Reproducible Switching Research Articles

Electrical Control of Valley Polarized Charged Exciton Species in Monolayer WS2.

Excitons are key to the optoelectronic applications of van der Waals semiconductors, with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation remains challenging due to inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic electrical control of valley polarization in charged excitonic states of monolayer tungsten disulfide, achieving up to a 6-fold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the charged exciton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible nonmonotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a resonant interplay between neutral and charged exciton states.

Field-free switching of perpendicular magnetic elements by using two orthogonal sub-nanosecond spin–orbit torque pulses

We propose a field-free switching mechanism that utilizes two spatially orthogonal spin–orbit torque (SOT) currents. Initially applied simultaneously, one of the currents is subsequently switched off. The superposition of these two currents results in an in-plane magnetization, which is not orthogonal to the remaining SOT current after the second one is deactivated. This symmetry-breaking procedure leads to reproducible and rapid switching, with field pulse durations as short as 0.25 ns.

Unraveling Conductive Filament Formation in High Performance Halide Perovskite Memristor

AbstractHalide perovskites (HPs) are promising materials for memristor devices because of their unique characteristics. In this study, nonvolatile resistive switching memory devices based on thick MAPbI3 perovskite (800 nm) films with structure FTO/MAPbI3/polymethyl methacrylate (PMMA)/Ag are presented. Reproducible and reliable bipolar switching characteristics are demonstrated with an ultra‐low operating voltage (−0.1 V), high ON/OFF ratio (106), endurance (>2 × 103 times) and a record retention time (>105 s). The I–V curve of the first cycle exhibits self‐formed conductive filaments. These are attributed to the presence of metallic Pb resulting from an excess of PbI2 in the perovskite film. The subsequent activation process involves the formation of conductive filaments, consisting of either iodide vacancies or migrated charged metals. Numerical simulations are then carried out to understand the nature of these conductive filaments and the role of the internal electric field in the migration of iodide ions, iodide vacancies, and Ag cations. Finally, an exhaustive model is proposed that explains the set and reset processes governing the first voltage cycle and the steady state, at different voltage ranges. In summary, this work offers a novel and thorough perspective of the complete resistive switching (RS) behavior in a MAPbI3/buffer/Ag memristor, supported by numerical simulations.

Novel coumarin-based ratiometric bifunctional fluorescent probe mimicking a set-reset memorized device

A novel coumarin-based fluorescent sensor CHE, incorporating 2-hydrazinylbenzothiazole and coumarin aldehyde, has been developed that demonstrated a preferential detection of Hg2+ and Ag+ in presence of interferences. Compared to previously prevalent intensity-based fluorescent probes, CHE exhibited a ratiometric fluorescence response to Hg2+ and Ag+, and further accurately differentiated Hg2+ and Ag + using the differential extractive ability of EDTA when interacting with ion-CHE complexes. Sensing mechanism was investigated and elucidated. The chemosensor CHE was successfully applied to detect Hg2+ and Ag+ in six distinct samples with satisfactory results. Additionally, combinatorial logic circuits were constructed utilizing three distinct logic gates (NOT, OR, and INH) based on the sensor's differential output signals in response to Hg2+/Ag+ and other cations. Interestingly, utilizing the reversible and reproducible switching behavior of the EDTA interaction with Hg2+, a conceptual 'Write–Read–Erase–Read' memory function with multi-write capability was proposed, offering a novel perspective for molecular-based memory systems.

Memristors Based on Ni(II)‐tetraaza[14]annulene Complexes: Toward an Unconventional Resistive Switching Mechanism

AbstractIn this work, a family of Ni‐based dibenzotetraaza[14]annulene (dtaa) complexes are investigated for their application in memristors (memory resistors). A series of four Ni(II) complexes with different peripheral substituents of the dtaa ligand are successfully synthesized. Based on these compounds, two‐terminal thin‐film devices are fabricated in planar architecture. Four metals with different work functions are tested: Mg, Cu, Ni, and Au. It is demonstrated that ITO|[Ni(Me4dtaa)]|Cu devices show hysteretic behavior and offer stable, robust, and reproducible switching between high‐ and low‐resistive states. An in‐depth spectroscopic characterization of the Ni complex is performed, using radiation from infrared, through visible and ultraviolet, to tender X‐rays. Operando X‐ray fluorescence spectroscopy is used to monitor redox and structural changes upon the polarization of the studied memristor with the external electric field. Density functional theory calculations are used to better understand the electronic structure of the studied material, as well as structural rearrangement after electron injection that may be responsible for the modulation of electric conductivity. Finding a unique case of filamentary‐type resistive switching involving redox reactions of stationary molecules within a molecular solid is postulated. Yet, the formation of these filaments is not related to any significant configurational changes at the atomic scale.

Efficient Spiking Neural Networks with Biologically Similar Lithium-Ion Memristor Neurons.

Benefiting from the brain-inspired event-driven feature and asynchronous sparse coding approach, spiking neural networks (SNNs) are becoming a potentially energy-efficient replacement for conventional artificial neural networks. However, neuromorphic devices used to construct SNNs persistently result in considerable energy consumption owing to the absence of sufficient biological parallels. Drawing inspiration from the transport nature of Na+ and K+ in synapses, here, a Li-based memristor (LixAlOy) was proposed to emulate the biological synapse, leveraging the similarity of Li as a homologous main group element to Na and K. The Li-based memristor exhibits ∼8 ns ultrafast operating speed, 1.91 and 0.72 linearity conductance modulation, and reproducible switching behavior, enabled by lithium vacancies forming a conductive filament mechanism. Moreover, these memristors are capable of simulating fundamental behaviors of a biological synapse, including long-term potentiation and long-term depression behaviors. Most importantly, a threshold-tunable leaky integrate-and-fire (TT-LIF) neuron is built using LixAlOy memristors, successfully integrating synaptic signals from both temporal and spatial levels and achieving an optimal threshold of SNNs. A computationally efficient TT-LIF-based SNN algorithm is also implemented for image recognition schemes, featuring a high recognition rate of 90.1% and an ultralow firing rate of 0.335%, which is 4 times lower than those of other memristor-based SNNs. Our studies reveal the ion dynamics mechanism of the LixAlOy memristor and confirm its potential in rapid switching and the construction of SNNs.

Transfer-Free Analog and Digital Flexible Memristors Based on Boron Nitride Films.

The traditional von Neumann architecture of computers, constrained by the inherent separation of processing and memory units, faces challenges, for instance, memory wall issue. Neuromorphic computing and in-memory computing offer promising paradigms to overcome the limitations of additional data movement and to enhance computational efficiency. In this work, transfer-free flexible memristors based on hexagonal boron nitride films were proposed for analog neuromorphic and digital memcomputing. Analog memristors were prepared; they exhibited synaptic behaviors, including paired-pulse facilitation and long-term potentiation/depression. The resistive switching mechanism of the analog memristors were investigated through transmission electron microscopy. Digital memristors were prepared by altering the electrode materials, and they exhibited reliable device performance, including a large on/off ratio (up to 106), reproducible switching endurance (>100 cycles), non-volatile characteristic (>60 min), and effective operating under bending conditions (>100 times).

Highly stable two-dimensional Ruddlesden-Popper perovskite-based resistive switching memory devices.

Resistive switching random-access memory (ReRAM) devices based on organic-inorganic halide perovskites (OIHPs) have emerged as a new class of data storage devices. Recently, two-dimensional (2D) OIHPs have attracted much attention for ReRAM applications because of their structural diversity and superior stability. Here, the RS characteristics of ReRAM devices fabricated utilizing pure 2D Ruddlesden-Popper (RP) perovskite crystals, namely (TEA)2PbBr4 and (TEA)2PbI4, are reported. The RS memory devices exhibit reliable and reproducible bipolar switching properties with high ON/OFF ratio (∼104), excellent data retention of over 104 s, and good endurance characteristics of 200 cycles. This study investigates temperature-dependent RS behaviors, elucidating the creation and annihilation of a conducting pathway in the presence of an external electric field. Additionally, the RS property of 2D RP perovskite-based memory devices is found to be retained over 45 days at ambient conditions under a relative humidity of 47% ± 4%. Our findings may accelerate the technological deployment of stable 2D perovskite-based RS memory devices for successful logic application.

Multibit, Lead‐Free Cs2SnI6 Resistive Random Access Memory with Self‐Compliance for Improved Accuracy in Binary Neural Network Application

AbstractIn the realm of neuromorphic computing, integrating Binary Neural Networks (BNN) with non‐volatile memory based on emerging materials can be a promising avenue for introducing novel functionalities. This study underscores the viability of lead‐free, air‐stable Cs2SnI6 (CSI) based resistive random access memory (RRAM) devices as synaptic weights in neuromorphic architectures, specifically for BNNs applications. Herein, hydrothermally synthesized CSI perovskites are explored as a resistive layer in RRAM devices either on the rigid or flexible substrate, highlighting reproducible multibit switching with self‐compliance, low‐ resistance‐state (LRS) variations, a decent On/Off ratio(or retention) of ≈103(or 104 s), and endurance exceeding 300 cycles. Moreover, a comprehensive evaluation with the 32 × 32 × 3 RGB CIFAR‐10 dataset reveals that binary convolutional neural networks (BCNN) trained solely on binary weight values can achieve competitive rates of accuracy comparable to those of their analog weight counterparts. These findings highlight the dominance of the LRS for CSI RRAM with self‐compliance in a weighted configuration and minimal influence of the high resistance state despite substantial fluctuations for flexible CSI RRAM under varying bending radii. With its unique electrical switching capabilities, the CSI RRAM is highly anticipated to emerge as a promising candidate for embedded AI systems, especially in IoT devices and wearables.

Nitrogen-doped carbon nanotubes for self-powered memristive systems

Memristive devices are one of the promising candidates for creating neuromorphic systems due to the possibility of multilevel switching, low operating voltages and high scalability. However, as with any passive element, the memristor requires an external bias voltage to operate, which requires the inclusion of a power source in the circuit. In this regard, of great interest are works on the creation of self-powered memristive systems consisting of connecting in series a memristor and a nanogenerator that converts the energy of the external environment into electrical energy [1, 2]. Such a memristive system has a high potential for applications in aerospace and implantable electronics. At the moment, the first self-powered memristive and sensor systems based on metal oxides and piezoelectric nanogenerators (PENG) have already been developed [2]. The main problems in this area are to reduce the size of the nanogenerator and to match the output parameters of the nanogenerator and the input parameters of the memristor. In the framework of this work, these problems are being resolved by creating a self-powered memristive system based on nitrogen-doped carbon nanotubes (N-CNTs). Previously, we studied the memristive properties of N-CNTs and showed that nanotubes demonstrate reproducible multilevel switching with a resistance ratio in the high- and low-resistance states (HRS/LRS) of about 4⋅105 [3, 4]. It was found that the memristive effect in N-CNTs is due to the incorporation of nitrogen atoms into the nanotube structure and the formation of an internal piezoelectric field [4]. As part of further studies, it was found that an array of vertically aligned N-CNTs is a promising material for creating PENG: the generated output voltage is hundreds of mV and the current generated by single nanotube reaches hundreds of nA [5]. The results obtained allow us to speak about the possibility of developing a self-powered memristive system by connecting in series a memristor and PENG based on N-CNTs. To optimize the output characteristics of the PENG, in particular, the amplitude of the generated voltage, and the input switching voltage of the N-CNT-based memristor, studies were carried out to increase the piezoelectric response and reduce the switching voltage of the N-CNT resistance by changing the concentration of the dopant nitrogen in the nanotube growth process. It was found that it is necessary to grow N-CNTs with a doping nitrogen concentration of up to 12% and a high aspect ratio of length to diameter (more than 60) to create PENG with an output voltage of up to 2 V. These N-CNT parameters are provided at a low growth temperature (500–550 C°) and high ratio of acetylene and ammonia flows (1:5 - 1:6). On the contrary, the N-CNTs with a small aspect ratio (less than 30) and doping nitrogen concentrations of 4–6% are required for the manufacture of memristors with a minimum switching voltage (about 2 V), These N-CNT parameters are provided by increasing the growth temperature to 615 C° and reduction in growth time. The obtained results can be used in the development of self-powered memristive and sensor systems based on nitrogen-doped carbon nanotubes.

Stereoelectronic Switches of Single-Molecule Junctions through Conformation-Modulated Intramolecular Coupling Approaches.

Stereoelectronic effects in single-molecule junctions have been widely utilized to achieve a molecular switch, but high-efficiency and reproducible switching remain challenging. Here, we demonstrate that there are three stable intramolecular conformations in the 9,10-diphenyl-9,10-methanoanthracen-11-one (DPMAO) systems due to steric effect. Interestingly, different electronic coupling approaches including weak coupling (through-space), decoupling, and strong coupling (through-bond) between two terminal benzene rings are accomplished in the three stable conformations, respectively. Theoretical calculations show that the molecular conductance of three stable conformations differs by more than 1 order of magnitude. Furthermore, the populations of the three stable conformations are highly dependent on the solvent effect and the external electric field. Therefore, an excellent molecular switch can be achieved using the DPMAO molecule junctions and external stimuli. Our findings reveal that modulating intramolecular electronic coupling approaches may be a useful manner to enable molecular switches with high switching ratios. This opens up a new route for building high-efficiency molecular switches in single-molecular junctions.

A MAPbBr3/PdSe2 Schottky junction-based optoelectronic sensor with self-powered and switchable photocurrents

Neuromorphic optoelectronic sensors with in-sensor computing architecture hold great promise for applications that require processing large amounts of redundant data, such as the Internet of Things, robotics, and environmental sciences, due to their advantages of low time latency and energy efficiency. Halide perovskites, known for their extraordinary optoelectronic properties and stimuli-responsive characteristics, offer excellent opportunities for developing switchable visual sensors with high sensitivity, fast response, low energy consumption, and wide adaptive range. In this work, we successfully realized a MAPbBr3-PdSe2 heterojunction-based optoelectronic sensor, demonstrating a responsivity of 28 mA/W and a high specific detectivity of 5.2 × 1011 Jones. A fast response time of ∼ 25 μs has also been achieved. Additionally, we investigated the role of voltage-induced ion migration in actively adjusting the device's photoresponse capacity. Under 0 V bias, the device exhibited a wide switchable range of optical responsivity from 137.5% to 27000%, significantly surpassing pure perovskite-based devices in previous reports. Moreover, we demonstrated the device's adaptive learning capabilities and reproducible switching characteristics by simulating Pavlovian classical conditioned reflex experiments using electrical modulation. These findings open up exciting possibilities for next-generation artificial vision systems that are energy-efficient, adaptive, and capable of learning and effectively responding to varying visual conditions.

Mg dopant induced ultra-high HRS resistance and striking switching window characteristics in amorphous Y2O3 film-based memristors

Y2O3 has attracted attention as the representative emerging candidate of a resistive switching (RS) medium in memristors due to its excellent electrical properties and good thermal stability. However, many challenges for Y2O3 film-based memristors remain to be resolved, particularly for the small switching window. Here, the doping engineering strategy is proposed, and in particular, the Mg doped amorphous Y2O3 film is adopted as the RS layer to construct memristors. The prepared Pt/Mg:Y2O3/Pt memristor exhibits a typical reproducible bipolar switching behavior with ultra-high HRS resistance and excellent switching window (>105), compared with the undoped counterparts (∼50). In addition, the multilevel storage capability is also achieved by controlling compliance current. Furthermore, the mechanisms and corresponding physical models for the striking RS characteristics for Pt/Mg:Y2O3/Pt memristors, stemming from the Mg dopant, are discussed and illustrated in detail. This work affords a deep understanding of RS mechanisms for Mg-doped Y2O3 film-based memristors and provides an effective strategy to enlarge the switching window for other transition metal oxide memristors.

Robust Single-Supermolecule Switches Operating in Response to Two Different Noncovalent Interactions.

Supramolecular electronics provide an opportunity to introduce molecular assemblies into electronic devices through a combination of noncovalent interactions such as [π···π] and hydrogen-bonding interactions. The fidelity and dynamics of noncovalent interactions hold considerable promise when it comes to building devices with controllable and reproducible switching functions. Here, we demonstrate a strategy for building electronically robust switches by harnessing two different noncovalent interactions between a couple of pyridine derivatives. The single-supermolecule switch is turned ON when compressing the junction enabling [π···π] interactions to dominate the transport, while the switch is turned OFF by stretching the junction to form hydrogen-bonded dimers, leading to a dramatic decrease in conductance. The robustness and reproducibility of these single-supermolecule switches were achieved by modulating the junction with Ångström precision at frequencies of up to 190 Hz while obtaining high ON/OFF ratios of ∼600. The research presented herein opens up an avenue for designing robust bistable mechanoresponsive devices which will find applications in the building of integrated circuits for microelectromechanical systems.

360° detection of linear ZnO@CFs photoelectrochemical-type ultraviolet photodetector

Applications of omnidirectional ultraviolet (UV) detectors are numerous and promising. There is an increasing demand for flexible UV detectors toward the realization of intelligent and weavable systems. Carbon fibers (CFs) are used in flexible omnidirectional UV detectors due to their linear structure and unique flexibility. In this work, a flexible photoanode for omnidirectional photoelectrochemical (PEC)-type UV photodetectors (PDs) based on wurtzite hexagonal-phase ZnO nanowires grown on CFs is developed. The PD based on ZnO@CFs has a good response for rotation angles in the range from 0° to 360°. After being bent and twisted several hundred times, the PD still exhibits a stable switching period. Furthermore, the detector shows stable photocurrents up to 185 μA cm−2, reproducible switching periods, fast rising and falling response times of 0.17 and 0.12 s, respectively, and excellent spectral selectivity of 300–400 nm. The proposed flexible photoanode has potential applications in wearable PEC UV detectors with 360° detection.

Multiple-State Nonvolatile Memory Based on Ultrathin Indium Oxide Film via Liquid Metal Printing.

In this work, the ultrathin two-dimensional (2D) indium oxide (InOx) with a large area of more than 100 μm2 and a high degree of uniformity was automatically peeled off from indium by the liquid-metal printing technique. Raman and optical measurements revealed that 2D-InOx has a polycrystalline cubic structure. By altering the printing temperature which affects the crystallinity of 2D-InOx, the mechanism of the existence and disappearance of memristive characteristics was established. The tunable characteristics of the 2D-InOx memristor with reproducible one-order switching was manifest from the electrical measurements. Further adjustable multistate characteristics of the 2D-InOx memristor and its resistance switching mechanism were evaluated. A detailed examination of the memristive process demonstrated the Ca2+ mimic dynamic in 2D-InOx memristors as well as the fundamental principles underlying biological and artificial synapses. These surveys allow us to comprehend a 2D-InOx memristor using the liquid-metal printing technique and could be applied to future neuromorphic applications and in the field of revolutionary 2D material exploration.

Exploring conductance modulation and implementation of convolutional neural network in Pt/ZnO/Al2O3/TaN memristors for brain-inspired computing

Oxide-based memristors have emerged as a promising electronic device for high-density memory and neuromorphic applications. In our study, we explored the tunable analog switching and biological synaptic functions of a Pt/ZnO/Al2O3/TaN memristive device. Using transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS), we confirmed the presence of a TaOxNy interface layer at the anode contact, believed to play a critical role in resistance transitions. The memristive device showed excellent performance, including a stable and reproducible analog switching memory with a low operating voltage (μ=̶2.0/+1.7V), good cycling endurance (2 × 102), a high on/off ratio (>103), and retention up to 104 s at 85 °C. Additionally, multi-state resistances were achieved by varying the reset voltage, enabling the creation of neuromorphic synapses and high-density memories. Direct-current mode set and reset transitions showed multi-state resistance changes similar to potentiation and depression behaviors in biological synapses. Further simulations, including long-term potentiation (LTP) and long-term depression (LTD), paired pulse facilitation (PPF), and convolutional neural network (CNN) simulations for handwritten digits, showed an accuracy of 86.5%. These results indicate that the memristive device is highly suitable for use in high-density memory and brain-inspired computer systems.

Redox Switching Behavior in Resistive Memory Device Designed Using a Solution-Processable Phenalenyl-Based Co(II) Complex: Experimental and DFT Studies.

We herein report a novel square-planar complex [CoIIL], which was synthesized using the electronically interesting phenalenyl-derived ligand LH2 = 9,9'-(ethane-1,2-diylbis(azanediyl))bis(1H-phenalen-1-one). The molecular structure of the complex is confirmed with the help of the single-crystal X-ray diffraction technique. [CoIIL] is a mononuclear complex where the Co(II) ion is present in the square-planar geometry coordinated by the chelating bis-phenalenone ligand. The solid-state packing of [CoIIL] complex in a crystal structure has been explained with the help of supramolecular studies, which revealed that the π···π stacking present in the [CoIIL] complex is analogous to the one present in tetrathiafulvalene/tetracyanoquinodimethane charge transfer salt, well-known materials for their unique charge carrier interfaces. The [CoIIL] complex was employed as the active material to fabricate a resistive switching memory device, indium tin oxide/CoIIL/Al, and characterized using the write-read-erase-read cycle. The device has interestingly shown a stable and reproducible switching between two different resistance states for more than 2000 s. Observed bistable resistive states of the device have been explained by corroborating the electrochemical characterizations and density functional theory studies, where the role of the CoII metal center and π-conjugated phenalenyl backbone in the redox-resistive switching mechanism is proposed.

Electrical switching of a bistable moiré superconductor.

Electrical control of superconductivity is critical for nanoscale superconducting circuits including cryogenic memory elements1-4, superconducting field-effect transistors (FETs)5-7 and gate-tunable qubits8-10. Superconducting FETs operate through continuous tuning of carrier density, but no bistable superconducting FET, which could serve as a new type of cryogenic memory element, has been reported. Recently, gate hysteresis and resultant bistability in Bernal-stacked bilayer graphene aligned to its insulating hexagonal boron nitride gate dielectrics were discovered11,12. Here we report the observation of this same hysteresis in magic-angle twisted bilayer graphene (MATBG) with aligned boron nitride layers. This bistable behaviour coexists alongside the strongly correlated electron system of MATBG without disrupting its correlated insulator or superconducting states. This all-van der Waals platform enables configurable switching between different electronic states of this rich system. To illustrate this new approach, we demonstrate reproducible bistable switching between the superconducting, metallic and correlated insulator states of MATBG using gate voltage or electric displacement field. These experiments unlock the potential to broadly incorporate this new switchable moiré superconductor into highly tunable superconducting electronics.

Reproducible switching between a walled and cell wall-deficient lifestyle of actinomycetes using gradient agar plates

The cell wall is a shape-defining structure that envelopes almost all bacteria, protecting them from biotic and abiotic stresses. Paradoxically, some filamentous actinomycetes have a natural ability to shed their cell wall under influence of hyperosmotic stress. These wall-deficient cells can revert to their walled state when transferred to a medium without osmoprotection but often lyse due to their fragile nature. Here, we designed plates with an osmolyte gradient to reduce cell lysis and thereby facilitating the transition between a walled and wall-deficient state. These gradient plates allow determining of the osmolyte concentration where switching takes place, thereby enabling careful and reproducible comparison between mutants affected by switching. Exploring these transitions could give valuable insights into the ecology of actinomycetes and their biotechnological applications.