Ferroelectricity in Epitaxial Perovskite Oxide Bi₂WO₆ Films with One-Unit-Cell Thickness

Retaining ferroelectricity in ultrathin films or nanostructures is crucial for miniaturizing ferroelectric devices, but it is a challenging task due to intrinsic depolarization and size effects. In this study, we have shown that it is possible to stably maintain in-plane polarization in an extremely thin, one-unit-cell thick epitaxial Bi₂WO₆ film. The use of a perfectly lattice-matched NdGaO₃ (110) substrate for the Bi₂WO₆ film minimizes strain and enhances stability. We attribute the residual polarization in this ultrathin film to the crystal stability of the Bi-O octahedral framework against structural distortions. Our findings suggest that ferroelectricity can surpass the critical thickness limit through proper strain engineering, and the Bi₂WO₆/NdGaO₃ (110) system presents a potential platform for designing low-energy consumption, nonvolatile ferroelectric memories.