Rice University
HOUSTON, USA – A new study led by Rice University materials scientist Lane Martin sheds light on how the extreme miniaturization of thin films affects the behaviour of relaxor ferroelectrics — materials with noteworthy energy-conversion properties used in sensors, actuators and nanoelectronics. The findings reveal that as the film shrinks to dimensions comparable to the materials’ internal polarization structures, their fundamental properties can shift in unexpected ways.
The focus of the study published in Nature
Nanotechnology is lead magnesium niobate-lead titanate, or PMN-PT, a widely used ceramic material found in applications ranging from medical imaging (ultrasounds) and energy harvesting to gas sensors and beyond. In their quest to shed light on how the internal polarization structure of PMN-PT evolves and acts at vanishingly small scales, the researchers made a surprising discovery: Before losing its special abilities, the material actually improved. This unexpected “sweet spot” could open the door to a new generation of nanoelectronic devices.
As a ferroelectric relaxor, PMN-PT excels at converting energy from one form to another. For instance, pressing on a thin film of this material generates a voltage, while applying an external voltage to it makes it change shape. At the atomic level, its structure is made up of negative and positive atoms which can move relative to each other to create local dipoles. These dipoles do not align uniformly across the material; instead, they are subject to competing energies — one that wants them pointing randomly and another that wants to align them pointing in the same direction. The result is that the material breaks up into polar nanodomains — tiny clusters no bigger than a small virus, wherein all the dipoles point in roughly the same direction.
“These self-assembled structures of polarization inside the material are highly responsive to external stimuli due to the chemical complexity of the material and the size of these regions — at their smallest, PMN-PT nanodomains are only 5-10 nanometers,” said Jieun Kim, assistant professor at the Korea Advanced Institute of Science and Technology and the study’s first author. “Nobody really knew what would happen if we shrunk the whole material down to their size.”
Understanding how materials behave at tiny scales is critical for advancing miniaturized electronics and other applications. As devices shrink, they require ultrathin films of materials like PMN-PT, but detailed studies mapping out the physics of relaxors at very small length scales had, Kim said, “never been done before.”
“We hypothesized that as PMN-PT films got thinner, their polar nanodomains would shrink and eventually disappear along with the material’s desirable properties,” said Martin, the Robert A. Welch Professor of Materials Science and Nanoengineering and director of the Rice Advanced Materials Institute. “The research confirmed this expectation, but we also found something unexpected.”
Instead of immediately deteriorating, PMN-PT actually performed better when shrunk down to a precise range of 25-30 nanometers — about 10,000 times thinner than a human hair. At this scale, the material’s phase stability — its ability to maintain its structure and functionality under varying conditions — was significantly enhanced.
To uncover this hidden behaviour, the researchers used some of the world’s most advanced scientific tools. At the Advanced Photon Source at Argonne National Laboratory, researchers fired ultrabright X-ray beams at the material to probe its atomic structure. This technique, known as synchrotron-based X-ray diffraction, allowed them to observe how the nanodomains evolved as the material was thinned.
“We correlated these findings with measurements of dielectric properties we performed in our lab and rounded out the picture using scanning transmission electron microscopy to map out polarization with atom-level resolution,” said Kim, who began the project four years ago as a doctoral student under Martin at University of California, Berkeley. “For the thinnest films, we also performed molecular-dynamics simulations — basically recreating the thin films in a computer — to study the structural evolution of the polar nanodomains.”
Together, these approaches provided the most detailed picture yet of how PMN-PT behaves at the nanoscale. While many materials lose their useful properties when they are made extremely small, PMN-PT exhibits what the researchers call a “Goldilocks zone” size effect where its properties actually improve before eventually deteriorating. Understanding this effect could pave the way for advanced applications such as nanoelectromechanical systems, capacitive-energy storage (pulsed-power), pyroelectric energy conversion, low-voltage magnetoelectrics and more.
Next, the researchers plan to explore how stacking ultrathin layers of PMN-PT and similar materials — like building a “pancake stack” of different functional layers — could create entirely new materials with properties that do not exist in nature. These engineered materials could revolutionize energy harvesting, low-power computing and next-generation sensors.
“Now we know that we could make devices that are smaller and better,” Kim said.
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