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

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