Intrinsic bending flexoelectric constants in two-dimensional materials

TL;DR

This paper reports the intrinsic flexoelectric constants of various 2D materials using atomistic modeling and a novel bending scheme, revealing that transition metal dichalcogenides exhibit the largest flexoelectric response.

Contribution

It introduces a method to measure flexoelectric constants in 2D materials by eliminating piezoelectric effects and provides the first comprehensive values for multiple 2D materials.

Findings

Graphene has low flexoelectric constants due to weak interactions.

Buckling increases flexoelectric constants in group-IV monolayers.

TMDCs like MoS₂ have the largest flexoelectric constants, ten times that of graphene.

Abstract

Flexoelectricity is a form of electromechanical coupling that has recently emerged because, unlike piezoelectricity, it is theoretically possible in any dielectric material. Two-dimensional (2D) materials have also garnered significant interest because of their unusual electromechanical properties and high flexibility, but the intrinsic flexoelectric properties of these materials remain unresolved. In this work, using atomistic modeling accounting for charge-dipole interactions, we report the intrinsic flexoelectric constants for a range of two-dimensional materials, including graphene allotropes, nitrides, graphene analogs of group-IV elements, and the transition metal dichalcogenides (TMDCs). We accomplish this through a proposed mechanical bending scheme that eliminates the piezoelectric contribution to the total polarization, which enables us to directly measure the flexoelectric constants. While flat 2D materials like graphene have low flexoelectric constants due to weak $\pi-\sigma$ interactions, buckling is found to increase the flexoelectric constants in monolayer group-IV elements. Finally, due to significantly enhanced charge transfer coupled with structural asymmetry due to bending, the TMDCs are found to have the largest flexoelectric constants, including MoS$_{2}$ having a flexoelectric constant ten times larger than graphene.