Flexible Nanoporous WO3-x Nonvolatile Memory Device

Nanostructured NiS2-based flexible smart sensors for human respiration monitoring

The growing demand for wearable healthcare devices has led to an urgent need for cost-effective, wireless and portable breath monitoring systems. However, it is essential to explore novel nanomaterials that combine state-of-the-art flexible sensors with high performance and sensing capabilities along with scalability and industrially acceptable processing. In this study, we demonstrate a highly efficient NiS2-based flexible capacitive sensor fabricated via a solution-processible route using a novel single-source precursor [Ni{S2P(OPr)2}2]. The developed sensor could precisely detect the human respiration rate and exhibit rapid responsiveness, exceptional sensitivity and selectivity at ambient temperatures, with an ultra-fast response and recovery. The device effectively differentiates the exhaled breath patterns including slow, fast, oral and nasal breath, as well as post-exercise breath rates. Moreover, the sensor shows outstanding bending stability, repeatability, reliable and robust sensing performance and is capable of contactless sensing. The sensor was further employed with a user-friendly wireless interface to facilitate smartphone-enabled real-time breath monitoring systems. This work opens up numerous avenues for cost-effective, sustainable and versatile sensors with potential applications for Internet of Things-based flexible and wearable electronics.This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

Oxygen Vacancies Trigger Rapid Charge Transport Channels at the Engineered Interface of S-Scheme Heterojunction for Boosting Photocatalytic Performance

Although oxygen vacancies (Ovs) have been intensively studied in single semiconductor photocatalysts, exploration of intrinsic mechanisms and in-depth understanding of Ovs in S-scheme heterojunction photocatalysts are still limited. Herein, a novel S-scheme photocatalyst made from WO3-Ov/In2S3 with Ovs at the heterointerface is rationally designed. The microscopic environment and local electronic structure of the S-scheme heterointerface are well optimized by Ovs. Femtosecond transient absorption spectroscopy (fs-TAS) reveals that Ovs trigger additional charge movement routes and therefore increase charge separation efficiency. In addition, Ovs have a synergistic effect on the thermodynamic and kinetic parameters of S-scheme photocatalysts. As a result, the optimal photocatalytic performance is significantly improved, surpassing that of single component WO3-Ov and In2S3 (by 35.5 and 3.9 times, respectively), as well as WO3/In2S3 heterojunction. This work provides new insight into regulating the photogenerated carrier dynamics at the heterointerface and also helps design highly efficient S-scheme photocatalysts.

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.

WO3-NP-activated WS2 layered heterostructures for efficient broadband (254 nm-940 nm) photodetection

Broadband photodetection including deep UV using Si is technically challenging due to its negligible optical absorption at 254 nm and the requirement of heterogeneous integration with very high bandgap photoactive materials. However, monolithic integration of high-bandgap semiconductors on Si is not possible due to CMOS fabrication incompatibility. Comprehensive experimental studies to achieve broadband photodetection including deep UV on Si are lacking in the literature. Here for the first time we have investigated 2D/0D heterojunctions of WS2/WO3 on a Si platform both experimentally and theoretically and established the charge transfer mechanism between them. Transient photocarrier decay experiments demonstrate effective quenching of excited photocarriers generated in WO3/WS2, signifying its utility in facilitating carrier transport, which is further evidenced by charge density calculation from DFT simulation. Our designed vertically aligned p-Si/WS2/WO3 heterojunction-based photodetector exhibits an excellent photosensitivity performance with a broad spectral response ranging from deep ultraviolet (254 nm) to near infrared (940 nm) wavelengths, and it not only provides a peak responsivity of 251 A W-1 and a specific detectivity of 1.89 × 014 Jones, but also possesses a rapid response speed with a rise/fall time of 0.64/0.48 s at 365 nm with a bias of 2 volt.

Flexible aluminum-doped hafnium oxide ferroelectric synapse devices for neuromorphic computing

The HfO2-based ferroelectric tunnel junction has received outstanding attention owing to its high-speed and low-power characteristics. In this work, aluminum-doped HfO2 (HfAlO) ferroelectric thin films are deposited on a muscovite substrate (Mica). We investigate the bending effect on the ferroelectric characteristics of the Au/Ti/HfAlO/Pt/Ti/Mica device. After 1000 bending times, the ferroelectric properties and the fatigue characteristics are largely degraded. The finite element analysis indicates that crack formation is the main reason for the fatigue damage under threshold bending diameters. Moreover, the HfAlO-based ferroelectric synaptic device exhibits excellent performance of neuromorphic computing. The artificial synapse can mimic the paired-pulse facilitation and long-term potentiation/depression of biological synapses. Meanwhile, the accuracy of digit recognition is 88.8%. This research provides a new research idea for the further development of hafnium-based ferroelectric devices.

Unveiling the Electronic Structure of Pseudotetragonal WO3 Thin Films

WO3 is a 5d compound that undergoes several structural transitions in its bulk form. Its versatility is well-documented, with a wide range of applications, such as flexopiezoelectricity, electrochromism, gating-induced phase transitions, and its ability to improve the performance of Li-based batteries. The synthesis of WO3 thin films holds promise in stabilizing electronic phases for practical applications. However, despite its potential, the electronic structure of this material remains experimentally unexplored. Furthermore, its thermal instability limits its use in certain technological devices. Here, we employ tensile strain to stabilize WO3 thin films, which we call the pseudotetragonal phase, and investigate its electronic structure using a combination of photoelectron spectroscopy and density functional theory calculations. This study reveals the Fermiology of the system, notably identifying significant energy splittings between different orbital manifolds arising from atomic distortions. These splittings, along with the system's thermal stability, offer a potential avenue for controlling inter- and intraband scattering for electronic applications.

Review of Electrochemically Synthesized Resistive Switching Devices: Memory Storage, Neuromorphic Computing, and Sensing Applications

Resistive-switching-based memory devices meet most of the requirements for use in next-generation information and communication technology applications, including standalone memory devices, neuromorphic hardware, and embedded sensing devices with on-chip storage, due to their low cost, excellent memory retention, compatibility with 3D integration, in-memory computing capabilities, and ease of fabrication. Electrochemical synthesis is the most widespread technique for the fabrication of state-of-the-art memory devices. The present review article summarizes the electrochemical approaches that have been proposed for the fabrication of switching, memristor, and memristive devices for memory storage, neuromorphic computing, and sensing applications, highlighting their various advantages and performance metrics. We also present the challenges and future research directions for this field in the concluding section.

Selective Dual-Ion Modulation in Solid-State Magnetoelectric Heterojunctions for In-Memory Encryption

Nanoionic technologies are identified as a promising approach to modulating the physical properties of solid-state dielectrics, which have resulted in various emergent nanodevices, such as nanoionic resistive switching devices and magnetoionic devices for memory and computing applications. Previous studies are limited to single-type ion manipulation, and the investigation of multiple-type ion modulation on the coupled magnetoelectric effects, for developing information devices with multiple integrated functionalities, remains elusive. Here, a dual-ion solid-state magnetoelectric heterojunction based on Pt/HfO_{2- x} /NiO_y /Ni with reconfigurable magnetoresistance (MR) characteristics is reported for in-memory encryption. It is shown that the oxygen anions and nickel cations can be selectively driven by voltages with controlled polarity and intensity, which concurrently change the overall electrical resistance and the interfacial magnetic coupling, thus significantly modulate the MR symmetry. Based on this device, a magnetoelectric memory prototype array with in-memory encryption functionality is designed for the secure storage of image and digit information. Along with the advantages including simple structure, multistate encryption, good reversibility, and nonvolatile modulation capability, this proof-of-concept device opens new avenues toward next-generation compact electronics with integrated information functionalities.

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 [Co^IIL], 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. [Co^IIL] 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 [Co^IIL] complex in a crystal structure has been explained with the help of supramolecular studies, which revealed that the π···π stacking present in the [Co^IIL] complex is analogous to the one present in tetrathiafulvalene/tetracyanoquinodimethane charge transfer salt, well-known materials for their unique charge carrier interfaces. The [Co^IIL] complex was employed as the active material to fabricate a resistive switching memory device, indium tin oxide/Co^IIL/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 Co^II metal center and π-conjugated phenalenyl backbone in the redox-resistive switching mechanism is proposed.

Femtosecond Laser-Induced Nano-Joining of Volatile Tellurium Nanotube Memristor

Nanowire/nanotube memristor devices provide great potential for random-access high-density resistance storage. However, fabricating high-quality and stable memristors is still challenging. This paper reports multileveled resistance states of tellurium (Te) nanotube based on the clean-room free femtosecond laser nano-joining method. The temperature for the entire fabrication process was maintained below 190 °C. A femtosecond laser joining technique was used to form nanowire memristor units with enhanced properties. Femtosecond (fs) laser-irradiated silver-tellurium nanotube-silver structures resulted in plasmonic-enhanced optical joining with minimal local thermal effects. This produced a junction between the Te nanotube and the silver film substrate with enhanced electrical contacts. Noticeable changes in memristor behavior were observed after fs laser irradiation. Capacitor-coupled multilevel memristor behavior was observed. Compared to previous metal oxide nanowire-based memristors, the reported Te nanotube memristor system displayed a nearly two-order stronger current response. The research displays that the multileveled resistance state is rewritable with a negative bias.

Ultralow-Power RRAM with a High Switching Ratio Based on the Large van der Waals Interstice Radius of TMDs

Low power and high switching ratio are the development direction of the next generation of resistive random access memory (RRAM). Previous techniques could not increase the switching ratio while reducing the SET power. Here, we report a method to fabricate low-power and high-switching-ratio RRAM by adjusting the interstice radius (r_g) between the van der Waals (vdW) layers of transitional-metal dichalcogenides (TMDs), which simultaneously increases the switching ratio and reduces the SET power. The SET voltage, SET power, switching ratio and endurance of the device are strongly correlated with r_g. When the ratio of r_g to the radius of the metal ions that form the conductive filaments (r_g/r_Ag⁺) is near 1, the SET voltage and SET power vertically decrease while the switching ratio vertically rises with increasing r_g/r_Ag⁺. For the fabricated Ag/[SnS₂/poly(methyl methacrylate)]/Cu RRAM with an r_g/r_Ag⁺ of 1.04, the SET voltage, SET power and switching ratio are 0.14 V, 10^-10 W and 10⁶, respectively. After 10⁴ switching cycles and a 10⁴ s retention time, the switching ratio of the device can still be stable above 10⁶. Bending has no influence on the performance of the device when the bending radius is not <2 mm.

Nonvolatile Ternary Memristor Based on Fluorene-Benzimidazole Copolymer/Au NP Composites

A donor-acceptor type polymer of poly [2,7-(9,9-dioctyl)-fluorene-alt-7H-benzimidazo-[2,1-a]benzo[de]isoquinolin-7-one] (PF-BBO) based on benzimidazole groups was synthesized. This material was incorporated into data storage devices that exhibited good data storage characteristics. In order to improve the storage properties of the device, Au NPs were compounded in this material. We observed an increase in the ratio of switching current for the device with the PF-BBO/Au NP composite as the active layer. The device comprising 8 wt% Au NPs demonstrated optimal storage performance with a switching current ratio of 1:3.4 × 10²:1.0 × 10⁵ and a threshold voltage of -0.40 V/-0.85 V, respectively. The number of cycle times of this device was over 3000, which indicates excellent stability. Thus, the devices containing PF-BBO/Au NP composite as active materials offer a new dimension for future application prospects of high-density data storage.

Optimization of the incubation parameters for biogenic synthesis of WO3 nanoparticles using Taguchi method

Green synthesis of metal nanoparticles is gathering attention due to eco-friendly processing. Tungsten oxide (WO₃) nanoparticles have immense applications as semiconductors, antimicrobials and photo thermal materials but their synthesis using biological systems is hitherto unpublicized. The paper discusses synthesis of WO₃ nanoparticles using Stenotrophomonas maltophilia and the optimization of physico-chemical parameters of incubation which influence the growth and metabolism of the bacterium and consequently the size of the WO₃ nanoparticles. The biogenic synthesis of WO₃ nanoparticles was confirmed by ATR-FTIR and X-ray diffraction analysis. Taguchi and analysis of variance method was applied to optimize the physico-chemical parameters (pH, temperature, time, aeration rate and concentration), considering particle size and poly dispersity index (PDI) of the nanoparticles as the experimental responses. Under the design of experiments technique, Taguchi's L27 array was selected to determine the optimal process parameters which could significantly reduce the particle size and PDI of WO₃ nanoparticles. Statistical analysis by signal-to-noise ratio, regression analysis and ANOVA (95% confidence level) on experimental responses confirmed pH and aeration as most influential while temperature and time as least influential parameters. pH 8, Temperature 40 °C, aeration 200 RPM, time 3 days and concentration of sodium tungstate at 1 mM (p3t3r3d3c1) was the most effective level and parameters combination for smallest particle size and PDI of WO₃ nanoparticles. Regression models developed for particle size and PDI exhibited a linear regression of 97.80% and 90.89% respectively, while the confirmation test validated the size and PDI of the experimental values against predicted results. SEM image of WO₃ nanoparticles illustrated the same particle size as that predicted, further validating the model. The study can be applied to optimize any process parameters in the industry or on biological systems.

Tungsten oxide polymorphs and their multifunctional applications

Owing to the natural abundance, easy availability, high stability, non-stoichiometry, and chemical diversity, considerable interest has been devoted to tungsten oxide (WO_3-x) nanomaterials, and many advances have been achieved ranging from traditional catalysts and electronics to emerging artificial intelligence. This review focuses on recent progress of WO_3-x polymorphs and their multifunctional applications. The structural diversity and crystal phase transitions of WO_3-x and recent advances on the general synthesis of various WO_3-x nanostructures are first summarized, since the crystal structure and morphology adjustment obviously affect the physiochemical merits of WO_3-x materials. Then, their applications and related mechanisms in different fields are demonstrated, such as gas sensing, chromogenic (electro-, photo-, gaso-, and thermochromic), photocatalytic (pollutant degradation and water splitting), and emerging applications (biomedical, antibiotic, and artificial intelligence). With the advances highlighted here and the ongoing research efforts, the continuous breakthrough in functionalized WO_3-x nanostructure and their attractive applications is foreseeable in the future.

Non-volatile ternary memristors based on a polymer containing a carbazole donor with CuO NPs embedded

A D–A-type polymer PCz–BMBI was synthesized and non-volatile ternary memory devices of ITO/PCz–BMBI:CuO/Al were fabricated with an ON2/ON1/OFF ratio of 105.3 : 102.3 : 1.

Resistance Switching Effect of Memory Device Based on All-Inorganic Cspbbri2 Perovskite

In this study, the CsPbBrI₂ perovskite film was prepared by the preparation of the sol-gel and the spin-coating method, and the cubic lattice was stabilized by introducing Br⁺ into the CsPbI₃ film, which solved the problem of instability of the traditional perovskite phase. Based on the CsPbBrI₂ perovskite film, the Ag/CsPbBrI₂/ITO memory device with a resistance switching effect was prepared. The morphology and phase compositions of the film were analyzed by scanning electron microscope and X-ray diffraction. The non-volatile and repeatable resistance switching effect of the Ag/CsPbBrI₂/ITO memory device was measured under open-air conditions. The experimental results show that the surface of the CsPbBrI₂ perovskite film is uniform and dense, and the Ag/CsPbBrI₂/ITO memory device has an order of magnitude resistance-on-off ratio after 500 cycles of cyclic voltage. This study shows that Ag/CsPbBrI₂/ITO memory devices based on CsPbBrI₂ perovskite films have potential applications in the field of non-volatile memory devices. At the same time, the transient properties of the CsPbBrI₂ film that can quickly dissolve in deionized water make it potentially useful in short-period data storage units and implantable electronic devices with human or environmental sensors.

Stimuli-Responsive Memristive Materials for Artificial Synapses and Neuromorphic Computing

Neuromorphic computing holds promise for building next-generation intelligent systems in a more energy-efficient way than the conventional von Neumann computing architecture. Memristive hardware, which mimics biological neurons and synapses, offers high-speed operation and low power consumption, enabling energy- and area-efficient, brain-inspired computing. Here, recent advances in memristive materials and strategies that emulate synaptic functions for neuromorphic computing are highlighted. The working principles and characteristics of biological neurons and synapses, which can be mimicked by memristive devices, are presented. Besides device structures and operation with different external stimuli such as electric, magnetic, and optical fields, how memristive materials with a rich variety of underlying physical mechanisms can allow fast, reliable, and low-power neuromorphic applications is also discussed. Finally, device requirements are examined and a perspective on challenges in developing memristive materials for device engineering and computing science is given.

A Review on the Properties and Applications of WO3 Nanostructure-Based Optical and Electronic Devices

Tungsten oxide (WO3) is a wide band gap semiconductor with unintentionally n-doping performance, excellent conductivity, and high electron hall mobility, which is considered as a candidate material for application in optoelectronics. Several reviews on WO3 and its derivatives for various applications dealing with electrochemical, photoelectrochemical, hybrid photocatalysts, electrochemical energy storage, and gas sensors have appeared recently. Moreover, the nanostructured transition metal oxides have attracted considerable attention in the past decade because of their unique chemical, photochromic, and physical properties leading to numerous other potential applications. Owing to their distinctive photoluminescence (PL), electrochromic and electrical properties, WO3 nanostructure-based optical and electronic devices application have attracted a wide range of research interests. This review mainly focuses on the up-to-date progress in different advanced strategies from fundamental analysis to improve WO3 optoelectric, electrochromic, and photochromic properties in the development of tungsten oxide-based advanced devices for optical and electronic applications including photodetectors, light-emitting diodes (LED), PL properties, electrical properties, and optical information storage. This review on the prior findings of WO3-related optical and electrical devices, as well as concluding remarks and forecasts will help researchers to advance the field of optoelectric applications of nanostructured transition metal oxides.

Flexible Nonvolatile Bioresistive Random Access Memory with an Adjustable Memory Mode Capable of Realizing Logic Functions

In this study, a flexible bioresistive memory with an aluminum/tussah hemolymph/indium tin oxide/polyethylene terephthalate structure is fabricated by using a natural biological material, tussah hemolymph (TH), as the active layer. When different compliance currents (Icc) are applied to the device, it exhibits different resistance characteristics. When 1 mA is applied in the positive voltage range and 100 mA is applied in the negative voltage range, the device exhibits bipolar resistive switching behavior. Additionally, when 1 mA is applied in both the positive- and negative-voltage ranges, the device exhibits write-once-read-many-times (WORM) characteristics. The device has good endurance, with a retention time exceeding 10⁴ s. After 10⁴ bending cycles, the electrical characteristics remain constant. This memory device can be applied for “AND” and “OR” logic operations in programmable logic circuits. The prepared flexible and transparent biomemristor made of pure natural TH provides a promising new approach for realizing environmentally friendly and biocompatible flexible memory, nerve synapses, and wearable electronic devices.

Solution-Processed Stretchable Ag2 S Semiconductor Thin Films for Wearable Self-Powered Nonvolatile Memory

Compared with the large plastic deformation observed in ductile metals and organic materials, inorganic semiconductors have limited plasticity (<0.2%) due to their intrinsic bonding characters, restricting their widespread applications in stretchable electronics. Herein, the solution-processed synthesis of ductile α-Ag₂ S thin films and fabrication of all-inorganic, self-powered, and stretchable memory devices, is reported. Molecular Ag₂ S complex solution is synthesized by chemical reduction of Ag₂ S powder, fabricating wafer-scale highly crystalline Ag₂ S thin films. The thin films show stretchability due to the intrinsic ductility, sustaining the structural integrity at a tensile strain of 14.9%. Moreover, the fabricated Ag₂ S-based resistive random access memory presents outstanding bipolar switching characteristics (I_on /I_off ratio of ≈10⁵ , operational endurance of 100 cycles, and retention time >10⁶ s) as well as excellent mechanical stretchability (no degradation of properties up to stretchability of 52%). Meanwhile, the device is highly durable under diverse chemical environments and temperatures from -196 to 300 °C, especially maintaining the properties for 168 h in 85% relative humidity and 85 °C. A self-powered memory combined with motion sensors for use as a wearable healthcare monitoring system is demonstrated, offering the potential for designing high-performance wearable electronics that are usable in daily life in a real-world setting.

Excellent optoelectronic applications and electrical transport behavior of the n-WO3nanostructures/p-diamond heterojunction: a new perspective

Nanostructured n-type metal oxides/p-type boron-doped diamond heterojunctions have demonstrated a typical rectification feature and/or negative differential resistance (NDR) potentially applied in wide fields. Recently, the fabrication and electronic transport behavior of n-WO₃nanorods/p-diamond heterojunction at high temperatures were studied by Wanget al(2017Appl. Phys. Lett.110052106), which opened the door for optoelectronic applications that can operate at high-temperatures, high-power, and in various harsh environments. In this perspective, an overview was presented on the future directions, challenges and opportunities for the optoelectronic applications based on the n-WO₃nanostructures/p-diamond heterojunction. We focus, in particular, on the prospects for its high temperature NDR, UV photodetector, field emission emitters, photocatalyst and optical information storage for a wide range of new optoelectronic applications.

Effect of isothermal holding time on hydrogen-induced structural transitions of WO3

Tungsten trioxide (WO3) has the ability to transform oxygen-deficient structures (WO3-x; 0 ≦ x ≦ 1) at high temperatures under hydrogen. Because the band gap of WO3-x depends on the amount of W5+ species resulting from oxygen vacancies, this material is expected to have unique applications. Herein, to elucidate the WO3 reduction mechanism, we investigated the crystallographic changes of monoclinic WO3 powder samples using X-ray and neutron diffraction measurements under different reduction conditions, namely, under hydrogen at 500 or 800 °C for isothermal holding times of 30 min or 22 h. During heating, the yellow color of WO3 changed to various other colors, suggesting that WO3 underwent different reactions with hydrogen depending on the temperature and isothermal holding time. The X-ray powder diffraction results indicated that the hydrogen-treated WO3 crystals formed various oxygen-deficient structures, including stoichiometric WO3-x, non-stoichiometric WO3-x, and W metal. However, the formation of a single WO3-x phase was extremely difficult. For the blue WO3 sample obtained at short isothermal holding times, the total scattering analysis suggested that the oxygen vacancies in WO3 gradually formed at local positions. Furthermore, the neutron powder diffraction measurements revealed that the reduction of WO3 under hydrogen occurred on the surface. These results obtained by diffraction measurements enhance the knowledge in the chemical and physical properties of WO3-x.

Room-Temperature, Solution-Processed SiOx via Photochemistry Approach for Highly Flexible Resistive Switching Memory

Due to its high versatility and cost-effectiveness, solution process has a remarkable advantage over physical or chemical vapor deposition (PVD/CVD) methods in developing flexible resistive random-access memory (RRAM) devices. However, the reported solution-processed binary oxides, the most promising active layer materials for their compatibility with silicon-based semiconductor technology, commonly require high-temperature annealing (>145 °C) and the RRAMs based on them encounter insufficient flexibility. In this work, an amorphous and uniform SiO_x active layer was prepared by irradiating an inorganic polymer, perhydropolysilazane, with a vacuum ultraviolet of 172 nm at room temperature. The corresponding RRAM showed typical bipolar resistance switching with a forming-free behavior. The device on polyimide film exhibited outstanding flexibility with a minimum bending radius of 0.5 mm, and no performance degradation was observed after bending 2000 times with a radius of 2.3 mm, which is the best among the reported solution-processed binary oxide-based RRAMs and can even rival the performance of PVD/CVD-based devices. This room-temperature solution process and the afforded highly flexible RRAMs have vast prospects for application in smart wearable electronics.

Superior photoresponse MIS Schottky barrier diodes with nanoporous:Sn–WO3 films for ultraviolet photodetector application

Nanoporous:Sn–WO₃ film based metal–insulator–semiconductor type Schottky diodes exhibit ultra-high responsivity with higher quantum efficiency and detectivity.

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.

Electrochromic Photodetectors: Toward Smarter Glasses and Nano Reflective Displays via an Electrolytic Mechanism

Electrochromic devices, serving as smart glasses, have not yet been intelligent enough to regulate lighting conditions independent of external photosensing devices. On the other hand, their bulky sandwich structures have been suffering setbacks utilized for reflective displays in an effort to compete with mature emissive displays. The key to resolve both problems lies in incorporating the photosensing function into electrochromic devices while simplifying their configuration via replacing ionic electrolytes. However, so far it has not yet been achieved because of the essential operating difference between the optoelectronic devices and the ionic devices. Herein, a concept of a smarter and thinner device: "electrochromic photodetector" is proposed to solve such problems. It is all-solid-state and electrolyte-free and operates with a simple thin metal-semiconductor-metal structure via an electrolytic mechanism. As a proof of concept, a configuration of the electrochromic photodetector is presented in this work based on a tungsten trioxide (WO₃) thin film deposited on Au electrodes via facile, low-cost solution processes. The electrochromic photodetector switches between its photosensing and electrochromic functions via voltage modulation within 5 V, which is the result of the semiconductor-metal transition. The transition mechanism is further analyzed to be the voltage-triggered reversible oxygen/water vapor adsorption/intercalation from ambient air.

Flexible artificial nociceptor using a biopolymer-based forming-free memristor

The development of electronic devices possessing the functionality of a nociceptor is a crucial step toward electronic receptors that can transfer the external stimuli to the internal nerve system. Of the various materials that have been used to realize artificial nociceptors, biopolymers have the advantages of being abundant, inexpensive, biocompatible, and flexible. In this study, nociceptor behaviors are demonstrated by the flexible Ag/carboxymethyl ι-carrageenan/ITO/PET forming-free memristors for the electronic receptors. The flexible carboxymethyl ι-carrageenan-based memristor showed threshold switching characteristics with a high ION/IOFF ratio of ∼104 and good switching endurance (>1.5 × 105 cycles). It also showed high bending endurance over 1000 cycles when measured in both the flat and the maximum bending conditions. More importantly, it differs from other common sensory receptors with its key features and functions, including threshold, relaxation, allodynia and hyperalgesia behaviors. Such nociceptive behaviors are attributed to the formation and spontaneous rupture of the Ag filament with diffusive dynamics. Finally, we built a pressure sensory alarm system by using our artificial nociceptor devices.

Highly Flexible and Transparent Memristive Devices Using Cross-Stacked Oxide/Metal/Oxide Electrode Layers

Flexible and transparent memristive (FT memristors) devices are considered to be among the promising candidates for future nonvolatile memories. To realize these devices, it is essential to achieve flexible and transparent conductive electrodes (TCEs). However, conventionally used TCEs such as indium tin oxide, gallium zinc oxide, and indium zinc oxide are not so flexible and even necessitate thermal annealing for high conductivity and optical transmittance. Here, we introduce Ag/ZnO/Ag- and Ag/Al₂O₃/Ag-based FT memristors using cross-stacked oxide/metal/oxide electrode layers (i.e., ZnO/Ag/ZnO + ZnO/Ag/ZnO and Al₂O₃/Ag/Al₂O₃ + Al₂O₃/Ag/Al₂O₃) without using any annealing process on poly(ethylene terephthalate) substrates (PETs). Both Ag/ZnO/Ag- and Ag/Al₂O₃/Ag-based FT memristors on PETs exhibited excellent properties, including high transmittance (>86% in the visible region), high on/off current ratios (>10³), and long retention times (>10⁵ s). In addition, they showed very stable and flexible characteristics on PETs even after 2500 bending cycles with a bending radius of 8.1 mm. Finally, we analyzed transmission electron microscopy images and time-of-flight secondary ion mass spectroscopy profiles to identify switching mechanisms in these devices.

Characteristics of flexible and transparent Eu2O3 resistive switching memory at high bending condition

The characteristics of ITO/Eu₂O₃/ITO/PET transparent and flexible resistive switching memory are studied. The device exhibits superior characteristics such as device area-independent and forming-free resistive switching behavior with a resistance on/off ratio of 10⁴, good retention of >10⁴ s and high AC endurance of >10⁷ cycles. The conduction mechanism of the high-resistance state is the Poole-Frenkel mechanism, while that of the low-resistance state is ohmic conduction. The electrical characteristics of the flexible device have shown excellent results up to 5 mm bending radius, at which a degradation in the on/off ratio of the memory window is observed, due to the change in the dielectric layer resistance. The resistive switching characteristics can be improved during bending up to the radius of 2 mm by the incorporation of an aluminum-doped zinc oxide layer in the device as the bottom electrode, proving its application in future flexible and transparent memory devices.

Reset Voltage-Dependent Multilevel Resistive Switching Behavior in CsPb1- xBi xI3 Perovskite-Based Memory Device

All-inorganic CsPb_{1- x}Bi _xI₃ perovskite film was successfully fabricated by incorporating Bi³⁺ in CsPbI₃ to stabilize the cubic lattice. Furthermore, the perovskite film was applied to manufacture a simple Ag/CsPb_{1- x}Bi _xI₃/indium tin oxide (ITO) memory device with a bipolar resistive switching behavior. Nonvolatile, reliable, and reproducible switching properties are demonstrated through retention and endurance test under fully open-air conditions. The memory device also presents highly uniform and long-term stable characteristics. Importantly, by modulating the reset stop voltages, multilevel high-resistance states are observed for the first time in lead halide perovskite memory device. The resistive switching behavior is proposed to explain the formation and partial rupture of conductive multifilament that are dominated by the migration of iodine ions and their corresponding vacancies in perovskite film. This study suggests Ag/CsPb_{1- x}Bi _xI₃/ITO device potential application for multilevel data storage in a nonvolatile memory device.

Excellent Resistive Switching Performance of Cu-Se-Based Atomic Switch Using Lanthanide Metal Nanolayer at the Cu-Se/Al2O3 Interface

The next-generation electronic society is dependent on the performance of nonvolatile memory devices, which has been continuously improving. In the last few years, many memory devices have been introduced. However, atomic switches are considered to be a simple and reliable basis for next-generation nonvolatile devices. In general, atomic switch-based resistive switching is controlled by electrochemical metallization. However, excess ion injection from the entire area of the active electrode into the switching layer causes device nonuniformity and degradation of reliability. Here, we propose the fabrication of a high-performance atomic switch based on Cu _x-Se_{1- x} by inserting lanthanide (Ln) metal buffer layers such as neodymium (Nd), samarium (Sm), dysprosium (Dy), or lutetium (Lu) between the active metal layer and the electrolyte. Current-atomic force microscopy results confirm that Cu ions penetrate through the Ln-buffer layer and form thin conductive filaments inside the switching layer. Compared with the Pt/Cu _x-Se_{1- x}/Al₂O₃/Pt device, the optimized Pt/Cu _x-Se_{1- x}/Ln/Al₂O₃/Pt devices show improvement in the on/off resistance ratio (10²-10⁷), retention (10 years/85 °C), endurance (∼10 000 cycles), and uniform resistance state distribution.

Nanogap-Engineerable Electromechanical System for Ultralow Power Memory

Nanogap engineering of low-dimensional nanomaterials has received considerable interest in a variety of fields, ranging from molecular electronics to memories. Creating nanogaps at a certain position is of vital importance for the repeatable fabrication of the devices. Here, a rational design of nonvolatile memories based on sub-5 nm nanogaped single-walled carbon nanotubes (SWNTs) via the electromechanical motion is reported. The nanogaps are readily realized by electroburning in a partially suspended SWNT device with nanoscale region. The SWNT memory devices are applicable for both metallic and semiconducting SWNTs, resolving the challenge of separation of semiconducting SWNTs from metallic ones. Meanwhile, the memory devices exhibit excellent performance: ultralow writing energy (4.1 × 10^-19 J bit^-1), ON/OFF ratio of 10⁵, stable switching ON operations, and over 30 h retention time in ambient conditions.

Highly Flexible Resistive Switching Memory Based on the Electronic Switching Mechanism in the Al/TiO2/Al/Polyimide Structure

A highly flexible resistive switching (RS) memory was fabricated in the Al/TiO₂/Al/polyimide structure using a simple and cost-effective method. An electronic-resistive-switching-based flexible memory with high performance that can withstand a bending strain of up to 3.6% was obtained. The RS properties showed no obvious degradation even after the bending tests that were conducted up to 10 000 times, and over 4000 writing/erasing cycles were confirmed at the maximally bent state. The superior electrical properties against the mechanical stress of the device can be ascribed to the electronic RS mechanism related to electron trapping/detrapping, which can prevent the inevitable degradation in the case of the RS related with the ionic defects.

Efficient hydrogen evolution from the hydrolysis of ammonia borane using bilateral-like WO3−x nanorods coupled with Ni2P nanoparticles

A ten times enhanced H₂ evolution from the hydrolysis of ammonia borane through bilateral-like plasmonic WO_3−x nanorods coupled with Ni₂P nanoparticles was realized.

Low-temperature crystallization of solution-derived metal oxide thin films assisted by chemical processes

Low-temperature chemical solution methods to prepare crystalline metal oxide thin films and to integrate them with flexible substrates are shown.

Controllable Switching Filaments Prepared via Tunable and Well-Defined Single Truncated Conical Nanopore Structures for Fast and Scalable SiOx Memory

The controllability of switching conductive filaments is one of the central issues in the development of reliable metal-oxide resistive memory because the random dynamic nature and formation of the filaments pose an obstacle to desirable switching performance. Here, we introduce a simple and novel approach to control and form a single silicon nanocrystal (Si-NC) filament for use in SiO_x memory devices. The filament is formed with a confined vertical nanoscale gap by using a well-defined single vertical truncated conical nanopore (StcNP) structure. The physical dimensions of the Si-NC filaments such as number, size, and length, which have a significant influence on the switching properties, can be simply engineered by the breakdown of an Au wire through different StcNP structures. In particular, we demonstrate that the designed SiO_x memory junction with a StcNP of pore depth of ∼75 nm and a bottom diameter of ∼10 nm exhibited a switching speed of up to 6 ns for both set and reset process, significantly faster than reported SiO_x memory devices. The device also exhibited a high ON-OFF ratio, multistate storage ability, acceptable endurance, and retention stability. The influence of the physical dimensions of the StcNP on the switching features is discussed based on the simulated temperature profiles of the Au wire and the nanogap size generated inside the StcNP structure during electromigration.

Electrochemical Tantalum Oxide for Resistive Switching Memories

Redox-based resistive switching memories (ReRAMs) are strongest candidates for the next-generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta₂ O₅ -based devices by anodizing. This method allows to grow high-quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical-oxide-based devices demonstrate superior properties, i.e., endurance of at least 10⁶ pulse cycles and/or 10³ I-V sweeps maintaining a good memory window with a low dispersion in R_OFF and R_ON values, nanosecond fast switching, and data retention of at least 10⁴ s. Multilevel programing capability is presented with both I-V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.

Structurally Engineered Nanoporous Ta2O5-x Selector-Less Memristor for High Uniformity and Low Power Consumption

A memristor architecture based on metal-oxide materials would have great promise in achieving exceptional energy efficiency and higher scalability in next-generation electronic memory systems. Here, we propose a facile method for fabricating selector-less memristor arrays using an engineered nanoporous Ta₂O_5-x architecture. The device was fabricated in the form of crossbar arrays, and it functions as a switchable rectifier with a self-embedded nonlinear switching behavior and ultralow power consumption (∼2.7 × 10^-6 W), which results in effective suppression of crosstalk interference. In addition, we determined that the essential switching elements, such as the programming power, the sneak current, the nonlinearity value, and the device-to-device uniformity, could be enhanced by in-depth structural engineering of the pores in the Ta₂O_5-x layer. Our results, on the basis of the structural engineering of metal-oxide materials, could provide an attractive approach for fabricating simple and cost-efficient memristor arrays with acceptable device uniformity and low power consumption without the need for additional addressing selectors.

Flexible All-Inorganic Perovskite CsPbBr3 Nonvolatile Memory Device

All-inorganic perovskite CsPbX₃ (X = Cl, Br, or I) is widely used in a variety of photoelectric devices such as solar cells, light-emitting diodes, lasers, and photodetectors. However, studies to understand the flexible CsPbX₃ electrical application are relatively scarce, mainly due to the limitations of the low-temperature fabricating process. In this study, all-inorganic perovskite CsPbBr₃ films were successfully fabricated at 75 °C through a two-step method. The highly crystallized films were first employed as a resistive switching layer in the Al/CsPbBr₃/PEDOT:PSS/ITO/PET structure for flexible nonvolatile memory application. The resistive switching operations and endurance performance demonstrated the as-prepared flexible resistive random access memory devices possess reproducible and reliable memory characteristics. Electrical reliability and mechanical stability of the nonvolatile device were further tested by the robust current-voltage curves under different bending angles and consecutive flexing cycles. Moreover, a model of the formation and rupture of filaments through the CsPbBr₃ layer was proposed to explain the resistive switching effect. It is believed that this study will offer a new setting to understand and design all-inorganic perovskite materials for future stable flexible electronic devices.