Ambient Stable All Inorganic CsCu2I3 Artificial Synapses for Neurocomputing.

In resistive switching memories or artificial synaptic devices, halide perovskites have attracted attention for their unusual features such as rapid ion migration, adjustable composition, and facile synthesis. Herein, the environmentally friendly and highly air stable CsCu2I3 perovskite films are used as the active layer in the Au/CsCu2I3/ITO/glass artificial synapses. The device shows variable synaptic plasticities such as long-term and short-term synaptic plasticity, paired-pulse facilitation, and spike-timing-dependent plasticity by combining potentiation and depression along the formation of conductive filaments. The performances of the devices are maintained for 160 days under ambient conditions. Additionally, the accuracy evaluation of the CsCu2I3-based artificial synapses performs exceptionally well with the MNIST and Fashion MNIST data sets, demonstrating high learning accuracy in deep neural networks. Using the novel B-site engineered halide perovskite material with extreme air stability, this study paves the way for artificial synaptic devices for next-generation in-memory hardware.

Lead-Free Dual-Phase Halide Perovskites for Preconditioned Conducting-Bridge Memory.

Organometallic and all-inorganic halide perovskites (HPs) have recently emerged as promising candidate materials for resistive switching (RS) nonvolatile memory due to their current-voltage hysteresis caused by fast ion migration. Lead-free and all-inorganic HPs have been researched for non-toxic and environmentally friendly RS memory devices. However, only HP-based devices with electrochemically active top electrode (TE) exhibit ultra-low operating voltages and high on/off ratio RS properties. The active TE easily reacts to halide ions in HP films, and the devices have a low device durability. Herein, RS memory devices based on an air-stable lead-free all-inorganic dual-phase HP (AgBi2 I7 -Cs3 Bi2 I9 ) are successfully fabricated with inert metal electrodes. The devices with Au TE show filamentary RS behavior by conducting-bridge involving Ag cations in HPs with ultra-low operating voltages (<0.15 V), high on/off ratio (>107 ), multilevel data storage, and long retention times (>5 × 104 s). The use of a closed-loop pulse switching method improves reversible RS properties up to 103 cycles with high on/off ratio above 106 . With an extremely small bending radius of 1 mm, the devices are operable with reasonable RS characteristics. This work provides a promising material strategy for lead-free all-inorganic HP-based nonvolatile memory devices for practical applications.

Two-Terminal Lithium-Mediated Artificial Synapses with Enhanced Weight Modulation for Feasible Hardware Neural Networks.

Recently, artificial synapses involving an electrochemical reaction of Li-ion have been attributed to have remarkable synaptic properties. Three-terminal synaptic transistors utilizing Li-ion intercalation exhibits reliable synaptic characteristics by exploiting the advantage of non-distributed weight updates owing to stable ion migrations. However, the three-terminal configurations with large and complex structures impede the crossbar array implementation required for hardware neuromorphic systems. Meanwhile, achieving adequate synaptic performances through effective Li-ion intercalation in vertical two-terminal synaptic devices for array integration remains challenging. Here, two-terminal Au/LixCoO2/Pt artificial synapses are proposed with the potential for practical implementation of hardware neural networks. The Au/LixCoO2/Pt devices demonstrated extraordinary neuromorphic behaviors based on a progressive dearth of Li in LixCoO2 films. The intercalation and deintercalation of Li-ion inside the films are precisely controlled over the weight control spike, resulting in improved weight control functionality. Various types of synaptic plasticity were imitated and assessed in terms of key factors such as nonlinearity, symmetricity, and dynamic range. Notably, the LixCoO2-based neuromorphic system outperformed three-terminal synaptic transistors in simulations of convolutional neural networks and multilayer perceptrons due to the high linearity and low programming error. These impressive performances suggest the vertical two-terminal Au/LixCoO2/Pt artificial synapses as promising candidates for hardware neural networks.

Halide Perovskites for Memristive Data Storage and Artificial Synapses.

Halide perovskites have been noted for their exotic properties such as fast ion migration, tunable composition, facile synthetic routes, and flexibility in addition to large light absorption coefficients, long carrier diffusion lengths, and high defect tolerance. These properties have made halide perovskites promising materials for memristors. Applications in the field of resistive switching memory devices and artificial synapses for neuromorphic computing are especially noteworthy. This Perspective covers state-of-the-art perovskite-based memristive devices. Moreover, the fundamental mechanisms and characteristics of perovskite-based memristors are elucidated. Interesting opportunities to improve the performance of perovskite-based memristors for commercialization are provided, including improving film uniformity and air stability, controlling the stoichiometry, finding new all-inorganic and lead-free halide perovskites, and making perovskites into single crystals or quantum dots. We expect our Perspective to be the foundation of realizing next-generation halide perovskite-based memristors.