Ionic control over ferroelectricity in 2D layered van der Waals capacitors

TL;DR

This study explores how ionic migration in 2D layered ferroelectric materials like CuInP2S6 can be controlled with electric pulses to modulate and stabilize multiple polarization states, enabling advanced functional behaviors.

Contribution

It demonstrates ionic control over ferroelectric switching in 2D materials, revealing new multi-state polarization capabilities driven by voltage pulse parameters.

Findings

Increased pulse duration enhances ionic currents and polarization.

Internal electric fields shift and increase polarization loops.

Hysteretic switching can be deactivated or reactivated, enabling three-state systems.

Abstract

The van der Waals layered material CuInP2S6 features interesting functional behavior, including the existence of four uniaxial polarization states, polarization reversal against the electric field through Cu ion migration, a negative-capacitance regime, and reversible extraction of Cu ions. At the heart of these characteristics lies the high mobility of Cu ions, which also determines the spontaneous polarization. Therefore, Cu migration across the lattice results in unusual ferroelectric behavior. Here, we demonstrate how the interplay of polar and ionic properties provides a path to ionically controlled ferroelectric behavior, achieved by applying selected DC voltage pulses and subsequently probing ferroelectric switching during fast triangular voltage sweeps. Using current measurements and theoretical calculations, we observe that increasing DC pulse duration results in higher ionic currents, the build-up of an internal electric field that shifts polarization loops, and an increase in total switchable polarization by ~50% due to the existence of a high polarization phase which is stabilized by the internal electric field. Apart from tuning ferroelectric behavior by selected square pulses, hysteretic polarization switching can even be entirely deactivated and reactivated, resulting in three-state systems where polarization switching is either inhibited or can be performed in two different directions.