Sunday, 21 December 2014

ORNL microscopy pencils patterns in polymers at the nanoscale

Scientists at the Department
of Energy’s Oak Ridge National Laboratory have used advanced microscopy
to carve out nanoscale designs on the surface of a new class of ionic
polymer materials for the first time. The study provides new evidence
that atomic force microscopy, or AFM, could be used to precisely
fabricate materials needed for increasingly smaller devices.

Polymerized ionic liquids have potential applications in technologies
such as lithium batteries, transistors and solar cells because of their
high ionic conductivity and unique structure. But many aspects of the
recently discovered materials are still not well understood.

When ORNL researchers used an atomic force microscope to begin
characterizing the properties of polymerized ionic liquid thin films,
the experiment yielded some surprising results.

“We were
expecting to measure ionic conductivity, and instead we found that we
were forming holes on the surface,” said ORNL’s Vera Bocharova,
corresponding author on the study published in Advanced Functional
Materials. “Then we started to think about how this might have great
applications in nanofabrication.”

Nanolithography is the
dominant technique used by industry for nanofabrication, but its size
limitations are leading researchers to explore other methods such as
AFM.

“This study is part of our search for alternative methods
and materials that can be used to create smaller sized objects,”
Bocharova said. “For example, our technique might be interesting for the
miniaturization of semiconductor technology.”

Similar AFM
techniques have been used to study and produce patterns in nonconductive
polymers, but the ORNL study uncovered several differences in the
application to polymerized ionic liquids.

“In comparison to
nonconductive polymers, we have to apply less bias — four volts instead
of 20 volts — to generate the holes, which is good in terms of energy
savings for future applications,” Bocharova said.

In
nonconductive polymers, the high voltage applied through the AFM tip
punctures the material’s surface by localized heating. In contrast, the
ORNL team used experiment and theory to determine that the holes formed
in the conductive polymer liquids resulted from negative ions migrating
to the positively charged microscope tip. The researchers plan to
continue refining the technique’s capabilities and their understanding
of the polymerized ionic liquids’ properties.

“Right now the
size of the formed features is in the range of 100 nanometers, but it’s
not the limit,” Bocharova said. “We believe it’s possible to change the
experimental setup to advance to lower scales.”

The paper is published as “Controlled Nanopatterning of a Polymerized Ionic Liquid in a Strong Electric Field.”
Coauthors are ORNL’s Vera Bocharova, Alexander Agapov, Alexander
Tselev, Rajeev Kumar, Alexander Kisliuk, Ivan Kravchenko, Bobby Sumpter,
Alexei Sokolov, Sergei Kalinin, and Evgheni Strelcov; Liam Collins of
the University College Dublin; and Stefan Berdzinski and Veronika
Strehmel of the Hochschule Niederrhein University of Applied Sciences.
Sokolov holds an ORNL-University of Tennessee Governor’s Chair
appointment.

This research was supported by ORNL’s Laboratory
Directed Research and Development program and was conducted in part at
ORNL’s Center for Nanophase Materials Sciences, a DOE Office of Science
User Facility. Parts of the research were supported by DOE’s Office of
Science and the National Science Foundation.

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