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Two-dimensional nanomaterials only a few atoms thick are being explored for a range of critical applications in biomedicine, electronics, nanodevices, energy storage and other areas, especially to enhance performance in extreme environments and ultra-demanding conditions.

But maintaining the order and stability that is vital for more widespread and predictably reliable nanomaterial applications is finicky; matter can exhibit unusual physical and chemical behavior at the nanoscale. That same quirky behavior, when understood and corralled, can provide many benefits through the ability to tailor material structure at extremely small scales to achieve customizable properties and performance capabilities.

Babak Anasori is the Reilly Rising Star Associate Professor of Materials and Mechanical Engineering at Purdue University. His research group studies the family of 2D materials known as MXenes (pronounced "max-eens"), which were discovered in 2011 and have since become the largest known family of 2D nanomaterials.

MXenes are 2D carbides and nitrides—imagine materials like titanium carbide or tungsten carbide but in ultrathin 1-nanometer sheets, which is about 100,000 times thinner than a human hair. Every nanometer sheet is made of only a few layers of atoms. Their layered construction offers a combination of properties—such as high electrical conductivity, hydrophilicity (readily soluble), compositional tunability and novel functionality—that make them ideal building blocks for a variety of uses in technology.

In Anasori's recent paper, "Order to Disorder Transition Due to Entropy in Layered and 2D Carbides," in Science, the limits for the construction of these ultrathin materials were tested.

Anasori and collaborators from Vanderbilt University; the University of Pennsylvania; Drexel University; Argonne National Laboratory; and the Institute of Microelectronics and Photonics in Warsaw, Poland, were able to put up to nine transition metals from the periodic table into a single 2D sheet of MXene, which is a major advancement in the synthesis of "high-entropy" MXenes.

By completing this complicated task, they were able to evaluate the true role of entropy (a measure of disorder or randomness in a system) versus enthalpy (the chemical preference for order) in these high-entropy materials, as it is critical to the successful design and implementation of nanomaterials in use cases.

The impact of this study goes beyond the design of a few high-entropy 2D phases. In their study, the research team designed, discovered and characterized nearly 40 different layered materials with varying numbers of metal combinations, two, four, five, all the way to nine metals.

"These are new layered carbides, basically new atomic sandwiches. A fascinating aspect is how atoms are arranged in these sandwiches," Anasori said. "Imagine making cheeseburgers with two to nine ingredients (layers). Imagine you put all the ingredients, including the beef patty, cheese, lettuce, tomato, pickles and buns into a magic box and give it a shake (providing a source of energy).

"When you open the box, a cheeseburger assembles itself into a nice sandwich. Even more fascinating is that every time you do it, the magic box always puts the layers in a set order. For example, the patty is always below the veggies.

"This is what happens to our phases when we use two to six metals; the resulting structures show a set order of atomic arrangements (enthalpic preference). However, if we add one or more ingredients, for example, making it a double or triple, or adding bacon or onion, then the magic box can only make the sandwich, but the layering is different each time.

"Similarly, when we use seven or more metals, the metals do not follow any preference for order, and true disorder (high entropy) is achieved. Our magic here is thermodynamics, and the box is a high-temperature furnace (1,600°C or approximately 3,000°F)."

His lab team first synthesized nearly 40 known and novel nanolayered structures of MAX phases, which are the "parent material" from which the MXenes are derived, with their structural covalent-metallic-covalent carbide interfaces. This was a critical step: Transforming all these MAX phases into 2D MXenes, they showed the effects of order versus disorder on their surface properties and electronic behavior—key to their potential suitability for a host of applications.

Brian Wyatt, a postdoctoral researcher in Anasori's lab and the first author on this article, believes in the importance of this work to the general scientific community.

"This study indicates that short-range ordering—the arrangement of atoms over a short distance of a few atomic diameters—in high-entropy materials determines the impact of entropy versus enthalpy on their structures and properties," Wyatt said.

"For the broad scientific community, this work represents major progress in understanding the role of enthalpy and entropy in the formation and order-disorder transitions in these high-entropy materials. Within layered ceramics and 2D material research, this expands the families of these materials and their potential applications."

That aligns closely with the central thrust of Anasori's lab—to discover entirely new MXene phases and related nanomaterials that have never existed before. The lab investigates how to harness the thermodynamics and kinetics of reactions to design novel structures with tailored properties.

It concentrates on developing materials capable of operating in extreme environments such as ultrahigh temperatures and radiation: Examples are in the design of structures that can interact with and shield electromagnetic waves, or that can serve as highly efficient, ultrathin antennae for next-generation communication technologies.

"We want to continue pushing the boundaries of what materials can do, especially in extreme environments where current materials fall short," Anasori said. "The ultimate objective is to create materials that can outperform anything currently known to humanity in these demanding conditions.

"Whether it is enabling clean energy, or longer EV range in extreme cold or extreme heat in aerospace, or crafting materials that function in space or deep-sea conditions, I hope our work can help enable the next generation of technologies."

In that next generation, he said, "Materials discovery will play a major role, where we can still ask the 'why not' questions—for example, 'Why not put atoms in a different form or combination to make novel materials with certain unique and outstanding properties?'"