Epitaxially ferroelectric hexagonal boron nitride on graphene

The only insulative two-dimensional (2D) material, hexagonal boron nitride (h-BN) plays an important role in 2D electronics. The insulative and atomically flat surface make it become the most used substrate and gate dielectric material. In recent year, h-BN is not just serving as supporting role in 2D electronics, more and more exotic quantum phenomenon was found in h-BN. In this study, we developed layer-controllable h-BN films on single-crystal graphene synthesized on a SiC (0001) substrate using nitrogen plasma-assisted molecular beam epitaxy (PA-MBE). Systematic angle-resolved photoemission spectroscopy (ARPES) studies and first-principles calculations revealed that the epitaxial mono-, bi-, and tri-layer h-BN films exhibit layer-number-dependent band dispersions due to an AB stacking sequence on a Bernal-stacked graphene substrate. Furthermore, our piezoelectric force microscopy (PFM) confirmed the coexistence of robust moiré and sliding ferroelectricity at the well-aligned h-BN/graphene heterojunction and in multilayer h-BN films, respectively. Ferroelectricity in two-dimensional (2D) van der Waals (vdW) layers has revolutionized our understanding of the vdW layered coupling, and has been introduced in the domains of moiré superlattice patterns generated by interlayer twisting or sliding. In the smallest limit of thickness, untwisted and epitaxial vdW stacking layers exhibiting ferroelectricity would clearly serve as building blocks for realizing 2D devices with nonvolatile and reconfigurable functionalities.