Persistent Photoconductivity and Gate‐Tunable Synaptic Plasticity in Few‐Layer MoS 2 /MnPSe 3 Vertical Heterojunctions

A few‐layer van der Waals MoS 2 /MnPSe 3 vertical heterostructure is demonstrated as an optoelectronic synaptic device featuring broadband photoresponse, persistent photocurrent, and gate‐tunable plasticity. Photoexcitation spanning from the ultraviolet to near‐infrared regime induces a conductance enhancement that persists after the removal of illumination. Repeated optical pulses produce paired‐pulse facilitation, a stimulus‐dependent transition from short‐term to long‐term plasticity, and learning–forgetting–relearning behavior. Gate voltage pulses regulate the relaxation of photoinduced conductance states, enabling electrical modulation of optically programmed states. Wavelength‐dependent measurements show that the accessible conductance modulation amplitude decreases with increasing wavelength, whereas normalized conductance accumulation and relaxation profiles remain largely analogous. This observation suggests wavelength governs the magnitude of the accessible conductance window rather than altering programming kinetics. The power‐dependent photoresponse, interfacial potential contrast characterized via Kelvin probe force microscopy, and multiscale relaxation dynamics support a trap‐assisted persistent photoconductivity mechanism involving interfacial charge separation and slow carrier recombination. A device‐informed multilayer perceptron built upon experimentally measured conductance update characteristics achieves an accuracy of 89.08% for MNIST handwritten digit recognition, in contrast to 96% obtained from an ideal weight baseline. Collectively, these findings verify versatile optoelectronic synaptic functionality within few‐layer MoS 2 /MnPSe 3 heterostructures.