Cross-Dimensional Control of Topological Defects and Critical Size Effect in Lead-Free Thin Films

Defect engineering has emerged as a powerful strategy for manipulating nontrivial topological defects in ferroelectric systems. However, the critical interaction between trivial planar defects and their role in mediating topological configurations remains insufficiently understood. Here, we stabilized a one-dimensional vortex topological structure through the interaction between two-dimensional trivial planar defects in lead-free BiFeO3(BFO) thin films and explored the critical issues of their interactions and the corresponding enhancement of piezoelectricity. Atomic-scale scanning transmission electron microscopy imaging reveals that an array of small-size charged oxygen vacancy plates (Vo plates) is distributed in the BFO films. Combined with phase-field simulation, we find that exotic polarization rotation patterns in BFO are strongly related to interactions between the interior and interfacial oxygen vacancy plates along the out-of-plane direction. Moreover, the topological vortex phases appear when their distances decrease to about 8-unit cells. Interferometric displacement sensor measurements combined with phase-field simulation results further confirm that these samples could exhibit an enhanced piezoelectric coefficient d33 more than 2 times that of common BFO films. These results offer a route to enhance functional responses in environmentally benign ferroelectrics.