Interface-Tuning of Ferroelectricity and Quadruple-Well State in CuInP 2 S 6 via Ferroelectric Oxide.

Ferroelectric van der Waals CuInP 2 S 6 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 CuInP 2 S 6 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 0.2 Ti 0.8 O 3 films. Piezoresponse force microscopy studies reveal that the polar domains in thin CuInP 2 S 6 fully conform to those of the underlying PbZr 0.2 Ti 0.8 O 3 , 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 CuInP 2 S 6 on PbZr 0.2 Ti 0.8 O 3 . 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 CuInP 2 S 6 . Our study provides an effective material strategy to engineer the polar properties of CuInP 2 S 6 for flexible nanoelectronic, optoelectronic, and mechanical applications.