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

Abstract

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 O2- and Se2- 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/Bi4+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. The development of two-dimensional memristors is limited by unclear links between crystal direction, ion motion, and switching. Qu et al. build orientation controlled Bi2O2Se devices and combine real time imaging and theory to reveal distinct direction dependent ion migration and switching pathways.