Researchers have demonstrated the ability to use van der Waals forces to tune the physical and electronic properties of ferroelectric thin films. The work opens the door to new techniques for engineering materials for use in smaller, more energy efficient electronic devices.

“Epitaxy is when you deposit a crystalline layer of material on top of another crystalline layer of material,” says Yin Liu, co-corresponding author of a paper on the work and an assistant professor of materials science and engineering at North Carolina State University. “When you are doing this with thin films, and the two layers are chemically bonded, the structure of those two layers have to match. However, if the two layers are bonded using van der Waals forces, the layers can have different orientations.

“We wanted to see whether the relative strength or weakness of the van der Waals force influences the physical and electrical properties of an epitaxial material.”

For this work, the researchers deposited a thin film of tin selenide (SnSe) on a monolayer of molybdenum disulfide (MoS₂). The researchers chose SnSe because it is widely studied in ferroelectric research; and MoS₂ because it is a material that can be easily integrated into electronic devices.

“We also chose these materials because MoS₂ has very close lattice matching with SnSe – meaning the crystalline structures are closely aligned,” says Liu. “This close matching means the van der Waals force is relatively strong. Previous work has looked at the properties of SnSe thin films deposited on other substrates, such as graphene, which had weaker van der Waals forces between the substrate and the SnSe. We wanted to see how the difference in van der Waals forces affected the epitaxial materials’ structure and properties.”

The researchers found that a combination of the lattice matching and strength of the van der Waals force affects three of the epitaxial material’s properties: its thickness, its strain state, and its domain architecture. Thickness refers to the number of crystalline layers in the material. The material’s strain state refers to how the material is stretched or compressed at the atomic level. And domain architecture refers to the polarization of discrete sections within the material.

“All of these characteristics have an effect on the material’s physical and electronic properties,” says Liu.

“Ultimately, this finding suggests we need to take a closer look at the role van der Waals force plays in influencing the properties of ferroelectric thin films, because it could be a valuable tool for engineering materials for a variety of applications.”

The researchers also found that using a monolayer of MoS₂ as a substrate allowed them to grow larger thin films of SnSe, in terms of lateral size or surface area, than has been demonstrated on previous substrates. And the thin films were higher quality, with fewer defects.

“This substrate is promising, and merits future investigation,” Liu says.

The paper, “Heteroepitaxial control of thickness, strain, and domain architecture in few-layer ferroelectric tin monochalcogenides” is published in the journal ACS Nano. First author of the paper is Yueyin Wang, a Ph.D. student at NC State. Co-corresponding author of the paper is Honggyu Kim of the University of Florida. Co-authors of the paper include Hana Jones, David Szympruch, Reza Ghanbari, Yusen Pei, Guanyue Chen, Franky So, Dali Sun and Ruijuan Xu of NC State; Garrett Baucom of UF; Pochun Hsieh, Albert Suceava and Venkatraman Gopalan of Pennyslvania State University; Tao Zhou, Tugba Isik and Rui Liu of Argonne National Laboratory; and Haoye Sun of Texas A&M University.

This work was done with support from the National Science Foundation under grants 2340751, 2338558, 2442399 and 2143642; the Department of Energy under grants DE-AC02-06CH11357 and DE-SC0021118; and the American Chemical Society Petroleum Research Fund under award 68244-DNI10.