Unveiling transient current response in bilayer oxide-based physical reservoirs for time-series data analysis.

Physical reservoirs employed to map time-series data and analyze extracted features have attracted interest owing to their low training cost and mitigated interconnection complexity. This study reports a physical reservoir based on a bilayer oxide-based dynamic memristor. The proposed device exhibits a nonlinear current response and short-term memory (STM), satisfying the requirements of reservoir computing (RC). These characteristics are validated using a compact model to account for resistive switching (RS) via the dynamic evolution of the internal state variable and the relocation of oxygen vacancies. Mathematically, the transient current response can be quantitatively described according to a simple set of equations to correlate the theoretical framework with experimental results. Furthermore, the device shows significant reliability and ability to distinguish 4-bit inputs and four diverse neural firing patterns. Therefore, this work shows the feasibility of implementing physical reservoirs in hardware and advances the understanding of the dynamic response.

Dual Light Temporal Coding Modes Enabled by Nanoparticle-Mediated Phototransistors via Gate Bias Modulation for Brain-Inspired Visual Perception.

The core integration and cooperation of the retina, neurons, and synapses in the visual systems enable humans to effectively sense and process visual information with low power consumption. To mimic the human visual system, an artificial sensory nerve, along with optical sensing─a paired-pulse ratio (PPR) of the light pulse stimulated currents─and neural coding has been developed. For performing the artificial visual perception functions, we consistently reveal the positive and negative correlations between the PPR index and light pulse time interval by applying two consecutive light stimuli with gate voltages of -10 and 5 V, respectively, to a phototransistor. This phototransistor contains a heterostructured channel layer composed of zinc-oxide nanoparticles (ZnO NPs) interconnected with a solution-processed zinc-tin oxide (ZTO) film. The oxygen adsorption and desorption on the ZnO NP surface under light illumination are responsible for the positive-sloped PPR; the electron trapping effect at the ZnO NP/SiO2 interface is attributed to the negative-sloped PPR. The various accountable light power densities and number of surface trap states are considered to be directly realizing these spike-timing interval-dependent characteristics. The actual benefit of these characteristics is the dual temporal coding modes based on multiplicative operation using a ZTO/ZnO NP phototransistor realized via the active gate voltage modulation. The contrary tendency of the PPR index and temporal coding─a major biological neural coding─is well demonstrated by the potential of ZTO/ZnO NP phototransistors to be implemented in sensor networks for an artificial visual perception.

Mimicking Pain-Perceptual Sensitization and Pattern Recognition Based on Capacitance- and Conductance-Regulated Neuroplasticity in Neural Network.

Neuromorphic computing, inspired by the biological neuronal system, is a high potential approach to substantially alleviate the cost of computational latency and energy for massive data processing. Artificial synapses with regulable synaptic weights are the basis of neuromorphic computation, providing an efficient and low-power system to overcome the constraints of the von Neumann architecture. Here, we report an ITO/TaOx-based synaptic capacitor and transistor. With the drift motion of mobile-charged ions in the TaOx, the capacitance and channel conductance can be tuned to exhibit synaptic weight modulation. Robust stability in the cycle-to-cycle (C2C) variation is found in capacitance and conductance potentiation/depression weight updating of 0.9 and 1.8%, respectively. Simulation results show a higher classification accuracy of handwritten digit recognition (95%) in capacitance synapses than that in conductance synapses (84%). Besides, the synaptic capacitor consumes much less energy than the synaptic transistor. Moreover, the ITO/TaOx-based capacitor successfully emulates the pain-perceptual sensitization on top of the superior performance, indicating its promising potential in applying the capacitive neural network.

Enhancement of H2S sensing performance of rGO decorated CuO thin films: experimental and DFT studies.

We demonstrate a highly selective and sensitive Cupric oxide (CuO) thin film-based low concentration Hydrogen sulfide (H2S) sensor. The sensitivity was improved around three times by decorating with reduced graphene oxide (rGO) nanosheets. CuO thin films were deposited by Chemical Vapor Deposition followed by inter-digital electrode fabrication by a thermal evaporations system. The crystal structure of CuO was confirmed by x-ray diffraction. The sensing response of pristine CuO was found around 54% at 100 °C to 100 ppm of H2S. In contrast, the sensing response was enhanced to 167% by decorating with rGO of 1.5 mg ml-1concentration solution. The sensing was improved due to the formation of heterojunctions between the rGO and CuO. The developed sensor was examined under various gas environments and found to be highly selective towards H2S gas. The improvement in sensing response has been attributed to increased hole concentration in CuO in the presence of rGO due to the Fermi level alignment and increased absorption of H2S molecules at the rGO/CuO heterojunction. Further, electronic structure calculations show the physisorption behavior of H2S molecules on the different adsorption sites. Detailed insight into the gas sensing mechanism is discussed based on experimental results and electronic structure calculations.

First-principles simulation of neutral and charged oxygen vacancies in m-ZrO2: an origin of filamentary type resistive switching.

Metal oxide ZrO2has been widely explored for resistive switching application due to excellent properties like high ON/OFF ratio, superior data retention, and low operating voltage. However, the conduction mechanism at the atomistic level is still under debate. Therefore, we have performed comprehensive insights into the role of neutral and charged oxygen vacancies in conduction filament (CF) formation and rupture, which are demonstrated using the atomistic simulation based on density functional theory (DFT). Formation energy demonstrated that the fourfold coordinated oxygen vacancy is more stable. In addition, the electronic properties of the defect included supercell confirm the improvement in electrical conductivity due to the presence of additional energy states near Fermi energy. The CF formation and rupture using threefold and fourfold oxygen vacancies are demonstrated through cohesive energy, electron localization function, and band structure. Cohesive energy analysis confirms the cohesive nature of neutral oxygen vacancies while the isolated behavior for +2 charged oxygen vacancies in the CF. In addition, nudged elastic band calculation is also performed to analyze the oxygen vacancy diffusion energy under different paths. Moreover, we have computed the diffusion coefficient and drift velocity of oxygen vacancies to understand the CF. This DFT study described detailed insight into filamentary type resistive switching observed in the experimentally fabricated device. Therefore, this fundamental study provides the platform to explore the switching mechanism of other oxide materials used for memristor device application.

Synaptic Emulation via Ferroelectric P(VDF-TrFE) Reinforced Charge Trapping/Detrapping in Zinc-Tin Oxide Transistor.

Brain inspired artificial synapses are highly desirable for neuromorphic computing and are an alternative to a conventional computing system. Here, we report a simple and cost-effective ferroelectric capacitively coupled zinc-tin oxide (ZTO) thin-film transistor (TFT) topped with ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) for artificial synaptic devices. Ferroelectric dipoles enhance the charge trapping/detrapping effect in ZTO TFT, as confirmed by the transfer curve (ID-VG) analysis. This substantiates superior artificial synapse responses in ferroelectric-coupled ZTO TFT because the current potentiation and depression are individually improved. The ferroelectric-coupled ZTO TFT successfully emulates the essential features of the artificial synapse, including pair-pulsed facilitation (PPF) and potentiation/depression (P/D) characteristics. In addition, the device also mimics the memory consolidation behavior through intensified stimulation. This work demonstrates that the ferroelectric-coupled ZTO synaptic transistor possesses great potential as a hardware candidate for neuromorphic computing.

Efficacious CO2 Photoconversion to C2 and C3 Hydrocarbons on Upright SnS-SnS2 Heterojunction Nanosheet Frameworks.

In this work, SnS-SnS2 heterostructured upright nanosheet frameworks are constructed on FTO substrates, which demonstrate promising photocatalytic performances for the conversion of CO2 and water to C2 (acetaldehyde) and C3 (acetone) hydrocarbons without H2 formation. With post annealing in designated atmospheres, the photocatalytic activity of the SnS-SnS2 heterostructured nanosheet framework is critically enhanced by increasing the fraction of crystalline SnS in nanosheets through partial transformation of the SnS2 matrix to SnS but not obviously influenced by improving the crystallinity of the SnS2 matrix. DFT calculations indicate that transformed SnS possesses the CO2 adsorption sites with significantly lower activation energy for the rate-determining step to drive efficient CO2 conversion catalysis. The experimental results and DFT calculations suggest that the SnS-SnS2 heterojunction nanosheet framework photocatalyst experiences Z-scheme charge transfer dynamic to allow the water oxidation and CO2 reduction reactions occurring on the surfaces of SnS2 and SnS, respectively. The Z-scheme SnS-SnS2 heterostructured nanosheet framework photocatalyst exhibits not only efficient charge separation but also highly catalytic active sites to boost the photocatalytic activity for CO2 conversion to C2 and C3 hydrocarbons.

Violet-light stimulated synaptic and learning functions in a zinc-tin oxide photoelectric transistor for neuromorphic computation.

In contrast to the commonly present UV light-stimulated synaptic oxide thin-film transistors, this study demonstrates a violet light (wavelength of 405 nm) stimulated zinc-tin oxide (ZTO) photoelectric transistor for potential application in optical neuromorphic computation. Owing to the light-induced oxygen vacancy ionization and persistent photoconductivity effect in ZTO, this device well imitates prominent synaptic functions, including photonic potentiation, electric depression, and short-term memory (STM) to long-term memory (LTM) transition. A highly linear and broad dynamic range of photonic potentiation can be achieved by modulating the light power density, while electric depression is realized by gate voltage pulsing. In addition, the brain-like re-learning experience with extended forgetting time (200 s) is well mimicked by the ZTO photoelectric transistor. As a result, the ZTO photoelectric transistor provides excessive synaptic function with multi-series of synaptic weight levels (90 levels for each given light power density), which makes it prevalent in the neuromorphic computation of massive data as well as in learning-driven artificial intelligence computation.

Aluminum electrode modulated bipolar resistive switching of Al/fuel-assisted NiOx/ITO memory devices modeled with a dual-oxygen-reservoir structure.

Bipolar resistive switching in Al/fuel-assisted NiO(x) (40 nm)/ITO devices is demonstrated in this work. XPS analysis reveals the simultaneous presence of metallic Ni, Ni(2)O(3), and NiO components in the fuel-assisted NiO(x). The concentration, as well as spreading of the metallic Ni and accompanying oxygen vacancies, are related to the Al/NiO(x) interfacial reaction, which is enhanced by the increasing thickness of the Al top electrode. Correspondingly, the preswitching-on voltage decreases while the preswitching-off voltage increases with increasing thickness (from 15 to 60 nm) of Al. However, in regular switching operation, set and reset voltages are considerably lowered for devices with an increased thickness of the Al top electrode. The bipolar resistive switching behaviors of Al/fuel-assisted NiO(x)/ITO devices are therefore discussed based on the formation of conductive paths and their correlation with the Al-electrode modulated composition in the fuel-assisted NiO(x). The Al/NiO(x) interfacial reaction region pairs with ITO to form a dual-oxygen-reservoir structure. Mechanisms of construction/destruction of conducting paths originating from the electrochemical redox reactions at the interface between NiO(x), and the dual oxygen reservoirs will also be explicated.

The importance of p-n junction interfaces for efficient small molecule-based organic solar cells.

The efficiency of small-molecule solar cells critically depends on the match of the junction of the donor and acceptor semiconductors used in these devices to create charged carriers and on the mobility of individual components to transport holes and electrons. In the present study, a 2% efficient bilayer organic solar cell consisting of a p-type semiconductor, pentacene, and an n-type semiconductor, N,N'-diheptyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C(7)), is fabricated. The morphology of PTCDI-C(7) interestingly follows pentacene due to the matched surface energy of these two active layers and the easily deposited PTCDI-C(7) monomers on an inclined plane of the pentacene grains. This condition results in the low trap states in the PTCDI-C(7) film and at the pentacene/PTCDI-C(7) interface for the enhancement of exciton dissociation and carrier transport compared with the photoactive layer comprised of pentacene and N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C(13)). The detailed exciton and carrier transport mechanisms are investigated using time-resolved photoluminescence and X-ray diffraction spectroscopy.

Resistive switching behavior and multiple transmittance states in solution-processed tungsten oxide.

In this work, a tungsten oxide (WO(x)) film is prepared using a thiourea-assisted solution process. We demonstrate a device composed of fluorine doped tin oxide (FTO)-glass/WO(x)/electrolyte/indium-tin oxide (ITO)-glass stacking electrochromic (EC) structure and Al electrodes that are locally patterned and interposed between the WO(x) film and electrolyte, which form an Al(top electrode)/WO(x)/FTO(bottom electrode) resistance random access memory (RRAM) unit. According to transmission electron microscopy and X-ray photoelectron spectroscopy analyses, the WO(x) film contains nanosize pores and metallic-tungsten nanoclusters which are scattered within the tungsten oxide layer and concentrated along the interface between the Al electrode and WO(x) film. With application of voltage to the ITO electrode, multiple transmittance states are achieved for the EC unit due to the different quantity of intercalated Li ions in the WO(x) film. As for the Al/WO(x)/FTO RRAM unit, a bipolar nonvolatile resistive switching behavior is attained by applying voltage on the Al top electrode, showing electrical bistability with an ON/OFF current ratio up to 1 × 10(4).

Schottky barrier mediated single-polarity resistive switching in Pt layer-included TiO(x) memory device.

A distinct unipolar but single-polarity resistive switching behavior is observed in a TiO(x)/Pt/TiO(x) trilayer structure, formed by thermal oxidation of a Ti/Pt/Ti stack. As a comparison, a memory device with a single TiO(x) active layer (without addition of Pt midlayer) is also fabricated but it cannot perform resistive switching. Energy band diagrams are illustrated to realize the modulation of Schottky barrier junctions and current conduction in TiO(x)-based devices under various biasing polarities. Introduction of the Pt midlayer creates two additional Schottky barriers, which mediate the band bending potential at each metal-oxide interface and attains a rectifying current conduction at the high-resistance state. The rectifying conduction behavior is also observed with an AFM-tip as the top electrode, which implies the rectifying property is still valid when miniaturizing the device to nanometer scale. The current rectification consequently leads to a single-polarity, unipolar resistive switching and electrically rewritable performance for the TiO(x)/Pt/TiO(x) device.

An efficient route to nanostructured tungsten oxide films with improved electrochromic properties.

An effective chemical route to nanostructured tungsten oxide films derived from a peroxopolytungstic acid (PTA)/thiourea precursor solution is demonstrated. The conventional procedure of preparing the precursor needs more than 24 h for well-mixing and refluxing the PTA-based solution, while the thiourea-assisted approach takes less than 1 h to prepare the precursor solution because the excess hydrogen peroxide can be efficiently eliminated by oxidation of thiourea. With the precursor solution, tungsten oxide films are deposited by spin coating followed by high temperature annealing. The film annealed at 400 °C possesses a porous nanostructure of nanocrystalline tungsten oxide embedded in an amorphous tungsten oxide matrix, which arises from the gaseous species released through decomposition of thiourea oxides during annealing. The 400 °C-annealed, thiourea-assisted tungsten oxide film exhibits electrochromic (EC) properties superior to those of the film prepared without thiourea, including large transmittance modulation and coloration efficiency, fast response time and adequate reliability. When increasing the annealing temperature to 450 °C, the thiourea-assisted tungsten oxide film is also porous but well-crystallized and shows inferior EC properties. Electrochemical impedance spectroscopy analysis indicates that, in addition to the porous structure, a fast charge-transport rate within the solid portion of the 400 °C-annealed nanostructured film plays a crucial role in enhancing EC performances of the thiourea-assisted tungsten oxide film.

Outperformed electrochromic behavior of poly(ethylene glycol)-template nanostructured tungsten oxide films with enhanced charge transfer/transport characteristics.

Porous tungsten oxide films of nanocrystalline tungsten oxide embedded in an amorphous tungsten oxide matrix have been synthesized via poly(ethylene glycol) (PEG)-template sol-gel technique with peroxopolytungstic acid precursor. The effects of PEG addition on the microstructure and electrochromic performance of the tungsten oxide films are investigated. Charge transfer/transport properties in the tungsten oxide films are studied by electrochemical impedance spectroscopy (EIS) as well. Triclinic tungsten oxide film is formed in the absence of PEG. The PEG-template tungsten oxide film demonstrates an electrochromic performance superior to that of the crystalline tungsten oxide film, including larger transmittance modulation and coloration/bleaching efficiency as well as faster response times. EIS measurements indicate that faster charge-transfer rates at the tungsten oxide/electrolyte interface and larger Li(+) diffusion coefficients in tungsten oxide are achieved in the PEG-template film. We suggest that the PEG-template tungsten oxide film with a porous crystalline/amorphous nanostructure provides an effective means for charge transfer/transport to encourage its superior electrochromic performance.

Fast-switching photovoltachromic cells with tunable transmittance.

In this study, we demonstrate a photovoltachromic cell (PVCC) which is a solar cell and able to take solar energy to stimulate chromic behavior with the characteristic of tunable transmittance. The cell is composed of a patterned WO(3)/Pt electrochromic electrode and a dye-sensitized TiO(2) nanoparticle photoanode. Compared to reported photoelectrochromic cells (PECC) with nonpatterned WO(3) electrochromic electrodes, PVCC achieves a much faster bleaching time of only 60 s by blocking the light at short circuit. When PVCC is bleached under illumination at open circuit, an exceedingly short bleaching time of 4 s is achieved. Furthermore, PVCC has photovoltaic characteristics comparable to those of dye-sensitized solar cells (with Pt as the counter-electrode). In contrast to conventional photochromic devices, the transmittance of PVCC under a constant illumination can be adjusted by the resistance of a load in series with the cell. These characteristics are a result of the patterned WO(3)/Pt electrode, which provides effective charge transfer pathways to facilitate the charging/discharging of Li ions and electrons via the photovoltaic potential and the Pt-electrolyte catalytic route, respectively.

Performance and electron transport properties of TiO(2) nanocomposite dye-sensitized solar cells.

TiO(2) nanowire (NW)/nanoparticle (NP) composite films have been fabricated by hybridizing various ratios of hydrothermal anatase NWs and TiO(2) NPs for use in dye-sensitized solar cells (DSSCs). Scanning electron microscopy (SEM) images reveal that uniform NW/NP composite films were formed on fluorine-doped tin oxide (FTO) substrates by the dip-coating method. The NWs are randomly but neither vertically nor horizontally oriented within the composite film. The TiO(2) NP DSSC possesses superior performance to those of the NW/NP composite and the pure NW cells, and the efficiency of the NW/NP composite DSSC increases on increasing the NP/NW ratio in the composite anode. All types of DSSC possess the same dependence of performance on the anode thickness that the efficiency increases with the anode thickness to a maximum value, then it decreases when the anode is thickened further. Electrochemical impedance spectroscopy analyses reveal that the NP DSSCs possess larger effective electron diffusion coefficients (D(eff)) in the photoanodes and smaller diffusion resistances of I(3)(-) in electrolytes compared to those in the NW/NP and the NW DSSCs. D(eff) decreases when NWs are added into the photoanode. These results suggest that the vertical feature of the NWs within the anodes is crucial for achieving a high electron transport rate in the anode.