Size-effect in layered ferrielectric CuInP2S6

We report on polarization switching properties of thin flakes of van der Waals ferrielectric CuInP2S6. We observe mesoscale polarization domains, ferroelectric switching, and the Curie temperature above 299 K down to a thickness of ∼50 nm. However, the electromechanical response is progressively suppressed below 50 nm, and vanishes at room temperature at a thickness of ∼10 nm. Though larger than a single layer, 10 nm is still a very small value compared to the expectations for an intrinsic ferroelectric semiconductor. We therefore propose a model for a doped surface layer that screens spontaneous polarization in this material. The charges in the screening layer may also participate in secondary chemical reactions, which may explain domain pinning observed in thermal cycling of the flakes above the Curie temperature. At the same time, ferroelectric switching is intertwined with ionic diffusion, resulting in erratic and damaging switching at room temperature. Owing to much stronger temperature dependence of ionic diffusion, the two phenomena can be decoupled allowing more reliable switching to be obtained at low temperatures.