UW Researchers Develop Novel Technology for Distinguishing Elements at Subnanometer Scale

A University of Wyoming research group has demonstrated a novel method to distinguish elements at the subnanometer scale on the surfaces of high entropy alloys.

High entropy alloys are alloys that are formed by mixing equal proportions of five or more elements randomly. High entropy alloys and materials exhibit great potential for many applications, including strong mechanical materials for heavy machines, bridges and aircraft building; high catalytic properties for important chemical reactions that tie to human society; and many uses yet to be identified and discovered.

“This is a scientific achievement for providing the first-ever tool to study high entropy materials at a subnanometer scale,” says TeYu Chien, an associate professor in the UW Department of Physics and Astronomy. “This work provides a pathway toward correlating the material properties to their local elemental arrangement at the atomic scale.”

Lauren Kim, a third-year Ph.D. student in Chien’s lab, is the lead author of a paper, titled “Distinguishing Elements at the Sub-Nanometer Scale on the Surface of a High Entropy Alloy,” that was published April 29 in Advanced Materials, a weekly, peer-reviewed scientific journal that covers materials science.

Kim, of Fairport, N.Y., conducted the experiments at the Center for Nanoscale Materials and the Advanced Photon Source at Argonne National Laboratory in Lemont, Ill. Teaming up with researchers from Argonne National Laboratory, Northwestern University and Lehigh University, the team used the synchrotron X-ray scanning tunneling microscopy -- which combines elemental sensitive X-ray adsorption spectroscopy and high spatial resolution scanning tunneling microscopy imaging -- in the Advanced Photon Source facility to achieve this goal.

“X-ray adsorption spectroscopy measurements can provide elemental fingerprints through measuring the atomic core-level energies, but it has the resolution of the X-ray beam size at about the micrometer scale,” says Chien, who is corresponding author of the paper. “On the other hand, scanning tunneling microscopy has atomic-scale resolution through quantum tunneling effects using an atomically sharp metallic tip. However, it lacks elemental sensitivity.

“Combining the two, the elemental sensitivity at the subnanometer scale imaging is achieved by a newly discovered X-ray-assisted tunneling mechanism,” he says.

Traditional alloys are synthesized by introducing additive elements into a matrix of a single principal element. Examples of traditional alloys include stainless steel, which is primarily made up of iron with some chromium, nickel and a little bit of carbon; and brass, which is mainly composed of copper with some zinc and other trace elements.

Starting in early 2000, a new concept of high entropy alloy was developed by mixing equal amounts of five elements, Chien says. High entropy alloys have been demonstrated to exhibit strong mechanical strength and high resistance to corrosion. Most recently, it has been revealed that high entropy alloys may have high catalytic, electronic and magnetic properties.

“Owing to the high entropy nature, this new type of material has shown a wide range of functionalities and tunability,” Chien says. “On the other hand, also due to the high entropy down to the atomic scale, it is very challenging to understand the relationship between the material properties and its elemental arrangements.”

The project was funded by a National Science Foundation grant.