Reversible resistive switching behaviour in CVD grown, large area MoOx

MoOx Synaptic Memristor with Programmable Multilevel Conductance for Reliable Neuromorphic Hardware

Memristor holds great potential for enabling next-generation neuromorphic computing hardware. Controlling the interfacial characteristics of the device is critical for seamlessly integrating and replicating the synaptic dynamic behaviors; however, it is commonly overlooked. Herein, we report the straightforward oxidation of a Mo electrode in air to design MoOx memristors that exhibit nonvolatile ultrafast switching (0.6-0.8 mV/decade, <1 mV/decade) with a high on/off ratio (>104), a long durability (>104 s), a low power consumption (17.9 μW), excellent device-to-device uniformity, ingeniously synaptic behavior, and finely programmable multilevel analog switching. The analyzed physical mechanism of the observed resistive switching behavior might be the conductive filaments formed by the oxygen vacancies. Intriguingly, upon organization into memristor-based crossbar arrays, in addition to simulated multipattern memorization, edge detection on random images can be implemented well by parallel processing of pixels using a 3 × 3 × 2 array of Prewitt filter groups. These are vital functions for neural system hardware in efficient in-memory computing neural systems with massive parallelism beyond a von Neumann architecture.

Low-Temperature Solution-Based Molybdenum Oxide Memristors

Solution-based memristors have gained significant attention in recent years due to their potential for the low-cost, scalable, and environmentally friendly fabrication of resistive switching devices. This study is focused on the fabrication and characterization of solution-based molybdenum trioxide (MoO3) memristors under different annealing temperatures (200 to 400 °C). A MoO3 ink recipe is developed using water as the main solvent, enabling a simplified and cost-effective fabrication process. Material analysis reveals the presence of a Mo6+ oxidation state and an amorphous structure in the films annealed up to 250 °C. Electrical tests confirm a bipolar resistive switching behavior in the memristors according to the valence change mechanism (VCM). Endurance tests demonstrate stable memristors, indicating their robust nature after multiple cycles. Memristors annealed at 250 °C exhibit a nonvolatile behavior with a retention time up to 105 s under ambient air conditions. The high reproducibility observed in these memristors highlights their potential for practical applications and scalability.

Spike rate dependent synaptic characteristics in lamellar, multilayered alpha-MoO3 based two-terminal devices - efficient way to control the synaptic amplification

Brain-inspired computing systems require a rich variety of neuromorphic devices using multi-functional materials operating at room temperature. Artificial synapses which can be operated using optical and electrical stimuli are in high demand. In this regard, layered materials have attracted a lot of attention due to their tunable energy gap and exotic properties. In the current study, we report the growth of layered MoO3 using the chemical vapor deposition (CVD) technique. MoO3 has an energy gap of 3.22 eV and grows with a large aspect ratio, as seen through optical and scanning electron microscopy. We used transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy for complete characterisation. The two-terminal devices using platinum (Pt/MoO3/Pt) exhibit superior memory with the high-resistance state (HRS) and low-resistance state (LRS) differing by a large resistance (∼MΩ). The devices also show excellent synaptic characteristics. Both optical and electrical pulses can be utilised to stimulate the synapse. Consistent learning (potentiation) and forgetting (depression) curves are measured. Transition from long term depression to long term potentiation can be achieved using the spike frequency dependent pulsing scheme. We have found that the amplification of postsynaptic current can be tuned using such frequency dependent spikes. This will help us to design neuromorphic devices with the required synaptic amplification.

An artificial optoelectronic synapse based on MoOxfilm

Artificial optoelectronic synapses have the advantages of large bandwidth, low power consumption and low crosstalk, and are considered to be the basic building blocks of neuromorphic computing. In this paper, a two-terminal optoelectronic synaptic device with ITO-MoO_x-Pt structure is prepared by magnetron sputtering. The performance of resistive switching (RS) and the photo plastic properties of the device are analyzed and demonstrated. Electrical characterization tests show that the device has a resistive HRS/LRS ratio of about 90, stable endurance, and retention characteristics of more than 10⁴s (85 °C). The physical mechanism of the device is elucidated by a conducting filament composed of oxygen vacancies. Furthermore, the function of various synaptic neural morphologies is successfully mimicked using UV light as the stimulation source. Including short-term/long-term memory, paired-pulse facilitation, the transition from short-term to long-term memory, and 'learning-experience' behavior. Integrated optical sensing and electronic data storage devices have great potential for future artificial intelligence, which will facilitate the rapid development of retina-like visual sensors and low-power neuromorphic systems.

Quantum Conductance in Memristive Devices: Fundamentals, Developments, and Applications

Quantum effects in novel functional materials and new device concepts represent a potential breakthrough for the development of new information processing technologies based on quantum phenomena. Among the emerging technologies, memristive elements that exhibit resistive switching, which relies on the electrochemical formation/rupture of conductive nanofilaments, exhibit quantum conductance effects at room temperature. Despite the underlying resistive switching mechanism having been exploited for the realization of next-generation memories and neuromorphic computing architectures, the potentialities of quantum effects in memristive devices are still rather unexplored. Here, a comprehensive review on memristive quantum devices, where quantum conductance effects can be observed by coupling ionics with electronics, is presented. Fundamental electrochemical and physicochemical phenomena underlying device functionalities are introduced, together with fundamentals of electronic ballistic conduction transport in nanofilaments. Quantum conductance effects including quantum mode splitting, stability, and random telegraph noise are analyzed, reporting experimental techniques and challenges of nanoscale metrology for the characterization of memristive phenomena. Finally, potential applications and future perspectives are envisioned, discussing how memristive devices with controllable atomic-sized conductive filaments can represent not only suitable platforms for the investigation of quantum phenomena but also promising building blocks for the realization of integrated quantum systems working in air at room temperature.

Highly Responsive Pd-Decorated MoO3 Nanowall H2 Gas Sensors Obtained from In-Situ-Controlled Thermal Oxidation of Sputtered MoS2 Films

Among transition metal oxides, MoO₃ is a promising material due to its layered structure and different oxidation states, making it suitable for different device applications. One of the methods used to grow MoO₃ is radio frequency magnetron sputtering (RFMS), which is the most compatible method in industry. However, obtaining nanostructures by RFMS for metal oxides is challenging because of compact morphology film formation. In this study, α-MoO₃ with vertical nanowalls is obtained by a two-step process; deposition of magnetron-sputtered MoS₂ vertical nanowalls and postoxidation of these structures without changing the morphology. In situ transmittance and electrical measurements are performed to control the oxidation process, which shed light on understanding the oxidation of MoS₂ nanowalls. The transition from MoS₂ to α-MoO₃ is investigated with partially oxidized MoS₂/MoO₃ samples with different thicknesses. It is also concluded that oxidation starts from nanowalls perpendicular to the substrate and lasts with oxidation of basal planes. Four different thicknesses of α-MoO₃ nanowall samples are fabricated for H₂ gas sensors. Also, the effect of Pd deposition on the H₂-sensing properties of sensors is deeply investigated. An outstanding response of 3.3 × 10⁵ as well as the response and recovery times of 379 and 304 s, respectively, are achieved from the thinnest Pd-loaded sample. Also, the gas-sensing mechanism is explored by gasochromic measurements to investigate the sensor behaviors under the conditions of dry air and N₂ gas as the carrier gas.

Wafer-Scalable Single-Layer Amorphous Molybdenum Trioxide

Molybdenum trioxide (MoO₃), an important transition metal oxide (TMO), has been extensively investigated over the past few decades due to its potential in existing and emerging technologies, including catalysis, energy and data storage, electrochromic devices, and sensors. Recently, the growing interest in two-dimensional (2D) materials, often rich in interesting properties and functionalities compared to their bulk counterparts, has led to the investigation of 2D MoO₃. However, the realization of large-area true 2D (single to few atom layers thick) MoO₃ is yet to be achieved. Here, we demonstrate a facile route to obtain wafer-scale monolayer amorphous MoO₃ using 2D MoS₂ as a starting material, followed by UV-ozone oxidation at a substrate temperature as low as 120 °C. This simple yet effective process yields smooth, continuous, uniform, and stable monolayer oxide with wafer-scale homogeneity, as confirmed by several characterization techniques, including atomic force microscopy, numerous spectroscopy methods, and scanning transmission electron microscopy. Furthermore, using the subnanometer MoO₃ as the active layer sandwiched between two metal electrodes, we demonstrate the thinnest oxide-based nonvolatile resistive switching memory with a low voltage operation and a high ON/OFF ratio. These results (potentially extendable to other TMOs) will enable further exploration of subnanometer stoichiometric MoO₃, extending the frontiers of ultrathin flexible oxide materials and devices.

Mechanically Durable Memristor Arrays Based on a Discrete Structure Design

Memristors constitute a promising functional component for information storage and in-memory computing in flexible and stretchable electronics including wearable devices, prosthetics, and soft robotics. Despite tremendous efforts made to adapt conventional rigid memristors to flexible and stretchable scenarios, stretchable and mechanical-damage-endurable memristors, which are critical for maintaining reliable functions under unexpected mechanical attack, have never been achieved. Here, the development of stretchable memristors with mechanical damage endurance based on a discrete structure design is reported. The memristors possess large stretchability (40%) and excellent deformability (half-fold), and retain stable performances under dynamic stretching and releasing. It is shown that the memristors maintain reliable functions and preserve information after extreme mechanical damage, including puncture (up to 100 times) and serious tearing situations (fully diagonally cut). The structural strategy offers new opportunities for next-generation stretchable memristors with mechanical damage endurance, which is vital to achieve reliable functions for flexible and stretchable electronics even in extreme and highly dynamic environments.

Resistance Switching and Failure Behavior of the MoOx/Mo2C Heterostructure

With the rapid demand for high-performance and power-efficient memristive and synaptic systems, more 2D heterostructures with improved resistance switching (RS) properties are still urgently in need for next-generation devices. Here, we report the RS behaviors of vertical MoO_x/Mo₂C heterostructures fabricated by controllable thermal oxidation and uncover the failure behavior for the first time. It is found that the MoO_x/Mo₂C heterostructure exhibits bipolar RS with a low set/reset voltage of +0.5/-0.3 V, an ultralow power consumption of 5 × 10^-8 W, and an on/off ratio of 10², which is ascribed to the transport of the internal oxygen ions of MoO_x. Furthermore, the failure behavior of RS behaviors of the MoO_x/Mo₂C heterostructure under a higher work voltage is revealed. It indicates that the amorphization of the pristine crystalline MoO_x layer could block the movement of the internal oxygen ions in the vertical direction. The excellent RS performance induced by the synergy of MoO_x and Mo₂C and the demonstration of the failure behavior enable the potential applications of the 2D heterostructure in related memory devices and biological neural networks.

Kinetically Stable Oxide Overlayers on Mo3 P Nanoparticles Enabling Lithium-Air Batteries with Low Overpotentials and Long Cycle Life

The main drawbacks of today's state-of-the-art lithium-air (Li-air) batteries are their low energy efficiency and limited cycle life due to the lack of earth-abundant cathode catalysts that can drive both oxygen reduction and evolution reactions (ORR and OER) at high rates at thermodynamic potentials. Here, inexpensive trimolybdenum phosphide (Mo₃ P) nanoparticles with an exceptional activity-ORR and OER current densities of 7.21 and 6.85 mA cm^-2 at 2.0 and 4.2 V versus Li/Li⁺ , respectively-in an oxygen-saturated non-aqueous electrolyte are reported. The Tafel plots indicate remarkably low charge transfer resistance-Tafel slopes of 35 and 38 mV dec^-1 for ORR and OER, respectively-resulting in the lowest ORR overpotential of 4.0 mV and OER overpotential of 5.1 mV reported to date. Using this catalyst, a Li-air battery cell with low discharge and charge overpotentials of 80 and 270 mV, respectively, and high energy efficiency of 90.2% in the first cycle is demonstrated. A long cycle life of 1200 is also achieved for this cell. Density functional theory calculations of ORR and OER on Mo₃ P (110) reveal that an oxide overlayer formed on the surface gives rise to the observed high ORR and OER electrocatalytic activity and small discharge/charge overpotentials.

Molybdenum-Suboxide Thin Films as Anode Layers in Planar Lithium Microbatteries

In this paper, we investigate the effects of operational conditions on structural, electronic and electrochemical properties on molybdenum suboxides (MoO3-δ) thin films. The films are prepared using pulsed-laser deposition by varying the deposition temperature (Ts), laser fluence (Φ), the partial oxygen pressure (PO2) and annealing temperature (Ta). We find that three classes of samples are obtained with different degrees of stoichiometric deviation without post-treatment: (i) amorphous MoO3-δ (δ < 0.05) (ii) nearly-stoichiometric samples (δ ≈ 0) and (iii) suboxides MoO3-δ (δ > 0.05). The suboxide films 0.05 ≤ δ ≤ 0.25 deposited on Au/Ti/SiO2/flexible-Si substrates with appropriate processing conditions show high electrochemical performance as an anode layer for lithium planar microbatteries. In the realm of simple synthesis, the MoO3-δ film deposited at 450 °C under oxygen pressure of 13 Pa is a mixture of α-MoO3 and Mo8O23 phases (15:85). The electrochemical test of the 0.15MoO3-0.85Mo8O23 film shows a specific capacity of 484 µAh cm−2 µm−1 after 100 cycles of charge-discharge at a constant current of 0.5 A cm−2 in the potential range 3.0-0.05 V.

Electrically Activated UV-A Filters Based on Electrochromic MoO3-x

Chromism-based optical filters is a niche field of research, due to there being only a handful of electrochromic materials. Typically, electrochromic transition metal oxides such as MoO₃ and WO₃ are utilized in applications such as smart windows and electrochromic devices (ECD). Herein, we report MoO_3-x-based electrically activated ultraviolet (UV) filters. The MoO_3-x grown on indium tin oxide (ITO) substrate is mechanically assembled onto an electrically activated proton exchange membrane. Reversible H⁺ injection/extraction in MoO_3-x is employed to switch the optical transmittance, enabling an electrically activated optical filter. The devices exhibit broadband transmission modulation (325-800 nm), with a peak of ∼60% in the UV-A range (350-392 nm). Comparable switching times of 8 s and a coloration efficiency of up to 116 cm² C^-1 are achieved.

Influence of an external electric field on the rapid synthesis of MoO3 micro- and nanostructures by Joule heating of Mo wires

The growth mechanism of layered α-MoO₃ nano- and microplates on the surface of Mo wires during Joule heating has been investigated by application of an external electric field to the current carrying wire. The observed rapid growth of the structures, involving enhanced diffusion processes associated to the intense electric current, is further enhanced by the external field leading to a near instantaneous formation of MoO₃ plates. Thermally assisted electromigration in the Mo wire with the additional effect of the electric field appears as a very time effective method to grow MoO₃ layered low dimensional structures. Other molybdenum oxide nanostructures, such as nanospheres and nanocrystallites with different shapes, have been found to grow by deposition from the Mo wire on the electrodes used to apply the external electric field. The growth on the electrodes takes place by a thermally assisted electric-field-driven process.

Differential Work-Function Enabled Bifunctional Switching in Strontium Titanate Flexible Resistive Memories

Multifunctional electronic memories capable of demonstrating both analog and digital switching on-demand are extremely attractive for miniaturization of electronics without significant drain on energy consumption. Simultaneously translating functionality onto mechanically conformable platforms will further enhance their suitability. Here, we demonstrate the ability to engineer multifunctionality in strontium titanate (STO)-based resistive random-access memories (ReRAM) on a flexible polyimide platform. By utilizing different bottom electrodes of various work functions while the top electrode is fixed, differential work functions are induced in STO, to induce bipolar or complementary switching behaviors whenever required. This work-function difference-induced bifunctional switching on the flexible platform reveals a streamlined route for achieving flexible artificial neural networks, high density integration, and logic operation using a single ReRAM.

A unipolar nonvolatile resistive switching behavior in a layered transition metal oxide

Two-dimensional layered materials have been considered as promising candidates for resistive random access memory, one of the most promising next-generation nonvolatile memories. However, due to the types of defects, most of the devices still suffer from poor environmental stability, defects inducing complexity, and uncontrollability. Here, we fabricate memory cells based on synthesized high-quality two-dimensional layered transition-metal oxide (α-MoO₃) nanosheets which can be thinned to 8.68 nm (∼6 layers) and find a unipolar nonvolatile resistive switching behavior. Driven by the migration of intrinsic oxygen vacancies, the devices show a large memory window (∼10⁵), good memory voltage stability, long-term endurance (for durations of over 3 days and 50 manual DC switching cycles) and multi-bit memory states. Furthermore, we find the devices with an excellent temperature tolerance of lower SET/RESET voltages and a larger memory window (>10⁴ at 380 K) at higher temperatures, suggesting their potential in practical applications. Finally, all 2D memory devices are demonstrated using graphene/α-MoO₃/graphene heterostructures.

Dual Selective Gas Sensing Characteristics of 2D α-MoO3-x via a Facile Transfer Process

Metal oxide-based gas sensor technology is promising due to their practical applications in toxic and hazardous gas detection. Orthorhombic α-MoO₃ is a planar metal oxide with a unique layered structure, which can be obtained in a two-dimensional (2D) form. In the 2D form, the larger surface area-to-volume ratio of the material facilitates significantly higher interaction with gas molecules while exhibiting exceptional transport properties. The presence of oxygen vacancies results in nonstoichiometric MoO₃ (MoO_3-x), which further enhances the charge carrier mobility. Here, we study dual gas sensing characteristics and mechanism of 2D α-MoO_3-x. Herein, conductometric dual gas sensors based on chemical vapor deposited 2D α-MoO_3-x are developed and demonstrated. A facile transfer process is established to integrate the material into any arbitrary substrate. The sensors show high selectivity toward NO₂ and H₂S gases with response and recovery rates of 295.0 and 276.0 kΩ/s toward NO₂ and 28.5 and 48.0 kΩ/s toward H₂S, respectively. These gas sensors also show excellent cyclic endurance with a variation in ΔR ∼ 112 ± 1.64 and 19.5 ± 1.13 MΩ for NO₂ and H₂S, respectively. As such, this work presents the viability of planar 2D α-MoO_3-x as a dual selective gas sensor.

Evolution map of the memristor: from pure capacitive state to resistive switching state

Memristors possess great application prospects in terabit nonvolatile storage devices, memory-in-logic algorithmic chips and bio-inspired artificial neural network systems. However, "what is the origin state of the memristor?" has remained an unanswered question for half a century. While many applications rely on the memristor, its origin state is becoming a fundamental issue. Herein, we reveal a new state, the pure capacitance state (PCS), which occurs before the memristor is triggered, and the origin state of the memristor can be verified in the memory cells through controlling the ambience parameters. Discovery of the PCS, a missing earlier stage of the memristor, completes the whole evolution map of the memristor from the very beginning to the final developed state.