Discovery of polarization in metals points to new opportunities in electronic devices and energy applications

Researchers at the University of Minnesota Twin Cities have uncovered a powerful new way to modulate the surface work function of a metal by inducing interfacial polarization. They have demonstrated that interfacial polarization can tune the surface work function of metallic ruthenium dioxide (RuO₂) by more than 1 eV simply by adjusting film thickness at the nanometer scale.

The details of the study are published in Nature Communications in a paper titled, “Strain-Stabilized Interfacial Polarization Tunes Work Function Over 1 eV in RuO₂/TiO₂Heterostructures.” 

Typically induced through means such as strain engineering, chemical modification, or structural asymmetry, interfacial polarization is a tool applied only to insulating or semiconducting oxides. Such polarization offers functionalities that have resulted in applications ranging from antennas to computers to spectroscopy technology. However, the emergence of polar metals has indicated that polar distortions can exist in metals under specific conditions, suggesting a broader design space for such materials. 

To explore the presence of interfacial polarization, the research team used hybrid molecular beam epitaxy (MBE) to synthesize a heterostructure of TiO₂/RuO₂/TiO₂ where the RuO₂ layer had a thickness of less than 4 nm. Using epitaxial strain and structural distortions at the atomic level at the interface RuO₂ and TiO₂, the scientists induced polarization within the conducting metal. The team made two key observations: interfacial polarization in metallic systems can be manipulated by epitaxial strain and the type of engineered material/heterostructure, and this polarization can impact the material’s work function and electronic transport properties. 

The observed modulation of work function by over 1 eV is significant. Work function, which is the energy required for an electron to move from a material surface is a key criterion for applications such as electronic devices and energy technologies. The tunability of the work function is significant as it offers a strategy that could lead to the design of materials whose electronic and structural responses can be controlled, significant for the development of oxide electronics, spintronics, and energy applications. 

Addressing the significance of their research, co-author of the paper and Paul Palmberg Professor of Electrical and Computer Engineering Tony Low says: “This study challenges the long-standing assumption that polarization and metallicity are incompatible. Our calculations show how symmetry, strain, and interface design can allow these effects to coexist. It provides a new platform for engineering quantum and electronic materials.”

Besides Low, the research team comprised members from the Department of Chemical Engineering and Materials Science at the University of Minnesota Twin Cities (including corresponding author Professor and Shell Chair Bharat Jalan; corresponding and first author Seung Gyo Jeong who is a researcher in Jalan’s group), Massachusetts Institute of Technology, Texas A&M University, Gwangyu Institute of Science and Technology and the School of Physics at the University of Minnesota Twin Cities.

Funding information: The research was funded by the U.S. Department of Energy and the Air Force Office of Scientific Research.

Read the complete paper at the Nature Communications website.