Band structure engineering of 2D materials using designer superlattice potentials

Abstract

Recent advances in precise van der Waals (vdW) assembly have enabled construction of superlattices based on two-dimensional (2D) materials to tailor the quantum properties of the constituent. It is either formed by moiré structure of two dissimilar or same materials, or by periodical perturbation of exterior structure, such as strain, intercalated compounds or lithographic engineered pattern. Thanks to the absence of dangling bonds, at the exfoliated surface of 2D materials, interfacial interactions in these vdW structures become strong enough to engineer original electronic band structure. Different types of superlattices have been realized with each having unique and new properties. Examples include, but not limited to, hexagonal boron nitride (hBN)/graphene moiré system, magic-angle twisted bilayer graphene, and graphene on patterned dielectric substrate.

In this work, we explore new types of superlattice structure and their effects on band structures of graphene. Firstly, we prompt an optical method to precisely determine the layer number of hBN. By combining a statistic optical contrast method under commercial optical microscope and second harmonic generation, the layer number of few-layer hBN can be unambiguously determined up to four layers, which is very important to fabricate hBN superlattice in different stacking configuration. Based on this method, we show that an aligned interface between two hBN flakes can be used not only to induce moiré potential at the target material on top but also to create an unconventional ferroelectric effect. Using graphene as the target material, we show that near 1° twisted hBN interface can induce corresponding moiré potential enough to engineer the band structures of graphene, with the moiré wavelength exceeding the maximum value in graphene-hBN moiré system. From electric transport measurement, we have clearly shown the multiple satellite Dirac peaks, Hofstadter butterfly effect and Brown-Zak oscillation. On the other hand, by twisting two bilayer hBN at 60°, we found a conventional sliding ferroelectricity (FE). More interestingly, in one of the samples with double bilayer hBN, we clearly observed an unconventional ferroelectric behavior, exhibiting resistance hysteresis in both top and back gate voltage sweep and a novel electronic ratchet effect.

In summary, by studying electrical transport through graphene, our studies have revealed rich new phenomena emerging at the insulating hBN interface. Since the graphene is only used as a detector, the study can be extended easily to other 2D materials. This broader exploration promises to yield further insights into the intricate interplay of electronic properties within layered structures, potentially unlocking novel avenues in moiré physics for technological innovation and fundamental scientific inquiry.