Venkatraman Gopalan
A goal of the CNMS facility is to enable researchers to answer basic questions about why materials behave as they do.
One goal of user facilities such as the Center for Nanophase
Materials Sciences (CNMS) is to enable researchers to answer
basic questions about why materials behave as they do. One such
project involves a ferroelectric material being studied by Venkatraman
Gopalan, a professor of materials science and engineering and associate
director for the Center for Optical Technologies at Penn State
University, and his student, Vasudeva Rao Aravind. Much of the research
conducted at the nanocenter involves new kinds of material; however
this material has been valued for its unique
properties for decades, yet retains a number
of fundamental mysteries.
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![Members of Sergei Kalinin's group receive high praise from users for their experimental techniques and for their ability to help users maximize their research.](images/a11_p17sm.jpg)
Members of Sergei Kalinin's group receive high praise from users for their experimental techniques and for their ability to help users maximize their research.
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Gopalan and Aravind used the scanning
probe microscopy facilities at CNMS
to study a specific feature, called a domain
wall, of a class of crystalline materials called
"ferroelectrics." Ferroelectric materials are
notable primarily because they are naturally
polarized. One end of the material can be
positively charged and the other negatively
charged, giving them a net polarization
direction. The polarization "up" and
polarization "down" domain states of the
material are separated by a boundary called
a domain wall. When an electrical current is
applied to these materials, the polarization
of the ends can be "flipped," or reversed.
This behavior makes the materials useful
in applications, such as small piezoelectric
motors, nonvolatile memory and other
electronic and optical devices.
"One of the interesting things about ferroelectrics," Gopalan says, "is that despite using them for 60 years we are still learning new
things." The research conducted by Gopalan and Aravind focused on
determining the structure of the domain wall. Gopalan points out
that, "We can determine some features about the wall from textbooks,
but there are still mysteries that remain unresolved. That's what this
project was about."
Working at Penn State, Gopalan and Aravind came to believe that the domain wall has properties that are significantly different from
those of the rest of the ferroelectric material. "In a ferroelectric material,
the wall is very 'sharp,' or narrow, one or two crystal cells wide," Gopalan
explains. "It is so narrow that researchers generally don't think about the
properties of the wall itself and are more concerned with the bulk properties
of the whole material." However, Gopalan and Aravind surmised
that the nanoscale properties of the wall might be very important to
understanding the material as a whole.
The two researchers did some preliminary experiments to test their theory that one of the distinctive properties of the wall is an
electrical "softness," meaning that only a small voltage is required to
flip the polarization of the material if the voltage is applied close to
the wall. A much larger voltage is needed to achieve the same result
if applied farther away. "Our results suggested this could be possible,
but we didn't have the kind of sophisticated scanning probe microscope that would be needed to achieve definitive results." That's where the CNMS scanning probe microscopy facilities came in. "At
the nanocenter," Gopalan says, "Sergei Kalinin's group has not only a
very sophisticated scanning probe microscope, but they have also
developed unique software that automates the process of scanning
and mapping the crystal."
Aravind made several multiweek trips to the Oak Ridge to study the structure of the wall. "After the first trip, we had a sense of where
this experiment would go," Gopalan recalls. "By the second trip, he was
seeing amazing results. He mapped beautifully how the wall has a life
of its own. We found that the area near the wall is 10 times electrically
softer than the areas further away. This is the first direct measurement
of such a thing. None of the textbooks describe anything like this."
Gopalan adds that while the structure and electrical nature of the ferroelectric domain wall had recently been theoretically predicted, no
direct measurements of the phenomenon existed until this experiment.
"Our research group has analyzed the results using several different
models to try to explain them because this behavior is so basic to
understanding ferroelectric materials," he says. "Now we have a model
that enables us to understand why the domain wall behaves as it does.
We have theories and predictions that match our experimental results."
Gopalan emphasizes that the nanocenter played a valuable role in unraveling a little more of the mystery of the ferroelectric domain wall.
"I don't know of a better facility, particularly in the area of piezoelectric
force microscopy." He adds, "Sergei Kalinin's group is doing pioneering
work in this field. They have not just developed their techniques, but
they have taken their approach to the research to a new level. These
guys are some of the best in the world in their field. We want to collaborate
with them, not just to get access to their facilities and collect
high-quality data but also for their intellectual inputs in interpreting
and understanding the data. That's where their real value comes in."
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