Advances in Flexible Memristors with Hybrid Perovskites

Capacitive and Inductive Characteristics of Volatile Perovskite Resistive Switching Devices with Analog Memory

With the increasing demands and complexity of the neuromorphic computing schemes utilizing highly efficient analog resistive switching devices, understanding the apparent capacitive and inductive effects in device operation is of paramount importance. Here, we present a systematic array of characterization methods that unravel two distinct voltage-dependent regimes demonstrating the complex interplay between the dynamic capacitive and inductive effects in volatile perovskite-based memristors: (1) a low voltage capacitance-dominant and (2) an inductance-dominant regime evidenced by the highly correlated hysteresis type with nonzero crossing, the impedance responses, and the transient current characteristics. These dynamic capacitance- and inductance-dominant regimes provide fundamental insight into the resistive switching of memristors governing the synaptic depression and potentiation functions, respectively. More importantly, the pulse width-dependent and long-term transient current measurements further demonstrate a dynamic transition from a fast capacitive to a slow inductive response, allowing for the tailored stimulus programming of memristor devices to mimic synaptic functionality.

2D Halide Perovskites for High-Performance Resistive Switching Memory and Artificial Synapse Applications

Metal halide perovskites (MHPs) are considered as promising candidates in the application of nonvolatile high-density, low-cost resistive switching (RS) memories and artificial synapses, resulting from their excellent electronic and optoelectronic properties including large light absorption coefficient, fast ion migration, long carrier diffusion length, low trap density, high defect tolerance. Among MHPs, 2D halide perovskites have exotic layered structure and great environment stability as compared with 3D counterparts. Herein, recent advances of 2D MHPs for the RS memories and artificial synapses realms are comprehensively summarized and discussed, as well as the layered structure properties and the related physical mechanisms are presented. Furthermore, the current issues and developing roadmap for the next-generation 2D MHPs RS memories and artificial synapse are elucidated.

Control-Etched Ti3C2Tx MXene Nanosheets for a Low-Voltage-Operating Flexible Memristor for Efficient Neuromorphic Computation

Hardware neural networks with mechanical flexibility are promising next-generation computing systems for smart wearable electronics. Overcoming the challenge of developing a fully synaptic plastic network, we demonstrate a low-operating-voltage PET/ITO/p-MXene/Ag flexible memristor device by controlling the etching of aluminum metal ions in Ti3C2Tx MXene. The presence of a small fraction of Al ions in partially etched MXene (p-Ti3C2Tx) significantly suppresses the operating voltage to 1 V compared to 7 V from fully Al etched MXene (f-Ti3C2Tx)-based devices. Former devices exhibit excellent non-volatile data storage properties, with a robust ∼103 ON/OFF ratio, high endurance of ∼104 cycles, multilevel resistance states, and long data retention measured up to ∼106 s. High mechanical stability up to ∼73° bending angle and environmental robustness are confirmed with consistent switching characteristics under increasing temperature and humid conditions. Furthermore, a p-Ti3C2Tx MXene memristor is employed to mimic the biological synapse by measuring the learning-forgetting pattern for ∼104 cycles as potentiation and depression. Spike time-dependent plasticity (STDP) based on Hebb's Learning rules is also successfully demonstrated. Moreover, a remarkable accuracy of ∼95% in recognizing modified patterns from the National Institute of Standards and Technology (MNIST) data set with just 29 training epochs is achieved in simulation. Ultimately, our findings underscore the potential of MXene-based flexible memristor devices as versatile components for data storage and neuromorphic computing.

Alkylammonium Halides for Phase Regulation and Luminescence Modulation of Cesium Copper Iodide Nanocrystals for Light-Emitting Diodes

All-inorganic cesium copper halide nanocrystals have attracted extensive attention due to their cost-effectiveness, low toxicity, and rich luminescence properties. However, controlling the synthesis of these nanocrystals to achieve a precise composition and high luminous efficiency remains a challenge that limits their future application. Herein, we report the effect of oleylammonium iodide on the synthesis of copper halide nanocrystals to control the composition and phase and modulate their photoluminescence (PL) quantum yields (QYs). For CsCu2I3, the PL peak is centered at 560 nm with a PLQY of 47.3%, while the PL peak of Cs3Cu2I5 is located at 440 nm with an unprecedently high PLQY of 95.3%. Furthermore, the intermediate-state CsCu2I3/Cs3Cu2I5 heterostructure shows white light emission with a PLQY of 66.4%, chromaticity coordinates of (0.3176, 0.3306), a high color rendering index (CRI) of 90, and a correlated color temperature (CCT) of 6234 K, indicating that it is promising for single-component white-light-emitting applications. The nanocrystals reported in this study have excellent luminescence properties, low toxicity, and superior stability, so they are more suitable for future light-emitting applications.

Piezo-Acoustic Resistive Switching Behaviors in High-Performance Organic-Inorganic Hybrid Perovskite Memristors

Memristors are regarded as promising candidates for breaking the problems including high off-chip memory access delays and the hash rate cost of frequent data moving induced by algorithms for data-intensive applications of existing computational systems. Recently, organic-inorganic halide perovskites (OIHPs) have been recognized as exceptionally favorable materials for memristors due to ease of preparation, excellent electrical conductivity, and structural flexibility. However, research on OIHP-based memristors focuses on modulating resistive switching (RS) performance through electric fields, resulting in difficulties in moving away from complex external circuits and wire connections. Here, a multilayer memristor has been constructed with eutectic gallium and indium (EGaIn)/ MAPbI3 /poly(3,4-ethylenedioxythiophene): poly(4-styrenesulphonate) (PEDOT: PSS)/indium tin oxide (ITO) structure, which exhibits reproducible and reliable bipolar RS with low SET/RESET voltages, stable endurance, ultrahigh average ON/OFF ratio, and excellent retention. Importantly, based on ion migration activated by sound-driven piezoelectric effects, the device exhibits a stable acoustic response with an average ON/OFF ratio greater than 103 , thus realizing non-contact, multi-signal, and far-field control in RS modulation. This study provides a single-structure multifunctional memristor as an integrated architecture for sensing, data storage, and computing.

Synthesis of Perovskite-Type BiScO3 Ceramics and their Dielectric and Infrared Characterization

BiScO₃ compound was obtained in the form of dense ceramic with a perovskite-type structure, and its complex characterization was determined for the first time. The corresponding synthesis procedure is described in detail. It is demonstrated that the temperature region of the phase stability at atmospheric pressure lies at T < 700 °C (973 K). It is shown that the crystal structure of the BiScO₃ ceramic is centrosymmetric. Dielectric measurements of the synthesized sample performed at frequencies 25 Hz to 1 MHz and at temperatures 10-340 K show no changes typical for phase transition. Room-temperature infrared (30-15600 cm^-1) and Raman (90-2000 cm^-1) spectra of the prepared BiScO₃ ceramic are measured, and information on the parameters of phonon resonances is obtained. The number of infrared modes exceeds that predicted by the factor group analysis of the noncentrosymmetric space group C2. The reason for selection rules violation can be associated with the disorder of the crystal structure and local distortions induced by the lone pair of electrons of Bi³⁺.

Ag/HfOx/Pt Unipolar Memristor for High-Efficiency Logic Operation

Unipolar resistive switching (URS) behavior, known as the SET and RESET operating in a single voltage sweep direction, has shown great potential in the simplification of the peripheral circuit. The URS memristor always involves complicated interfacial engineering and structural design. In this work, a reliable URS behavior is realized using a simple Ag/HfO_x/Pt memristor structure. The memristor displays a retention time of >10⁴ s, an ON/OFF ratio of >10³, and a good operation voltage. Synergy and competition between the Ag conductive filament formed by redox reaction and the migration of an oxygen vacancy are responsible for the observed URS. By comparison, a 35% power consumption is reduced during the logical operation from 0 to 1 to 0. The operation strategy is demonstrated by exhibiting the ACSII code of the capital letter denoted by eight logic states. This work provides a low-power concept for ultrahigh data storage using the URS memristor.

High-Performance Resistive Random Access Memories Based on Two-Dimensional HAPbI4 Organic-Inorganic Hybrid Perovskite

Organic-inorganic hybrid perovskites have attracted extensive attention for potential memory applications because of their excellent properties, such as high charge carrier mobility and fast ion migration. Herein, the two-dimensional HAPbI₄ perovskite with an octahedral structure and high stability was prepared by a facile solution method. Moreover, the resistive random access memory (RRAM) with the Ag/PMMA/HAPbI₄/ITO structure has been successfully fabricated by spin coating and vacuum thermal evaporation. The as-prepared RRAM device based on HAPbI₄ demonstrated superior resistive switching performance. The on/off ratio is as high as 10⁵, and the corresponding retention of the device exceeds 10 000 s; furthermore, the RRAM device could be kept stable after being kept in the air for 24 weeks.

Dynamic Instability and Time Domain Response of a Model Halide Perovskite Memristor for Artificial Neurons

Memristors are candidate devices for constructing artificial neurons, synapses, and computational networks for brainlike information processing and sensory-motor autonomous systems. However, the dynamics of natural neurons and synapses are challenging and cannot be well reproduced with standard electronic components. Halide perovskite memristors operate by mixed ionic-electronic properties that may lead to replicate the live computation elements. Here we explore the dynamical behavior of a halide perovskite memristor model to evaluate the response to a step perturbation and the self-sustained oscillations that produce analog neuron spiking. As the system contains a capacitor and a voltage-dependent chemical inductor, it can mimic an action potential in response to a square current pulse. Furthermore, we discover a property that cannot occur in the standard two-dimensional model systems: a three-dimensional model shows a dynamical instability that produces a spiking regime without the need for an intrinsic negative resistance. These results open a new pathway to create spiking neurons without the support of electronic circuits.

Chemical Inductor

A multitude of chemical, biological, and material systems present an inductive behavior that is not electromagnetic in origin. Here, it is termed a chemical inductor. We show that the structure of the chemical inductor consists of a two-dimensional system that couples a fast conduction mode and a slowing down element. Therefore, it is generally defined in dynamical terms rather than by a specific physicochemical mechanism. The chemical inductor produces many familiar features in electrochemical reactions, including catalytic, electrodeposition, and corrosion reactions in batteries and fuel cells, and in solid-state semiconductor devices such as solar cells, organic light-emitting diodes, and memristors. It generates the widespread phenomenon of negative capacitance, it causes negative spikes in voltage transient measurements, and it creates inverted hysteresis effects in current–voltage curves and cyclic voltammetry. Furthermore, it determines stability, bifurcations, and chaotic properties associated to self-sustained oscillations in biological neurons and electrochemical systems. As these properties emerge in different types of measurement techniques such as impedance spectroscopy and time-transient decays, the chemical inductor becomes a useful framework for the interpretation of the electrical, optoelectronic, and electrochemical responses in a wide variety of systems. In the paper, we describe the general dynamical structure of the chemical inductor and we comment on a broad range of examples from different research areas.

Flexible and Stretchable Memristive Arrays for in-Memory Computing

With the tremendous progress of Internet of Things (IoT) and artificial intelligence (AI) technologies, the demand for flexible and stretchable electronic systems is rapidly increasing. As the vital component of a system, existing computing units are usually rigid and brittle, which are incompatible with flexible and stretchable electronics. Emerging memristive devices with flexibility and stretchability as well as direct processing-in-memory ability are promising candidates to perform data computing in flexible and stretchable electronics. To execute the in-memory computing paradigm including digital and analogue computing, the array configuration of memristive devices is usually required. Herein, the recent progress on flexible and stretchable memristive arrays for in-memory computing is reviewed. The common materials used for flexible memristive arrays, including inorganic, organic and two-dimensional (2D) materials, will be highlighted, and effective strategies used for stretchable memristive arrays, including material innovation and structural design, will be discussed in detail. The current challenges and future perspectives of the in-memory computing utilizing flexible and stretchable memristive arrays are presented. These efforts aim to accelerate the development of flexible and stretchable memristive arrays for data computing in advanced intelligent systems, such as electronic skin, soft robotics, and wearable devices.

Impedance Spectroscopy of Metal Halide Perovskite Solar Cells from the Perspective of Equivalent Circuits

Impedance spectroscopy (IS) provides a detailed understanding of the dynamic phenomena underlying the operation of photovoltaic and optoelectronic devices. Here we provide a broad summary of the application of IS to metal halide perovskite materials, solar cells, electrooptic and memory devices. IS has been widely used to characterize perovskite solar cells, but the variability of samples and the presence of coupled ionic-electronic effects form a complex problem that has not been fully solved yet. We summarize the understanding that has been obtained so far, the basic methods and models, as well as the challenging points still present in this research field. Our approach emphasizes the importance of the equivalent circuit for monitoring the parameters that describe the response and providing a physical interpretation. We discuss the possibilities of models from the general perspective of solar cell behavior, and we describe the specific aspects and properties of the metal halide perovskites. We analyze the impact of the ionic effects and the memory effects, and we describe the combination of light-modulated techniques such as intensity modulated photocurrent spectroscopy (IMPS) for obtaining more detailed information in complex cases. The transformation of the frequency to time domain is discussed for the consistent interpretation of time transient techniques and the prediction of features of current-voltage hysteresis. We discuss in detail the stability issues and the occurrence of transformations of the sample coupled to the measurements.

Resistance Switching Behavior of a Perhydropolysilazane-Derived SiOx-Based Memristor

SiO_x is an important dielectric material layer for resistive switching memory due to its compatibility with complementary metal-oxide semiconductor (CMOS) technology. Here we propose a solution process for a SiO_x dielectric layer based on perhydropolysilazane (PHPS). A series of SiO_x layers with different compositions are prepared by controlling the conversion process from PHPS, then the resistance switching behaviors of typical Ag/SiO_x/Au memristors are analyzed. The effect of oxygen vacancies and Si-OH groups on the formation and rupture of Ag conducting filaments (CFs) in the SiO_x layer was thoroughly investigated. Ultimately, we achieved a high-performance memristor with a coefficient of variation (σ/μ) as low as 0.16 ± 0.08 and an on/off ratio as high as 10⁶, which can rival the performance of the SiO_x memristors based on the high-vacuum and high-cost vapor deposition methods. These findings demonstrate the high promise of the PHPS-derived SiO_x dielectric layer in the field of memristors.