Ion adsorption-induced reversible polarization switching of a van der Waals layered ferroelectric

Abstract Solid-liquid interface is a key concept of many research fields, enabling numerous physical phenomena and practical applications. For example, electrode-electrolyte interfaces with electric double layers have been widely used in energy storage and regulating physical properties of functional materials. Creating a specific interface allows emergent functionalities and effects. Here, we show the artificial control of ferroelectric-liquid interfacial structures to switch polarization states reversibly in a van der Waals layered ferroelectric CuInP2S6. We discover that upward and downward polarization states can be induced by spontaneous physical adsorption of anions [DDBS] and cations [DEME], respectively, at the ferroelectric-liquid interface. This distinctive approach circumvents the structural damage of CIPS caused by Cu-ion conductivity during electrical switching process. Moreover, the polarized state features super-long retention time (> 1 year). The interplay between ferroelectric dipoles and adsorbed organic ions has been studied systematically by comparative experiments and first-principles calculations. Such ion adsorption-induced reversible polarization switching in a van der Waals ferroelectric enriches the functionalities of solid-liquid interfaces, offering opportunities for liquid-controlled two-dimensional ferroelectric-based devices.