Two-dimensional Element Ferroelectricity with large negative piezoelectric coefficients and intrinsically stable charged domain walls

Ferroelectric materials, particularly in two-dimensional (2D) systems, have drawn significant attention in recent years. However, most known ferroelectric materials are compounds consisting of different atomic species. In a counterintuitive discovery, we have revealed spontaneous polarization and ferroelectricity in 2D elemental group-V materials with a buckled lattice structure, similar to black phosphorus, based on first-principles calculations, and confirmed by experimental measurements. Furthermore, we have developed a general model to facilitate the understanding and identification of potential 2D ferroelectric and antiferroelectric materials. Through this model, we also found that ferroelectric and antiferroelectric phases can occur in group-IV and group-VI elemental 2D systems. Notably, these 2D elemental ferroelectric materials display unusual negative piezoelectric coefficients and stable charged domain walls, making them ideal candidates for ultrathin ferroelectric devices with wide-ranging applications. The insights gained from studying elemental ferroelectric systems point to the possibility of realizing unconventional ferroelectric effects in other materials, offering substantial potential for experimental verification and practical technological advances.