Atomic-scale mechanism of anisotropic ion migration in 2D Bi₂O₂Se nanodevices

Progress in 2D memristive technologies is increasingly constrained by a limited understanding of how crystallographic anisotropy governs ion migration and resistive switching. Bi₂O₂Se offers a compelling model system in which in-plane and out-of-plane devices display strikingly different electrical behaviors, yet the atomic-scale origins of this disparity remain unknown. Here, we engineer orientation-defined Bi₂O₂Se nanodevices using focused ion beam fabrication coupled with in situ aberration-corrected transmission electron microscopy, enabling simultaneous electrical probing and real-time imaging of structural evolution under bias. Supported by density functional theory (DFT) calculations, we demonstrate that anisotropic migration barriers for O^2- and Se^2- ions give rise to two fundamentally distinct switching pathways. Vertical fields, constrained by strong interlayer electrostatic locking, lead to localized vertical migration and the formation of a reversible, ordered conductive D-Bi₂O₂Se phase, producing abrupt, threshold-type switching. By contrast, lateral fields enable long-range ion diffusion, generating extended Bi/Bi_4+2nSe₃/Bi₂O₂Se heterostructures through a topotactic sequence with continuously evolving Se concentration, yielding smooth and linear conductance modulation. These results establish the microscopic principles that underpin direction-dependent transport and phase transformation in Bi₂O₂Se memristors. By revealing how crystallographic orientation dictates functionality, our work provides a mechanistic foundation for the rational design of directionally engineered 2D neuromorphic and memory systems with enhanced versatility and integration potential.