Interface-Tuning of Ferroelectricity and Quadruple-Well State in CuInP₂S₆ via Ferroelectric Oxide

Ferroelectric van der Waals CuInP₂S₆ possesses intriguing quadruple-well states and negative piezoelectricity. Its technological implementation has been impeded by the relatively low Curie temperature (bulk T_C ∼ 42 °C) and the lack of precise domain control. Here we show that CuInP₂S₆ can be immune to the finite size effect and exhibits enhanced ferroelectricity, piezoelectricity, and polar alignment in the ultrathin limit when it is interfaced with ferroelectric oxide PbZr_0.2Ti_0.8O₃ films. Piezoresponse force microscopy studies reveal that the polar domains in thin CuInP₂S₆ fully conform to those of the underlying PbZr_0.2Ti_0.8O₃, where the piezoelectric coefficient changes sign and increases sharply with reducing thickness. High temperature in situ domain imaging points to a significantly enhanced T_C of >200 °C for 13 nm CuInP₂S₆ on PbZr_0.2Ti_0.8O₃. Density functional theory modeling and Monte Carlo simulations show that the enhanced polar alignment and T_C can be attributed to interface-mediated structure distortion in CuInP₂S₆. Our study provides an effective material strategy to engineer the polar properties of CuInP₂S₆ for flexible nanoelectronic, optoelectronic, and mechanical applications.