Interface and Bending Effects on Ferroelectricity in Ultrathin CuInP₂S₆ Probed by Piezoresponse Force Microscopy

Van der Waals (vdW) ferroelectrics have attracted intense interest recently due to their potential in realizing ferroelectricity in the ultrathin limit. When the thickness of a ferroelectric vdW material becomes very thin, the competition between depolarization field, surface chemistry, and bending strain will affect the electric polarization. In addition, chemistry at the interface between the vdW material and the electrode may also modify the ferroelectric properties. In this work, we investigate the ferroelectric domains of in nanoflakes of CuInP2S6 (CIPS), a vdW material that is ferroelectric at room temperature, focusing on the effect of surface and interface conditions, as well as the vdW layer bending curvature. A vdW CIPS nanoflake is layer transferred onto monolayer graphene which is in turn placed on a SiO₂/Si substrate. To investigate the effect of electrodes on the ferroelectricity of CIPS, we also prepared a Pt electrode on the CIPS nanoflake. We performed band excitation piezoresponse force microscopy to visualize the ferroelectric domain structures and band excitation piezoresponse spectroscopy to investigate the polarization switching dynamics. We observed a large effect of the interface and the layer bending curvature on the domain structure and polarization of CIPS flakes. The importance of the interface and layer bending curvature will be explored both experimentally and theoretically.