A critical routine for memristors applied to neuromorphic computing is to approximate synaptic dynamic behaviors as closely as possible. A type of homogenous bilayer memristor with a structure of W/HfO y /HfO x /Pt is designed and constructed in this paper. The memristor replicates the structure and oxygen vacancy (V O ) distribution of a complete synapse and its Ca 2+ distribution, respectively, after the forming process. The detailed characterizations of its atomic structure and phase transformation in and near the conductive channel demonstrate that the crystallite kinetics are adaptively coupled with the V O migration prompted by directional external bias. The extrusion (injection) of the V O s and the subsequent crystallite coalescence (separation), phase transformation, and alignment (misalignment) resemble closely the Ca 2+ flux and neurotransmitter dynamics in chemical synapses. Such adaptation and similarity allow the memristor to emulate diverse synaptic plasticity. This study supplies a kinetic process of conductive channel theory for bilayer memristors. In addition, our memristor has very low energy consumption (5-7.5 fJ per switching for a 0.5 µm diameter device, compatible with a synaptic event) and is therefore suitable for large-scale integration used in neuromorphic networks.

A type of memristor with structure of Pd/Nb : AlNO/Pd is designed to mimic synaptical plasticity and kinetics.

A series of silica supported Pd (Pd/SiO 2 ) catalysts were prepared in various HCl concentrations ( C HCl ) of the impregnation solution with different electrostatic interactions between Pd precursor and support, and their catalytic properties were evaluated by the selective hydrogenation of nitrile butadiene rubber (NBR). The results show that with the C HCl increasing from 0.1 to 5 M, the particle size of Pd nanoparticles dramatically decreases from 24.2 to 5.1 nm and stabilizes at ∼5 nm when C HCl is higher than 2 M. Using the catalysts prepared with a high C HCl (>2 M), an excellent hydrogenation degree (HD) of ∼94% with 100% selectivity to C=C can be acquired under mild conditions. Interestingly, the HD could be remarkably increased from 65 to 92% by increasing only C Cl – from 0.1 to 2 M with the addition of NaCl while keeping C H + at 0.1 M. This is because PdCl 4 2– is the predominant existing form of precursor at high C Cl – , which has a strong electrostatic attraction with the positively charged support favorable for the formation of small-sized Pd nanoparticles over silica. Notably, Pd leaching behavior during the hydrogenation reaction is closely related to C H + , and the higher the C H + , the less Pd residues are detected in the hydrogenated NBR. Our contribution is to provide a facile strategy to synthesize effective and stable Pd/SiO 2 catalysts via adjusting the electrostatic interaction, which exhibits a high activity and selectivity for NBR hydrogenation.

A selector device based on threshold switching is a potential candidate for preventing leaky current from nearby units in cross-point memristor arrays during operation, which is required for low-cost operation and good reliability. A simple selector structure, i.e., Ag/HfO y /HfO x /Pt (AHHP), was constructed in this work. The device exhibits electroforming-free properties, a low threshold switching voltage of ∼0.28 V, a wide range of operating current from 1 nA to 300 μA, and an extremely sharp switching slope of ∼0.6 mV/dec. Both selector and memristor can be achieved by simple and compatible technology in comparison with a memristor structure of Ag/HfO x /Pt. In light of the detailed microstructure characteristics, a coupling mechanism between crystallite kinetics and activated Ag atom migration in the electrolyte is presented and discussed in detail. Stress-assisted crystallite rotation of hafnium oxide in AHHP interferes with the bonding and evolution feature of Ag conductive filaments and then induces selective effects during threshold switching. We also demonstrate the AHHP device can serve for logic circuit functions and expect the feasibility of the device for fabricating integrated array unit. The results suggest a new mechanism of threshold switching potential for application in memristor array.

Pt/Ca2+–polyethylene oxide/polymer poly[3-hexylthiophene-2,5-diyl]/Pt devices were fabricated, and their pulse responses were studied. The discharging peak, represented by the postsynaptic current (PSC), first increases and then decreases with increasing input number in a pulse train. The weight of the PSC decreased for low-frequency stimulations but increased for high-frequency stimulations. However, the peak of the negative differential resistance during the charging process varied following the opposite trend. These behaviors suggested the ability for transferring the signal bidirectionally, confirming the equivalence between the ionic kinetics of our device and the transmitter kinetics of one kind of synapse. A facilitation (F)–depression (D) interplay model corresponding to the ionic polarization and doping interplay at the electrolyte/semiconducting polymer interface was adopted to successfully mimic the weight modification of the PSC. The simulation results showed that the observed synaptic plasticity was caused by the great disparity between the recovery time constants of F and D (τF and τD). Moreover, such an interplay could inspire the features of responses to post-tetanic stimulations. Our study suggested a means to realize synaptic computation.

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Ionic migration in organic electrolytes resembles that in neural system involving signal transportation. Here, ionic dynamic simulations are applied to explore pulse responses of lithium‐doped polyethyleneoxide complexes. Two main interactions were considered: diffusion of ions and directional movement guided by an applied electric field. Frequency responses are simulated using arbitrary wave shape. It is found that redistribution of ions results in accumulation of charge and establishes a reverse inbuilt electric field controlling the discharging process and the frequency response. The charging current's wave shape is controlled mainly by contribution from the diffusion process, which strengthens the charging current in the first millisecond before weakening it. For stimulation with higher energy density (higher frequency), activation of ion channels should be considered and an active ion number is introduced to describe the process. The weights of calculated discharging current agree well with the experimental results. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 831–837

Spike‐rate dependent plasticity, one of the conventional learning protocols in neuroscience, has been achieved in semiconducting polymer/electrolyte cells. The frequency threshold θm of spike‐rate dependent plasticity is sliding in requirement of stability. In this work, various prior signal inputs are applied to poly[2‐methoxy‐5‐(2‐ethylhexyloxy)−1,4‐phenylenevinylene]/polyethylene oxide ‐Nd3+ cells to explore their effects on θm.The study find that a prior inhibitory input, i.e., a weak stimulation, moves θm in the low frequency direction, while a prior excitatory input, i.e., an intensive stimulation, moves θm in the high frequency direction. Our results demonstrate the adaptivity of ionic kinetics in macromolecules systems for signal handling and learning. We suggest that a sliding threshold is due to the variable active ionic domains depending on stimulation strength and history. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2412–2417