General strategy for activating ferroelectricity in bilayer 2D materials with intercalating inert atoms

Activating ferroelectricity in two-dimensional (2D) bilayer materials is essential for enabling non-volatile memory and logic functionalities. While interlayer sliding driven by van der Waals interactions can break inversion symmetry, achieving out-of-plane (OOP) polarization through this mechanism remains challenging in highly symmetric 2D materials—particularly those that are centrosymmetric, such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs). Here, we propose a general strategy to activate OOP ferroelectricity by intercalating inert atoms into the interlayer space. Using bilayer graphene, hBN, and MoS2 as model systems, we reveal that such intercalation lowers the symmetry from nonpolar D3d to polar C3v, enabling reversible polarization switching via lateral displacement of the intercalants. This resulting semi-sliding ferroelectricity preserves the structural and electronic integrity of the host materials, and features ultralow switching barriers along with atomic-scale dipole control—where each intercalated atom acts as an independent, reversible memory bit. Importantly, the polarization magnitude scales linearly with the electrostatic potential difference across the bilayer, providing a quantitative and tunable design rule. Our findings establish a universal and material-agnostic framework for realizing low-power, ultrahigh-density 2D ferroelectric devices on otherwise nonpolar platforms.