A 2D Van Der Waals Molecular Ferroelectric.

2D van der Waals (vdW) ferroelectrics have aroused great interest for their novel structures and functionalities in many applications such as in-memory computing and ferroelectric detectors. However, the documented 2D ferroelectrics remain very rare, mainly limited to inorganic ones represented by transition metal thio/selenophosphates. Constructing new 2D vdW ferroelectric categories may bring interesting phenomena, but it has always been a challenge. Here, a molecular 2D vdW ferroelectric sodium 2,2,3,3-tetrafluoropropionate (2,2,3,3-TFPS) is successfully designed through H/F substitution, which adopts a unique stable layered structure where each coordination bonded molecule-based 2,2,3,3-TFPS monolayers knitted together by weak vdW forces between protruding (-CHF2) cantilevers in the adjacent two monolayers. More strikingly, it can show robust room-temperature ferroelectricity even in a mechanically exfoliated ultrathin flake as thin as 9 nm, which has never been achieved in traditional molecular ferroelectrics with ferroelectricity mainly in the bulk form and micron-scale thin film. To the best of the knowledge, 2,2,3,3-TFPS is the first molecular 2D vdWs ferroelectric showing robust ferroelectricity in sub-10 nm thickness. This work expands the diversity of 2D vdW ferroelectrics and sheds light on the exploration of molecular 2D vdW ferroelectrics with promising low-dimensional ferroelectricity for applications in nanoscale devices.