If a three-dimensional physical electronic system emulating synapse networks could be built, that would be a significant step toward neuromorphic computing. However, the fabrication complexity of complementary metal-oxide-semiconductor architectures impedes the achievement of three-dimensional interconnectivity, high-device density, or flexibility. Here we report flexible three-dimensional artificial chemical synapse networks, in which two-terminal memristive devices, namely, electronic synapses (e-synapses), are connected by vertically stacking crossbar electrodes. The e-synapses resemble the key features of biological synapses: unilateral connection, long-term potentiation/depression, a spike-timing-dependent plasticity learning rule, paired-pulse facilitation, and ultralow-power consumption. The three-dimensional artificial synapse networks enable a direct emulation of correlated learning and trainable memory capability with strong tolerances to input faults and variations, which shows the feasibility of using them in futuristic electronic devices and can provide a physical platform for the realization of smart memories and machine learning and for operation of the complex algorithms involving hierarchical neural networks.

Nonvolatile memory devices based on hybrid inorganic/organic nanocomposites have emerged as excellent candidates for promising applications in next-generation electronic and optoelectronic devices. Among the various types of nonvolatile memory devices, organic bistable devices fabricated utilizing hybrid organic/inorganic nanocomposites have currently been receiving broad attention because of their excellent performance with high-mechanical flexibility, simple fabrication and low cost. The prospect of potential applications of nonvolatile memory devices fabricated utilizing hybrid nanocomposites has led to substantial research and development efforts to form various kinds of nanocomposites by using various methods. Generally, hybrid inorganic/organic nanocomposites are composed of organic layers containing metal nanoparticles, semiconductor quantum dots (QDs), core-shell semiconductor QDs, fullerenes, carbon nanotubes, graphene molecules or graphene oxides (GOs). This review article describes investigations of and developments in nonvolatile memory devices based on hybrid inorganic/organic nanocomposites over the past 5 years. The device structure, fabrication and electrical characteristics of nonvolatile memory devices are discussed, and the switching and carrier transport mechanisms in the hybrid nonvolatile memory devices are reviewed. Furthermore, various flexible memory devices fabricated utilizing hybrid nanocomposites are described and their future prospects are discussed.