Cornell researchers have developed a new way to create moir\u00e9 patterns \u2013 atomic-scale structures that can give materials unusual quantum behaviors \u2013 without relying on the difficult-to-control twisting and stacking methods traditionally used.<\/p>\n
Since the 2018 discovery that slightly twisted layers of graphene can exhibit superconductivity, moir\u00e9 materials have become of great interest to researchers. Moir\u00e9 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.<\/p>\n
Microscopy of molybdenum disulfide showing how controlled strain applied above the dotted line with a patterned thin film generated different moir\u00e9 geometries across the material. Above the dotted line, biaxial strain generated distorted hexagonal patterns, while uniaxial strain below the line formed stripe-like domains, resulting in localized electric polarization.<\/p>\n
But engineering those moir\u00e9 patterns has largely depended on manually rotating and stacking 2D flakes of materials, a process with limited reproducibility and scalability.<\/p>\n
A new type of moir\u00e9 engineering, published May 27<\/a> 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 moir\u00e9 superlattices across the material. The approach offers a predictable and scalable path for creating quantum materials using common fabrication techniques.<\/p>\n \u201cStrain engineering is already a standard part of semiconductor manufacturing,\u201d said\u00a0Judy Cha<\/a>, the Rick and Betty Tsai Ph.D. 1981 Professor in Materials Science and Engineering in the Cornell Duffield College of Engineering. \u201cFor decades, companies have used approaches like silicon-germanium alloys and stressed metal coatings to deliberately strain silicon and boost transistor performance.\u201d<\/p>\n Cha said the inspiration to use this technique to generate moir\u00e9 effects came after researchers demonstrated that metal stressor films could strain 2D materials. She realized the approach might generate moir\u00e9 patterns because the upper atomic layers would deform differently from the layers beneath them.<\/p>\n 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 moir\u00e9 geometries.<\/p>\n The different strain environments generated localized electric polarization in molybdenum disulfide, a material that is normally nonpolar. Along the boundaries between moir\u00e9 domains, tiny shifts in atomic registry created in-plane polarization textures whose orientation depended on the underlying strain geometry.<\/p>\n The researchers are now exploring whether the polar domains can be incorporated into functioning electronic devices.<\/p>\n \u201cBecause the polarization can potentially be switched with an electric field, the effect could eventually be used to tune electrical resistance at the nanoscale,\u201d said Cha, who is also the Lester B. Knight Director of the\u00a0Cornell NanoScale Facility<\/a>, adding that the work could lower the barrier for researchers wanting to study moir\u00e9 physics.<\/p>\n \u201cThis is a very standard lithography step and everybody who makes devices does this every day,\u201d Cha said. \u201cI hope that this opens the door so that people who have not really done extensive stacking to make moir\u00e9 potentials can explore this approach.\u201d<\/p>\n