Distorting the atomic arrangement of materials could lead to cleaner, smarter devices

What’s the best way to precisely manipulate a material’s properties to the desired state? According to a team led by researchers at Penn State University, it may be putting stress on the material’s very atoms. Researchers have discovered that by “spray painting” atoms of potassium niobate, a material used in advanced electronics, they can tailor the resulting thin film with exquisite control.

The discovery, published in the journal Advanced Materials, could accelerate green advances in consumer electronics, medical devices and quantum computing, the researchers said.

This process, called strain tuning, changes the properties of the material by stretching or compressing the atoms. The researchers are using molecular beam epitaxy (MBE), a technique that deposits layers of atoms onto a substrate to form thin films. In this case, they produced thin films of strain-tuned potassium niobate.

“This was the first time that MBE was used to grow potassium niobate,” Gopalan said. “This technique is like spray painting atoms onto a surface.”

According to the researchers, the new MBE technique itself creates the strains needed to tune the material.

“This method allows the atoms in the thin film to adapt to the structure of the underlying material, creating a strain,” said co-author Sankalpa Hazra, a doctoral candidate in materials science and engineering.

“Even a small elongation of about 1% can create a pressure that cannot be achieved just by pulling or pushing the material from the outside. This pressure can significantly improve the behavior of the material from a ferroelectric perspective. .”

Potassium niobate is a ferroelectric, a type of material that has a natural charge that can be reversed by applying an external electric field, just as a magnet has a magnetic field that can be reversed.

“Ferroelectrics are essentially like mini-batteries that are already permanently charged,” says Venkatraman “Venkat,” a professor of materials science and engineering at Penn State University and corresponding author of the study.・Mr. Gopalan said.

“Although not a household name, ferroelectrics are used throughout major technologies that we take for granted in our daily lives. For example, the Internet uses electrical signals to convert optical signals into , which relies on converting to This is done by ferroelectric crystals. These materials can reverse their electrical polarity when exposed to an external electric field, making it a precision instrument for ultrasound equipment, infrared cameras, and advanced machinery. This property is also essential for devices such as actuators. ”

To “spray paint” the potassium niobate in the study, Gopalan turned to Darrell Schlom, a former colleague at Penn State. Schlom is currently a Tisch Professor in the Department of Materials Science and Engineering at Cornell University. They grew the films at the Platform for Accelerated Realization, Analysis, and Discovery of Interfacial Materials (PARADIM) thin film growth facility, which Schlom co-directs at Cornell University. Schlom said both he and Gopalan worked on the first-ever strain tuning of ferroelectric materials about 15 years ago at Penn State.

“Our role was to help Venkat and Sankalpa realize this material they had dreamed of for decades,” Shlom said. “Venkat created thin films of this material during his doctoral research at Penn State, so he knows how difficult it is to grow them. In this research, my student Tobias Schweigert and I helped grow this material.”

Schlom explained that strain engineering works by layering two materials with slightly different sizes. Imagine atoms landing on a surface made up of the same type of atoms but with slightly different spacing. If the added layer is thin enough, it will stretch or compress slightly to fit the surface beneath it.

Small changes in spacing cause the material to strain, much like a rubber band stretches when pulled. This strain is controlled by the size and spacing of atoms on the surface and leads to changes in the material’s properties, such as increased temperature limits and improved ferroelectric performance.

“Compared to other ferroelectrics, potassium niobate’s superior strain-polarization coupling strength provides a unique opportunity to significantly tune both the ferroelectric structure and its polarization with relatively small amounts of strength. ,” Hazra said.

“A key consequence of this superior strain sensitivity is that potassium niobate’s ferroelectric performance exceeds even that of lead titanate and lead zirconate titanate, which are considered the industry standard level of ferroelectricity for device applications. It will be significantly strengthened.”

The demonstration of strain control in potassium niobate is particularly noteworthy because potassium niobate does not contain lead, Hazra said. Although lead poses human toxicity and environmental concerns, the best ferroelectric materials, such as lead titanate and lead zirconate titanate, tend to contain lead.

Without strain conditioning, potassium niobate’s ferroelectric properties tend not to be as strong as its lead counterpart, but Hazra said this study shows that potassium niobate is a powerful yet environmentally friendly and safe ferroelectric material. He said that this shows its potential as a material.

Hazra said the research team also found that the ferroelectric performance of the strain-tuned potassium niobate was stable at high temperatures. Ferroelectric materials typically lose their polarization when heated, making them unable to switch charges.

“Our study showed that adding strain can increase the temperature at which the material loses its ferroelectricity,” Gopalan said. “What’s even more amazing is that with just 1% strain, we can increase the temperature to over 975 Kelvin, which is close to the point where the material begins to degrade.”

Next, the researchers need to grow these thin films on silicon, which is widely used in the electronics industry, in what the researchers call a “significant hurdle” for practical application. Gopalan’s team is also working to improve the electrical properties of materials by fine-tuning the film growth process. This allows strain-tuned potassium niobate to be used in practical devices such as high-temperature memory storage for space exploration, quantum computing, and greener high-tech devices.

“With further development, this new version of the material could become a central player in the next generation of green, high-performance technologies that will impact everything from our personal devices to space exploration.” Yes,” Gopalan said.