The energy system of the future will require sustainable catalysts that, for example, enable the efficient production of green hydrogen. Materials consisting of mixtures of five chemical elements show great promise for enabling ideal catalysts in the future. Researchers from DFG Collaborative Research Center CRC 1625, “Atomic-Scale Understanding and Design of Multifunctional Compositionally Complex Solid Solution Surfaces”, Germany, have succeeded in growing so-called complex solid solutions in a nearly single-crystal state on a sapphire wafer and examining them in great detail using four different microscopy methods. They report their findings the journal Materials Horizons from July 14, 2026.

To produce complex solid solutions, the researchers use a combinatorial coating system (co-sputtering) that fires atoms of the five selected source elements onto a wafer from five different directions. “There, they deposit randomly and form chemically disordered nanocrystals with countless interfaces,” explains Professor Alfred Ludwig of Ruhr University Bochum, spokesperson of CRC 1625. “While this is beneficial for applications, it also poses a challenge when it comes to studying these new materials in basic research at resolutions as high as the atomic level. However, to achieve this is important for their future targeted design.”

To maintain the mixture of the five atomic components while introducing more structural order into the materials, Dr. Satykam Kar from the Bochum research group chose a sapphire single-crystal wafer as a base, which was coated with an extremely thin layer of platinum as a mediator. "As a result, the incident atoms deposit themselves in such a way that they extend the crystal structure of the substrate over micrometer-scale areas," Ludwig explains, describing the process known as epitaxial layer growth. This results in a very well-defined crystalline surface that is extremely smooth, thereby enabling high-resolution analyses. "For this to work, the sputter deposition process must take place at high temperatures ranging from 400 to 600 degrees Celsius; the rate of incoming atoms must be neither too high nor too low; and the correct intermediate layer between the single-crystal substrate and the actual layer must be selected," explains Satyam Kar, who now works as an assistant professor at the Indian Institute of Technology Gandhinagar.

The research team tested the method on a mixture of iridium, palladium, platinum, rhodium, and ruthenium. They used the resulting layers as an experimental platform: Through microscopic analysis, they observed that, despite the high degree of structural order, the five starting elements had deposited in a highly chemically mixed state. "What we can also see is that, during the film growth, two different orientations form, between which there are also interfaces—but far fewer and better defined than in the case of nanocrystals" says Ludwig. Using a tiny diamond tip, the researchers made markings on the wafer so they could use them as reference points during further microscopic examinations, locate the various areas, and compare them with one another. Nanoelectrochemical analyses in the group of Professor Corina Andronescu (University Duisburg-Essen) showed, that of the one of the orientations is electrochemically advantageous.

Further investigations in the group of Professor Christina Scheu of Max-Planck-Institute for Sustainable Materials showed, that the epitaxial films feature an interesting defect structure. In some areas, the crystals continue to grow perfectly in accordance with the sapphire substrate. In other areas, however, stacking structures form that are visible under a transmission electron microscope as transverse stripes. “Future work will show what significance these defects have for the desired properties of catalysts,” says Alfred Ludwig.