Strain-tuned potassium niobate may enable cleaner, smarter devices

What's the best way to precisely manipulate a material's properties to the desired state? It may be straining the material's atomic arrangement, according to a team of researchers from Cornell and Penn State, among other institutions.

The team discovered that "atomic spray painting" of potassium niobate, a material used in advanced electronics, could tune the resulting thin films with exquisite control. The finding, published in Advanced Materials, could drive environmentally friendly advancements in consumer electronics, medical devices and quantum computing, the researchers said.

The process, called strain tuning, alters a material's properties by stretching or compressing its atomic unit cell, which is the repeating motif of atoms that builds up its crystal structure. The researchers use molecular beam epitaxy (MBE), a technique that involves depositing a layer of atoms on a substrate to form a thin film. In this case, they produced a thin film of strain-tuned potassium niobate.

"This was the first time potassium niobate has been grown using MBE," said Venkatraman "Venkat" Gopalan, a professor of materials science and engineering at Penn State and corresponding author of the study. "The technique is like spray-painting atoms onto a surface."

According to the researchers, the novel MBE technique - in combination with a crystal that serves as a substrate template - creates the strain needed to tune the material.

Potassium niobate is ferroelectric, or a class of materials with a natural electric polarization that can be reversed by applying an external electric field, much like how magnets have a magnetic polarization that can be flipped with an external magnetic field. Ferroelectrics are vital for devices like ultrasound equipment, infrared cameras and precision actuators for advanced microdevices.

To "spray paint" the potassium niobate for the study, Gopalan turned to Darrell Schlom, the Tisch University Professor in the Department of Materials Science and Engineering at Cornell University. They grew the thin films at the U.S. National Science Foundation-funded Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM) thin film growth facility, which Schlom co-directs at Cornell. Schlom noted that both he and Gopalan worked at Penn State on the first-ever strain tuning of ferroelectric materials approximately 20 years ago.

"Our role was to help Venkat and Sankalpa realize this material that Venkat has been dreaming about for decades now," Schlom said. "Venkat synthesized unstrained thin films of this material during his doctoral work at Cornell three decades ago, so he knows just how challenging it can be to grow it. For this work, my student Tobias Schwaigert and I helped them grow this material."

Schlom explained that strain engineering works by layering two materials of slightly dissimilar sizes. Imagine raining down atoms onto a surface comprising the same type of atoms but spaced a little differently. If the layer being added is thin enough, it will stretch or compress slightly to match the surface below it. The small change in spacing creates a strain in the material, similar to how a rubber band stretches when pulled. This strain, controlled by the size and spacing of the atoms on the surface, is what leads to changes in the material's properties, like increasing its temperature limits or improving its ferroelectric performance.

The research team also discovered that strain tuned-potassium niobate's ferroelectric performance remained stable even at high temperatures. Typically, ferroelectric materials, when heated, lose their polarization - meaning they are no longer able to switch their electrical charge.

"With further development, this novel version of the material could become a key player in the next generation of green, high-performance technologies that impact everything from our personal devices to space exploration," Gopalan said.

Along with Gopalan, Hazra, Schwaigert and Schlom, other authors of the study from the Penn State Department of Materials Science and Engineering are Aiden Ross, doctoral candidate; Utkarsh Saha, Tatiana Kuznetsova and Saugata Sarker, all graduate research assistants; Betul Akkopru-Akgun, assistant research professor; Susan Trolier-McKinstry, Evan Pugh University Professor and Flaschen Professor of Ceramic Science and Engineering; Vladimir A. Stoica, associate research professor; and Long-Qing Chen, Hamer Professor of Materials Science and Engineering, professor of engineering science and mechanics and of mathematics. Other co-authors include Haidong Lu, Xin Li, Xiaoshan Xu and Alexei Gruverman, University of Nebraska; Victor Trinquet and Gian-Marco Rignanese, Institute of Condensed Matter and Nanosciences in Belgium; Benjamin Z. Gregory, Suchismita Sarker, Matthew R. Barone, Andrej Singer and David A. Muller, Cornell University; Anudeep Mangu and Aaron M. Lindenberg, Stanford University; John W. Freeland, Argonne National Laboratory; Roman Engel-Herbert, Paul Drude Institute for Solid State Electronics; and Salva Salmani-Rezaie, Ohio State University.

The U.S. Department of Energy and the U.S. National Science Foundation, among others, supported this research.

This article was adapted with permission from a version written by Jamie Oberdick and published by Penn State.

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