A research team led by Professor Taesung Kim of the School of Mechanical Engineering at Sungkyunkwan University (SKKU, President Ji-beom Yoo) developed an optoelectronic synaptic device that mimics the functions of human neurons and synapses at the device scale. The researchers designed a designable van der Waals (vdW) crystal through a single-step sulfurization process using mixed plasma. The developed device operates under optical stimuli, offering a structural solution to configure semiconductor materials for brain-inspired computing. Rapid advancements in artificial intelligence and hyper-connectivity require neuromorphic vision systems capable of sensing and processing vast amounts of visual data in real time. Optoelectronic synapses, which exhibit conductance variations in response to light signals, serve as core components of these systems. Layered vdW materials attracted significant attention as promising candidates due to their excellent optical properties and atomic-scale thickness. However, conventional vdW materials faced technical challenges, including the difficulty of precisely controlling grain boundaries and intercalation, polymer residue accumulation, mechanical warpage at interfaces, and poor large-area crystalline uniformity. To overcome these limitations, the research team focused on the structural similarity between light-sensitive ion channels in biological membranes and layered vdW lattices. The researchers applied an argon and hydrogen sulfide (Ar + H₂S) plasma sulfurization process to bulk van der Waals rhenium selenide (ReSe₂). This single-step process transformed the upper portion of the material into a nano-crystalline ReSe₂ layer composed of nano-sized grains, while preserving the underlying bulk single-crystalline ReSe₂ layer without damaging the interlayer interfaces. These two integrated layers structurally correspond to the light-sensitive ion channels of a neuronal cell membrane and the intracellular environment, respectively, and were fabricated without additional deposition or patterning steps. The research team utilized scanning probe microscopy (SPM) to resolve the pathways of S²⁻ (sulfur) ionic migration. The grain boundaries in the nano-crystalline ReSe₂ layer confined the sulfur ionic