Co nanoparticles induced resistive switching and magnetism for the electrochemically deposited polypyrrole composite films

Review of Electrochemically Synthesized Resistive Switching Devices: Memory Storage, Neuromorphic Computing, and Sensing Applications

Resistive-switching-based memory devices meet most of the requirements for use in next-generation information and communication technology applications, including standalone memory devices, neuromorphic hardware, and embedded sensing devices with on-chip storage, due to their low cost, excellent memory retention, compatibility with 3D integration, in-memory computing capabilities, and ease of fabrication. Electrochemical synthesis is the most widespread technique for the fabrication of state-of-the-art memory devices. The present review article summarizes the electrochemical approaches that have been proposed for the fabrication of switching, memristor, and memristive devices for memory storage, neuromorphic computing, and sensing applications, highlighting their various advantages and performance metrics. We also present the challenges and future research directions for this field in the concluding section.

Deterministic Conductive Filament Formation and Evolution for Improved Switching Uniformity in Embedded Metal-Oxide-Based Memristors─A Phase-Field Study

The extreme device-to-device variation of switching performance is one of the major obstacles preventing the applications of metal-oxide-based memristors in large-scale memory storage and resistive neural networks. Recent experimental works have reported that embedding metal nano-islands (NIs) in metal oxides can effectively improve the uniformity of the memristors, but the underlying role of the NIs is not fully understood. Here, to address this specific problem, we develop a physical model to understand the origin of the variability and how the embedded NIs can improve the performance and uniformity of memristors. We find that due to the dimension confinement effect, embedding metal NIs can modulate the electric field distribution and lead to a more deterministic formation of the conductive filament (CF) from their vicinity, in contrast to the random growth of CFs without embedded NIs. This deterministic CF formation, via vacancy nucleation, further reduces the forming, reset, and set voltages and enhances the uniformity of the operation voltages and current ON/OFF ratios. We further demonstrate that modifying the shapes of the metal NIs can modulate the field strengths/distributions around the NIs and that choosing the NI metal composition and shape that chemically facilitate vacancy formations can further optimize the CF morphology, reduce the operation voltages, and improve the switching performance. Our work thus provides a fundamental understanding of how embedded metal NIs improve the resistive switching performance in oxide-based memristors and could potentially guide the selection of embedded NIs to realize a more uniform memristor that also operates at a higher efficiency than present materials.

Optical and structural properties of gradient (Ti,Co)Ox thin-film coatings with a resistive switching effect

In this work, the optical and structural properties of gradient (Ti,Co)Ox coatings with a resistive switching effect have been outlined. They were prepared using multi-magnetron sputtering and, despite the high cobalt content, they were transparent and had a high refractive index. The gradient Co-addition resulted in the receiving of fine crystalline T i O ₂-anatase and C o ₃ O ₄ forms in the amorphous surrounding. Observed resistance switching was a fully repeatable effect, and its occurrence in gradient (Ti,Co)Ox coatings has not reported earlier. The prepared gradient coatings exhibit great potential as transparent electronic devices with the resistance switching effect. Such memory effects in transparent thin-film coatings open new possibilities for the manufacturing of innovative memory elements in the future.

Tunable Si Dangling Bond Pathway Induced Forming-Free Hydrogenated Silicon Carbide Resistive Switching Memory Device

With the coming of the big data age, the resistive switching memory (RSM) of three-dimensional (3D) high density shows a significant application in information storage and processing due to its simple structure and size-scalable characteristic. However, an electrical initialization process makes the peripheral circuits of 3D integration too complicated to be realized. Here a new forming-free SiC_x:H-based device can be obtained by tuning the Si dangling bond conductive channel. It is discovered that the forming-free behavior can be ascribed to the Si dangling bonds in the as-deposited SiC_x:H films. By tuning the number of Si dangling bonds, the forming-free SiC_x:H RSM exhibits a tunable memory window. The fracture and connection of the Si dangling bond conduction pathway induces the switching from the high-resistance state (HRS) to the low-resistance state (LRS). Our discovery of forming-free SiC_x:H resistive switching memory with tunable pathway opens a way to the realization of 3D high-density memory.

Toward Controlling Filament Size and Location for Resistive Switches via Nanoparticle Exsolution at Oxide Interfaces

Memristive devices are among the most prominent candidates for future computer memory storage and neuromorphic computing. Though promising, the major hurdle for their industrial fabrication is their device-to-device and cycle-to-cycle variability. These occur due to the random nature of nanoionic conductive filaments, whose rupture and formation govern device operation. Changes in filament location, shape, and chemical composition cause cycle-to-cycle variability. This challenge is tackled by spatially confining conductive filaments with Ni nanoparticles. Ni nanoparticles are integrated on the bottom La_0.2 Sr_0.7 Ti_0.9 Ni_0.1 O_{3- δ} electrode by an exsolution method, in which, at high temperatures under reducing conditions, Ni cations migrate to the perovskite surface, generating metallic nanoparticles. This fabrication method offers fine control over particle size and density and ensures strong particle anchorage in the bottom electrode, preventing movement and agglomeration. In devices based on amorphous SrTiO₃ , it is demonstrated that as the exsolved Ni nanoparticle diameter increases up to ≈50 nm, the ratio between the ON and OFF resistance states increases from single units to 180 and the variability of the low resistance state reaches values below 5%. Exsolution is applied for the first time to engineer solid-solid interfaces extending its realm of application to electronic devices.

Hybrid conjugated polymer/magnetic nanoparticle composite nanofibers through cooperative non-covalent interactions

Hybrid organic-inorganic composites possessing both electronic and magnetic properties are promising materials for a wide range of applications. Controlled and ordered arrangement of the organic and inorganic components is key for synergistic cooperation toward desired functions. In this work, we report the self-assemblies of core-shell composite nanofibers from conjugated block copolymers and magnetic nanoparticles through the cooperation of orthogonal non-covalent interactions. We show that well-defined core-shell conjugated polymer nanofibers can be obtained through solvent induced self-assembly and polymer crystallization, while hydroxy and pyridine functional groups located at the shell of nanofibers can immobilize magnetic nanoparticles via hydrogen bonding and coordination interactions. These precisely arranged nanostructures possess electronic properties intrinsic to the polymers and are simultaneously responsive to external magnetic fields. We applied these composite nanofibers in organic solar cells and found that these non-covalent interactions led to controlled thin film morphologies containing uniformly dispersed nanoparticles, although high loadings of these inorganic components negatively impact device performance. Our methodology is general and can be utilized to control the spatial distribution of functionalized organic/inorganic building blocks, and the magnetic responsiveness and optoelectronic activities of these nanostructures may lead to new opportunities in energy and electronic applications.

Nanoscale mechanical control of surface electrical properties of manganite films with magnetic nanoparticles

Mechanical control of electrical properties in complex heterostructures, consisting of magnetic FeO x nanoparticles on top of manganite films, is achieved using atomic force microscope (AFM) based methods. Under applied pressure of the AFM tip, drop of the electrical conductivity is observed inducing an electrically insulating state upon a critical normal load. Current and surface potential maps suggest that the switching process is mainly governed by the flexoelectric field induced at the sample surface. The relaxation process of the electrical surface potential indicates that the diffusion of oxygen vacancies from the bulk of the manganite films towards the sample surface is the dominant relaxation mechanism. The magnetic FeO x nanoparticles, staying attached to the sample surface after the rubbing, protect the underlying manganite films and provide stability of the observed resistive switching effect. The employed mechanical control gives a new freedom in the design of resistive switching devices since it does not depend on the film thickness, and biasing is not needed.

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.

Ag-NPs doping enhanced resistive switching performance and induced changes in magnetic properties of NiFe2O4 thin films

The resistive switching performance of NiFe₂O₄ thin films can be enhanced by controlling Ag-NPs doping concentration.

Effect of Electronegativity on Bipolar Resistive Switching in a WO3-Based Asymmetric Capacitor Structure

This study investigates the transport and switching time of nonvolatile tungsten oxide based resistive-switching (RS) memory devices. These devices consist of a highly resistive tungsten oxide film sandwiched between metal electrodes, and their RS characteristics are bipolar in the counterclockwise direction. The switching voltage, retention, endurance, and switching time are strongly dependent on the type of electrodes used, and we also find quantitative and qualitative evidence that the electronegativity (χ) of the electrodes plays a key role in determining the RS properties and switching time. We also propose an RS model based on the role of the electronegativity at the interface.

Epitaxial Brownmillerite Oxide Thin Films for Reliable Switching Memory

Resistive switching memory, which is mostly based on polycrystalline thin films, suffers from wide distributions in switching parameters-including set voltage, reset voltage, and resistance-in their low- and high-resistance states. One of the most commonly used methods to overcome this limitation is to introduce inhomogeneity. By contrast, in this paper, we obtained uniform resistive switching parameters and sufficiently low forming voltage by maximizing the uniformity of an epitaxial thin film. To achieve this result, we deposited an SrFeOx/SrRuO3 heteroepitaxial structure onto an SrTiO3 (001) substrate by pulsed laser deposition, and then we deposited an Au top electrode by electron-beam evaporation. This device exhibited excellent bipolar resistance switching characteristics, including a high on/off ratio, narrow distribution of key switching parameters, and long data retention time. We interpret these phenomena in terms of a local, reversible phase transformation in the SrFeOx film between brownmillerite and perovskite structures. Using the brownmillerite structure and atomically uniform thickness of the heteroepitaxial SrFeOx thin film, we overcame two major hurdles in the development of resistive random-access memory devices: high forming voltage and broad distributions of switching parameters.

Conductance Quantization in Resistive Random Access Memory

The intrinsic scaling-down ability, simple metal-insulator-metal (MIM) sandwich structure, excellent performances, and complementary metal-oxide-semiconductor (CMOS) technology-compatible fabrication processes make resistive random access memory (RRAM) one of the most promising candidates for the next-generation memory. The RRAM device also exhibits rich electrical, thermal, magnetic, and optical effects, in close correlation with the abundant resistive switching (RS) materials, metal-oxide interface, and multiple RS mechanisms including the formation/rupture of nanoscale to atomic-sized conductive filament (CF) incorporated in RS layer. Conductance quantization effect has been observed in the atomic-sized CF in RRAM, which provides a good opportunity to deeply investigate the RS mechanism in mesoscopic dimension. In this review paper, the operating principles of RRAM are introduced first, followed by the summarization of the basic conductance quantization phenomenon in RRAM and the related RS mechanisms, device structures, and material system. Then, we discuss the theory and modeling of quantum transport in RRAM. Finally, we present the opportunities and challenges in quantized RRAM devices and our views on the future prospects.