In this study, a memristor with the simple structure Ag/poly(3,4-ethylenedioxythiophene):poly (styrenesulphonate) (PEDOT:PSS)/Ta was fabricated. Essential synaptic plasticity and learning behaviours were emulated using this memristor, including short-term plasticity, long-term plasticity, spike-timing-dependent plasticity and spike-rate-dependent plasticity. Important time constants were extracted from these synaptic modifications, which are associated with brain learning and memory functions. It was clearly demonstrated that the movement of the Ag interface upon the initiation of a redox reaction accounts for the resistive switching mechanism of our memristor. The conducting path in the polymer layer and the elastic effect of the polymer matrix were suggested to be considered in the memory and learning processes. Moreover, the energy band diagram of our memristor was drawn after the cross-sectional transmission electron microscopy images were analysed. It was found that a natural p–n junction in the PEDOT:PSS/Ta compound was formed. This resulted in rectifying, high resistance and low power consumption. Our device structure may be considered a feasible prototype for integrating memristors into a large-scale neuromorphic circuit.

The modulation of oxygen vacancies in ZnO epilayers and its effect on optical band gap have been investigated. The evolution of band gap is correlated with the oxygen vacancy variations induced by readily varying reactive oxygen partial pressure. The oxygen vacancy increased with the decreased oxygen partial pressure, accompanied with a red shift of absorption band edge and of ultraviolet emission peak. The optical band gap was roughly linear dependent on reactive oxygen partial pressure. The band gap narrowing is attributed to the overlap of the nonlocalized oxygen vacancy states with valence band.

Resistive switching and conductance quantization are systematically studied in a Ag/poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester/indium–tin oxide sandwich structure. The observed bipolar switching behavior can be attributed to the formation and dissolution of Ag filaments during positive and negative voltage sweeps, respectively. More importantly, conductance quantization with both integer and half integer multiples of single atomic point contact can be realized by slowing down the voltage sweep speed as well as by pulse measurement. The former may reflect the formed Ag filaments with different atomic point contacts, while the latter probably originates from the interaction between the Ag filaments and the elemental hydrogen provided by the organic storage medium. With appropriate current compliances, low resistance states with desired quantized conductance values are successfully achieved, thus showing the potential for ultrahigh density memory applications. Besides, 100 successive switching cycles with densely distributed resistance values of each resistance state and extrapolated retention properties over ten years are also demonstrated.

Resistive switching mechanism of Ti/HfO_x/Pt memory devices was studied using X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy images. Spatial distributions of valence of Hf demonstrated that the fraction of Hf⁴⁺ increased from Ti/HfO_x interface to HfO_x/Pt interface in high resistance state (HRS), but it maintained a constant level in low resistance state (LRS). Rupture of oxygen vacancies formed conducting paths occurred near the HfO_x/Pt interface. The cross sectional images of active switching region also varied with HRS and LRS. A dynamic model of interface processes was proposed to interpret interfaces migration of oxygen vacancies near both the top and bottom electrodes.

The Ag/SiO₂/indium tin oxide (ITO) devices exhibit bipolar resistive switching with a large memory window of ∼10², satisfactory endurance of >500 cycles, good retention property of >2000 s, and fast operation speed of <100 ns, thus being a type of promising resistive memory. Under slow voltage sweep measurements, conductance plateaus with a conductance value of integer or half-integer multiples of single atomic point contact have been observed, which agree well with the physical phenomenon of conductance quantization. More importantly, the Ag/SiO₂/ITO devices exhibit more distinct quantized conductance plateaus under pulse measurements, thereby showing the potential for realizing ultra-high storage density.

Through the one-step plasma oxidation of TiN thin films at room temperature (a simple semiconductor technology compatible method), a partly oxidised structure of titanium oxynitride (TiN_xO_y) with a TiO_2−x nanolayer on top has been prepared for non-volatile resistive switching memory devices. The fabricated Pt/TiO_2−x/TiN_xO_y/TiN memory devices demonstrate complementary resistive switching behaviours within an operation voltage of 1 V. The complementary resistive switching behaviours can be explained by redistribution of the oxygen vacancies between the Pt/TiO_2−x top interface and the TiO_2−x/TiN_xO_y bottom interface in the TiO_2−x nanolayer. A model concerning the resistive switching mechanism as well as a recover program of a failed device is also proposed. Our work provides a possible cost-efficient solution to suppress the sneak-path problem in nanoscale crossbar memory arrays.