Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary-condition Engineering

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancements of modern electronic devices. Here, we report the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO₃ ferroelectric films. Using aberration-corrected scanning transmission electron microscopy (STEM), it is found that the spontaneous polarization in a 2 nm thick ultrathin BiFeO₃ film is abnormally increased up to ~90-100 μC/cm² in the out-of-plane direction and a pecuiliar rumpled nanodomain structure with very large variation in c/a ratios, which is in analog to morphotropic phase boundaries (MPB), is formed. By a combination of density functional and phase-field calculations, it’s shown that it’s the unique single atomic Bi₂O_3-x layer at the surface that leads to the enhanced polarization and appearance of MPB-like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.