Programming twist angle and strain profiles in 2D materials

Moiré superlattices in twisted two-dimensional materials have generated tremendous excitement as a platform for achieving quantum properties on demand. However, the moiré pattern is highly sensitive to the interlayer atomic registry, and current assembly techniques suffer from imprecise control of the average twist angle, spatial inhomogeneity in the local twist angle, and distortions caused by random strain. We manipulated the moiré patterns in hetero- and homobilayers through in-plane bending of monolayer ribbons, using the tip of an atomic force microscope. This technique achieves continuous variation of twist angles with improved twist-angle homogeneity and reduced random strain, resulting in moiré patterns with tunable wavelength and ultralow disorder. Our results may enable detailed studies of ultralow-disorder moiré systems and the realization of precise strain-engineered devices. Description Editor’s summary Moiré superlattices formed by stacking single-layer materials in a twisted configuration can host many exotic states. However, such structures have been found to be disordered, with the twist angle and strain varying unpredictably within a single sample. Kapfer et al. found an elegant method for controlling the twist angle and decreasing the disorder (see the Perspective by Park). The researchers placed a ribbon-shaped graphene layer on top of hexagonal boron nitride and bent one end of the ribbon using the tip of an atomic force microscope. The resulting structure had a twist angle that increased continuously from the point at which the ribbon started bending to its end. Such control is expected to improve reproducibility and understanding of this class of materials. —Jelena Stajic The tip of an atomic force microscope was used to control the twist angle of a moiré superlattice.