Direct Observation of Dipole Interlocking Effect Occurrence in Two-Dimensional Ferroelectricity.

The electric dipole in materials is closely associated with their electronic transport, optical properties, and mechanical behavior. Here, we have employed the differential phase contrast (DPC) technique of the scanning transmission electron microscopy technique (STEM) to directly analyze the local electric dipole at the sub-Angstrom scale. By utilizing DPC-STEM technology, we successfully visualized the ferroelectric polarization of van der Waals material 3R α-InSe and directly confirmed the dipole interlocking effect (DIE) between in-plane (IP) and out-of-plane (OOP) polarizations. Through density functional theory (DFT) calculations and structural analysis, we discovered that this DIE is caused by the central asymmetry of the middle Se atoms of each monolayer and that the reversal of polarization is accompanied by the emergence of an intermediate phase, β-InSe. Leveraging the DIE, we developed a multidirectional ferroelectric memristor that can effectively modulate the IP polarization by applying an OOP pulse voltage.