Flexible organic bistable devices based on graphene embedded in an insulating poly(methyl methacrylate) polymer layer

Memristors with Monolayer Graphene Electrodes Grown Directly on Sapphire Wafers

The development of the memristor has generated significant interest due to its non-volatility, simple structure, and low power consumption. Memristors based on graphene offer atomic monolayer thickness, flexibility, and uniformity and have attracted attention as a promising alternative to contemporary field-effect transistor (FET) technology in applications such as logic and memory devices, achieving higher integration density and lower power consumption. The use of graphene as electrodes in memristors could also increase robustness against degradation mechanisms, including oxygen vacancy diffusion to the electrode and unwanted metal ion diffusion. However, to realize this technological transformation, it is necessary to establish a scalable, robust, and cost-effective device fabrication process. Here we report the direct growth of high-quality monolayer graphene on sapphire wafers in a mass-producible, contamination-free, and transfer-free manner, using a commercially available metal-organic chemical vapor deposition (MOCVD) system. By taking advantage of this approach, graphene-electrode based memristors are developed, and all the processes used in the device fabrication incorporating graphene electrodes can be performed at wafer scale. The graphene electrode-based memristor demonstrates promising characteristics in terms of endurance, retention, and ON/OFF ratio. This work presents a possible and viable route to achieving robust graphene-based memristors in a commercially and technologically sustainable manner, paving the way for the realization of more powerful and compact integrated graphene electronics in the future.

Flame-Retardant Nonaqueous Electrolytes for High-Safety Potassium-Ion Batteries

Potassium-ion batteries (PIBs) with conventional organic-based flammable electrolytes suffer from serious safety issues with a high risk of ignition and burning especially under harsh conditions, which significantly limits their widespread applications. Flame-retardant electrolytes (FREs) are considered as one of the most effective strategies to address these safety issues. Therefore, it's much necessary to summarize the challenges, recent progress, and design principles of flame-retardant nonaqueous electrolytes for PIBs to guide their development and future applications. In this review, an in-depth introduction and explanation of the origins of electrolyte flammability are first presented. Particularly, the state-of-the-art design principles of FREs for PIBs are extensively summarized and emphasized, including the electrolyte flame-retardant solvents/additives, highly concentrated electrolytes (HCEs), localized high-concentration electrolytes (LHCEs), ionic liquids-based electrolytes and solid-state electrolytes. Moreover, the advantages and drawbacks of each approach are systematically presented and discussed, following by proposed perspectives to guide the rational development of next-generation high-safety PIBs for practical applications.

Surface Plasmon Resonance Enhanced Photoelectrochemical Sensing of Cysteine Based on Au Nanoparticle-Decorated ZnO@graphene Quantum Dots

In this work, Au nanoparticle-decorated ZnO@graphene core-shell quantum dots (Au-ZnO@graphene QDs) were successfully prepared and firstly used to modify an ITO electrode for the construction of a novel photoelectrochemical biosensor (Au-ZnO@graphene QDs/ITO). Characterization of the prepared nanomaterials was conducted using transmission electron microscopy, steady-state fluorescence spectroscopy and the X-ray diffraction method. The results indicated that the synthesized ternary nanomaterials displayed excellent photoelectrochemical performance, which was much better than that of ZnO@graphene QDs and pristine ZnO quantum dots. The graphene and ZnO quantum dots formed an effective interfacial electric field, enhancing photogenerated electron-hole pairs separation and leading to a remarkable improvement in the photoelectrochemical performance of ZnO@graphene QDs. The strong surface plasmon resonance effect achieved by directly attaching Au nanoparticles to ZnO@graphene QDs led to a notable increase in the photocurrent response through electrochemical field effect amplification. Based on the specifical recognition between cysteine and Au-ZnO@graphene QDs/ITO through the specificity of Au-S bonds, a light-driven photoelectrochemical sensor was fabricated for cysteine detection. The novel photoelectrochemical biosensor exhibited outstanding analytical capabilities in detecting cysteine with an extremely low detection limit of 8.9 nM and excellent selectivity. Hence, the Au-ZnO@graphene QDs is a promising candidate as a novel advanced photosensitive material in the field of photoelectrochemical biosensing.

Resistive random access memory: introduction to device mechanism, materials and application to neuromorphic computing

The modern-day computing technologies are continuously undergoing a rapid changing landscape; thus, the demands of new memory types are growing that will be fast, energy efficient and durable. The limited scaling capabilities of the conventional memory technologies are pushing the limits of data-intense applications beyond the scope of silicon-based complementary metal oxide semiconductors (CMOS). Resistive random access memory (RRAM) is one of the most suitable emerging memory technologies candidates that have demonstrated potential to replace state-of-the-art integrated electronic devices for advanced computing and digital and analog circuit applications including neuromorphic networks. RRAM has grown in prominence in the recent years due to its simple structure, long retention, high operating speed, ultra-low-power operation capabilities, ability to scale to lower dimensions without affecting the device performance and the possibility of three-dimensional integration for high-density applications. Over the past few years, research has shown RRAM as one of the most suitable candidates for designing efficient, intelligent and secure computing system in the post-CMOS era. In this manuscript, the journey and the device engineering of RRAM with a special focus on the resistive switching mechanism are detailed. This review also focuses on the RRAM based on two-dimensional (2D) materials, as 2D materials offer unique electrical, chemical, mechanical and physical properties owing to their ultrathin, flexible and multilayer structure. Finally, the applications of RRAM in the field of neuromorphic computing are presented.

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.

Flexible Memristor Devices Using Hybrid Polymer/Electrodeposited GeSbTe Nanoscale Thin Films

We report on the development of hybrid organic-inorganic material-based flexible memristor devices made by a fast and simple electrochemical fabrication method. The devices consist of a bilayer of poly(methyl methacrylate) (PMMA) and Te-rich GeSbTe chalcogenide nanoscale thin films sandwiched between Ag top and TiN bottom electrodes on both Si and flexible polyimide substrates. These hybrid memristors require no electroforming process and exhibit reliable and reproducible bipolar resistive switching at low switching voltages under both flat and bending conditions. Multistate switching behavior can also be achieved by controlling the compliance current (CC). We attribute the switching between the high resistance state (HRS) and low resistance state (LRS) in the devices to the formation and rupture of conductive Ag filaments within the hybrid PMMA/GeSbTe matrix. This work provides a promising route to fabricate flexible memory devices through an electrodeposition process for application in flexible electronics.

Ternary Flash Memory with a Carbazole-Based Conjugated Copolymer: WS2 Composites as Active Layers

For nonvolatile memory devices, the design and synthesis of their substrate materials are very important. Due to the versatility and large-area fabrication of the low-temperature spin coating process, organic/inorganic nanomaterials as active layers of memory devices have been deeply studied. Inorganic nanoparticles can engage in interactions with polymers via external voltage. WS₂ NPs have a large specific surface area and good conductivity. They can be used as the charge trap center in the active layer, which is conducive to the charge transfer in the active layer. Poly[2,7-9-(9-heptadecanyl)-9H-carbazole-co-benzo[4,5] imidazole[2,1-α] isoindol-11-one] (PIIO) was synthesized via the Suzuki coupling reaction. ITO/PIIO/Al and ITO/PIIO:WS₂ NP/Al devices were prepared by the spin coating method and vacuum evaporation technology. All devices showed tristable switching behavior. The influence of the WS₂ mass fraction on memory performance was studied. The device composite with 6 wt % WS₂ NPs showed the best storage features. The OFF/ON1/ON2 current ratio was 1: 1.11 × 10¹: 2.03 × 10⁴, and the threshold voltage V_th1/V_th2 was -0.60 V/-1.05 V. The device is steady for 12,000 s in three states-high-resistance state (HRS), intermediate state (IRS), and low-resistance state (LRS). After reading 3500 times, the switch-state current displayed no obvious attenuation. This work shows that the polymer and its composites have broad prospects in next-generation nonvolatile storage.

Standards for the Characterization of Endurance in Resistive Switching Devices

Resistive switching (RS) devices are emerging electronic components that could have applications in multiple types of integrated circuits, including electronic memories, true random number generators, radiofrequency switches, neuromorphic vision sensors, and artificial neural networks. The main factor hindering the massive employment of RS devices in commercial circuits is related to variability and reliability issues, which are usually evaluated through switching endurance tests. However, we note that most studies that claimed high endurances >10⁶ cycles were based on resistance versus cycle plots that contain very few data points (in many cases even <20), and which are collected in only one device. We recommend not to use such a characterization method because it is highly inaccurate and unreliable (i.e., it cannot reliably demonstrate that the device effectively switches in every cycle and it ignores cycle-to-cycle and device-to-device variability). This has created a blurry vision of the real performance of RS devices and in many cases has exaggerated their potential. This article proposes and describes a method for the correct characterization of switching endurance in RS devices; this method aims to construct endurance plots showing one data point per cycle and resistive state and combine data from multiple devices. Adopting this recommended method should result in more reliable literature in the field of RS technologies, which should accelerate their integration in commercial products.

Sustainable resistance switching performance from composite-type ReRAM device based on carbon Nanotube@Titania core-shell wires

A novel nanocomposite-based non-volatile resistance switching random access memory device introducing single-walled carbon nanotube (SWCNT)@TiO2 core-shell wires was proposed for flexible electronics. The SWCNT was de-bundled by ultrasonication with sodium dodecylbenzene sulfonate (SDBS), and then the TiO2 skin layer on the SWCNT surface was successfully introduced by adding benzyl alcohol as a weak surfactant. The nanocomposite resistance switching layer was composed of the SWCNT@TiO2 core-shell wires and poly(vinyl alcohol) (PVA) matrix by a simple spin-coating method. The device exhibited reproducible resistance switching performance with a remarkably narrow distribution of operating parameters (VSET and VRESET were 2.63 ± 0.16 and 0.95 ± 0.11 V, respectively) with a large RON/ROFF ratio of 105 for 200 consecutive switching cycles. Furthermore, the excellent resistance switching behavior in our device was maintained against mechanical stress up to 105 bending test. We believe that the nanocomposite memory device with SWCNT@TiO2 core-shell wires would be a critical asset to realize practical application for a flexible non-volatile memory field.

Interlayer of PMMA Doped with Au Nanoparticles for High-Performance Tandem Photodetectors: A Solution to Suppress Dark Current and Maintain High Photocurrent

Currently, colloidal quantum dots (CQDs)-based photodetectors are widely investigated due to their low cost and easy integration with optoelectronic devices. The requirements for a high-performance photodetector are a low dark current and a high photocurrent. Normally, photodetectors with a low dark current also possess a low photocurrent, or photodetectors with reduced dark current possess a reduced photocurrent, resulting in low detectivity. In this paper, a solution to suppress dark current and maintain a high photocurrent, i.e., use of poly(methyl methacrylate) doped with Au nanoparticles (NPs) (i.e., PMMA:Au) as an interlayer for enhanced-performance tandem photodetectors, is presented. Our experimental data showed that the dark current through the tandem photodetector ITO/PEDOT:PSS/PbS:CsSnBr₃/ZnO/PMMA:Au/CuSeN/PbS:CsSnBr₃/ZnO/Ag is suppressed significantly; meanwhile, a high photocurrent is maintained after a PMMA:Au interlayer has been inserted between two subdetectors. The inserted PMMA:Au interlayer acts as storage nodes for electrons, reducing the dark current through the device; meanwhile, the photocurrent can be enhanced under illumination. As a result, the specific detectivity of the tandem photodetector with 35 nm PMMA:Au interlayer was enhanced significantly from 5.01 × 10¹² to 2.7 × 10¹⁵ Jones under 300 μW/cm² 532 nm illumination at a low voltage of -1 V as compared to the device without a PMMA:Au interlayer. Further, the physical mechanism of enhanced performance is discussed in detail.

Highly Stable and Flexible Memristive Devices Based on Polyvinylpyrrolidone: WS2 Quantum Dots

Tungsten disulfide (WS₂) quantum dots (QDs) embedded in polyvinylpyrrolidone (PVP) based flexible memristive devices were prepared, and those devices exhibited typical bistable electrical switching and remarkable nonvolatile memristive behaviors. Maximum electricity ON/OFF ratio obtained from the current-voltage (I-V) curves of the device is close to 10⁴. The set voltage of the device is +0.7 V, which effectively reduced the energy consumption. The retention times extracted from data for the devices were as large as 1 × 10⁴ s, which points to these devices having nonvolatile characteristics. Moreover, the highly flexible characteristics of the devices were demonstrated by bending the devices. The carrier transport mechanisms were explained by fitting the I-V curves, and possible operating mechanisms of the devices can be described based on the electron trapping and detrapping processes. WS₂ QDs uniformly dispersed in pure transparent N, N-Dimethylformamide (DMF) were obtained by using ultrasonication and a hydrothermal process in this work.

Decade of 2D-materials-based RRAM devices: a review

Two dimensional (2D) materials have offered unique electrical, chemical, mechanical and physical properties over the past decade owing to their ultrathin, flexible, and multilayer structure. These layered materials are being used in numerous electronic devices for various applications, and this review will specifically focus on the resistive random access memories (RRAMs) based on 2D materials and their nanocomposites. This study presents the device structures, conduction mechanisms, resistive switching properties, fabrication technologies, challenges and future aspects of 2D-materials-based RRAMs. Graphene, derivatives of graphene and MoS2 have been the major contributors among 2D materials for the application of RRAMs; however, other members of this family such as hBN, MoSe2, WS2 and WSe2 have also been inspected more recently as the functional materials of nonvolatile RRAM devices. Conduction in these devices is usually dominated by either the penetration of metallic ions or migration of intrinsic species. Most prominent advantages offered by RRAM devices based on 2D materials include fast switching speed (<10 ns), less power losses (10 pJ), lower threshold voltage (<1 V) long retention time (>10 years), high electrical endurance (>108 voltage cycles) and extended mechanical robustness (500 bending cycles). Resistive switching properties of 2D materials have been further enhanced by blending them with metallic nanoparticles, organic polymers and inorganic semiconductors in various forms.

High-performance non-volatile resistive switching memory based on a polyimide/graphene oxide nanocomposite

Chemical structure of PI-GO, schematic structure of the ITO/PI-GO/Al device and its memory characteristics.

A Universal Strategy for Stretchable Polymer Nonvolatile Memory via Tailoring Nanostructured Surfaces

Building stretchable memory is an effective strategy for developing next-generation memory technologies toward stretchable and wearable electronics. Here we demonstrate a universal strategy for the fabrication of high performance stretchable polymer memory via tailoring surface morphology, in which common conjugated polymers and sharp reduced graphene oxide (r-rGO) films are used as active memristive layers and conductive electrodes, respectively. The fabricated devices feature write-once-read-many-times (WORM) memory, with a low switching voltage of 1.1 V, high ON/OFF current ratio of 10⁴, and an ideal long retention time over 12000 s. Sharp surface-induced resistive switching behavior has been proposed to explore the electrical transition. Moreover, the polymer memory show reliable electrical bistable properties with a stretchability up to 30%, demonstrating their great potential candidates as high performance stretchable memory in soft electronics.

Flexible, transparent patterned electrodes based on graphene oxide/silver nanowire nanocomposites fabricated utilizing an accelerated ultraviolet/ozone process to control silver nanowire degradation

We developed flexible, transparent patterned electrodes, which were fabricated utilizing accelerated ultraviolet/ozone (UV/O₃)-treated graphene oxide (GO)/silver nanowire (Ag-NW) nanocomposites via a simple, low-cost pattern process to investigate the feasibility of promising applications in flexible/wearable electronic and optoelectronic devices. The UV/O₃ process of the GO/Ag-NW electrode was accelerated by the pre-heat treatment, and the degradation interruption of Ag NWs was removed by the GO treatment. After the deposition of the GO-treated Ag NW electrodes, the sheet resistance of the thermally annealed GO-treated Ag-NW electrodes was significantly increased by using the UV/O₃ treatment, resulting in a deterioration of the GO-treated Ag NWs in areas exposed to the UV/O₃ treatment. The degradation of the Ag NWs caused by the UV/O₃ treatment was confirmed by using the sheet resistances, scanning electron microscopy images, X-ray photoelectron microscopy spectra, and transmittance spectra. While the sheet resistance of the low-density Ag-NW electrode was considerably increased due to the pre-thermal treatment at 90 °C for 10 min, that of the high-density Ag-NW electrode did not vary significantly even after a UV/O₃ treatment for a long time. The degradation interference phenomenon caused by the UV/O₃ treatment in the high-density Ag NWs could be removed by using a GO treatment, which resulted in the formation of a Ag-NW electrode pattern suitable for promising applications in flexible organic light-emitting devices. The GO treatment decreased the sheet resistance of the Ag-NW electrode and enabled the pattern to be formed by using the UV/O₃ treatment. The selective degradation of Ag NWs due to UV/O₃ treatment decreased the transparency of the Ag-NW electrode by about 8% and significantly increased its sheet resistance more than 100 times.

Highly flexible memristive devices based on MoS2 quantum dots sandwiched between PMSSQ layers

This paper reports a facile, cost effective method that uses an aqueous hydrothermal process for synthesizing two-dimensional molybdenum disulphide (MoS2) monolayer quantum dots (QDs) and their potential applications in flexible memristive devices. High-resolution transmission electron microscopy and atomic force microscopy images confirmed that the diameters of the synthesized MoS2 QDs with irregular shapes were in the range between 3 and 6 nm; their thicknesses were confirmed to lie between 1.0 and 0.8 nm, a clear indication that a monolayer of MoS2 QDs had been synthesized. Photoluminescence (PL) and time-resolved PL spectra of the MoS2 QDs revealed a strong emission in the blue region with a slower decay constant. Memristive devices fabricated by incorporating MoS2 QDs between poly(methylsilsesquioxane) ultrathin layers, which had been deposited on poly(ethylene terephthalate), demonstrated a high ON-OFF current ratio of ∼104, stable retention, and excellent endurance in the relaxed state; these devices were also demonstrated to function properly during bending and in a bent state. The flexible memristive devices demonstrated an OFF state with a very low current of 10-6 A. These results clearly show that ultrathin two-dimensional QDs have promising applications in high-performance flexible memristive devices.

Electron transport phenomena at the interface of Al electrode and heavily doped degenerate ZnO nanoparticles in quantum dot light emitting diode

ZnO nanoparticles (NPs) of 4-5 nm, widely adopted as an electron transport layer (ETL) in quantum dot light emitting diodes (QD-LEDs), were synthesized using the solution-precipitation process. It is notable that synthesized ZnO NPs are highly degenerate intrinsic semiconductors and their donor concentration can be increased up to N _D = 6.9 × 10²¹ cm^-3 by annealing at 140 °C in air. An optical bandgap increase of as large as 0.16-0.33 eV by degeneracy is explained well by the Burstein-Moss shift. In order to investigate the influence of intrinsic defects of ZnO NP ETLs on the performance of QD-LED devices without a combined annealing temperature between ZnO NP ETLs and the emissive QD layer, pre-annealed ZnO NPs at 60 °C, 90 °C, 140 °C, and 180 °C were spin-coated on the annealed QD layer without further post-annealing. As the annealing temperature increases from 60 °C to 180 °C, the defect density related to oxygen vacancy (V _O) in ZnO NPs is reduced from 34.4% to 17.8%, whereas the defect density of interstitial Zn (Zn_i) is increased. Increased Zn_i reduces the width (W) of the depletion region from 0.21 to 0.12 nm and lowers the Schottky barrier (Ф_B) between ZnO NPs and the Al electrode from 1.19 to 0.98 eV. We reveal for the first time that carrier conduction between ZnO NP ETLs and the Al electrode is largely affected by the concentration of Zn_i above the conduction band minimum, and effectively described by space charge limited current and trap charge limited current models.

Flexible memristive devices based on polyimide:mica nanosheet nanocomposites with an embedded PEDOT:PSS layer

Flexible memristive devices with a structure of Al/polyimide:mica/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/indium-tin-oxide/polyethylene glycol naphthalate showed electrical bistability characteristics. The maximum current margin of the devices with mica nanosheets was much larger than that of the devices without mica nanosheets. For these devices, the current vs. time curves showed nonvolatile characteristics with a retention time of more than 1 × 10⁴ s, and the current vs. number-of-cycles curves demonstrated an endurance for high resistance state/low resistance state switchings of 1 × 10² cycles. As to the operation performance, the “reset” voltage was distributed between 2.5 and 3 V, and the “set” voltage was distributed between −0.7 and −0.5 V, indicative of high uniformity. The electrical characteristics of the devices after full bendings with various radii of curvature were similar to those before bending, which was indicative of devices having ultra-flexibility. The carrier transport and the operation mechanisms of the devices were explained based on the current vs. voltage curves and the energy band diagrams.

Highly-stable write-once-read-many-times switching behaviors of 1D-1R memristive devices based on graphene quantum dot nanocomposites

One diode and one resistor (1D–1R) memristive devices based on inorganic Schottky diodes and poly(methylsilsesquioxane) (PMSSQ):graphene quantum dot (GQD) hybrid nanocomposites were fabricated to achieve stable memory characteristics. Current-voltage (I-V) curves for the Al/PMSSQ:GQDs/Al/p-Si/Al devices at room temperature exhibited write-once, read-many-times memory (WORM) characteristics with an ON/OFF ratio of as large as 10⁴ resulting from the formation of a 1D–1R structure. I-V characteristics of the WORM 1D–1R device demonstrated that the memory and the diode behaviors of the 1D–1R device functioned simultaneously. The retention time of the WORM 1D–1R devices could be maintained at a value larger than 10⁴ s under ambient conditions. The operating mechanisms of the devices were analyzed on the basis of the I–V results and with the aid of the energy band diagram.

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.

Effects of a Nb nanopin electrode on the resistive random-access memory switching characteristics of NiO thin films

We report the effects of bottom electrode shapes on resistive random-access memory (RRAM) devices composed of Nb (bottom electrode)/NiO (dielectric)/Nb (top electrode) structures. By adopting a nano-fabrication process using an anodic aluminum oxide (AAO) nanotemplate, a well-aligned Nb nanopin array bottom electrode was formed on the surface of a Si substrate. For comparison, a Nb thin film was employed as a different type of bottom electrode. Then, a NiO thin film dielectric was prepared on both the Nb bottom electrodes via a spin coating method, followed by Nb sputtering for the Nb top electrode. Both the RRAM devices with Nb nanopin and thin film bottom electrodes exhibited typical unipolar resistive switching behavior. However, a lower SET/RESET voltage was observed for the Nb nanopin electrode compared to the Nb thin film electrode by virtue of an enhanced electric field induced by the nanopin-shaped electrode. More significantly, on the basis of endurance and retention characteristics, the Nb nanopin electrode played a key role in minimizing the dispersion of the low- and high-resistance state currents and the variation in the SET/RESET voltage by developing more-concise conducting filaments in the conducting path. Therefore, we foresee that this approach can provide an insight into the optimal design of RRAM devices.

Flexible Memristive Devices Based on InP/ZnSe/ZnS Core-Multishell Quantum Dot Nanocomposites

The effects of the ZnS shell layer on the memory performances of flexible memristive devices based on quantum dots (QDs) with an InP/ZnSe/ZnS core-multishell structure embedded in a poly(methylmethacrylate) layer were investigated. The on/off ratios of the devices based on QDs with an InP/ZnSe core-shell structure and with an InP/ZnSe/ZnS core-multishell structure were approximately 4.2 × 10² and 8.5 × 10³, respectively, indicative of enhanced charge storage capability in the latter. After bending, the memory characteristics of the memristive devices based on QDs with the InP/ZnSe/ZnS structure were similar to those before bending. In addition, those devices maintained the same on/off ratios for retention time of 1 × 10⁴ s, and the number of endurance cycles was above 1 × 10². The reset voltages ranged from -2.3 to -3.1 V, and the set voltages ranged from 1.3 to 2.1 V, indicative of reliable electrical characteristics. Furthermore, the possible operating mechanisms of the devices are presented on the basis of the electron trapping and release mode.

Bistable electrical switching and nonvolatile memory effects by doping different amounts of GO in poly(9,9-dioctylfluorene-2,7-diyl)

The device shows different conductive behavior: electric bistable nonvolatile flash memory behavior and conductor behavior.

Flexible bio-memristive devices based on chicken egg albumen:Au@SiO2 core-shell nanoparticle nanocomposites

Flexible bio-memristive (FBM) devices utilizing chicken egg albumen (CEA):Au@SiO₂ core-shell nanoparticle nanocomposites were fabricated on indium-tin-oxide (ITO) coated polyethylene naphthalate (PEN) substrates. Current-voltage (I-V) curves for the Al/CEA:Au@SiO₂ core-shell nanoparticle/ITO/PEN devices showed clockwise current hysteresis behaviors due to the existence of the CEA:Au@SiO₂ core-shell nanoparticle nanocomposites. The endurance number of the ON/OFF switching for the FBM devices was above 10² cycles. An ON/OFF current ratio of 1 × 10⁵ was maintained for retention times longer than 1 × 10⁴ s. The memory characteristics of the FBM devices after bending were similar to those before bending. The memory margin and the stability of FBM devices were enhanced due to the embedded Au@SiO₂ core-shell nanoparticles. The switching mechanisms occurring in the Al/CEA:Au@SiO₂ core-shell nanoparticle/ITO-coated PEN devices are described on the basis of the I-V results and the filament mechanisms.

Black Phosphorus Quantum Dots with Tunable Memory Properties and Multilevel Resistive Switching Characteristics

Solution-processed black phosphorus quantum-dot-based resistive random access memory is demonstrated with tunable characteristics, multilevel data storage, and ultrahigh ON/OFF ratio. Effects of the black phosphorous quantum dots layer thickness and the compliance current setting on resistive switching behavior are systematically studied. Our devices can yield a series of SET voltages and current levels, hence having the potential for practical applications in the flexible electronics industry.

The memristive effect as a novelty in drug monitoring

Nanoscale devices exhibiting memristive properties show great potential in a plethora of applications. In this work, memristive nanowires are presented for the first time as ideal candidates for absolutely novel, ultrasensitive, highly specific and selective drug-biosensors, also paving the way for real-time monitoring applications, in coupling with the restoration properties of DNA-aptamers. The hysteretic properties exhibited by the hereby-presented special nanodevices, modified via surface treatments, are leveraged along the complete cycle consisting of DNA-aptamer immobilization, target binding, and DNA-aptamer regeneration for successful and effective detection of Tenofovir, an antiviral drug for HIV treatment, in buffer as well as in non-diluted human serum. This results in ultrasensitive, label-free monitoring of the therapeutic compound with a limit of detection of 3.09 pM in buffer and 1.38 nM in full serum. These LODs demonstrate 10 times higher sensitivity for the in-buffer drug detection, and twice better performance for drug sensing in full human serum, ever obtained. The selectivity of the memristive biosensor for Tenofovir detection was verified through both positive and negative controls in full human serum. In addition, the DNA-aptamer regeneration character is portrayed for the first time through a memristive effect, and scanning electron microscopy throws more light on the binding mechanism efficiency through the variation of the nanodevice surface properties at the nanoscale.The results presented in this work demonstrate that the coupling of the memristive effect and aptamer regeneration provides the best ever realized nano-biosensor for drug detection also in full human serum.

Mimicking Classical Conditioning Based on a Single Flexible Memristor

The mimicking of classical conditioning, including acquisition, extinction, recovery, and generalization, can be efficiently achieved by using a single flexible memristor. In particular, the experiment of Pavlov's dog is successfully demonstrated. This demonstration paves the way for reproducing advanced neural processes and provides a frontier approach to the design of artificial-intelligence systems with dramatically reduced complexity.

Synthesis and optical and electrochemical memory properties of fluorene–triphenylamine alternating copolymer

A fluorene–triphenylamine copolymer (PF–TPA) was designed and synthesized under Suzuki coupling reaction conditions in this work. It exhibited a typical electrical conductance switching behavior and non-volatile flash memory effects.

Coupling effects of the electric field and bending on the electronic and magnetic properties of penta-graphene nanoribbons

The magnetic configuration transitions for P-GNRs vs. critical electric field strength on applying different bending strains.

Unique visible-light-assisted field emission of tetrapod-shaped ZnO/reduced graphene-oxide core/coating nanocomposites

The electronic and the optoelectronic properties of graphene-based nanocomposites are controllable, making them promising for applications in diverse electronic devices. In this work, tetrapod-shaped zinc oxide (T-ZnO)/reduced graphene oxide (rGO) core/coating nanocomposites were synthesized by using a hydrothermal-assisted self-assemble method, and their optical, photoelectric, and field-emission properties were investigated. The ZnO, an ideal ultraviolet-light-sensitive semiconductor, was observed to have high sensitivity to visible light due to the rGO coating, and the mechanism of that sensitivity was investigated. We demonstrated for the first time that the field-emission properties of the T-ZnO/rGO core/coating nanocomposites could be dramatically enhanced under visible light by decreasing the turn-on field from 1.54 to 1.41 V/μm and by increasing the current density from 5 to 12 mA/cm² at an electric field of 3.5 V/μm. The visible-light excitation induces an electron jump from oxygen vacancies on the surface of ZnO to the rGO layer, resulting in a decrease in the work function of the rGO and an increase in the emission current. Furthermore, a field-emission light-emitting diode with a self-enhanced effect was fabricated making full use of the photo-assisted field-emission process.

Oxygen Effects on Performance of Electrically Bistable Devices Based on Hybrid Silver Sulfide Poly(N-vinylcarbazole) Nanocomposites

An organic/inorganic bistable device is fabricated by using a simple spin-coating technique, in which the hybrid silver sulfide (Ag2S) poly(N-vinylcarbazole) (PVK) nanocomposite film is sandwiched between two electrodes. An obvious electrical hysteresis is observed in the current-voltage (I-V) curve of the device measured in the presence of different oxygen concentrations, and the magnitude of the electrical hysteresis is decreased with a decrease of the oxygen concentrations. The electrical bistability of the device exhibits a strong dependence on the oxygen concentrations, and the current variation of the OFF state is higher than that of the ON state with the gas atmosphere changing from N2 to air. Different theoretical models have been employed to describe the carrier transport mechanisms of the device in the OFF and ON states measured in different gas atmospheres on the basis of the experimental I-V results, and the carrier transport of the device in the ON state measured in air is very different from that measured in N2 and low O2 concentrations due to the participation of oxygen vacancies in the trapping and de-trapping processes of electrons into and out of the Ag2S/PVK heterointerface.

Flexible Nanoporous WO3-x Nonvolatile Memory Device

Flexible resistive random access memory (RRAM) devices have attracted great interest for future nonvolatile memories. However, making active layer films at high temperature can be a hindrance to RRAM device fabrication on flexible substrates. Here, we introduced a flexible nanoporous (NP) WO3-x RRAM device using anodic treatment in a room-temperature process. The flexible NP WO3-x RRAM device showed bipolar switching characteristics and a high ION/IOFF ratio of ∼10(5). The device also showed stable retention time over 5 × 10(5) s, outstanding cell-to-cell uniformity, and bending endurance over 10(3) cycles when measured in both the flat and the maximum bending conditions.

Organolead Halide Perovskites for Low Operating Voltage Multilevel Resistive Switching

Organolead halide perovskites are used for low-operating-voltage multilevel resistive switching. Ag/CH3 NH3 PbI3 /Pt cells exhibit electroforming-free resistive switching at an electric field of 3.25 × 10(3) V cm(-1) for four distinguishable ON-state resistance levels. The migration of iodine interstitials and vacancies with low activation energies is responsible for the low-electric-field resistive switching via filament formation and annihilation.

Wearable Electricity Generators Fabricated Utilizing Transparent Electronic Textiles Based on Polyester/Ag Nanowires/Graphene Core-Shell Nanocomposites

The technological realization of wearable triboelectric generators is attractive because of their promising applications in wearable self-powered intelligent systems. However, the low electrical conductivity, the low electrical stability, and the low compatibility of current electronic textiles (e-textiles) and clothing restrict the comfortable and aesthetic integration of wearable generators into human clothing. Here, we present high-performance, transparent, smart e-textiles that employ commercial textiles coated with silver nanowire/graphene sheets fabricated by using a scalable, environmentally friendly, full-solution process. The smart e-textiles show superb and stable conduction of below 20 Ω/square as well as excellent flexibility, stretchability, foldability, and washability. In addition, wearable electricity-generating textiles, in which the e-textiles act as electrodes as well as wearable substrates, are presented. Because of the high compatibility of smart e-textiles and clothing, the electricity-generating textiles can be easily integrated into a glove to harvest the mechanical energy induced by the motion of the fingers. The effective output power generated by a single generator due to that motion reached as high as 7 nW/cm(2). The successful demonstration of the electricity-generating glove suggests a promising future for polyester/Ag nanowire/graphene core-shell nanocomposite-based smart e-textiles for real wearable electronic systems and self-powered clothing.

Label-Free Ultrasensitive Memristive Aptasensor

We present the very first worldwide ever-reported electrochemical biosensor based on a memristive effect and DNA aptamers. This novel device is developed to propose a completely new approach in cancer diagnostics. In this study, an affinity-based technique is presented for the detection of the prostate specific antigen (PSA) using DNA aptamers. The hysteretic properties of memristive silicon nanowires functionalized with these DNA aptamers provide a label-free and ultrasensitive biodetection technique. The ultrasensitive detection is hereby demonstrated for PSA with a limit of detection down to 23 aM, best ever published value for electrochemical biosensors in PSA detection. The effect of polyelectrolytes on our memristive devices is also reported to further show how positive or negative charges affect the memristive hysteresis. With such an approach, combining memristive nanowires and aptamers, memristive aptamer-based biosensors can be proposed to detect a wide range of cancer markers with unprecedent ultrasensitivities to also address the issue of an early detection of cancer.

Multilevel Nonvolatile Memristive and Memcapacitive Switching in Stacked Graphene Sheets

We fabricated devices consisting of single and double graphene sheets embedded in organic polymer layers. These devices had binary and ternary nonvolatile resistive switching behaviors, respectively. Capacitance-voltage (C-V) curves and scanning capacitance microscopy (SCM) images were obtained to investigate the switching mechanism. The C-V curves exhibited a large hysteresis, implying that the graphene sheets acted as charging and discharging layers and that resistive switching was caused by charges trapped in the graphene layers. In addition, binary capacitive switching behaviors were observed for the device with a single graphene sheet, and ternary capacitive switching behaviors were observed for the device with the double graphene sheets. These results demonstrated that devices consisting of graphene sheets embedded in the polymer layers can be applied to multilevel nonvolatile memcapacitive devices as well as memristive devices.

Hybrid Flexible Resistive Random Access Memory-Gated Transistor for Novel Nonvolatile Data Storage

Here, a single-device demonstration of novel hybrid architecture is reported to achieve programmable transistor nodes which have analogies to flash memory by incorporating a resistive switching random access memory (RRAM) device as a resistive switch gate for field effect transistor (FET) on a flexible substrate. A high performance flexible RRAM with a three-layered structure is fabricated by utilizing solution-processed MoS2 nanosheets sandwiched between poly(methyl methacrylate) polymer layers. Gate coupling with the pentacene-based transistor can be controlled by the RRAM memory state to produce a nonprogrammed state (inactive) and a programmed state (active) with a well-defined memory window. Compared to the reference flash memory device based on the MoS2 floating gate, the hybrid device presents robust access speed and retention ability. Furthermore, the hybrid RRAM-gated FET is used to build an integrated logic circuit and a wide logic window in inverter logic is achieved. The controllable, well-defined memory window, long retention time, and fast access speed of this novel hybrid device may open up new possibilities of realizing fully functional nonvolatile memory for high-performance flexible electronics.

Ferroelectric-like hysteresis effect observed in carbon quantum dots sandwiched between PMMA and PEDOT:PSS hybrid film

(a) The linear I–V curves in three cyclic multiple-valued voltage sweepings, respectively. (b) The I_sc values measured at t = 0 s and 0.06 s at the write voltage of 2 V and read voltage of 0 V.

Nonvolatile electrical switching behavior and mechanism of functional polyimides bearing a pyrrole unit: influence of different side groups

Influence of side groups to the nonvolatile electrical switching behaviors and its mechanism of polyimides bearing pyrrole unit were systematically studied.

Study on the bio-functionalization of memristive nanowires for optimum memristive biosensors

Semiconductor nanowires are emerging as promising building blocks for biosensors enabling direct electrical detection of various biomolecules.

Organic Electronic Memory Devices

With the rapid development of the electronics industry in recent years, information technology devices, such as personal computers, mobile phones, digital cameras and media players, have become an essential part of our daily life. From both the technological and economic points of view, the development of novel information storage materials and devices has become an emergent issue facing the electronics industry. Due to the advantages of good scalability, flexibility, low cost, ease of processing, 3D-stacking capability and high capacity for data storage, organic-based electrical memory devices have been promising alternatives or supplementary devices to conventional inorganic semiconductor-based memory technology. The basic concepts and historical development of electronic memory devices are first presented. The following section introduces the structures and switching mechanisms of organic electronic memory devices classified as transistors, capacitors and resistors. Subsequently, the progress in the field of organic-based memory materials and devices is systematically summarized and discussed. Finally, the challenges posed to the development of novel organic electronic memory devices are summarized.

Reduced Water Vapor Transmission Rate of Graphene Gas Barrier Films for Flexible Organic Field-Effect Transistors

Preventing reactive gas species such as oxygen or water is important to ensure the stability and durability of organic electronics. Although inorganic materials have been predominantly employed as the protective layers, their poor mechanical property has hindered the practical application to flexible electronics. The densely packed hexagonal lattice of carbon atoms in graphene does not allow the transmission of small gas molecules. In addition, its outstanding mechanical flexibility and optical transmittance are expected to be useful to overcome the current mechanical limit of the inorganic materials. In this paper, we reported the measurement of the water vapor transmission rate (WVTR) through the 6-layer 10 × 10 cm(2) large-area graphene films synthesized by chemical vapor deposition (CVD). The WVTR was measured to be as low as 10(-4) g/m(2)·day initially, and stabilized at ∼0.48 g/m(2)·day, which corresponds to 7 times reduction in WVTR compared to bare polymer substrates. We also showed that the graphene-passivated organic field-effect transistors (OFETs) exhibited excellent environmental stability as well as a prolonged lifetime even after 500 bending cycles with strain of 2.3%. We expect that our results would be a good reference showing the graphene's potential as gas barriers for organic electronics.

Ultra-flexible nonvolatile memory based on donor-acceptor diketopyrrolopyrrole polymer blends

Flexible memory cell array based on high mobility donor-acceptor diketopyrrolopyrrole polymer has been demonstrated. The memory cell exhibits low read voltage, high cell-to-cell uniformity and good mechanical flexibility, and has reliable retention and endurance memory performance. The electrical properties of the memory devices are systematically investigated and modeled. Our results suggest that the polymer blends provide an important step towards high-density flexible nonvolatile memory devices.

Vertically architectured stack of multiple graphene field-effect transistors for flexible electronics

Vertically architectured stack of multiple graphene field-effect transistors (GFETs) on a flexible substrate show great mechanical flexibility and robustness. The four GFETs are integrated in the vertical direction, and dually gated GFETs with graphene channel, PMMA dielectrics, and graphene gate electrodes are realized.

Low-temperature fabrication of alkali metal-organic charge transfer complexes on cotton textile for optoelectronics and gas sensing

A generalized low-temperature approach for fabricating high aspect ratio nanorod arrays of alkali metal-TCNQ (7,7,8,8-tetracyanoquinodimethane) charge transfer complexes at 140 °C is demonstrated. This facile approach overcomes the current limitation associated with fabrication of alkali metal-TCNQ complexes that are based on physical vapor deposition processes and typically require an excess of 800 °C. The compatibility of soft substrates with the proposed low-temperature route allows direct fabrication of NaTCNQ and LiTCNQ nanoarrays on individual cotton threads interwoven within the 3D matrix of textiles. The applicability of these textile-supported TCNQ-based organic charge transfer complexes toward optoelectronics and gas sensing applications is established.