Research progress of novel properties in several van der Waals ferroelectric materials

Ferroelectric (FE) materials possess electrically switchable spontaneous polarizations, showing broad applications in various functional devices. For the miniaturization of electronic devices, two-dimensional (2D) van der Waals (vdW) ferroelectric materials and the corresponding bulk counterparts have aroused more interest of researchers. Recently, several kinds of 2D vdW ferroelectrics have been fabricated in experiment. These 2D vdW FEs, as well as their bulk counterparts, exhibit novel properties as demonstrated in experiment or predicted in theory. This paper is to review the recent progress of novel properties of several vdW ferroelectrics. In Section II, we introduce the unusual ferroelectric property—a uniaxial quadruple potential well for Cu displacements—enabled by the van der Waals gap in copper indium thiophosphate (CuInP₂S₆). The electric field drives the Cu atoms to unidirectionally cross the vdW gaps, which is distinctively different from dipole reorientation, resulting in an unusual phenomenon that the polarization of CuInP₂S₆ aligns against the direction of the applied electric field. The potential energy landscape for Cu displacements is strongly influenced by strain, accounting for the origin of the negative piezoelectric coefficient and making CuInP₂S₆ a rare example of a uniaxial multi-well ferroelectric. In Section III, we introduce the distinct geometric evolution mechanism of the newly reported M₂Ge₂Y₆ (M = metal, X = Si, Ge, Sn, Y = S, Sn, Te) monolayers and a high throughput screening of 2D ferroelectric candidates based on this mechanism. The ferroelectricity of M₂Ge₂Y₆ originates from the vertical displacement of Ge-dimer in the same direction driven by a soft phonon mode of the centrosymmetric configuration. Another centrosymmetric configuration is also dynamically stable but higher in energy than the ferroelectric phase. The metastable centrosymmetric phase of M₂Ge₂Y₆ monolayers allows a new two-step ferroelectric switching path and may induce novel domain behaviors. In Section IV, a new concept about constructing 2D ferroelectric QL-M₂O₃/graphene heterostructure to realize monolayer-based FE tunnel junctions or potentially graphene p-n junctions is reviewed. These findings provide new perspectives of the integration of graphene with monolayer FEs, as well as related functional devices. Finally, the challenge and prospect of vdW ferroelectrics are discussed, providing some perspective for the field of ferroelectrics.