Vacancy-Induced Synaptic Behavior in 2D WS2 Nanosheet-Based Memristor for Low-Power Neuromorphic Computing.

Memristors with nonvolatile memory characteristics have been expected to open a new era for neuromorphic computing and digital logic. However, existing memristor devices based on oxygen vacancy or metal-ion conductive filament mechanisms generally have large operating currents, which are difficult to meet low-power consumption requirements. Therefore, it is very necessary to develop new materials to realize memristor devices that are different from the mechanisms of oxygen vacancy or metal-ion conductive filaments to realize low-power operation. Herein, high-performance and low-power consumption memristors based on 2D WS2 with 2H phase are demonstrated, which show fast ON (OFF) switching times of 13 ns (14 ns), low program current of 1 µA in the ON state, and SET (RESET) energy reaching the level of femtojoules. Moreover, the memristor can mimic basic biological synaptic functions. Importantly, it is proposed that the generation of sulfur and tungsten vacancies and electron hopping between vacancies are dominantly responsible for the resistance switching performance. Density functional theory calculations show that the defect states formed by sulfur and tungsten vacancies are at deep levels, which prevent charge leakage and facilitate the realization of low-power consumption for neuromorphic computing application.

Flexible Transparent Organic Artificial Synapse Based on the Tungsten/Egg Albumen/Indium Tin Oxide/Polyethylene Terephthalate Memristor.

As artificial synapses in biomimetics, memristors have received increasing attention because of their great potential in brain-inspired neuromorphic computing. The use of biocompatible and degradable materials as the active resistive layer is promising in memristor fabrication. In this work, we select egg albumen as the resistive layer to fabricate flexible tungsten/egg albumen/indium tin oxide/polyethylene terephthalate devices, which can operate normally under mechanical bending without significant performance degradation. This proposed memristor device exhibits a transparency of more than 90% under visible light with a wavelength range of 230-850 nm. Moreover, by changing amplitudes of pulse voltage instead of intervals, paired-pulse facilitation can be transmitted to paired-pulse depression, which can faithfully mimic dynamical balance of Ca2+ concentration shaped by voltage-sensitive calcium channels. The device resistance can be modulated gradually by applied pulse trains to mimic certain neural bionic behaviors, including excitatory postsynaptic current, short-term plasticity (STP) and long-term plasticity (LTP), and transitions between STP and LTP. The reasons behind these behaviors are analyzed through power consumption calculation. Excellent biosimulation characteristics have been demonstrated in this egg albumen-based memristor device, which is desirable in biocompatible and dissolvable electronics for flexible artificial neuromorphic systems.

Robust Ag/ZrO2/WS2/Pt Memristor for Neuromorphic Computing.

The development of the information age has made resistive random access memory (RRAM) a critical nanoscale memristor device (MD). However, due to the randomness of the area formed by the conductive filaments (CFs), the RRAM MD still suffers from a problem of insufficient reliability. In this study, the memristor of Ag/ZrO2/WS2/Pt structure is proposed for the first time, and a layer of two-dimensional (2D) WS2 nanosheets was inserted into the MD to form 2D material and oxide double-layer MD (2DOMD) to improve the reliability of single-layer devices. The results indicate that the electrochemical metallization memory cell exhibits a highly stable memristive switching and concentrated ON- and OFF-state voltage distribution, high speed (∼10 ns), and robust endurance (>109 cycles). This result is superior to MDs with a single-layer ZrO2 or WS2 film because two layers have different ion transport rates, thereby limiting the rupture/rejuvenation of CFs to the bilayer interface region, which can greatly reduce the randomness of CFs in MDs. Moreover, we used the handwritten recognition dataset (i.e., the Modified National Institute of Standards and Technology (MNIST) database) for neuromorphic simulations. Furthermore, biosynaptic functions and plasticity, including spike-timing-dependent plasticity and paired-pulse facilitation, have been successfully achieved. By incorporating 2D materials and oxides into a double-layer MD, the practical application of RRAM MD can be significantly enhanced to facilitate the development of artificial synapses for brain-enhanced computing systems in the future.

Self-Assembled Networked PbS Distribution Quantum Dots for Resistive Switching and Artificial Synapse Performance Boost of Memristors.

With the advent of the era of big data, resistive random access memory (RRAM) has become one of the most promising nanoscale memristor devices (MDs) for storing huge amounts of information. However, the switching voltage of the RRAM MDs shows a very broad distribution due to the random formation of the conductive filaments. Here, self-assembled lead sulfide (PbS) quantum dots (QDs) are used to improve the uniformity of switching parameters of RRAM, which is very simple comparing with other methods. The resistive switching (RS) properties of the MD with the self-assembled PbS QDs exhibit better performance than those of MDs with pure-Ga2 O3 and randomly distributed PbS QDs, such as a reduced threshold voltage, uniformly distributed SET and RESET voltages, robust retention, fast response time, and low power consumption. This enhanced performance may be attributed to the ordered arrangement of the PbS QDs in the self-assembled PbS QDs which can efficiently guide the growth direction for the conducting filaments. Moreover, biosynaptic functions and plasticity, are implemented successfully in the MD with the self-assembled PbS QDs. This work offers a new method of improving memristor performance, which can significantly expand existing applications and facilitate the development of artificial neural systems.

Efficient charge-transport in hybrid lead iodide perovskite solar cells.

Recently, highly efficient solar cells based on organic-inorganic perovskites have been intensively studied for developing fabricating methods and device structures. To improve the performance of perovskite film devices, delicate control of charge transfer material interconnectivity is required. Here, controlling the mesoporous TiO2 structure improves their charge collection and injection rate, and allows substantial enhancement of the corresponding device performance. We found that increasing the TiCl4 processing time deteriorates the device performance by introducing a large amount of excessively large perovskite particles, surface roughness and charge recombination. Proper TiCl4 processing dramatically improves the charge transport within the electron transfer layer, explaining the efficient performance of meso-superstructured solar cells.

Enhanced photocurrent by the co-sensitization of ZnO with dye and CuInSe nanocrystals.

ZnO nanocrystals with a particle size of 20-30 nm have been synthesised for the first time using a template-free method. Chalcopyrite Cu0.28In1.72Se2.72 nanocrystals (5-10 nm) were directly anchored on ZnO nanocrystals by a vacuum one-pot-nanocasting process without any long ligands. We further investigated Cu0.28In1.72Se2.72 quantum dots and dye bilayer-sensitized solar cells, which exhibited power conversion efficiency of 57.4% higher than the single-dye-sensitized solar cells.

Facile synthesis of CuInGaS2 quantum dot nanoparticles for bilayer-sensitized solar cells.

CuIn(0.7)Ga(0.3)S2 quantum dots (QDs) with particle size of 2-5 nm were directly synthesised by a vacuum one-pot-nanocasting process and homogeneously anchored on TiO2 nanocrystals (<50 nm) for the first time. We further present CuIn(0.7)Ga(0.3)S2 quantum dots and dye bilayer-sensitized solar cells with a power conversion efficiency 36.3% higher than mono-dye sensitized solar cells.