Researchers make moiré 2D materials without stacking, twisting

Cornell researchers have developed a new way to create moiré patterns - atomic-scale structures that can give materials unusual quantum behaviors - without relying on the difficult-to-control twisting and stacking methods traditionally used.

Since the 2018 discovery that slightly twisted layers of graphene can exhibit superconductivity, moiré materials have become of great interest to researchers. Moiré patterns arise when ultra-thin layers of materials are stacked slightly out of alignment, creating structural changes in atomic lattices that can alter how electrons move through a material. This can produce quantum behaviors such as correlated insulating states, magnetism and superconductivity.

But engineering those moiré patterns has largely depended on manually rotating and stacking 2D flakes of materials, a process with limited reproducibility and scalability.

A new type of moiré engineering, published May 27 in the Proceedings of the National Academy of Sciences, uses a coating of thin films to apply controlled strain to layers of molybdenum disulfide, generating moiré superlattices across the material. The approach offers a predictable and scalable path for creating quantum materials using common fabrication techniques.

"Strain engineering is already a standard part of semiconductor manufacturing," said Judy Cha, the Rick and Betty Tsai Ph.D. 1981 Professor in Materials Science and Engineering in the Cornell Duffield College of Engineering. "For decades, companies have used approaches like silicon-germanium alloys and stressed metal coatings to deliberately strain silicon and boost transistor performance."

Cha said the inspiration to use this technique to generate moiré effects came after researchers demonstrated that metal stressor films could strain 2D materials. She realized the approach might generate moiré patterns because the upper atomic layers would deform differently from the layers beneath them.

To induce that strain, the researchers deposited lithographically patterned stressor films onto molybdenum disulfide flakes. The films locally pulled and compressed the upper atomic layers, creating different strain environments across the material. Near the edges of the patterned films, the strain was primarily biaxial, while regions farther away experienced mostly uniaxial strain, producing entirely different moiré geometries.

The different strain environments generated localized electric polarization in molybdenum disulfide, a material that is normally nonpolar. Along the boundaries between moiré domains, tiny shifts in atomic registry created in-plane polarization textures whose orientation depended on the underlying strain geometry.

The researchers are now exploring whether the polar domains can be incorporated into functioning electronic devices.

"Because the polarization can potentially be switched with an electric field, the effect could eventually be used to tune electrical resistance at the nanoscale," said Cha, who is also the Lester B. Knight Director of the Cornell NanoScale Facility, adding that the work could lower the barrier for researchers wanting to study moiré physics.

"This is a very standard lithography step and everybody who makes devices does this every day," Cha said. "I hope that this opens the door so that people who have not really done extensive stacking to make moiré potentials can explore this approach."

The research was supported by the U.S. Department of Energy. Sample fabrication was performed in part at the Cornell NanoScale Facility and some microscopy characterization made use of the Cornell Center for Materials Research - both facilities are supported by the National Science Foundation.

Syl Kacapyr is associate director of marketing and communications for Duffield Engineering.