Creation of a Monolayer Ferroelectric using Substitutional Impurities

Ferroelectricity with out-of-plane polarization has so far been found in several two-dimensional (2D) materials, including monolayers comprising three to five planes of atoms (MoTe₂, In₂Se₃). Here, we explore creating the first single-atom-thick monolayer ferroelectric using hexagonal boron nitride. We performed density-functional-theory calculations to explore inducing ferroelectricity through incorporation of isovalent substitutional impurities that are larger than the host atoms. This disparity in bond lengths causes a buckling of the h-BN, either up or down, which amounts to a dipole with two equivalent orientations. We tested several impurities and optimized the centrosymmetric and distorted structures. Using these structures, we then determined the magnitude of the induced dipole and the switching energy barrier for dipole inversion. The effects of strain and vertical heterostructures with graphene were further explored. We are exploring how dense the impurities can be placed within the h-BN while maintaining sufficiently localized distortions and dipoles. We expect that this work will help foster new ways to include functionality in layered 2D-material-based applications.