Ferri-ionic Coupling in CuInP$_2$S$_6$ Nanoflakes: Polarization States and Controllable Negative Capacitance

TL;DR

This paper investigates the polarization states and negative capacitance phenomena in CuInP$_2$S$_6$ nanoflakes, revealing controllable phase transitions and potential applications in nano-electronic devices.

Contribution

It introduces a comprehensive model of ferrielectric and ionic states in CuInP$_2$S$_6$ nanoflakes, highlighting controllable phase transitions and negative capacitance effects.

Findings

High polarization (~5 μC/cm²) and stored charge (~10 μC/cm²) observed.

Transitions between paraelectric, antiferroelectric, ferrielectric, and ferri-ionic states induced by strain and ionic screening.

Prediction of controllable negative capacitance in CuInP$_2$S$_6$ nanoflakes.

Abstract

We consider nanoflakes of van der Waals ferrielectric CuInP$_2$S$_6$ covered by an ionic surface charge and reveal the appearance of polar states with relatively high polarization ~5 microC/cm$^2$ and stored free charge ~10 microC/cm$%2$, which can mimic "mid-gap" states associated with a surface field-induced transfer of Cu and/or In ions in the van der Waals gap. The change in the ionic screening degree and mismatch strains induce a broad range of the transitions between paraelectric phase, antiferroelectric, ferrielectric, and ferri-ionic states in CuInP$_2$S$_6$ nanoflakes. The states' stability and/or metastability is determined by the minimum of the system free energy consisting of electrostatic energy, elastic energy, and a Landau-type four-well potential of the ferrielectric dipole polarization. The possibility to govern the transitions by strain and ionic screening can be useful for controlling the tunneling barrier in thin film devices based on CuInP$_2$S$_6$ nanoflakes. Also, we predict that the CuInP$_2$S$_6$ nanoflakes reveal features of the controllable negative capacitance effect, which make them attractive for advanced electronic devices, such as nano-capacitors and gate oxide nanomaterials with reduced heat dissipation.