Flexible Photonic Synapses Using Vertical ZnO Nanotubes on Graphene Films

We report the fabrication of flexible photonic synapses using vertically grown ZnO nanotubes on graphene layers. Position- and morphology-controlled ZnO nanotubes were heteroepitaxially grown on multilayer graphene layers and served as flexible channels for synaptic devices. Current–voltage characterizations under different light illumination conditions, as well as a study of the temperature-dependent transient photocurrent, revealed the origin of persistent photoconductivity of the device. Importantly, the device exhibited excellent synaptic properties upon application of light pulses, such as excitatory post-synaptic currents and paired pulse facilitations. For the long-term plasticity, potentiation-depression characteristics with high linearity were successfully implemented with good reliability. These features were observed even under different bending conditions, confirming the flexibility of the device. A simulation study suggested that the device will exhibit excellent performance when integrated into neural network structures.