Ultrathin ferroelectrics have great potential in the creation of non-volatile memory devices with compact volume and low energy consumption. Different from the conventional top-down approach of thinning down bulk polar materials, the bottom-up approach based on van der Waals assembly can engineer 2D ferroelectrics out of non-ferroelectric parent compounds.
By cutting one monolayer boron nitride (BN) in half and stacking them in parallel, we engineer an inversion-symmetry broken bilayer BN that hosts out-of-plane polarization. This polarization can be switched by an out-of-plane electric field through the in-plane sliding motion between the BN layers[1]. We further generalize this sliding ferroelectricity concept to bilayer transition metal dichalcogenides[2]. The BN sliding ferroelectric has ultrafast switching speed and high endurance, comparable to state-of-the-art ferroelectric field effect transistors [3]. Besides real space ferroelectricity engineering, we demonstrate the ability to engineer material band structures in the reciprocal space. When a small twist angle is introduced to the ferroelectric BN bilayer, the staggered out-of-plane polarizations in twisted BN constitute a moiré ferroelectric substrate that can modify band structures of the target material which senses the moiré potential [4].
[1] K. Yasuda et al., Science 372, 1458–1462 (2021).
[2] X. Wang et al., Nat. Nano. 17, 367–371 (2022).
[3] K. Yasuda et al., Science 385, 53-56 (2024).
[4] X. Wang et al., arXiv : 2405.03761 (2024).