Trap-state depth distribution as the physical origin of multi-timescale synaptic plasticity in MoS2 optoelectronic synapses: A perspective

Optoelectronic synapses based on two-dimensional MoS2 can emulate both short-term and long-term synaptic plasticity using light stimuli. However, the physical parameter governing accessible plasticity timescales remains unclear, which prevents the design of predictive devices. From this perspective, we propose that the depth distribution of trap states arising from sulfur vacancies, interface defects, and synthesis-dependent disorder is the primary physical origin of multi-timescale synaptic behavior in MoS2 optoelectronic devices. We synthesize experimental photoconductivity data, first-principles defect calculations, and optoelectronic synapse demonstrations to construct a unified framework connecting trap energy (Et) to retention time (τ) via the thermal emission relation τ = τ0 exp(Et/kBT). Shallow traps (0.2–0.4 eV) enable short-term plasticity with retention of seconds to minutes, while deep traps (0.5–1.0 eV) provide the slow relaxation necessary for long-term plasticity exceeding 104 s. We identify a critical discrepancy between simple thermal emission predictions and observed retention times, which we attribute to distributed trap depths, re-trapping dynamics, and percolation transport. Based on this framework, we outline engineering strategies including synthesis-controlled defect density, heterostructure band alignment, and substrate selection for tuning plasticity timescales. Finally, we discuss key challenges, including defect variability and incomplete trap characterization, and propose directions for future research toward brain-inspired optoelectronic computing.