Two-Dimensional Ferroelectricity in a Single-Atom Adsorbed BiI3 Monolayer

Two-dimensional (2D) ferroelectric materials have received tremendous attention in recent years for their potential applications in high-performance electronic nanodevices, such as ferroelectric random-access memories. To increase the density of a ferroelectric memory, a 2D ferroelectric material with multiple ferroelectric states controllable via an external electric field is needed. However, 2D ferroelectric materials are still rare and most of them only have two equivalent ferroelectric states with opposite electric polarization directions. Here, we propose that a 2D ferroelectric material with multiple ferroelectric states can be realized by single-atom adsorption on a metal halide monolayer. Using first-principles calculations, we demonstrate that the 2D BiI3 monolayer adsorbed by Pd single atoms (denoted as Pd@BiI3) is a stable ferroelectric material, which possesses six equivalent ferroelectric states with different electric polarization directions. These findings provide a potential route to achieve multi-state 2D ferroelectric materials for multi-functional device applications.