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.
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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