Flexoelectricity in hexagonal boron nitride monolayers

Two-dimensional (2D) materials are highly bendable due to their atomic thickness and, thus, can readily attain a large strain gradient hard to achieve in bulk materials. This attribute provides a unique platform to explore the coupling between strain gradient and electric polarization, referred to as flexoelectric effect. Here, we perform an intensive first-principles study of the flexoelectric effect in hexagonal boron nitride (h-BN) monolayer for its potential applications in harsh environments. Despite being an isoelectronic isomorph of graphene, h-BN exhibits a distinctly higher flexoelectric coefficient than graphene does, along with a stronger nonlinearity. The enhanced flexoelectricity is ascribed to bending-induced structural buckling that separates B and N atoms into two coaxially curved surfaces, whose effect is well quantified by a model analysis. This behavior also helps rationalize why the three-atom-thick MoS2 has the largest flexoelectric coefficient among these 2D materials. We also find that the flexoelectricity results in a staggered band gap in all double-walled BN nanotubes that can favor charge separation in photovoltaics, in contrast to the size-and chirality-dependent band alignment in double-walled carbon nanotubes. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This paper focuses on the flexoelectric effect in thin 2D materials. Through intensive first-principles study, the authors found that hexagonal boron nitride (h-BN) monolayer has a higher flexoelectric coefficient and nonlinearity compared to graphene, and explained the underlying mechanism. The flexoelectric effect was also found to induce a staggered band gap in double-walled BN nanotubes, with potential applications in photovoltaics.