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.
"This method allows the atoms in the thin films to adjust to the underlying substrate's atomic structure, causing strain," said co-author Sankalpa Hazra, doctoral candidate in materials science and engineering. "Even a tiny stretch of about 1% can create pressure that would be impossible to achieve by simply pulling or pressing on such a brittle material from the outside. This pressure can significantly improve how the material works from a ferroelectric perspective."
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.
"A ferroelectric is sort of like a mini battery that is already charged up permanently by nature," said Gopalan. "Despite not being a household name, ferroelectrics are everywhere in key technologies we take for granted in our daily lives. The internet, for example, relies on converting electrical to optical signals, which is performed by a ferroelectric crystal. These materials can reverse their electric polarity when an external electric field is applied, a quality that also makes them 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 a former Penn State colleague, Darrell Schlom, who is currently 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 superior strength of coupling between strain and polarization in potassium niobate compared to other ferroelectrics allows for a unique opportunity where relatively small amounts of strain can result in colossal tuning of both the ferroelectric structure and its polarization," Hazra said. "A primary consequence of this superior strain sensitivity is that the ferroelectric performance of potassium niobate can be remarkably enhanced even surpassing those of lead titanate or lead zirconate titanate which are considered to be industrial standard levels of ferroelectricity for device applications."
Demonstrating the strain tuning of potassium niobate is particularly noteworthy, Hazra said, because potassium niobate is lead-free. While lead raises human toxicity and environmental concerns, the best ferroelectric materials - such as lead titanate and lead zirconate titanate - tend to include lead. Without strain tuning, potassium niobate's ferroelectric properties tend not to be as strong as its lead counterparts, but Hazra said the current study shows the potential of potassium niobate as a strong, yet environmentally friendly and safe, ferroelectric material.
According to Hazra, 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.
"In our work, we've shown that applying strain can increase the temperature at which the material loses its ferroelectric properties," Gopalan said. "What's even more impressive is that with just a 1% strain, we can push that temperature to over 975 degrees Kelvin, which is close to the point where the material starts to degrade."
Next, the researchers need to overcome what they called a "serious hurdle" for practical applications: growing these thin films on silicon, which is widely used in the electronics industry. Gopalan's team is also working on improving the electrical properties of the material by fine-tuning the film growth process. This would enable the use of strain-tuned potassium niobate in practical devices, such as high-temperature memory storage for space exploration, quantum computing and more environmentally friendly high-tech devices.
"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.
Research Report:Colossal Strain Tuning of Ferroelectric Transitions in KNbO3 Thin Films
Related Links
Materials Research Institute
Understanding Time and Space
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