Ferroelectric order in van der Waals layered materials

Structurally different from conventional oxide ferroelectrics with rigid lattices, van der Waals (vdW) ferroelectrics have stable layered structures with a combination of strong intralayer and weak interlayer forces. These special atomic arrangements, in combination with the ferroelectric order, give rise to fundamentally new phenomena and functionalities, including downscaling limits, origin of the polarization and switching mechanisms. Furthermore, their easily stackable nature means that vdW ferroelectrics are readily integrable with highly dissimilar materials, such as industrial silicon substrates, without interfacial issues, and are thus regarded as attractive building blocks for post-Moore’s law electronics. In this Review, we consider the experimentally verified vdW ferroelectric systems by discussing their unique characteristics, such as quadruple-well potentials, metallic ferroelectricity and dipole-locking effects. We highlight the emerging field of engineered vdW ferroelectricity, created by artificially breaking centrosymmetry in stacks of otherwise nonpolar parent materials. Additionally, innovative device applications harnessing vdW ferroelectricity are showcased, including transistors able to beat the Boltzmann tyranny, nonvolatile memories and optoelectronic and flexible devices. Recent progress and existing challenges provide a perspective on future research directions and applications.