Atomic-scale mechanism of anisotropic ion migration in 2D Bi(2)O(2)Se nanodevices.

Progress in 2D memristive technologies is increasingly constrained by a limited understanding of how crystallographic anisotropy governs ion migration and resistive switching. BiOSe 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 BiOSe 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 and Se 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-BiOSe phase, producing abrupt, threshold-type switching. By contrast, lateral fields enable long-range ion diffusion, generating extended Bi/BiSe/BiOSe 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 BiOSe 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.