Scientists at the U.S. Department of Energy's Argonne National Laboratory have achieved atom-by-atom control over MXenes, a promising class of two-dimensional materials that could transform energy storage, electronics, biomedicine, and space systems. The breakthrough, detailed in two recent publications, shows researchers can precisely dictate which atoms form the material, where those atoms sit within its structure, and what chemical groups attach to its ultra-thin surfaces.
This granular control matters enormously because MXenes—pronounced "max-eens"—are sheet-like materials typically just a few atoms thick, made from transition metals like titanium, vanadium, or molybdenum bonded to carbon and nitrogen. Unlike graphene, which consists only of carbon, MXenes can be engineered from many different elemental combinations, giving scientists far more flexibility to tune performance for specific applications. "I like to imagine MAX phases as a textbook with all the pages glued shut and MXenes as a single page you want to extract," explained Brian Wyatt, a Maria Goeppert Mayer Fellow at Argonne. "You have to dissolve the glue and coax that page out."
The research team pushed the boundaries of what's possible by creating 40 different MAX phase compositions—the layered precursors from which MXenes are derived—nearly doubling the known chemical space available for making these materials. Some of these new MAX phases contained as many as nine different metals in a single structure, letting the team explore a fundamental question: how many different elements can fit into one material before atoms stop arranging themselves in an orderly pattern?
The answer proved striking. Atomic order persists when a material contains up to six different metals, but at seven or more metals, the pattern breaks down into true disorder. This transition from organization to randomness is governed by entropy—nature's tendency toward chaos. "This is where entropy, the natural tendency toward randomness, wins," Wyatt said. "Nature likes some kinds of order, but once we add enough different ingredients, it becomes too hard for the atoms to stay organized."
Computational models had predicted that MAX phases with seven, eight, or nine metals should be unstable and essentially impossible to create. The Argonne team proved otherwise. Using secondary ion mass spectrometry, a technique that measures a material's composition layer by layer, they demonstrated that highly complex, multi-metal MAX phases can actually be synthesized and stabilized by entropy itself. "That's a key finding," noted Argonne materials scientist Sixbert Muhoza, "it means entropy can enable materials that were thought to be impractical or unstable."
This discovery carries real implications for how MXenes perform. When MAX phases transform into MXenes, atoms and chemical groups from the surrounding solution attach to the newly exposed surfaces. These surface groups critically influence electrical conductivity, energy storage capacity, and catalytic ability—determining whether a material works for a given application. By understanding how atomic order shapes surface chemistry, scientists can now engineer MXenes far more precisely for specific jobs.
The work represents a major step forward in rational materials design. Rather than stumbling upon useful materials by chance, researchers can now navigate a vastly expanded chemical landscape with intention, selecting exact compositions and structures to match the demands of real-world technology. That controlled, deliberate approach to crafting matter—building it almost atom by atom—opens doors to materials that solve problems we haven't yet imagined.