Interface-tuning of ferroelectricity and quadruple-well state in CuInP$_2$S$_6$ via ferroelectric oxide

TL;DR

This study demonstrates that interfacing ultrathin CuInP$_2$S$_6$ with ferroelectric PbZr$_{0.2}$Ti$_{0.8}$O$_3$ enhances its ferroelectric properties, raises its Curie temperature, and allows precise domain control, overcoming size limitations.

Contribution

It introduces a method to enhance ferroelectricity and control domains in CuInP$_2$S$_6$ by interface engineering with ferroelectric oxides, significantly improving its functional properties.

Findings

Enhanced ferroelectricity and piezoelectricity in ultrathin CuInP$_2$S$_6$

Curie temperature exceeds 200°C in 13 nm films on PbZr$_{0.2}$Ti$_{0.8}$O$_3$

Polar domains conform to underlying ferroelectric oxide

Abstract

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 {\deg}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 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 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 $T_C$ exceeding 200 {\deg}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.