Atomically Sharp but Three‐Dimensionally Rough Ferroelectric Domain Walls in BiFeO ₃ Nano‐islands

ABSTRACT

Ferroelectric domain walls (DWs) have attracted extensive interest because of their emergent physical properties and potential functionalities in nanoelectronic devices. Although DWs are traditionally proposed to be atomically sharp interfaces, experimental observations frequently report apparent broadening, roughness, and even complex three‐dimension, suggesting structural variations beyond the conventional 2D picture. However, direct experimental characterization of such 3D structures remains challenging because of the intrinsic limitations of projection‐based imaging. Here, we employ energy‐filtered multislice electron ptychography with ∼2 nm depth resolution to directly resolve the 3D morphology of ferroelectric DWs in BiFeO ₃ nano‐islands grown on SrTiO ₃ substrates. We reveal that domain walls can exhibit irregular 3D morphologies while remaining intrinsically atomically sharp, with widths confined to approximately one unit cell. The apparent broadening observed in conventional imaging is shown to originate predominantly from depth‐dependent structural variations rather than from a genuine increase in wall width. Furthermore, depth‐resolved strain analysis demonstrates that interfacial strain gradients and dislocations play an important role in stabilizing and modulating 3D domain‐wall configurations. Our findings uncover the 3D nature of ferroelectric domain walls and provide a structural foundation for understanding domain‐wall functionalities.