Ferri-ionic coupling in CuIn P2 S6 nanoflakes: Polarization states and controllable negative capacitance

Abstract

We consider nanoflakes of van der Waals ferrielectric CuInP2S6 covered by an ionic surface charge and reveal the appearance of polar states with a relatively large polarization of approximately 5 μC/cm2 and stored free charge of around 10 μC/cm2, which can mimic "midgap"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 CuInP2S6 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 of governing the transitions by strain and ionic screening can be useful for controlling the tunneling barrier in thin-film devices based on CuInP2S6 nanoflakes. Additionally, we predict that the CuInP2S6 nanoflakes reveal features of the controllable negative capacitance effect, which make them attractive for advanced electronic devices, such as nanocapacitors and gate oxide nanomaterials with reduced heat generation.