Memristive Performance Research Articles

Polyfluorene Bearing Quinoline and Ferrocene as the Pendant Groups for Bioelectronics

Abstract Analog-type polymer memristors are preeminent candidates for neuromorphic computation and synaptic biomimicking. A novel polyfluorene bearing quinoline and ferrocene as the pendant groups (PQF) is synthesized. The PQF film sandwiched between the Al and ITO electrodes exhibits outstanding analog-type memristive performance at a small sweep voltage range of ±1V, with 42 distinguishable conductance states. As compared, poly{[9,9-di(2,4-diphenylquinoline)-fluorene]-alt-[9,9-bis(6-bromohexyl)-9H-fluorene]}, and poly {[9H-fluorene]-alt-[fluorene with triazole and ferrocene moieties in the sidechains] only show 12 and 15 distinguishable conductance states, respectively. The former only involves intramolecular charge transfer from the polymer backbone to the quinolone moieties, while the latter mainly concern redox effect from the ferrocene moieties. These results demonstrate that the combined action of intramolecular electron transfer and redox effect could dramatically increase the number of distinguishable conductive states of the resultant materials. Associated with the exceptional memristive performance, the PQF-based memristor can accurately recognize human emotions. After 150 training sessions, the accuracy of emotion recognition reaches up to 98.74%.

Non-Volatile Resistive Switching in Nanoscaled Elemental Tellurium by Vapor Transport Deposition on Gold.

Two-dimensional (2D) materials are promising for resistive switching in neuromorphic and in-memory computing, as their atomic thickness substantially improve the energetic budget of the device and circuits. However, many 2D resistive switching materials struggle with complex growth methods or limited scalability. 2D tellurium exhibits striking characteristics such as simplicity in chemistry, structure, and synthesis making it suitable for various applications. This study reports the first memristor design based on nanoscaled tellurium synthesized by vapor transport deposition (VTD) at a temperature as low as 100°C fully compatible with back-end-of-line processing. The resistive switching behavior of tellurium nanosheets is studied by conductive atomic force microscopy, providing valuable insights into its memristive functionality, supported by microscale device measurements. Selecting gold as the substrate material enhances the memristive behavior of nanoscaled telluriumin terms of reduced values of set voltage and energy consumption. In addition, formation of conductive paths leading to resistive switching behavior on the gold substrate is driven by gold-tellurium interface reconfiguration during the VTD process as revealed by energy electron loss spectroscopy analysis. These findings reveal the potential of nanoscaled tellurium as a versatile and scalable material for neuromorphic computing and underscore the influential role of gold electrodes in enhancing its memristive performance.

Black phosphorus quantum dots functionalized with photochromic poly(vinylspiropyran)-grafted polydopamine for transient digital-type memristors

Abstract The covalent functionalization of black phosphorus quantum dots (BPQDs) with organic species or polymers will inevitably change or damage their electronic structure and intrinsic structure. To address this problem and explore the application of BPQDs in transient digital-type memristors, a polydopamine (PDA) thin film is first synthesized in situ onto the surface of BPQDs to produce a donor–acceptor-type BPQDs@PDA composite that is directly used to react with 2-bromoisobutyryl bromide to give BPQDs@PDA-Br. By using BPQDs@PDA-Br as an atom transfer radical polymerization agent, a large number of polyvinylspiropyran (PSP) chains are in situ grown from the PDA surface to yield BPQDs@PDA-PSP. Upon ultraviolet (UV)–visible light illumination, the 2 isomers of the spiropyran (ring-closed spiropyran form and ring-opened merocyanine) in the PSP moieties will interconvert into each other rapidly. As expected, the as-fabricated indium tin oxide (ITO)/BPQDs@PDA-PSP/ITO device exhibits typical nonvolatile digital-type memristive performance under visible irradiation, with a small turn-on voltage of −1.52 V, a turn-off voltage of +1.16 V, and an ON/OFF ratio current ratio of 1.02 × 104. Upon UV illumination, the information stored in the device is quickly and completely erased within 6 s. By utilizing a simple memristor-based convolutional neural network, one can easily realize handwritten digit recognition. After 10 epochs of training, numeral recognition accuracy can reach up to 96.21%.

Cyclopentadithiophene-based conjugated polymer artificial synapses for dual information encryption

High-performance polymer memristors with analog-type switching behaviors and low power consumption, which is a perfect candidate for brainoid intelligence, have triggered the research campaign of the novel in-memory computing paradigm. A novel poly[cyclopentadithiophene- alt-thiophene] with triazole and ferrocene moieties in the sidechains (PCTF) is synthesized by copper-catalyzed azide-alkyne Click chemistry reaction of poly[4,4-bis(6-azidohexyl)-4H- cyclopenta(1,2-b: 5,4-b′) dithiophene-alt-thiophene], which is prepared by the reaction of poly[thiophene-alt-4,4- bis(6-bromohexyl)- 4H-cyclopenta(1,2-b:5,4-b′)dithiophene](PTT-Br) with NaN3, with ethynylferrocene. The PTT-Br film sandwiched between the Al and ITO electrodes shows a typical history-dependent memristive switching performance at a small sweep voltage range of ± 1 V, with 6 distinguishable conductance states. The device current can be modulated continuously when being subjected to consecutive positive and negative voltage sweeps. In contrast to PTT-Br, PCTF exhibits excellent nonvolatile memristive performance, with 16 distinguishable conductance states due to the combined action of charge transfer between the triazole and ferrocene moieties, and redox activity from both the ferrocene and polymer backbone. As an important component related to human memory, the short-term synaptic plasticity to long-term potentiation/plasticity transition is, for the first time, successfully used to realize dual information encryption at a high information security level. Only by inputting a given number of voltage pulses and at a specific time can classified information be correctly read out.

High-Performance 2D Ambipolar MoTe2 Lateral Memristors by Mild Oxidation.

2D transition metal dichalcogenides (TMDCs) have been intensively explored in memristors for brain-inspired computing. Oxidation, which is usually unavoidable and harmful in 2D TMDCs, could also be used to enhance their memristive performances. However, it is still unclear how oxidation affects the resistive switching behaviors of 2D ambipolar TMDCs. In this work, a mild oxidation strategy is developed to greatly enhance the resistive switching ratio of ambipolar 2H-MoTe2 lateral memristors by more than 10 times. Such an enhancement results from the amplified doping due to O2 and H2O adsorption and the optimization of effective gate voltage distribution by mild oxidation. Moreover, the ambipolarity of 2H-MoTe2 also enables a change of resistive switching direction, which is uncommon in 2D memristors. Consequently, as an artificial synapse, the MoTe2 device exhibits a large dynamic range (≈200) and a good linearity (1.01) in long-term potentiation and depression, as well as a high-accuracy handwritten digit recognition (>96%). This work not only provides a feasible and effective way to enhance the memristive performance of 2D ambipolar materials, but also deepens the understanding of hidden mechanisms for RS behaviors in oxidized 2D materials.

Geometrically Scalable Iontronic Memristors: Employing Bipolar Polyelectrolyte Gels for Neuromorphic Systems.

Iontronics that are capable of mimicking the functionality of biological systems within an artificial fluidic network have long been pursued for biomedical applications and ion-based intelligence systems. Here, we report on facile and robust realization of iontronic bipolar memristors featuring a three-layer polyelectrolyte gel structure. Significant memristive hysteresis of ion currents was successfully accomplished, and the memory time proved geometrically scalable from 200 to 4000 s. These characteristics were enabled by the ion concentration polarization-induced rectification ratio within the polyelectrolyte gels. The memristors exhibited memory dynamics akin to those observed in unipolar devices, while the bipolar structure notably enabled prolonged memory time and enhanced the ion conductance switching ratio with mesoscale (10-1000 μm) geometry precision. These properties endow the devices with the capability of effective neuromorphic processing with pulse-based input voltage signals. Owing to their simple fabrication process and superior memristive performance, the presented iontronic bipolar memristors are versatile and can be easily integrated into small-scale iontronic circuits, thereby facilitating advanced neuromorphic computing functionalities.

Synchronous Regulation of Hydrophobic Molecular Architecture and Interface Engineering for Robust WORM‐Type Memristor

AbstractIn the context of human active response to exponentially increasing data volumes, memristors have emerged as a focal point in systematic problem‐solving approaches. Particularly, organic memristors, characterized by excellent scalability, flexibility, and 3D stacking capabilities, have shown its tremendous potential in innovative electronic applications. However, the modulation of molecular assembly and interface interaction at the electrode surface poses a challenging task, despite their crucial role in determining the memristive performance. Herein, a collaborative method involving hydrophobic molecular design and interface engineering is proposed to generate high‐quality nanofilms with optimal photoelectric and electrochemical properties. By subjecting the bottom electrode to O2 plasma treatment and chemical modification using a hydrophobic ionic liquid poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PTFSI), the hydrophobic compound DN demonstrates exceptional affinity and stronger intermolecular interactions with the electrode surface. Consequently, the two‐terminal device array comprising Al/DN@PTFSI/ITO exhibits robust non‐volatile write‐once‐read‐many times (WORM) memory behavior, resulting in a tripling production yield from 30% to 92% along with a lower threshold voltage of 1.5 V, higher ON/OFF current ratio of 103 and excellent stability. This study presents a versatile approach to optimize the film assembly and material/electrode interface, thereby accelerating the development of organic memristors toward future applications.

Molecular-Potential and Redox Coregulated Cathodic Electrosynthesis toward Ionic Azulene-Based Thin Films for Organic Memristors.

Organic memristors as promising electronic units are attracting significant attention owing to their simplicity of molecular structure design. However, fabricating high-quality organic films via novel synthetic technologies and exploring unprecedented chemical structures to achieve excellent memory performance in organic memristor devices are highly challenging. In this work, we report a cathodic electropolymerization to synthesize an ionic azulene-based memristive film (PPMAz-Py+Br-) under the molecular-potential and redox coregulation. During the cathodic electropolymerization process, electropositive pyridinium salts migrate to the cathode under an electric field, undergo a reduction-coupling deprotonation reaction, and polymerize into a uniform film with a controllable thickness on the electrode surface. The prepared Al/PPMAz-Py+Br-/ITO devices not only exhibit a high ON/OFF ratio of 1.8 × 103, high stability, long memory retention, and endurance under a wide range of voltage scans, but also achieve excellent multilevel storage and history-dependent memristive performance. In addition, the devices can mimic important biosynaptic functions, such as learning/forgetting function, synaptic enhancement/inhibition, paired-pulse facilitation/depression, and spiking-rate-dependent plasticity. The tunable memristive performances are attributed to the capture of free electrons on pyridinium cations, the migration of the aluminum ions (Al3+), and the form of Al conductive filaments under voltage scans.

Enhanced Memristive Performance via a Vertically Heterointerface in Nanocomposite Thin Films for Artificial Synapses.

Memristors can be used to mimic synaptic behavior in artificial neural networks, which makes them a key component in neuromorphic computing and holds promise for advancing the field. In this study, a memory artificial synaptic device based on ZnO-BaTiO3 (ZnO-BTO) vertically aligned nanocomposite thin films was prepared. The vertical interface between the two phases can be used as a conduit for oxygen vacancy (OV) accumulation and a channel for OV movement, which greatly optimizes the resistive switching performance of the device and has the potential for multistage storage. By applying different pulse sequences to the device, the conductance of the device is adjusted from multiple angles, and a variety of synaptic functions are simulated, such as paired-pulse facilitation, spike-timing-dependent plasticity, short-term plasticity to long-term plasticity (STP-LTP), and long-term potentiation/depression (LTP/LTD). Finally, we construct a neural network for image recognition, and the recognition accuracy can reach 91%. Our study demonstrates the feasibility of using composite thin-film vertical interface to regulate the resistive performance of memristors and its great potential in artificial synaptic simulation and neuromorphic computing.

Polyfluorene Conjugated Polyelectrolytes-Covalently Modified Black Phosphorus Nanosheets for Bioinspired Electronics.

As some of the most promising candidates for constituting bioinspired electronics, polymer memristors with analog-type switching behavior exhibit great potential for synaptic mimicking and neuromorphic computing systems. By using highly soluble conjugated polyelectrolyte poly[9,9-bis(6-(3-methyl-1-imidazolium-yl)hexyl)fluorene]-covalently modified black phosphorus (BP) nanomaterial (BP-PF-NMI+Br-) as the active layer, an electronic device with the BP-PF-NMI+Br- film sandwiched between the aluminum and indium tin oxide electrodes is successfully fabricated. This device exhibits an excellent amount of electricity-dependent memristive performance at a small sweep voltage range of ±1 V. With increasing amount of electricity flowing through the device, the device resistance gradually decreased in a linear pattern. The changes in frequency, amplitude, and duration of voltage pulses do not affect the linear relationship between the amount of electricity passing through the device and the resistance value achieved after each state reached equilibrium at different numbers of the same voltage pulse stimulations. Both the synaptic potentiation/depression and learning/memorizing/forgetting functions of biological systems have been emulated. In contrast to BP-PF-NMI+Br-, the pure PF-NMI+Br--based device shows a write-once-read-many-times effect at the same scanning voltage range, while the BP:PF-NMI+Br- blends exhibit very unstable memristive performances.

Logic Gate Circuits Based on CeOx/WOy Memristor for the Odd/Even Checker and Encryption/Decryption of Image Applications

AbstractDue to its powerful brain‐like parallel computing and efficient data processing capabilities, memristors are considered to be the core components for building the next generation of artificial intelligence systems. In this study, the CeOx/WOy heterojunction is employed as the functional layer, and various metal materials are utilized as the top electrode to fabricate the memristor. The results indicate that the memristive performance of the Ag/CeOx/WOy/ITO device can be improved by using Ag as the top electrode. By studying the conductivity mechanism of the device, a conductivity model is established that regulates oxygen vacancies and Ag conductive filaments. Furthermore, using the as‐prepared memristor, it is constructed four basic digital logic circuits: OR, AND, XOR, and XNOR, as well as a half adder and a full adder that can be used for digital arithmetic operations. Specifically, an odd/even checker is developed based on XOR and XNOR logic circuits to verify the correctness of data transmission. Finally, it is also designed and implemented a cryptographic array based on a memristor, which can be applied to encrypt and decrypt a series of numbers and images. Therefore, this work extends the application of memristor toward digital circuits, information transmission, data processing and image security encryption.

Evaluation of Nanostructured NiS2 Thin Films from a Single-Source Precursor for Flexible Memristive Devices.

Herein, we report the first demonstration of a single-step, in situ growth of NiS2 nanostructures from a single-source precursor onto a flexible substrate as a versatile platform for an effective nonvolatile memristor. The low temperature, solution-processed deposition of NiS2 thin films exhibits a wide band gap range, spherical-flower-like morphology with high surface area and porosity, and negligible surface roughness. Moreover, the fabricated Au/NiS2/ITO/PET memristor device reveals reproducible bipolar resistive switching (RS) at low operational voltages under both flat and bending conditions. The flexible device shows stable RS behavior for multiple cycles with a good memory window (∼102) and data retention of up to 104 s. The switching of a device between a high-resistance state and a low-resistance state is attributed to the filamentary conduction based on sulfur ion migration and sulfur vacancies and plays a key role in the outstanding memristive performance of the device. Consequently, this work provides a simple, scalable, solution-processed route to fabricate a flexible device with potential applications in next-generation neuromorphic computing and wearable electronics.

Effect of heterojunction order between CaTiO3 and Mn doped SrTiO3 on memristive performance and its mechanism analysis

It is well-known that memristive devices can be applied to brain-like parallel computing, but the interface effects have an indelible impact on memristive performance. In this work, two memristive devices with Ag/CTO/SMTO/Ti and Ag/SMTO/CTO/Ti structures were fabricated based on the heterojunctions between CaTiO3 (CTO) and Mn-doped SrTiO3 (SMTO). By comparison, the Ag/CTO/SMTO/Ti device shows relatively improved memristive characteristics with the optimum sweeping voltage of 1.2 V and the switching ratio of ∼12. That is to say, changing the heterojunction order between CTO and SMTO can regulate the memristive behaviors of the device because the interface effect caused by heterojunction contact has influence on the particle migration, resulting in different rates of conductive filaments formation, so that the memristive characteristics of the device can be regulated. Finally, the resistive switching (RS) characteristics of the device are explained by in-depth mechanism analysis based on fitting experimental data. Therefore, this work not only provides a feasible method for improving the memristive characteristics of the device, but also helps to understand the influence of the interface effect on the particles motion, thereby laying the foundation for the preparation of high-capacity and efficient memristive device in artificial intelligence (AI) applications.

Direct Observation of Oxygen Ion Dynamics in a WO3‐x based Second‐Order Memristor with Dendritic Integration Functions

AbstractDirect observation of oxygen dynamics in an oxide‐based second‐order memristor can provide the valid evidence to clarify the memristive mechanism, however, which is still limited for now. In this study, the migration and diffusion of oxygen ions in the region of Pt/WO3‐x Schottky interface are observed in the WO3‐x second‐order memristor by using the technique of in situ transmission electron microscopy (TEM) and the electron energy loss spectroscopy. Interestingly, the coexistence of memristive and memcapacitive switching can be implemented in this memristor. Combined with the analysis of depth‐profile X‐ray photoelectron spectroscopy (XPS), an interface‐barrier‐modulation second‐order memristive model is proposed based on the above results. Notably, temporally correlative oxygen dynamics in the memristor offers the platform to integrate signals from multiple inputs, enabling the realization of the dendritic functions of synchronous and asynchronous integration for the application of logic operations with fault‐tolerance capability and associative learning. These findings provide the experimental evidence to in‐depth understanding of oxygen dynamics and switching mechanism in second‐order memristor, which can support the optimization of memristive performance and the achievement of biorealistic synaptic functions.

Solution-processed small-molecular organic memristor with a very low resistive switching set voltage of 0.38 V

Organic memristors are considered to be the next-generation storage element due to their unique advantages of flexibility, transparency, and good solution processability. In this Letter, a Zn-porphyrin based small-molecular organic memristor is prepared by spin-coating with an ultralow resistive switching set voltage of 0.38 V. It is found that the zinc atom in the porphyrin molecule plays a very important role in improving the resistance switching characteristics of organic memristors. By tracking the change in oxygen valence in the vertical dimension, we demonstrate that Zn atom located in the core of porphyrin helps to enhance the oxygen ion migration across the active layer, clearly revealing the memory mechanism of low-cost solution-processed Zn-porphyrin based small-molecular organic memristors. This organic memristor shows excellent memristive performance resulting from rational material design and appropriate device structure engineering.

Redox-Active Azulene-based 2D Conjugated Covalent Organic Framework for Organic Memristors.

As a conjugated and unsymmetric building block composed of an electron-poor seven-membered sp2 carbon ring and an electron-rich five-membered carbon ring, azulene and its derivatives have been recognized as one of the most promising building blocks for novel electronic devices due to its intrinsic redox activity. By using 1,3,5-tris(4-aminophenyl)-benzene and azulene-1,3-dicarbaldehyde as the starting materials, an azulene(Azu)-based 2D conjugated covalent organic framework, COF-Azu, is prepared through liquid-liquid interface polymerization strategy for the first time. The as-fabricated Al/COF-Azu/indium tin oxide (ITO) mem-ristor shows typical non-volatile resistive switching performance due to the electric filed induced intramolecular charge transfer effect. Associated with the unique memristive performance, a simple convolutional neural network is built for image recognition. After 8 epochs of training, image recognition accuracy of 80% for a neutral network trained on a larger data set is achieved.

Redox‐Active Azulene‐based 2D Conjugated Covalent Organic Framework for Organic Memristors

AbstractAs a conjugated and unsymmetric building block composed of an electron‐poor seven‐membered sp2 carbon ring and an electron‐rich five‐membered carbon ring, azulene and its derivatives have been recognized as one of the most promising building blocks for novel electronic devices due to its intrinsic redox activity. By using 1,3,5‐tris(4‐aminophenyl)‐benzene and azulene‐1,3‐dicarbaldehyde as the starting materials, an azulene(Azu)‐based 2D conjugated covalent organic framework, COF‐Azu, is prepared through liquid‐liquid interface polymerization strategy for the first time. The as‐fabricated Al/COF‐Azu/indium tin oxide (ITO) memristor shows typical non‐volatile resistive switching performance due to the electric filed induced intramolecular charge transfer effect. Associated with the unique memristive performance, a simple convolutional neural network is built for image recognition. After 8 epochs of training, image recognition accuracy of 80 % for a neutral network trained on a larger data set is achieved.

Solvent-assisted sulfur vacancy engineering method in MoS2 for a neuromorphic synaptic memristor.

Recently, two-dimensional transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) have attracted great attention due to their unique properties. To modulate the electronic properties and structure of TMDs, it is crucial to precisely control chalcogenide vacancies and several methods have already been suggested. However, they have several limitations such as plasma damage by ion bombardment. Herein, we introduced a novel solvent-assisted vacancy engineering (SAVE) method to modulate sulfur vacancies in MoS2. Considering polarity and the Hansen solubility parameter (HSP), three solvents were selected. Sulfur vacancies can be modulated by immersing MoS2 in each solvent, supported by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy analyses. The SAVE method can further expand its application in memory devices representing memristive performance and synaptic behaviors. We represented the charge transport mechanism of sulfur vacancy migration in MoS2. The non-destructive, scalable, and novel SAVE method controlling sulfur vacancies is expected to be a guideline for constructing a vacancy engineering system of TMDs.

Facile synthesis of MXene−Polyvinyl alcohol hybrid material for robust flexible memristor

MXenes are emerging two-dimensional (2D) nanomaterials composed of transition metal carbides/nitrides/carbonitrides with several atomic layers. However, due to the analogous metallic conductivity, their development in the field of electronic devices is restricted. Recently, various etching methods of MXenes lead to the surface terminals containing different functional groups, which provides opportunities for further modification of MXene nanosheets and makes it possible to apply them in the field of electronic memristors. At present, there are few researches about memristors on the basis of MXene-polymer hybrid materials. Here, we synthesize Ti3C2Tx-MXenes and adjust their content in an insulating polyvinyl alcohol (PVA) polymer for triggering memristive performance, which affords outstanding bipolar resistive switching behavior. The memristor exhibits the characteristics of low operating voltage, long retention time, high flexibility, and acceptable mechanical stability. In order to examine the switching mechanism of the device, the charge transport model simulation is implemented, which indicates that charge trapping effect is responsible for the non-volatile memristor performance of Ti3C2Tx-PVA hybrid material. This work provides a simple strategy for preparing MXene-polymer hybrid materials and broadens the prospects of 2D MXenes in the field of non-volatile memory applications.

Rainer Waser – A Pioneer of Fundamentals of Resistive Switching Memories

Rainer Waser – A Pioneer of Fundamentals of Resistive Switching Memories