Flexo-induced ferroelectricity in low dimensional transition metal dichalcogenides

TL;DR

This paper develops a theoretical framework to understand flexoelectric-induced ferroelectricity in low-dimensional transition metal dichalcogenides, revealing bistable polarization states and implications for spectroscopic detection.

Contribution

It introduces a Landau theory for flexoelectric polarization in TMDs, highlighting the role of surface rippling and substrate interactions in inducing and controlling ferroelectricity.

Findings

Out-of-plane polarization is bistable and reversible with electric fields.

Misfit strain and surface piezoelectricity induce polarization that cannot be reversed.

Spectral analysis shows enhanced dielectric susceptibility critical for experimental observation.

Abstract

We developed a Landau type theory for the description of polar phenomena in low-dimensional transition metal dichalcogenides (TMDs), specifically exploring flexoelectric origin of the polarization induced by a spontaneous bending and by inversion symmetry breaking due to the interactions with substrate. We consider the appearance of the spontaneous out-of-plane polarization due to the flexoelectric coupling with the strain gradient of the spontaneous surface rippling and surface-induced piezoelectricity. Performed calculations proved that the out-of-plane spontaneous polarization, originated from flexoelectric effect in a rippled TMD, is bistable and reversible by a non-uniform electric field. In contrast, the spontaneous polarization induced by a misfit strain and symmetry-sensitive surface-induced piezoelectric coupling, cannot be reversed by an external electric field. The special attention is paid to the spectral analysis of the linear dielectric susceptibility and gain factor, which enhancement is critically important for the observation of the polar phenomena in low-dimensional TMDs by the surface-enhanced vibrational spectroscopy.