Sliding ferroelectricity in bilayer honeycomb structures: A first-principles study

The sliding ferroelectricity is engineered by artificially stacking the nonpolar two-dimensional (2D) materials, which greatly broadens the 2D ferroelectrics from very few candidate materials to a large family of 2D materials. However, the electric polarizations are generally small due to the weak van der Waals interlayer interaction. The search for 2D sliding ferroelectrics with large polarization presents an ongoing challenge. Here we systematically investigate the sliding ferroelectricity in the bilayer honeycomb structures of $\mathrm{B}X$ $(X=\mathrm{P}, \mathrm{As}, \mathrm{Sb})$, $Y\mathrm{N}$ $(Y=\mathrm{Al}, \mathrm{Ga}, \mathrm{In})$, and $Z\mathrm{C}$ $(Z=\mathrm{Si}, \mathrm{Ge}, \mathrm{Sn})$ based on first-principles calculations. It is shown that the electric polarization decreases with the increase of the interlayer distance, and increases with the difference in electronegativity of the two constituent elements. Such dependence is further corroborated by a simple model. It is interesting to see that GeC can be an ideal sliding ferroelectric material with high polarization and energetically favorable polar stacking. Our results reveal the key factors in determining the electric polarization, which could facilitate the search and design of 2D sliding ferroelectrics with large out-of-plane polarization.