Resistive switching and role of interfaces in memristive devices based on amorphous NbOx grown by anodic oxidation.

Memristive devices based on the resistive switching mechanism are continuously attracting attention in the framework of neuromorphic computing and next-generation memory devices. Here, we report on a comprehensive analysis of the resistive switching properties of amorphous NbOx grown by anodic oxidation. Besides a detailed chemical, structural and morphological analysis of the involved materials and interfaces, the mechanism of switching in Nb/NbOx/Au resistive switching cells is discussed by investigating the role of metal-metal oxide interfaces in regulating electronic and ionic transport mechanisms. The resistive switching was found to be related to the formation/rupture of conductive nanofilaments in the NbOx layer under the action of an applied electric field, facilitated by the presence of an oxygen scavenger layer at the Nb/NbOx interface. Electrical characterization including device-to-device variability revealed an endurance >103 full-sweep cycles, retention >104 s, and multilevel capabilities. Furthermore, the observation of quantized conductance supports the physical mechanism of switching based on the formation of atomic-scale conductive filaments. Besides providing new insights into the switching properties of NbOx, this work also highlights the perspective of anodic oxidation as a promising method for the realization of resistive switching cells.

Resistivity control by the electrochemical removal of dopant atoms from a nanodot.

Doping impurity atoms into semiconductor materials changes the resistance of the material. Selecting the atomic species of a dopant and the precise control of the number of dopant atoms in a unit volume can control the resistance to a desired value. The number of dopant atoms is usually controlled when the materials are synthesized. It can also be controlled after synthesizing by injecting dopant atoms using an ion implantation technique. This physical method has now enabled atom by atom implantation at the desired position. Here, we propose an additional technique, based on the electrochemical potential of dopant atoms in a material. The technique enables the dynamic control of the number of dopant atoms through the application of bias to the material. We demonstrate the controlled removal of dopant atoms using Ag2+δS and Ag-doped Ta2O5 as model materials. The change in resistance accompanying the removal of dopant atoms is also observed.

Spring-Like Pseudoelectroelasticity of Monocrystalline Cu2S Nanowire.

Prediction from the dual-phase nature of superionic conductors-both solid and liquid-like-is that mobile ions in the material may experience reversible extraction-reinsertion by an external electric field. However, this type of pseudoelectroelasticity has not been confirmed in situ, and no details on the microscopic mechanism are known. Here, we in situ monitor the pseudoelectroelasticity of monocrystalline Cu2S nanowires (NWs) using transmission electron microscopy (TEM). Specifically, we reveal the atomic scale details including phase transformation, migration and redox reactions of Cu+ ions, nucleation, growth, as well as spontaneous shrinking of Cu protrusion. Caterpillar-diffusion-dominated deformation is confirmed by the high-resolution transmission electron microscopy (HRTEM) observation and  ab initio calculation, which can be driven by either an external electric field or chemical potential difference. The observed spring-like behavior was creatively adopted for electric nanoactuators. Our findings are crucial to elucidate the mechanism of pseudoelectroelasticity and could potentially stimulate in-depth research into electrochemical and nanoelectromechanical systems.

Volatile resistance states in electrochemical metallization cells enabling non-destructive readout of complementary resistive switches.

Redox-based resistive memory cells exhibit changes of OFF or intermediate resistance values over time and even ON states can be completely lost in certain cases. The stability of these resistance states and the time until resistance loss strongly depends on the materials system. On the basis of electrical measurements and chemical analysis we found a viable explanation for these volatile resistance states (VRSs) in Ag-GeSx-based electrochemical metallization memory cells and identified a technological application in the field of crossbar memories. Complementary resistive switches usually suffer from the necessity of a destructive read-out procedure increasing wear and reducing read-out speed. From our analysis we deduced a solution to use the VRS as an inherent selector mechanism without the need for additional selector devices.

Physical origins and suppression of Ag dissolution in GeS(x)-based ECM cells.

Electrochemical metallisation (ECM) memory cells potentially suffer from limited memory retention time, which slows down the future commercialisation of this type of data memory. In this work, we investigate Ag/GeSx/Pt redox-based resistive memory cells (ReRAM) with and without an additional Ta barrier layer by time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray absorption spectroscopy (XAS) and synchrotron high-energy X-ray diffractometry (XRD) to investigate the physical mechanism behind the shift and/or loss of OFF data retention. Electrical measurements demonstrate the effectiveness and high potential of the diffusion barrier layer in practical applications.

Electrochemical rewiring through quantum conductance effects in single metallic memristive nanowires.

Memristive devices have been demonstrated to exhibit quantum conductance effects at room temperature. In these devices, a detailed understanding of the relationship between electrochemical processes and ionic dynamic underlying the formation of atomic-sized conductive filaments and corresponding electronic transport properties in the quantum regime still represents a challenge. In this work, we report on quantum conductance effects in single memristive Ag nanowires (NWs) through a combined experimental and simulation approach that combines advanced classical molecular dynamics (MD) algorithms and quantum transport simulations (DFT). This approach provides new insights on quantum conductance effects in memristive devices by unravelling the intrinsic relationship between electronic transport and atomic dynamic reconfiguration of the nanofilment, by shedding light on deviations from integer multiples of the fundamental quantum of conductance depending on peculiar dynamic trajectories of nanofilament reconfiguration and on conductance fluctuations relying on atomic rearrangement due to thermal fluctuations.

Atomically controlled electrochemical nucleation at superionic solid electrolyte surfaces.

Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg(4)I(5) superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems.

Switching kinetics of electrochemical metallization memory cells.

The strongly nonlinear switching kinetics of electrochemical metallization memory (ECM) cells are investigated using an advanced 1D simulation model. It is based on the electrochemical growth and dissolution of a Ag or Cu filament within a solid thin film and accounts for nucleation effects, charge transfer, and cation drift. The model predictions are consistent with experimental switching results of a time range of 12 orders of magnitude obtained from silver iodide (AgI) based ECM cells. By analyzing the simulation results the electrochemical processes limiting the switching kinetics are revealed. This study provides new insights into the understanding of the limiting electrochemical processes determining the switching kinetics of ECM cells.

Electrochemical-Memristor-Based Artificial Neurons and Synapses-Fundamentals, Applications, and Challenges.

Artificial neurons and synapses are considered essential for the progress of the future brain-inspired computing, based on beyond von Neumann architectures. Here, a discussion on the common electrochemical fundamentals of biological and artificial cells is provided, focusing on their similarities with the redox-based memristive devices. The driving forces behind the functionalities and the ways to control them by an electrochemical-materials approach are presented. Factors such as the chemical symmetry of the electrodes, doping of the solid electrolyte, concentration gradients, and excess surface energy are discussed as essential to understand, predict, and design artificial neurons and synapses. A variety of two- and three-terminal memristive devices and memristive architectures are presented and their application for solving various problems is shown. The work provides an overview of the current understandings on the complex processes of neural signal generation and transmission in both biological and artificial cells and presents the state-of-the-art applications, including signal transmission between biological and artificial cells. This example is showcasing the possibility for creating bioelectronic interfaces and integrating artificial circuits in biological systems. Prospectives and challenges of the modern technology toward low-power, high-information-density circuits are highlighted.

Chemical Influence of Carbon Interface Layers in Metal/Oxide Resistive Switches.

Thin layers introduced between a metal electrode and a solid electrolyte can significantly alter the transport of mass and charge at the interfaces and influence the rate of electrode reactions. C films embedded in functional materials can change the chemical properties of the host, thereby altering the functionality of the whole device. Using X-ray spectroscopies, here we demonstrate that the chemical and electronic structures in a representative redox-based resistive switching (RS) system, Ta2O5/Ta, can be tuned by inserting a graphene or ultrathin amorphous C layer. The results of the orbitalwise analyses of synchrotron Ta L3-edge, C K-edge, and O K-edge X-ray absorption spectroscopy showed that the C layers between Ta2O5 and Ta are significantly oxidized to form COx and, at the same time, oxidize the Ta layers with different degrees of oxidation depending on the distance: full oxidation at the nearest 5 nm Ta and partial oxidation in the next 15 nm Ta. The depth-resolved information on the electronic structure for each layer further revealed a significant modification of the band alignments due to C insertion. Full oxidation of the Ta metal near the C interlayer suggests that the oxygen-vacancy-related valence change memory mechanism for the RS can be suppressed, thereby changing the RS functionalities fundamentally. The knowledge on the origin of C-enhanced surfaces can be applied to other metal/oxide interfaces and used for the advanced design of memristive devices.

Effect of electrode materials on resistive switching behaviour of NbOx-based memristive devices.

Memristive devices that rely on redox-based resistive switching mechanism have attracted great attention for the development of next-generation memory and computing architectures. However, a detailed understanding of the relationship between involved materials, interfaces, and device functionalities still represents a challenge. In this work, we analyse the effect of electrode metals on resistive switching functionalities of NbOx-based memristive cells. For this purpose, the effect of Au, Pt, Ir, TiN, and Nb top electrodes was investigated in devices based on amorphous NbOx grown by anodic oxidation on a Nb substrate exploited also as counter electrode. It is shown that the choice of the metal electrode regulates electronic transport properties of metal-insulator interfaces, strongly influences the electroforming process, and the following resistive switching characteristics. Results show that the electronic blocking character of Schottky interfaces provided by Au and Pt metal electrodes results in better resistive switching performances. It is shown that Pt represents the best choice for the realization of memristive cells when the NbOx thickness is reduced, making possible the realization of memristive cells characterised by low variability in operating voltages, resistance states and with low device-to-device variability. These results can provide new insights towards a rational design of redox-based memristive cells.

Microwave synthesis of molybdenene from MoS2.

Dirac materials are characterized by the emergence of massless quasiparticles in their low-energy excitation spectrum that obey the Dirac Hamiltonian. Known examples of Dirac materials are topological insulators, d-wave superconductors, graphene, and Weyl and Dirac semimetals, representing a striking range of fundamental properties with potential disruptive applications. However, none of the Dirac materials identified so far shows metallic character. Here, we present evidence for the formation of free-standing molybdenene, a two-dimensional material composed of only Mo atoms. Using MoS2 as a precursor, we induced electric-field-assisted molybdenene growth under microwave irradiation. We observe the formation of millimetre-long whiskers following screw-dislocation growth, consisting of weakly bonded molybdenene sheets, which, upon exfoliation, show metallic character, with an electrical conductivity of ~940 S m-1. Molybdenene when hybridized with two-dimensional h-BN or MoS2, fetch tunable optical and electronic properties. As a proof of principle, we also demonstrate applications of molybdenene as a surface-enhanced Raman spectroscopy platform for molecular sensing, as a substrate for electron imaging and as a scanning probe microscope cantilever.

Recent Advances and Future Prospects for Memristive Materials, Devices, and Systems.

Memristive technology has been rapidly emerging as a potential alternative to traditional CMOS technology, which is facing fundamental limitations in its development. Since oxide-based resistive switches were demonstrated as memristors in 2008, memristive devices have garnered significant attention due to their biomimetic memory properties, which promise to significantly improve power consumption in computing applications. Here, we provide a comprehensive overview of recent advances in memristive technology, including memristive devices, theory, algorithms, architectures, and systems. In addition, we discuss research directions for various applications of memristive technology including hardware accelerators for artificial intelligence, in-sensor computing, and probabilistic computing. Finally, we provide a forward-looking perspective on the future of memristive technology, outlining the challenges and opportunities for further research and innovation in this field. By providing an up-to-date overview of the state-of-the-art in memristive technology, this review aims to inform and inspire further research in this field.

Design of Materials Configuration for Optimizing Redox-Based Resistive Switching Memories.

Redox-based resistive random access memories (ReRAMs) are based on electrochemical processes of oxidation and reduction within the devices. The selection of materials and material combinations strongly influence the related nanoscale processes, playing a crucial role in resistive switching properties and functionalities. To date, however, comprehensive studies on device design accounting for a combination of factors such as electrodes, electrolytes, and capping layer materials related to their thicknesses and interactions are scarce. In this work, the impact of materials' configuration on interfacial redox reactions in HfO2 -based electrochemical metallization memory (ECM) and valence-change memory (VCM) systems is reported. The redox processes are studied by cyclic voltammetry, and the corresponding resistive switching characteristics are investigated. In ECM cells, the overall cell resistance depends on the electrocatalytic activity of the counter electrode. Nonetheless, the capping layer material further influences the cell resistance and the SET and RESET voltages. In VCM systems, the influence of the electrode material configuration is also pronounced, and is capable of modulating the active resistive switching interface. For both types of memory cells, the switching behavior changes significantly with variation of the oxide thickness. The results present important materials selection criteria for rationale design of ReRAM cells for various memristive applications.