A controllable and reversible transition of volatile and non-volatile resistive switching is presented in Ag/indium-gallium-zinc oxide (IGZO)/manganese oxide (MnO)/Pt bilayer resistive memory devices. The coexistence of volatile and non-volatile switching characteristics was demonstrated by controlling the current compliance during the SET process. With lower current compliance (<50 μA), the formation of an unstable conducting filament presented typical diode-like non-volatile switching. A reversible transition from volatile to non-volatile switching could be obtained by applying a higher current compliance (≥50 μA). Moreover, highly uniform multistate memory characteristics were achieved by modulating the current compliance in the non-volatile switching region. The coexistence of volatile and multistate non-volatile resistive switching behaviors with a reversible transition demonstrates the capability of developing a selector element for crossbar arrays and the application for next generation multistate high-density storage with the same Ag/IGZO/MnO/Pt device.

In this work 3 × 3 crossbar arrays of titanium oxide were fabricated and tested for non-volatile memory applications and neuromorphic pattern recognition. The non-volatile memory characteristics of the memristor were examined using retention tests for each memristor. In order to test neuromorphic pattern recognition, the memristor crossbar array was programmed to store ‘111’, ‘100’ and ‘010’ at the first, second and third columns of the array, where ‘0’ and ‘1’ represent the high-resistance state (HRS) and low-resistance state (LRS), respectively. The three similar input patterns of ‘111’, ‘100’ and ‘010’ were applied to the crossbar array, for pattern recognition. Using a twin memristor crossbar array mechanism all three input patterns were recognized.

We demonstrate strong, analog, reversible, and nonvolatile memcapacitance in a Si-based MOS (metal-oxide-semiconductor) memcapacitor with an ITO (In-Sn-O)/HfO_x/Si structure. Both accumulation and depletion capacitances change sequentially and reversibly upon repeating voltage application with respect to voltage polarity. This memcapacitance is thought to be induced by oxygen ions' migration between ITO and HfO_x layers, which changes the HfOx permittivity and the depletion states in Si and ITO. The Si-based memcapacitor has potential to be applied to the gate stack of the MOS field-effect-transistor for nonvolatile memory and synaptic transistors through modulating drain current determined by the capacitance change of the MOS gate stack.

Analog synaptic weight modulation that is linear, symmetric, and exhibits long-term stability is demonstrated by the resistance changes in a Pt/indium-tin-oxide (ITO)/CeO₂/Pt memristor. Distinct from a Pt/CeO₂/Pt memristor without the ITO layer, which shows highly nonlinear and asymmetric resistance changes, the Pt/ITO/CeO₂/Pt memristor exhibits linear and symmetric resistance changes in proportion to the number of voltage applications with opposite polarities for potentiation and depression behaviors. The Pt/CeO₂/Pt memristor also displays high long-term stability of modulated synaptic weight over time, which originates from the ITO layer acting as a reservoir of oxygen ions drifted from the CeO₂ layer to retain the resistance change. Comparison of the results for the Pt/CeO₂/Pt and Pt/ITO/CeO₂/Pt memristors confirms the role of ITO in the linearity, symmetry, and long-term stability of the resistance change in CeO₂-based memristors for use as artificial synapses in neuromorphic systems.

Toluene gas was successfully measured at room temperature using a device microfabricated by a nanoimprinting method. A highly uniform nanoporous thin film was produced with a dense array of titania (TiO₂) pores with a diameter of 70~80 nm using this method. This thin film had a Pd/TiO₂ nanoporous/SiO₂/Si MIS layered structure with Pd-TiO₂ as the catalytic sensing layer. The nanoimprinting method was useful in expanding the TiO₂ surface area by about 30%, as confirmed using AFM and SEM imaging. The measured toluene concentrations ranged from 50 ppm to 200 ppm. The toluene was easily detected by changing the Pd/TiO₂ interface work function, resulting in a change in the I-V characteristics.