All oxide based flexible multi-folded invisible synapse as vision photo-receptor.

All oxide-based transparent flexible memristor is prioritized for the potential application in artificially simulated biological optoelectronic synaptic devices. SnOx memristor with HfOx layer is found to enable a significant effect on synaptic properties. The memristor exhibits good reliability with long retention, 104 s, and high endurance, 104 cycles. The optimized 6 nm thick HfOx layer in SnOx-based memristor possesses the excellent synaptic properties of stable 350 epochs training, multi-level conductance (MLC) behaviour, and the nonlinearity of 1.53 and 1.46 for long-term potentiation and depression, respectively, and faster image recognition accuracy of 100% after 23 iterations. The maximum weight changes of -73.12 and 79.91% for the potentiation and depression of the synaptic device, respectively, are observed from the spike-timing-dependent plasticity (STDP) characteristics making it suitable for biological applications. The flexibility of the device on the PEN substrate is confirmed by the acceptable change of nonlinearities up to 4 mm bending. Such a synaptic device is expected to be used as a vision photo-receptor.

CMOS-Compatible Memristor for Optoelectronic Neuromorphic Computing.

Optoelectronic memristor is a promising candidate for future light-controllable high-density storage and neuromorphic computing. In this work, light-tunable resistive switching (RS) characteristics are demonstrated in the CMOS process-compatible ITO/HfO2/TiO2/ITO optoelectronic memristor. The device shows an average of 79.24% transmittance under visible light. After electroforming, stable bipolar analog switching, data retention beyond 104 s, and endurance of 106 cycles are realized. An obvious current increase is observed under 405 nm wavelength light irradiation both in high and in low resistance states. The long-term potentiation of synaptic property can be achieved by both electrical and optical stimulation. Moreover, based on the optical potentiation and electrical depression of conductances, the simulated Hopfield neural network (HNN) is trained for learning the 10 × 10 pixels size image. The HNN can be successfully trained to recognize the input image with a training accuracy of 100% in 13 iterations. These results suggest that this optoelectronic memristor has a high potential for neuromorphic application.

Ion accumulation-induced capacitance elevation in a microporous graphene-based supercapacitor.

High-performance porous 3D graphene-based supercapacitors are one of the most promising and challenging directions for future energy technologies. Microporous graphene has been synthesized by the pyrolysis method. The fabricated lightweight graphene with a few layers (FLG) has an ultra-high surface area of 2266 m2 g-1 along with various-sized micropores. The defect-induced morphology and pore size distribution of the fabricated graphene are examined, and the results show that the micropores vary from 0.85 to 1.9 nm and the 1.02 nm pores contribute 30% of the total surface area. The electrochemical behaviour of the electrode fabricated using this graphene has been studied with various concentrations of the KOH electrolyte. The highest specific capacitance of the graphene electrode of 540 F g-1 (close to the theoretical value, ∼550 F g-1) can be achieved by using the 1 M KOH electrolyte. This high specific capacitance contribution involves the counter ion adsorption, co-ion desorption, and ion permutation mechanisms. The formation of a Helmholtz layer, as well as the diffusion of the electrolyte ions, confirms this phenomenon. The symmetrical solid-state supercapacitor fabricated with the graphene electrodes and PVA-KOH gel as the electrolyte exhibits excellent energy and power densities of 18 W h kg-1 and 10.2 kW kg-1, respectively. This supercapacitor also shows a superior 100% coulombic efficiency after 6000 cycles.

Negative Effects of Annealed Seed Layer on the Performance of ZnO-Nanorods Based Nitric Oxide Gas Sensor.

Nitric oxide (NO) is a toxic gas, which is dangerous for human health and causes many respiratory infections, poisoning, and lung damage. In this work, we have successfully grown ZnO nanorod film on annealed ZnO seed layer in different ambient temperatures, and the morphology of the nanorods sensing layer that affects the gas sensing response to nitric oxide (NO) gas were investigated. To acknowledge the effect of annealing treatment, the devices were fabricated with annealed seed layers in air and argon ambient at 300 °C and 500 °C for 1 h. To simulate a vertical device structure, a silver nanowire electrode covered in ZnO nanorod film was placed onto the hydrothermal grown ZnO nanorod film. We found that annealing treatment changes the seed layer's grain size and defect concentration and is responsible for this phenomenon. The I-V and gas sensing characteristics were dependent on the oxygen defects concentration and porosity of nanorods to react with the target gas. The resulting as-deposited ZnO seed layer shows better sensing response than that annealed in an air and argon environment due to the nanorod morphology and variation in oxygen defect concentration. At room temperature, the devices show good sensing response to NO concentration of 10 ppb and up to 100 ppb. Shortly, these results can be beneficial in the NO breath detection for patients with chronic inflammatory airway disease, such as asthma.

Pentafluoropyridine functionalized novel heteroatom-doped with hierarchical porous 3D cross-linked graphene for supercapacitor applications.

The fabrication with high energy density and superior electrical/electrochemical properties of hierarchical porous 3D cross-linked graphene-based supercapacitors is one of the most urgent challenges for developing high-power energy supplies. We facilely synthesized a simple, eco-friendly, cost-effective heteroatoms (nitrogen, phosphorus, and fluorine) co-doped graphene oxide (NPFG) reduced by hydrothermal functionalization and freeze-drying approach with high specific surface areas and hierarchical pore structures. The effect of different heteroatoms doping on the energy storage performance of the synthesized reduced graphene oxide is investigated extensively. The electrochemical analysis performed in a three-electrode system via cyclic voltammetry (CV), galvanostatic charging-discharging (GCD), and electrochemical impedance spectroscopy (EIS) demonstrates that the nitrogen, phosphorous, and fluorine co-doped graphene (NPFG-0.3) synthesized with the optimum amount of pentafluoropyridine and phytic acid (PA) exhibits a notably enhanced specific capacitance (319 F g-1 at 0.5 A g-1), good rate capability, short relaxation time constant (τ = 28.4 ms), and higher diffusion coefficient of electrolytic cations (Dk+ = 8.8261 × 10-9 cm2 s-1) in 6 M KOH aqueous electrolyte. The density functional theory (DFT) calculation result indicates that the N, F, and P atomic replacement within the rGO model could increase the energy value (G T) from -673.79 eV to -643.26 eV, demonstrating how the atomic level energy could improve the electrochemical reactivity with the electrolyte. The improved performance of NPFG-0.3 over NFG, PG, and pure rGO is mainly ascribed to the fast-kinetic process owing to the well-balanced electron/ion transport phenomenon. A symmetric coin cell supercapacitor device fabricated using NPFG-0.3 as the anode and cathode material with 6 M KOH aqueous electrolyte exhibits maximum specific energy of 38 W h kg-1, a maximum specific power of 716 W kg-1, and ∼88.2% capacitance retention after 10 000 cycles. The facile synthesis approach and promising electrochemical results suggest this synthesized NPFG-0.3 material has high potential for future supercapacitor application.

Neutral oxygen irradiation enhanced forming-less ZnO-based transparent analog memristor devices for neuromorphic computing applications.

Surface oxidation employing neutral oxygen irradiation significantly improves the switching and synaptic performance of ZnO-based transparent memristor devices. The endurance of the as-irradiated device is increased by 100 times, and the operating current can be lowered by 10 times as compared with the as-deposited device. Moreover, the performance-enhanced device has an excellent analog behavior that can exhibit 3 bits per cell nonvolatile multistate characteristics and perform 15 stable epochs of synaptic operations with highly linear weight updates. A simulated artificial neural network comprising 1600 synapses confirms the superiority of the enhanced device in processing a 40 × 40 pixels grayscale image. The irradiation effectively decreases the concentration of oxygen vacancy donor defects and promotes oxygen interstitial acceptor defects on the surface of the ZnO films, which consequently modulate the redox process during rupture and rejuvenation of the filament. This work not only proposes the potential of ZnO-based memristor devices for high-density invisible data storage and in-memory computing application but also offers valuable insight in designing high-performance memristor devices, regardless of the oxide system used, by taking advantage of our neutral oxygen irradiation technique.

Role of precursors mixing sequence on the properties of CoMn2O4 cathode materials and their application in pseudocapacitor.

In this study, the effect of oxygen vacancy in the CoMn2O4 on pseudocapacitive characteristics was examined, and two tetragonal CoMn2O4 spinel compounds with different oxygen vacancy concentrations and morphologies were synthesized by controlling the mixing sequence of the Co and Mn precursors. The mixing sequence was changed; thus, morphologies were changed from spherical nanoparticles to nanoflakes and oxygen vacancies were increased. Electrochemical studies have revealed that tetragonal CoMn2O4 spinels with a higher number of oxygen vacancies exhibit a higher specific capacitance of 1709 F g-1 than those with a lower number of oxygen vacancies, which have a higher specific capacitance of 990 F g-1. Oxygen vacancies create an active site for oxygen ion intercalation. Therefore, oxidation-reduction reactions occur because of the diffusion of oxygen ions at octahedral/tetrahedral crystal edges. The solid-state asymmetric pseudocapacitor exhibits a maximum energy density of 32 Wh-kg-1 and an excellent cyclic stability of nearly 100%.

Synthesis of Free-Standing Flexible rGO/MWCNT Films for Symmetric Supercapacitor Application.

Herein, we report a novel, simple, and cost-effective way to synthesize flexible and conductive rGO and rGO/MWCNT freestanding films. The effects of MWCNT addition on the electrochemical performance of rGO/MWCNT nanocomposite films are investigated in some strong base aqueous electrolytes, such as KOH, LiOH, and NaOH via three-electrode system. The supercapacitor behavior of the films is probed via cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy. The structural and morphological studies of the films are performed by X-ray diffractometer, Raman spectrometer, surface area analyzer, thermogravimetric analysis, field emission scanning electron microscope and transmission electron microscope. The rGO/MWCNT film synthesized with 10 wt% MWCNTs (GP10C) exhibits high specific capacitance of 200 Fg-1, excellent cyclic stability with 92% retention after 15,000 long cycle test, small relaxation time constant (~ 194 ms), and high diffusion coefficient (7.8457 × 10-9 cm2 s-1) in 2 M KOH electrolyte. Furthermore, the symmetric supercapacitor coin cell with GP10C as both anode and cathode using 2 M KOH as electrolyte demonstrates high energy density of 29.4 Whkg-1 and power density of 439 Wkg-1 at current density 0.1 Ag-1 and good cyclic stability with 85% retention of the initial capacitance at 0.3 Ag-1 after 10,000 cycles. Such a high performance of the GP10C film in the supercapacitor can be ascribed to the large surface area and small hydration sphere radius and high ionic conductivity of K+ cations in KOH electrolyte.

Improving linearity by introducing Al in HfO2 as a memristor synapse device.

Artificial synapse having good linearity is crucial to achieve an efficient learning process in neuromorphic computing. It is found that the synaptic linearity can be enhanced by engineering the doping region across the switching layer. The nonlinearity of potentiation and depression of the pure device is 36% and 91%, respectively; meanwhile, the nonlinearity after doping can be suppressed to be 22% (potentiation) and 60% (depression). Henceforth, the learning accuracy of the doped device is 91% with only 13 iterations; meanwhile, the pure device is 78%. A detailed conduction mechanism to understand this phenomenon is proposed.

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.

Highly sensitive nitric oxide gas sensor based on ZnO-nanorods vertical resistor operated at room temperature.

We successfully demonstrated a simple and low-cost nitric oxide (NO) gas sensor to deliver parts-per-billion (ppb) regime detection at room temperature operation. A vertical-channel ZnO nanorods resistor is fabricated by putting silver nanowire electrode onto the hydrothermal ZnO nanorods film. With suitable process condition, the nanorods film exhibits a uniform morphology to enable the formation of gas-permeable nanowire top electrode while also the nanorods provide good surface-to-volume ratio to deliver strong reaction with NO gas. A detection limit to 10 ppb NO is observed while the sensing dynamic range from 10 ppb to 100 ppb is obtained. The sensor is reversible and the real-time sensing response is within 30 s. The results benefit the NO breath detection for patients with chronic inflammatory airway disease, such as asthma.

Switching Failure Mechanism in Zinc Peroxide-Based Programmable Metallization Cell.

The impact of peroxide surface treatment on the resistive switching characteristics of zinc peroxide (ZnO2)-based programmable metallization cell (PMC) devices is investigated. The peroxide treatment results in a ZnO hexagonal to ZnO2 cubic phase transformation; however, an excessive treatment results in crystalline decomposition. The chemically synthesized ZnO2 promotes the occurrence of switching behavior in Cu/ZnO2/ZnO/ITO with much lower operation current as compared to the Cu/ZnO/ITO (control device). However, the switching stability degrades as performing the peroxide treatment for a longer time. We suggest that the microstructure of the ZnO2 is responsible for this degradation behavior and fine tuning on ZnO2 properties, which is necessary to achieve proper switching characteristics in ZnO2-based PMC devices.

A Collective Study on Modeling and Simulation of Resistive Random Access Memory.

In this work, we provide a comprehensive discussion on the various models proposed for the design and description of resistive random access memory (RRAM), being a nascent technology is heavily reliant on accurate models to develop efficient working designs and standardize its implementation across devices. This review provides detailed information regarding the various physical methodologies considered for developing models for RRAM devices. It covers all the important models reported till now and elucidates their features and limitations. Various additional effects and anomalies arising from memristive system have been addressed, and the solutions provided by the models to these problems have been shown as well. All the fundamental concepts of RRAM model development such as device operation, switching dynamics, and current-voltage relationships are covered in detail in this work. Popular models proposed by Chua, HP Labs, Yakopcic, TEAM, Stanford/ASU, Ielmini, Berco-Tseng, and many others have been compared and analyzed extensively on various parameters. The working and implementations of the window functions like Joglekar, Biolek, Prodromakis, etc. has been presented and compared as well. New well-defined modeling concepts have been discussed which increase the applicability and accuracy of the models. The use of these concepts brings forth several improvements in the existing models, which have been enumerated in this work. Following the template presented, highly accurate models would be developed which will vastly help future model developers and the modeling community.

Conductive bridge random access memory characteristics of SiCN based transparent device due to indium diffusion.

In this work, the transparent bipolar resistive switching characteristics of a SiCN-based ITO/SiCN/AZO structure due to In diffusion from ITO is studied. The SiCN based device is found to be 80% transparent in the visible wavelength region. This device, with AZO as both top and bottom electrodes, does not show any RRAM property due to deposition of the high quality O2-free SiCN film. Replacing the AZO top electrode with ITO in this device results in good resistive switching (RS) characteristics with a high on/off ratio and long retention. Replacing the SiCN film with ZrO2 also results in excellent RS characteristics due to the formation of an oxygen vacancies filament inside the ZrO2 film. A resistance ratio of on/off is found to be higher in the SiCN based device compared to that of the ZrO2 device. Diffusion of In from ITO into the SiCN film on application of high positive voltage during forming can be attributed to the occurrence of RS in the device, which is confirmed by the analyses of energy dispersive spectroscopy and secondary-ion mass spectrometry. This study shows a pathway for the fabrication of CBRAM based transparent devices for non-volatile memory application.

An ultra-compact blackbody using electrophoretic deposited carbon nanotube films.

Carbon nanotubes (CNTs) possesses decent optical properties and thus can be considered as a candidate for perfect absorbers due to their close-to-air refractive index and minimal extinction. However, weak absorption in porous materials, due to the low extinction coefficients, requires an inevitably thick absorption layer (∼100 µm) for the perfect opaque absorbers. Thus, the requirement of large thicknesses of CNTs prohibits them from being used as miniaturized integrated photonic devices. Here, we propose an electrophoretic deposited (EPD) CNT resonant cavity structure on tantalum (Ta) to enhance optical absorption. Efficient random light scattering along with the resonant cavity structure using Ti/SiO2 stacking enhances the absorption in our proposed EPD-CNT film while maintaining the total device thickness to <1 µm. The experiment results reveal that the absorption band covers the entire UV-VIS-NIR spectrum (λ = 0.3-2.6 µm), using resonant-cavity EPD-CNT design. The EPD deposition process is done at relatively low temperature < 120 °C. We believe that this proposal is very promising for sensing, antenna, and thermophotovoltaics (TPV), in terms of bandwidth, compactness and cost.

Flexible solid-like electrolytes with ultrahigh conductivity and their applications in all-solid-state supercapacitors.

All-solid-state supercapacitors (ASSS) with solid-state electrolytes (SSEs) can be used to overcome the liquid leakage problem in devices. However, ionic conduction in solid electrolytes is one of the barriers to further improvements in ASSS. This paper describes the fabrication of a flexible SSE composed of poly(vinylidene fluoride-co-hexafluoropropylene), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and ethylene carbonate, which demonstrates an ultrahigh conductivity of 8.52 mS cm-1 and a wide 5 V operation voltage window of -2 to +3 V. Electrodes composed of active carbon, multiwall carbon nanotubes, and polyvinylidene fluoride were used as both anode and cathode to assemble a symmetrical supercapacitor. The resultant supercapacitor exhibits a maximum power density of 3747 W kg-1 at an energy density of 7.71 W h kg-1 and a maximum energy density 17.1 W h kg-1 at a power density of 630 W kg-1. It displays excellent cycling stability with 91.3% of the initial specific capacitance after 3000 charging/discharging cycles. This flexible SSE in this study demonstrates a high potential for use in energy storage, conversion, and wearable device applications.

In Situ TEM Investigation of the Electrochemical Behavior in CNTs/MnO2-Based Energy Storage Devices.

Transition metal oxides have attracted much interest owing to their ability to provide high power density in lithium batteries; therefore, it is important to understand the electrochemical behavior and mechanism of lithiation-delithiation processes. In this study, we successfully and directly observed the structural evolution of CNTs/MnO2 during the lithiation process using transmission electron microscopy (TEM). CNTs/MnO2 were selected due to their high surface area and capacitance effect, and the lithiation mechanism of the CNT wall expansion was systematically analyzed. Interestingly, the wall spacings of CNTs/MnO2 and CNTs were obviously expanded by 10.92% and 2.59%, respectively. The MnO2 layer caused structural defects on the CNTs surface that could allow penetration of Li+ and Mn4+ through the tube wall and hence improve the ionic transportation speed. This study provided direct evidence for understanding the role of CNTs/MnO2 in the lithiation process used in lithium ion batteries and also offers potential benefits for applications and development of supercapacitors.

Peroxide induced volatile and non-volatile switching behavior in ZnO-based electrochemical metallization memory cell.

We explore the use of cubic-zinc peroxide (ZnO2) as a switching material for electrochemical metallization memory (ECM) cell. The ZnO2 was synthesized with a simple peroxide surface treatment. Devices made without surface treatment exhibits a high leakage current due to the self-doped nature of the hexagonal-ZnO material. Thus, its switching behavior can only be observed when a very high current compliance is employed. The synthetic ZnO2 layer provides a sufficient resistivity to the Cu/ZnO2/ZnO/ITO devices. The high resistivity of ZnO2 encourages the formation of a conducting bridge to activate the switching behavior at a lower operation current. Volatile and non-volatile switching behaviors with sufficient endurance and an adequate memory window are observed in the surface-treated devices. The room temperature retention of more than 104 s confirms the non-volatility behavior of the devices. In addition, our proposed device structure is able to work at a lower operation current among other reported ZnO-based ECM cells.

Status and Prospects of ZnO-Based Resistive Switching Memory Devices.

In the advancement of the semiconductor device technology, ZnO could be a prospective alternative than the other metal oxides for its versatility and huge applications in different aspects. In this review, a thorough overview on ZnO for the application of resistive switching memory (RRAM) devices has been conducted. Various efforts that have been made to investigate and modulate the switching characteristics of ZnO-based switching memory devices are discussed. The use of ZnO layer in different structure, the different types of filament formation, and the different types of switching including complementary switching are reported. By considering the huge interest of transparent devices, this review gives the concrete overview of the present status and prospects of transparent RRAM devices based on ZnO. ZnO-based RRAM can be used for flexible memory devices, which is also covered here. Another challenge in ZnO-based RRAM is that the realization of ultra-thin and low power devices. Nevertheless, ZnO not only offers decent memory properties but also has a unique potential to be used as multifunctional nonvolatile memory devices. The impact of electrode materials, metal doping, stack structures, transparency, and flexibility on resistive switching properties and switching parameters of ZnO-based resistive switching memory devices are briefly compared. This review also covers the different nanostructured-based emerging resistive switching memory devices for low power scalable devices. It may give a valuable insight on developing ZnO-based RRAM and also should encourage researchers to overcome the challenges.

Overview of emerging nonvolatile memory technologies.

Nonvolatile memory technologies in Si-based electronics date back to the 1990s. Ferroelectric field-effect transistor (FeFET) was one of the most promising devices replacing the conventional Flash memory facing physical scaling limitations at those times. A variant of charge storage memory referred to as Flash memory is widely used in consumer electronic products such as cell phones and music players while NAND Flash-based solid-state disks (SSDs) are increasingly displacing hard disk drives as the primary storage device in laptops, desktops, and even data centers. The integration limit of Flash memories is approaching, and many new types of memory to replace conventional Flash memories have been proposed. Emerging memory technologies promise new memories to store more data at less cost than the expensive-to-build silicon chips used by popular consumer gadgets including digital cameras, cell phones and portable music players. They are being investigated and lead to the future as potential alternatives to existing memories in future computing systems. Emerging nonvolatile memory technologies such as magnetic random-access memory (MRAM), spin-transfer torque random-access memory (STT-RAM), ferroelectric random-access memory (FeRAM), phase-change memory (PCM), and resistive random-access memory (RRAM) combine the speed of static random-access memory (SRAM), the density of dynamic random-access memory (DRAM), and the nonvolatility of Flash memory and so become very attractive as another possibility for future memory hierarchies. Many other new classes of emerging memory technologies such as transparent and plastic, three-dimensional (3-D), and quantum dot memory technologies have also gained tremendous popularity in recent years. Subsequently, not an exaggeration to say that computer memory could soon earn the ultimate commercial validation for commercial scale-up and production the cheap plastic knockoff. Therefore, this review is devoted to the rapidly developing new class of memory technologies and scaling of scientific procedures based on an investigation of recent progress in advanced Flash memory devices.