Memristive Performance Research Articles

Low-Power Memristor Based on Two-Dimensional Materials.

The memristor is an excellent candidate for nonvolatile memory and neuromorphic computing. Recently, two-dimensional (2D) materials have been developed for use in memristors with high-performance resistive switching characteristics, such as high on/off ratios, low SET/RESET voltages, good retention and endurance, fast switching speed, and low power and energy consumption. Low-power memristors are highly desired for recent fast-speed and energy-efficient artificial neuromorphic networks. This Perspective focuses on the recent progress of low-power memristors based on 2D materials, providing a condensed overview of relevant developments in memristive performance, physical mechanism, material modification, and device assembly as well as potential applications. The detailed research status of memristors has been reviewed based on different 2D materials from insulating hexagonal boron nitride, semiconducting transition metal dichalcogenides, to some newly developed 2D materials. Furthermore, a brief summary introducing the perspectives and challenges is included, with the aim of providing an insightful guide for this research field.

Structural Defects Improve the Memristive Characteristics of Epitaxial La0.8Sr0.2MnO3‐Based Devices

AbstractInterface‐type valence change memories (VCMs) are exciting candidates for multilevel storage in resistive random access memories (RRAM) and as artificial synapses for neuromorphic computing. Several materials have been proposed as VCM candidates and, depending on the materials and electrodes of choice, different switching mechanisms take place leading to the change in resistance. Here, the focus is on La0.8Sr0.2MnO3–δ (LSM) perovskite and, particularly, the role of its nanostructure on the memristive device performance. The nanostructural details of the layers are modified by growing LSM epitaxial thin films on different substrates, i.e., SrTiO3 (STO) and LaAlO3 (LAO), by metal‐organic chemical vapor deposition (MOCVD). An interface‐type memristive response is observed using Ti as active electrode and Pt as inert electrode. The modifications in the nanostructure of LSM (strain and dislocations) determine the memristive performance, leading to differences in cycle to cycle reproducibility and multilevel capabilities by the modification of the LSM's oxygen migration properties. The results show that nanostructure engineering is a promising approach for optimizing the performance of memristive devices, an approach which can also be extended and applied to other nanoionic electrochemical devices.

Intrinsically ionic conductive nanofibrils for ultra-thin bio-memristor with low operating voltage

Memristors integrated with low operating voltage, good stability, and environmental benignity play an important role in data storage and logical circuit technology, but their fabrication still faces challenges. This study reports an ultra-thin bio-memristor based on pristine environment-friendly silk nanofibrils (SNFs). The intrinsic ionic conductivity, combined with high dielectric performance and nanoscale thickness, lowers the operation voltage down to 0.1–0.2 V, and enables stable switching and retention time over 180 times and 105 s, respectively. Furthermore, the SNF-based memristor device in a crossbar array achieves stable memristive performance, and thus realizes the functions of memorizing image and logic operation. By carrying out variable-temperature electrical experiments and Kelvin probe force microscopy, the space charge-limited conduction mechanism is revealed. Integrating with low operating voltage, good stability, and ultra-thin thickness makes the SNF-based memristors excellent candidates in bioelectronics.

Regulation of oxygen vacancy on behaviors of memristors based on amorphous ZnTiSnO films

However, it is insufficient and oversimplified to attribute memristive performance to oxygen vacancy only.

Significance of electrode contact area on memristive parameters of silver iodide

Here we report the dependence of the memristive properties of silver iodide on the electrode contact area. The threshold voltage (Vth) reduces significantly by 60% and the ON-OFF ratio (RON/ROFF) increases by ∼ 102 fold with a decrease in electrode contact size. The results are accounted for electric field enhanced drift-diffusion of Ag+ ions in bulk. These observations demonstrate that the smaller contact area significantly improves the memristive performance, which is an important aspect in miniaturization of these memory devices to achieve a high density of memory elements.

Combinatorial Screening of Anodic Memristors Based on Hafnium-tantalum Mixed Oxides

Among valve metals, Ta and Hf are the most studied high-κ dielectric materials for development of the next-generation non-volatile electronic components, commonly known as memristors. Redox-based resistive switching memories (ReRAMs) and artificial neural networks are applications examples of metal-insulator-metal memory types. An insulating layer, interposed between the bottom and the top conducting electrodes, is the determining factor in memristive performance. From this aspect, oxidized forms of Hf and Ta are promising candidates to be used as solid-state electrolytes, allowing conductive filaments formation (CFs) inside the oxide layer. There is a strongly motivated scientific interest for the implementation of anodic oxides in memristive devices due to its simple, cost-efficient and controllable fabrication. However, properties of anodic memristors based on Hf and Ta are not yet fully investigated and several parameters still have to be examined in detail. Herein, influence of the electrolyte selection on Hf and Ta anodic memristors was studied. Consequently, anodization parameters were optimized and applied for the fabrication of memristors based on Hf-Ta alloys spread across a thin film combinatorial library. Parent metals and their mixtures served as bottom electrodes and, identical to the case of Pt top electrode, they were deposited by sputtering. The total compositional spread of Hf-Ta library was ranging from 5 to 92 at.% Hf and was distributed across three Si wafers. The memristive properties of devices based on mixed anodic oxides were screened with a resolution of 1 at.%, thus offering a comparative investigation of a high number of compositionally different devices. Generally, the ratios between resistance states for Ta and Hf memristors were approximately in the same range, as well as their number of switching voltage levels. Nevertheless, in both cases memory and electrical characteristics were dictated by the incorporation of electrolyte species, which was confirmed by XPS. Therefore, for devices based on pure metals, the memristive behavior varied only as depending on the electrochemical conditions used for device fabrication. However, no electrolyte incorporation was observed in case of devices based on Hf-Ta library. Memristive properties could have been related only to different compositions which were grouped in three zones containing alloys showing similar behaviors. An improvement of resistance states ratios in some zones by orders of magnitude was achieved, as compared to ratios obtained for devices based on pure metals. Additionally, variations between the zones of unipolar and bipolar switching behavior were observed. HRTEM and hard X-ray XPS were done in order to compare the CFs within the mixed oxide layers for memristors from different zones. Clearly, the novelty of this work is the found spectra of different memristive properties for anodic Hf and Ta memristors and anodic memristors based on numerous Hf-Ta alloys. Such approach is offering the possibility of better understanding whether the switching mechanism is based on electrolyte species, oxygen vacancies, metallic species movement or formation of additional vacancies by redox reactions or thermal field for each specific composition or electrochemical parameters. Conclusively, this may be an important step towards defect engineered memristors fabrication, as intrinsically offered by the anodic process of memristive formation.

Low-Power and Tunable-Performance Biomemristor Based on Silk Fibroin.

Biomemristors have attracted significant attention because of their potential applications in logic operations, nonvolatile memory, and synaptic emulators, thus leading to the urgent need to improve memristive performance. In this work, a silk fibroin (SF)-based memristor, integrated with both low power and low operating current simultaneously, has been reported. Doping the SF with Ag and an ethanol-based post-treatment promote microcrystal formation in the bulk of the SF. This induces carrier transport along fixed, short paths and results in a low set voltage, low operating current, and high memristive stability. Such performances can greatly reduce power consumption and heat generation, beneficial for the accuracy and durability of memristor devices. The memristive mechanism of SF-based memristors with different Ag contents is the space-charge-limited conduction (SCLC) mechanism. In addition, the nonlinear transmission property of SF-based memristors suggests useful applications in bioelectronics.

Flexible Ta/TiO x /TaO x /Ru memristive synaptic devices on polyimide substrates

It is very urgent to build memristive synapses and even wearable devices to simulate the basic functions of biological synapses. The linear conductance modulation is the basis of analog memristor for neuromorphic computing. By optimizing the interface engineering wherein Ta/TiO x /TaO x /Ru was fabricated, all the memristor devices with different TiO x thickness showed electroforming-free property. The short-term and long-term plasticity in both potentiation and depression behaviors can be mimicked when TiO x was fixed at 25 nm. The presented memristive synapses simulated the stable paired-pulse facilitation and spike-timing dependent plasticity performance. The potentiation and depression in linearity and symmetry improved with the TiO x thickness increasing, which provides the feasibility for the application of artificial neural network. In addition, the device deposited on polyimide (PI) still exhibits the synaptic performance until the bending radii reaches 6 mm. By carefully tuning the interface engineering, this study can provide general revelation for continuous improvement of the memristive performance in neuromorphic applications.

Electrolyte-Dependent Modification of Resistive Switching in Anodic Hafnia.

Anodic HfO2 memristors grown in phosphate, borate, or citrate electrolytes and formed on sputtered Hf with Pt top electrodes are characterized at fundamental and device levels. The incorporation of electrolyte species deep into anodic memristors concomitant with HfO2 crystalline structure conservation is demonstrated by elemental analysis and atomic scale imaging. Upon electroforming, retention and endurance tests are performed on memristors. The use of borate results in the weakest memristive performance while the citrate demonstrates clear superior memristive properties with multilevel switching capabilities and high read/write cycling in the range of 106. Low temperature heating applied to memristors shows a direct influence on their behavior mainly due to surface release of water. Citrate-based memristors show remarkable properties independent on device operation temperatures up to 100 °C. The switching dynamic of anodic HfO2 memristors is discussed by analyzing high resolution transmission electron microscope images. Full and partial conductive filaments are visualized, and apart from their modeling, a concurrency of filaments is additionally observed. This is responsible for the multilevel switching mechanism in HfO2 and is related to device failure mechanisms.

In Situ Resistive Switching Effect Scrutinization on Co-Designed Graphene Sensor.

Resistive switching (RS), an electric property based on the forming and rupture of conductive filaments in metal-insulator-metal structures, has attracted intensive attention due to its potential application in next generation energy-efficient and area-efficient memory devices. In situ studies of the RS effect are urgently needed for its mechanism understanding and memristive performance improvement. Here investigations of both the RS effect as well as the gate tunable conductance quantization effect are realized by co-designing an Ag/SiO2 based memory structure on a graphene local sensor. This design enables self-monitoring of the working states of the memristor in real-time by virtue of the graphene sensor. These findings pave the way for further investigations of on-chip electronics and quantum physics.

Bio-memristors based on silk fibroin.

Bio-memristors constitute candidates for the next generation of non-volatile storage and bionic synapses due to their biocompatibility, environmental benignity, sustainability, flexibility, degradability, and impressive memristive performance. Silk fibroin (SF), a natural and abundant biomaterial with excellent mechanical, optical, electrical, and structure-adjustable properties as well as being easy to process, has been utilized and shown to have potential in the construction of bio-memristors. Here, we first summarize the fundamental mechanisms of bio-memristors based on SF. Then, the latest achievements and developments of pristine and composited SF-based memristors are highlighted, followed by the integration of memristive devices. Finally, the challenges and insights associated with SF-based bio-memristors are presented. Advances in SF-based bio-memristors will open new avenues in the design and integration of high-performance bio-integrated systems and facilitate their application in logic operations, complex circuits, and neural networks.

Memristive effect with non-zero-crossing current-voltage hysteresis behavior based on Ag doped Lophatherum gracile Brongn

The memristive effect become a bio-electronics research focus with the development of sustainable systems and the application of multifunctional electronic devices. In this paper, the natural Lophatherum gracile Brongn (LGB) was employed as active layer to prepare Ag/LGB/fluorine-doped tin oxide (FTO) bio-memristor. Further, different doping ratios nano silver powders (Ag nanoparticle) were doped into LGB functional layers at 0 wt%, 0.2 wt%, 0.33 wt%, 1.0 wt%, 2.0 wt%, 10.0 wt% and 20.0 wt%. It is found that the capacitive-coupled memristive performance of the Ag/Ag-doped-LGB/FTO devices was markedly changed. Finally, it is confirmed that Ag ions transferring and Ag filament formation under the applied voltage are responsible for capacitive-coupled memristive behavior. This research has revealed the realization of capacitive-coupled memristive effect of bio-memristor, which opens a new way to achieve multifunctional performance for electronic device.

Flexible full two-dimensional memristive synapses of graphene/WSe2-xOy/graphene.

van der Waals heterostructures realized by stacking different two-dimensional materials offer the possibility to design new devices with atomic-level precision. The realization of memristive synapses based on full two-dimensional materials provides an avenue for developing neuromorphic computing systems with excellent mechanical flexibility in future. Herein, we first develop the flexible full two-dimensional memristive synapse of graphene/WSe2-xOy/graphene exhibiting stable volatile resistive switching. The oxidation of WSe2 through O2 plasma treatment plays a key role in improving the memristive performance of the devices. And the switching behavior is associated with the migration of oxygen vacancies at the graphene/WSe2-xOy interface. Versatile synaptic functions have been realized, including short-term plasticity, long-term plasticity, the transition from short-term plasticity to long-term plasticity and high-pass temporal filtering, with ultralow energy consumption of ∼16.1 pJ per spike. The flexible full two-dimensional devices fabricated on flexible polyimide substrate show excellent mechanical flexibility, including a good endurance against repeatable bending of 1000 times and a bending strain of 2.5%.

The pH-controlled memristive effect in a sustainable bioelectronic device prepared using lotus root

With the emergence of environmental problems, the green, degradable, sustainable, and environmentally friendly electronic device has become one of the current research hotspots. In this work, an organic biomemristor device was prepared using the lotus root as a functional layer on a fixable substrate, which shows an excellent memristive memory performance. Further, we studied the effect of acidity and alkalinity (pH) on the memristive performance of the device, in which it can be found that the pH can regulate the memristive behavior and memory window. Moreover, an apparent memristive behavior and stable reading-writing characteristic in neutral condition (pH = 7.0) were observed, and the high-resistance state/low-resistance state resistance ratio under the neutral condition is significantly larger than that under acid and alkali conditions. Therefore, our device can be used in lightweight and portable wearable device. In a word, this research can provide meaningful insights for developing sustainable bioelectronic devices with potential applications in information technology.

Vertical MoS2 Double-Layer Memristor with Electrochemical Metallization as an Atomic-Scale Synapse with Switching Thresholds Approaching 100 mV.

Atomically thin two-dimensional (2D) materials-such as transition metal dichalcogenide (TMD) monolayers and hexagonal boron nitride (hBN)-and their van der Waals layered preparations have been actively researched to build electronic devices such as field-effect transistors, junction diodes, tunneling devices, and, more recently, memristors. Two-dimensional material memristors built in lateral form, with horizontal placement of electrodes and the 2D material layers, have provided an intriguing window into the motions of ions along the atomically thin layers. On the other hand, 2D material memristors built in vertical form with top and bottom electrodes sandwiching 2D material layers may provide opportunities to explore the extreme of the memristive performance with the atomic-scale interelectrode distance. In particular, they may help push the switching voltages to a lower limit, which is an important pursuit in memristor research in general, given their roles in neuromorphic computing. In fact, recently Akinwande et al. performed a pioneering work to demonstrate a vertical memristor that sandwiches a single MoS2 monolayer between two inert Au electrodes, but it could neither attain switching voltages below 1 V nor control the switching polarity, obtaining both unipolar and bipolar switching devices. Here, we report a vertical memristor that sandwiches two MoS2 monolayers between an active Cu top electrode and an inert Au bottom electrode. Cu ions diffuse through the MoS2 double layers to form atomic-scale filaments. The atomic-scale thickness, combined with the electrochemical metallization, lowers switching voltages down to 0.1-0.2 V, on par with the state of the art. Furthermore, our memristor achieves consistent bipolar and analogue switching, and thus exhibits the synapse-like learning behavior such as the spike-timing dependent plasticity (STDP), the very first STDP demonstration among all 2D-material-based vertical memristors. The demonstrated STDP with low switching voltages is promising not only for low-power neuromorphic computing, but also from the point of view that the voltage range approaches the biological action potentials, opening up a possibility for direct interfacing with mammalian neuronal networks.

Modulating memristive performance of hexagonal WO3 nanowire by water-oxidized hydrogen ion implantation

In a two-terminal Au/hexagonal WO3 nanowire/Au device, ions drifting or carriers self-trapping under external electrical field will modulate the Schottky barriers between the nanowire and electrodes, and then result in memristive effect. When there are water molecules adsorbed on the surface of WO3 nanowire, hydrogen ions will generate near the positively-charged electrode and transport in the condensed water film, which will enhance the memristive performance characterized by analogic resistive switching remarkably. When the bias voltage is swept repeatedly under high relative humidity level, hydrogen ions will accumulate on the surface and then implant into the lattice of the WO3 nanowire, which leads to a transition from semiconducting WO3 nanowire to metallic HxWO3 nanowire. This insulator-metal transition can be realized more easily after enough electron-hole pairs being excited by laser illumination. The concentration of hydrogen ions in HxWO3 nanowire will decrease when the device is exposed to oxygen atmosphere or the bias voltage is swept in atmosphere with low relative humidity. By modulating the concentration of hydrogen ions, conductive hydrogen tungsten bronze filament might form or rupture near electrodes when the polarity of applied voltage changes, which will endow the device with memristive performance characterized by digital resistive switching.

Donor‐Induced Performance Tuning of Amorphous SrTiO3 Memristive Nanodevices: Multistate Resistive Switching and Mechanical Tunability

Metal–oxide valence‐change memristive devices are the key contenders for the development of multilevel nonvolatile analog memories and neuromorphic computing architectures. Reliable low energy performance and tunability of nonlinear resistive switching dynamics are essential to streamline the high‐density circuit level integration of these devices. Here, manipulation of room temperature‐synthesized defect chemistry is employed to enhance and tune the switching characteristics of high‐performance amorphous SrTiO3 (a‐STO) memristors. Substitutional donor (Nb) doping with low concentrations in the a‐STO oxide structure allows extensive improvements in energy requirements, stability, and controllability of the memristive performance, as well as field‐dependent multistate resistive switching. Evidence is presented that room temperature donor doping results in a modified insulator oxide where dislocation sites act as charge carrier modulators for low energy and multilevel operation. Finally, the performance of donor‐doped a‐STO‐based memristive nanodevices is showcased, with the possibility of mechanical modulation of the nonlinear memristive characteristics of these devices demonstrated. These results highlight the potential of donor‐doped a‐STO nanodevices for high‐density integration as analog memories and multifunctional alternative logic elements.

Dithienopyrrole‐/Benzodithiophene‐Based Donor–Acceptor Polymers for Memristor

AbstractTwo new donor (D)–acceptor (A) copolymers, poly({4,4′‐[4,4′‐(9H‐fluorene‐9,9‐diyl)bis(4,1‐phenylene)]bis(oxy)diphthalonitrile}‐alt‐[dithieno [3,2‐b:2′,3′‐d]pyrrole]) (P1) and poly({4,4′‐[4,4′‐(9H‐fluorene‐9,9‐diyl)bis(4,1‐phenylene)]bis(oxy)diphthalonitrile}‐alt‐([1,2‐b:4,5‐b′]dithiophene)) (P2), have been designed and synthesized by the Stille coupling reaction. The dipole moment of P1 (10.71 Debye) is larger than that of P2 (6.59 Debye). A strong dipole moment helps to sustain the conductive charge‐transfer state. To evaluate the nonvolatile memristive performance of P1 and P2, the corresponding memory device can be fabricated with the configuration of platinum (50 nm)/polymer (100 nm)/platinum (150 nm)/silicon. In contrast with the P2‐based device with almost negligible switching and memristive behavior, the P1‐based memristor exhibits a maximum ILRS/IHRS ratio of about 10 (ILRS and IHRS are the current values in the low‐resistance state (LRS) and high‐resistance state (HRS), respectively) at ±2.0 V. Distinguishable from the bistable resistive switching, showing abrupt resistance or conductance jumps, the electrical transition observed in the memristor demonstrates a smoother tuning of the sample conductance during the voltage sweeping processes. In addition, changes in the surface morphology of P1 and P2 are also observed under an applied bias voltage of 100 mV.

Enhanced memristive performance of individual hexagonal tungsten trioxide nanowires by water adsorption based on Grotthuss mechanism

The electrical transport properties of Au/WO3 nanowire/Au sandwich structures were studied under different levels of relative humidity at room temperature. Experimental results indicate that the memristive performance of the structures is enhanced remarkably when the relative humidity is above a critical level, which might be attributed to H+ drifting based on Grotthuss mechanism. H+ (produced by oxidizing water molecules with holes) drifting results in not only changes in the barrier heights but also an increase in electrical current. The Au/WO3 nanowire/Au sandwich structures, whose memristive performance can be modulated by gas molecule adsorption, might be a good candidate for exploiting next generation memory devices.

Bioengineered Tunable Memristor Based on Protein Nanocage

Bioengineered protein-based nanodevices with tunable and reproducible memristive performance are fabricated by combining the unique high loading capacity of Archaeoglobus fulgidus ferritin with OWL-generated nanogaps. By tuning the iron amount inside ferritin, the ON/OFF ratio of conductance switching can be modulated accordingly. Higher molecular loading exhibits better memristive performance owing to the higher electrochemical activity of the ferric complex core.