By keeping the ferroelectric “bubbles” intact, re


Special ferroelectric characteristics hold promise for microelectronic and energy applications.

When a magician suddenly removes a tablecloth from a table laden with plates and glasses, there is a moment of suspense as audiences wonder if the stage will soon be littered with broken glass. Until now, a similar dilemma has faced scientists working with special electric bubbles to create the next generation of flexible microelectronic and energy storage devices.

Scientists at the US Department of Energy’s (DOE) Argonne National Laboratory have discovered a new way to achieve an atomic-scale version of the water table trick by peeling off heterostructured thin films containing electric bubbles of a particular underlying material, or substrate, while keeping them fully intact. The discovery may bring us closer to a multitude of applications that rely on these unusual and fragile structures.

“You can think of it as trying to take a house off its foundation. Normally you would think the house would collapse, but we found that it retained all of its properties. – Saidur Bakaul, materials scientist from Argonne

“Bubbles are very fragile and initially need special underlying materials, called substrates, and specific conditions in order to develop films with them,” said Argonne materials scientist Saidur Bakaul. “There are many materials that interest us for which these bubbles could be extremely useful, such as plastics. However, we have not been able to grow them directly on these materials. Our research is the first step in making bubbles possible there. “

The electric bubbles are found in an ultrathin three-layer structure with alternating electric properties: ferroelectric, then dielectric, then ferroelectric again. The bubbles of this multi-layered structure are made up of specially ordered dipoles or twinned electrical charges. The orientation of these dipoles is based on the local stress in the material and the surface charges that cause the dipoles to seek their relative lowest energy state. Eventually, electric bubbles (bubble domains) form, but only when certain conditions are met. They are also easily deformed by even weak forces.

In the experiment, Bakaul’s colleagues at the University of New South Wales first grew the bubbles in an ultra-thin heterostructured film on a substrate of strontium titanate, one of the easiest materials to grow. use to create them. Next, Bakaul took on the challenge of removing the heterostructure from the substrate while retaining the bubbles. “You can think of it as trying to take a house off its foundation,” he said. “Normally you would think the house would collapse, but we found that it retained all of its properties. “

The bubble domains are tiny. They only have a radius of about 4 nanometers, which is as wide as a strand of human DNA. Therefore, they are difficult to see. In Argonne’s Materials Science division, advanced scanning probe microscopy techniques with Fourier transform analysis allow scientists not only to see them, but also to quantify their properties in self-supporting films.

To establish that the bubble domains remained intact, Bakaul measured their electronic (capacitance) and piezoelectric properties using two microscopy techniques: scanning microwave impedance microscopy and piezoelectric response force microscopy. . If the bubbles had disintegrated, the capacitance would have changed under applied voltage, but Bakaul saw that it remained relatively stable up to a fairly high voltage.

These experiments validated numerical capacitance estimates obtained from theoretical analyzes that Bakaul and his student developed by combining atomistic simulations with circuit theory. “The combination of experiment and simulation has conclusively proven that these bubbles are able to live even when removed from the original substrate. It was something that we had hoped to achieve for a long time, ”said Bakaul.

When the bubbles were removed, the heterostructure film – which was previously flat like a web – suddenly took on a wavy appearance. While Bakaul noted that many might assume that this change would alter the properties of the bubbles, he found that the bubbles were in fact protected by a change in the built-in tension of the materials. Atomistic simulations performed by Bakaul’s colleagues at the University of Arkansas have suggested that elastic energy at free interfaces is responsible for the formation of ripples.

The result is exciting, according to Bakaul, because these bubbles have unusual and intriguing electrical and mechanical properties. “Ferroelectric bubbles are newly discovered nanoscale objects,” he said. “There is a consensus in the community that they can have a lot of applications. For example, the transformation of these bubbles results in an unusually high electromechanical response, which can have applications in a wide range of devices in microelectronics and energy applications.

While it was physics and not magic that created a potential new path for the integration of these bubbles, Bakaul indicated that new technologies based on them could have a transformative impact. “Whether we’re talking about energy harvesters or supercomputers, these bubbles could make a big difference for many different materials and applications,” he said.

The research was funded by the Office of Basic Energy Sciences (DOE) Office of Science. The authors took advantage of the nanofabrication and cleanroom facilities at the Center for Nanoscale Materials at Argonne, a user facility of the DOE Office of Science, to prepare samples for electrical characterization.

A research-based article, “Autonomous ferroelectric bubble domains, ”Appeared in the September 19 issue of Advanced materials. Other authors of the article include Amanda Petford-Long of Argonne, as well as Sergei Prokhorenko, Yousra Nahas and Laurent Bellaiche of the University of Arkansas, as well as Qi Zhang and Nagarajan Valanoor of the University of New Wales. from the South and Yushi Hu from the University of Chicago (intern at Argonne).

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