Anisotropic Memristive Switching in NbOCl₂ Enabled by Directional Oxygen Ion Migration

Directional ion transport in anisotropic two-dimensional (2D) materials provides a powerful yet underexplored pathway for engineering memristive functionalities. Here, we demonstrate strongly orientation-dependent memristive switching in layered NbOCl₂ lateral devices, where resistive switching occurs exclusively along the in-plane c-axis, while devices aligned along the b-axis exhibit no hysteresis. The c-axis devices show forming-free volatile memristive behavior with a stable on/off ratio and cycle-to-cycle reproducibility. Combined experiments and first-principles calculations reveal that the anisotropic switching originates from direction-dependent oxygen-ion migration and vacancy propagation, which modulate the Schottky barrier at the Pd/NbOCl₂ interface. The resulting ion-driven barrier modulation enables key synaptic functions, including excitatory postsynaptic current, paired-pulse facilitation, spike-rate-dependent plasticity, and spike-amplitude-dependent plasticity, demonstrating short-term synaptic plasticity. Leveraging the intrinsic nonlinear and self-relaxation dynamics, the NbOCl₂ memristor is further implemented as a physical reservoir, achieving 94.3% accuracy in handwritten digit recognition using the MNIST dataset. These results reveal NbOCl₂ as a promising anisotropic memristive material and highlight directional ion migration in 2D systems as a versatile strategy for neuromorphic hardware and reservoir computing applications.