(a) Schematic of an optoelectronic synaptic device based on the heterostructure of bulk Si and HfO₂. (b) Schematic of electrical spikes for the demonstration of STDP. (c) STDP under different illumination conditions. The postsynaptic current was measured when the presynaptic electrical spike was 5 min before (I₁) or after (I₂) the postsynaptic electrical spike. The relative change of synaptic weight (ΔSW%) was defined as (I₂ – I₁)/I₁. (d) C–V characteristics under different illumination conditions. (e) Multilevel photocurrent (I_ph) output after the device was regulated by electrical spikes under different illumination conditions. Reproduced with permission from Ref. [83]. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.