molecular organic
crystals at the March 2005 meeting of the American Physical Society.
Brookhaven materials scientist Vladimir Butko will describe the
experimental techniques and key findings on Monday, March 21, at 3:42
p.m. in room 152 of the Los Angeles Convention Center.
Organic materials are particularly attractive for potential
applications such as flexible displays, or so-called "electronic
paper," because they are inherently flexible. "Imagine a computer
screen that you could crumple or fold like a sheet of plastic film,"
Butko says. Yet for this and any other electronics application, the
materials must also be able to carry an electric current.
"These organic materials, by themselves, have almost no charge
carriers -- electrons or "holes" [the absence of electrons] -- to
carry current," Butko says. "They act as insulators. But if we inject
charge carriers, we can sometimes create organic devices such as
field-effect transistors [FETs], through which charge will flow."
To find out which materials have the best potential for carrying
current, Butko has been studying single crystals of molecular organic
materials such as pentacene and rubrene. Though these crystals
themselves may not have direct applications, they provide the
simplest form in which to study the materials' intrinsic electronic
properties -- unaffected by factors that might play a role in larger
samples such as polycrystalline thin films.
The key, says Butko, is to know whether the injected charge carriers
will have a high mobility or stay localized. The most stringent test
of localization is to cool such a device to very low temperatures:
somewhat close to absolute zero, which is approximately -273 degrees
Celsius. At these low temperatures the mobility edge can be probed
without the complication of thermal activation -- a process that
assists charge carrier transport in semiconductors due to large
thermal energy at high temperatures. The studies were done using a
physical properties measurement system (PPMS) and electrometers at
the Los Alamos National Laboratory.
In his talk, Butko will present first evidence for low-temperature,
quasi-temperature-independent transport of injected charge in a
crystalline organic FET. "These materials, which also have the
highest charge mobility at room temperature among organic FETs, can
be most useful for electronic applications," Butko says.
Once scientists identify the best crystals, they will use thin-film
methods to test their applicability for electronic devices from
e-paper to large-format display screens.
This research was done in collaboration with Arthur Ramirez, David
Lang and Xiaoliu Chi from Bell Laboratories, and Jason Lashley from
Los Alamos National Laboratory, and was funded in part by the Office
of Basic Energy Sciences within the U.S. Department of Energy's
Office of Science.
One of the ten national laboratories overseen and funded primarily by
the Office of Science of the U.S. Department of Energy (DOE),
Brookhaven National Laboratory conducts research in the physical,
biomedical, and environmental sciences, as well as in energy
technologies and national security. Brookhaven Lab also builds and
operates major scientific facilities available to university,
industry and government researchers. Brookhaven is operated and
managed for DOE's Office of Science by Brookhaven Science Associates,
a limited-liability company founded by Stony Brook University, the
largest academic user of Laboratory facilities, and Battelle, a
nonprofit, applied science and technology organization. Visit
Brookhaven Lab's electronic newsroom for links, news archives,
graphics, and more: www.bnl.gov/newsroom
Karen McNulty Walsh
Mona S. Rowe
Copyright © BNL
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