Two-Dimensional Multiferroic δ-PbO Monolayer with a Large In-Plane Negative Poisson’s Ratio

Two-dimensional (2D) multiferroic materials offer a unique platform for the development of next-generation multifunctional devices. Based on first-principles calculations, a stable 2D configuration of a lead oxide (δ-PbO) monolayer is proposed to realize the multiferroic coupling between ferroelectricity (FE) and ferroelasticity (FA). The δ-PbO monolayer has a spontaneous in-plane polarization of about 2.64 × 10–10 C/m whose orientation can be reversed by overcoming a low energy barrier of 27.5 meV/atom. The interplay between FE and FA enables the switch of the electric polarization direction by 90°. Moreover, the mechanical properties of the δ-PbO monolayer exhibit significant in-plane anisotropy accompanied by a large negative Poisson’s ratio of about −0.68. The coexistence of auxeticity and multiferroicity in this 2D material renders a promising strategy to regulate the mechanical and electronic properties and the interplay between them and thereby concepts for nanoscale devices.