All-Optical Synapse Enabled by CdS/PbS Nano-PN Junctions for Visual Information Processing

In novel neuromorphic computations, it is essential to achieve bidirectional modulation of synaptic plasticity by external signal stimulation. However, conventional optoelectronic co-modulation leads to the complexity of external devices as well as operational drudgery. Therefore, it is significant that the modulation of synaptic plasticity in neuromorphic devices is realized entirely by optical signal stimulation. Thus, an all-optical memristor based on CdS nanorods/PbS quantum dots PN junctions and a PMMA defect capture layer is proposed. In particular, the CdS nanorods can be modulated in size by controlling the reaction conditions. The size of the nanorods affects the UV response current of the CdS-based memristor. The all-optical synaptic device exhibits EPSC under UV light (365 nm) irradiation and IPSC under red light (635 nm) irradiation. Transient absorption spectroscopy results indicate that the presence of a PN junction and defect trapping are responsible for bidirectional photomodulation. The “AND” and “XOR” logic operations are realized by selecting the input of light signals of different wavelengths. In addition, the feature enhancement operation of the image is realized by using the bidirectional photoresponse of the device. Finally, an artificial neural network is constructed for image recognition on the MNIST data set with a recognition accuracy of 90%. This work enriches the choice of materials for optoelectronic synaptic memristors and the design of device structures for neuromorphic computing.