(a) Schematic of a biological synapse and an optoelectronic synaptic device based on monolayer MoS₂ and bulk Si. (b) Emulation of short-term potentiation and long-term potentiation by using optical spikes. The frequency of the optical spikes was 0.1 Hz or 1 Hz. The power density of the optical spikes was 0.12 mW⋅cm⁻². (c) Emulation of short-term potentiation and long-term potentiation by using –8 V electrical spikes. The frequency of the electrical spikes was 1 Hz or 10 Hz, Reproduced with permission from Ref. [85]. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.