  2D ferroelectrics with robust polarization down to atomic thicknesses provide
novel building blocks for functional heterostructures. Experimental reports,
however, remain scarce because of the requirement of a layered polar crystal.
Here, we demonstrate a rational design approach to engineering 2D
ferroelectrics from a non-ferroelectric parent compound via employing van der
Waals assembly. Parallel-stacked bilayer boron nitride is shown to exhibit
out-of-plane electric polarization that reverses depending on the stacking
order. The polarization switching is probed via the resistance of an
adjacently-stacked graphene sheet. Furthermore, twisting the boron nitride
sheets by a small-angle changes the dynamics of switching due to the formation
of moir\'e ferroelectricity with staggered polarization. The ferroelectricity
persists to room temperature while keeping the high mobility of graphene,
paving the way for potential ultrathin nonvolatile memory applications.
