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Artificial synapses with ideal functionalities are essential in hardware neural networks to allow for energy-efficient analog computing. However, the realization of linear and symmetric weight updates in real synaptic devices has proven challenging and ultimately limits the online training capabilities of neural network systems. Herein, we investigate the effect of Mg doping on a LiCoO2(LCO) channel in a Li ion-gated synaptic transistor, so as to improve long-term and short-term plasticity. Two transistor structures, based on a lithium phosphorus oxynitride electrolyte, were examined by using undoped LCO and Mg-doped LCO as the channel material between the source and drain electrodes. It was found that Mg doping increased the initial channel conductance by 3 orders of magnitude, which is probably due to the substitution of Co3+by Mg2+and the compensation of hole creation. It was further found that the doped channel transistor showed good retention characteristics and better linearity of long-term potentiation and depression when voltage pulses were applied to the gate electrode. The improved retention and linearity are attributed to an extended range of the insulator-to-conductor transition by Mg doping and Li-ion extraction/insertion cooperated in the LCO channel. Using the obtained synaptic weight update, artificial neural network simulations demonstrated that the doped channel transistor shows an image recognition accuracy of ∼80% for handwritten digits, which is higher than ∼65% exhibited by the undoped channel transistor. Mg doping also improved short-term plasticity such as paired-pulse facilitation/depression and Hebbian spike timing-dependent plasticity. These results indicate that elemental doping to the channel of Li ion-gated synaptic transistors could be a useful procedure for realizing robust neuromorphic systems based on analog computing.

Neuromorphic processors using artificial neural networks are the center of attention for energy-efficient analog computing. Artificial synapses act as building blocks in such neural networks for parallel information processing and data storage. Herein we describe the fabrication of a proton-gated synaptic transistor using a Nafion electrolyte thin film, which is patterned by electron-beam lithography (EBL). The device has an active channel of indium–zinc-oxide (IZO) between the source and drain electrodes, which shows Ohmic behavior with a conductance level on the order of 100 μS. Under voltage applications to the gate electrode, the channel conductance is changed due to the injection and extraction of protons between the IZO channel and the Nafion electrolyte, emulating various synaptic functions with short-term and long-term plasticity. When positive (negative) gate voltage pulses are consecutively applied, the device exhibits long-term potentiation (depression) at the same number of steps as the number of input pulses. Based on these characteristics, an artificial neural network using this transistor shows ∼84% image recognition accuracy for handwritten digits. The subject transistor also successfully mimics paired-pulse facilitation and depression, Hebbian spike-timing-dependent plasticity, and Pavlovian associative learning followed by extinction activities. Finally, dynamical pattern image memorization is demonstrated in a 5 × 5 array of these synaptic transistors. The results indicate that EBL patternable Nafion electrolytes have great potential for use in the fabrication and circuit-level integration of synaptic devices for neuromorphic computing applications.

Center-surround antagonism, a key mechanism in the retina, contributes to the encoding of edge contrast rather than of the overall information on a visual image. Here, a neuromorphic system consisting of multiple ionic devices is built, where each device has a lithium cobalt oxide channel arranged on a common lithium phosphorus oxynitride electrolyte. Because of the migration of Li ions between the channels through the electrolyte, the devices are highly interactive, as is seen with retinal neurons. On the basis of the excitation of single devices and device-to-device inhibition, the system successfully emulates the antagonistic center-surround receptive field and the Mach band effect in which perceived contrast is enhanced at the edges between dark and bright regions. Furthermore, a two-dimensional array system is simulated to implement edge detection for real images. This scheme enables computer vision tasks with simple and effective operations, owing to the intrinsic properties of the materials employed.

The effects of plasma reactants on the plasma-assisted atomic layer deposition (ALD) of lithium phosphate are investigated in relation to the fabrication of high-quality lithium phosphorus oxynitride (LiPON) thin films for potential use as a solid-state electrolyte (SSE) in both microbatteries and neuromorphic devices. Our ALD processes enable the incorporation of nitrogen into a lithium phosphate matrix, using lithium tert-butoxide and tris(dimethylamino)phosphine as the lithium and phosphorus precursors, respectively, in a deposition temperature window of 220–300 °C. With O2plasma, polycrystalline lithium phosphate films, with a relatively well-arranged pyrophosphate, are deposited. Amorphous LiPON films, with a mixture of pyrophosphates and orthophosphates, are obtained when Ar or NH3plasma is used. When the NH3flow rate increases, the nitrogen composition increases up to ∼13%, while residual carbon is kept below a few percent. For a Li2.5PO1.9N0.8film deposited at 300 °C with NH3plasma, the ionic conductivity is measured as 1.65 ± 0.42 × 10–6S/cm at 25 °C, with an activation energy of 0.66 eV. This conductivity is the highest value of any ALD LiPON film reported to date. Our ALD processes exhibit a high level of controllability of the molecular structures of the phosphorus oxynitride matrix with high ionic conductivity, which makes them suitable for realizing high-performance Li SSE thin films.

Atomic layer deposition (ALD) was used to fabricate magnesium phosphate thin films as a magnesium-ion conducting solid electrolyte. The deposition was carried out at lower deposition temperatures, ranging from 125 to 300 °C. The film exhibits an amorphous nature and excellent step coverage, even in narrow trenches with a higher aspect ratio. The growth rate and the ionic conductivity were found to increase with a decrease of the deposition temperature. This can be explained by increased disordering of the phosphate matrix, giving rise to enhanced hopping conduction of magnesium ions. The film deposited at 125 °C showed an ionic conductivity of 1.6 × 10–7S cm–1at an ambient temperature of 500 °C, with an activation energy of 1.37 eV. These values are better than those of the film deposited at 250 °C as well as of a sputtered film. Our results indicate that ALD has great potential for the fabrication of magnesium-based solid electrolyte films.

Here we report the synthesis of new mesoporous crystalline fullerene (C70) cubes (MCFCs) at a liquid–liquid interface between tert-butyl alcohol (TBA) and a solution of C70in mesitylene under mild heat treatment. The highly crystalline pore walls of the MCFCs result in enhanced electrochemical and photoluminescence properties, which differ from pristine C70and crystalline C70cubes lacking mesopores.

Solid state ionics is a field that elucidates and applies the phenomena of ion transport in solids, in contrast to electronics which elucidates and applies the phenomena to electron transport. In recent years, unique functional solid state ionics devices have been developed for information and communication equipment, using not only solid electrolyte but also mixed conductors that transport ions and electrons in solids, and the fused field of solid state ionics devices and electronics devices is gaining importance. Herein we provide an overview of solid state ionics and various solid state ionics devices that have been developed to date, with emphasis on recently introduced solid state ionics devices made of solid electrolytes and mixed conductors. Such devices are expected to be indispensable for the future development of information and communication equipment, especially in relation to artificial intelligence, where electronic devices have been predominant.

We present the surface characterization and the local electronic properties of archetypical p-type perylene-based semiconductor organic molecule of Perylene Tetra Carboxylic Dianhydride (PTCDA) thermally evaporated on a transparent conducting metal oxide surface. A modified indium tin oxide (ITO) surface was successfully obtained by employing a subsequent chemical and physical treatment. Physisorbed PTCDA molecules exhibited a stacked-grain structure covering completely ITO surface. Scanning tunneling spectroscopy (STS) spectra of physisorbed PTCDA molecules were performed. In contrast to the previous studies of the homolog n-type perylene derivative thin films, here we successfully extracted both of the outmost frontier energy levels by measuring the current-voltage characteristics of PTCDA molecules in an estimated tunneling resistance from 4.17 to 100 GΩ at room temperature. Using numerical derivative of the I-V spectra, we extracted the series of transport gap of PTCDA molecule are lies in the region of 4.70-4.87 eV.

In situand nonvolatile tuning of photoluminescence (PL) has been achieved based on graphene oxide (GO), the PL of which is receiving much attention because of various potential applications of the oxide (e.g., display, lighting, and nano-biosensor). The technique is based on in situand nonvolatile tuning of the sp2domain fraction to the sp3domain fraction (sp2/sp3fraction) in GO through an electrochemical redox reaction achieved by solid electrolyte thin films. The all-solid-state variable PL device was fabricated by GO and proton-conducting mesoporous SiO2thin films, which showed an extremely low PL background. The device successfully tuned the PL peak wavelength in a very wide range from 393 to 712 nm, covering that for chemically tuned GO, by adjusting the applied DC voltage within several hundred seconds. We also demonstrate the sp2/sp3fraction tuning using a conductive atomic force microscope. The device achieved not only writing, but also erasing of the sp2/sp3-fraction-tuned nanodomain (both directions operation). The combination of these techniques is applicable to a wide range of nano-optoelectronic devices including nonvolatile PL memory devices and on-demand rewritable biosensors that can be integrated into nano- and microtips which are transparent, ultrathin, flexible, and inexpensive.

We conducted light irradiation experiments in air to clarify influence of atmosphere on the operation of a photo-assisted atomic switch. In air, Pt-Ag2S/Ag nanogap electrodes with a PTCDI thin layer in their nanogaps showed current fluctuations with an applied bias of from 1.5 V to 6 V regardless of the bias polarity and with or without light irradiation. This is in contrast to the fact that only two things cause an increase in current that result in the formation of a silver bridge and switching behavior under vacuum, namely, light irradiation and the application of positive bias to the Ag2S/Ag electrode [. In addition, photocurrent caused by irradiating a PTCDI thin layer was found to be sensitive to air and to N2. These results indicate that moisture or other gas molecules in air and in N2 have an influence on the photo-assisted atomic switching behavior.

A redox-based three-terminal device was fabricated using Ta2O5 as the ionic transfer material, and its operation was investigated. We found that application of a negative polarity gate bias, which increases oxygen anions in the channel regions, can make a conductive path between a source electrode and a drain electrode. The insulating state of the pristine device is turned on to a semiconductor state by the application of a negative polarity gate bias. Since turning off to the insulating state could not be achieved, the switching process resembles the soft breakdown of the first turning-on process of oxygen vacancy controlled resistive random access memories, although the polarity of the bias is opposite to that used in the first turning-on process. Further application of a gate bias causes a transition from the semiconductor state to a metal state. Accordingly, there are two types of on-state. It is possible to switch between the semiconductor and metal states.

Resistive switching memories (ReRAMs) are the major candidates for replacing the state-of-the-art memory technology in future nanoelectronics. These nonvolatile memory cells are based on nanoionic redox processes and offer prospects for high scalability, ultrafast write and read access, and low power consumption. The interfacial electrochemical reactions of oxidation and reduction of ions necessarily needed for resistive switching result inevitably in nonequilibrium states, which play a fundamental role in the processes involved during device operation. We report on nonequilibrium states in SiO2-based ReRAMs being induced during the resistance transition. It is demonstrated that the formation of metallic cations proceeds in parallel to reduction of moisture, supplied by the ambient. The latter results in the formation of an electromotive force in the range of up to 600 mV. The outcome of the study highlights the hitherto overlooked necessity of a counter charge/reaction to keep the charge electroneutrality in cation-transporting thin films, making it hard to analyze and compare experimental results under different ambient conditions such as water partial pressure. Together with the dependence of the electromotive force on the ambient, these results contribute to the microscopic understanding of the resistive switching phenomena in cation-based ReRAMs.

A potential route to extend Moore’s law beyond the physical limits of existing materials and device architectures is to achieve nanotechnology breakthroughs in materials and device concepts. Here, we discuss an on-demand WO3–x-based nanoionic device where electrical and neuromorphic multifunctions are realized through externally induced local migration of oxygen ions. The device is found to possess a wide range of time scales of memorization, resistance switching, and rectification varying from volatile to permanent in a single device, and these can furthermore be realizable in both two- or three-terminal systems. The gradually changing volatile and nonvolatile resistance states are experimentally demonstrated to mimic the human brain’s forgetting process for short-term memory and long-term memory.We propose this nanoionic device with its on-demand electrical and neuromorphic multifunction has a unique paradigm shifting potential for the fabrication of configurable circuits, analog memories, digital–neural fused networks, and more in one device architecture.

It is demonstrated that a Cu2S gap‐type atomic switch, referred to as a Cu2S inorganic synapse, emulates the synaptic plasticity underlying the sensory, short‐term, and long‐term memory formations in the human brain. The change in conductance of the Cu2S inorganic synapse is considered analogous to the change in strength of a biological synaptic connection known as the synaptic plasticity. The plasticity of the Cu2S inorganic synapse is controlled depending on the interval, amplitude, and width of an input voltage pulse stimulation. Interestingly, the plasticity is influenced by the presence of air or moisture. Time‐dependent scanning tunneling microscopy images of the Cu‐protrusions grown in air and in vacuum provide clear evidence of the influence of air on their stability. Furthermore, the plasticity depends on temperature, such that a long‐term memory is achieved much faster at elevated temperatures with shorter or fewer number of input pulses, indicating a close analogy with a biological synapse where elevated temperature increases the degree of synaptic transmission. The ability to control the plasticity of the Cu2S inorganic synapse justifies its potential as an advanced synthetic synapse with air/temperature sensibility for the development of artificial neural networks.

Resistive switching memories based on the formation and dissolution of a metal filament in a simple metal/oxide/metal structure are attractive because of their potential high scalability, low‐power consumption, and ease of operation. From the standpoint of the operation mechanism, these types of memory devices are referred to as gapless‐type atomic switches or electrochemical metallization cells. It is well known that oxide materials can absorb moisture from the ambient air, which causes shifts in the characteristics of metal‐oxide‐semiconductor devices. However, the role of ambient moisture on the operation of oxide‐based atomic switches has not yet been clarified. In this work, current–voltage measurements were performed as a function of ambient water vapor pressure and temperature to reveal the effect of moisture on the switching behavior of Cu/oxide/Pt atomic switches using different oxide materials. The main findings are: i) the ionization of Cu at the anode interface is likely to be attributed to chemical oxidation via residual water in the oxide layer, ii) Cu ions migrate along grain boundaries in the oxide layer, where a hydrogen‐bond network might be formed by moisture absorption, and iii) the stability of residual water has an impact on the ionization and migration processes and plays a major role in determining the operation voltages. These findings will be important in the microscopic understanding of the switching behavior of oxide‐based atomic switches and electrochemical metallization cells.

Studies on a resistive switching memory based on a silver‐ion‐conductive solid polymer electrolyte (SPE) are reported. Simple Ag/SPE/Pt structures containing polyethylene oxide–silver perchlorate complexes exhibit bipolar resistive switching under bias voltage sweeping. The switching behavior depends strongly on the silver perchlorate concentration. From the results of thermal, transport, and electrochemical measurements, it is concluded that the observed switching originates from formation and dissolution of a silver metal filament inside the SPE film caused by electrochemical reactions. This is the first report of an electrochemical “atomic switch” realized using an organic material. The devices also show ON/OFF resistance ratios greater than 105, programming speeds higher than 1 μs, and retention times longer than 1 week. These results suggest that SPE‐based electrochemical devices might be suitable for flexible switch and memory applications.

Sulfur vacancies formed on a MoS2surface have been predicted to have electronic states at the Fermi level, and to work as conductive atomic scale structures. We made sulfur vacancies on a MoS2surface by removing sulfur atoms using scanning tunneling microscopy (STM) induced field evaporation, and measured the current--voltage ($I/V$) characteristics of the vacancies. The $I/V$ curve measured at the vacancies showed a linear increase at a zero bias region, indicating the existence of electronic states at the Fermi level. On the other hand, the $I/V$ curve measured at a clean surface showed a gap of about 1 eV around the Fermi level, as was expected from the theoretical calculation. We also successfully carried out manipulation of Au nanoislands, which will be used as nanopads to be connected to a sulfur vacancy chain.

By injecting low-energy minority carriers from the tip of a scanning tunneling microscope (STM) and analyzing the light emitted from the tip-sample gap of the STM, it is possible to study the optical and electronic properties of individual semiconductor nanostructures with an extremely high spatial resolution close to the atomic scale. This technique has been applied to investigate the transport properties of hot electrons injected into AlGaAs/GaAs quantum well structures and the optical properties of single self-assembled InAs/AlGaAs quantum dots. The physical principles, usefulness and future expectations of this novel technique are discussed.

We have developed a system for measuring the complex refractive index of liquid- and solid-phase chalcogenide around their melting points. The system consists of a spectroscopic ellipsometer, an infrared heating system, and prism optics. As a container for the chalcogenide, we use a customized quartz cell, evacuated to several pascal level to avoid sample degradation. We adopted a measurement configuration that uses access from the bottom side, because a mirror-like surface which is necessary for optical measurement was naturally and easily created at the container bottom by gravity. We succeeded in observing the remarkable difference on the indices between liquid- and solid-phase Sb2Te3.

The oxidation state of the phenol-substituted porphyrin TDtBHPP is coupled with its structure so that its 2-electron oxidation leads to a coplanarization of the molecule and a substantial stabilization of its surface self-assembled structures adsorbed at metal substrates.

The rate limiting process in first resistive switching, called the "forming process", is determined by measuring the switching time of an as-fabricated Cu/Ta2O5/Pt atomic switch as a function of the ambient temperature and the Ta2O5 thickness. The temperature dependence is well fitted by the Arrhenius equation, suggesting that a certain activation process dominates the switching phenomenon. The switching time increases linearly as the Ta2O5 thickness increases. The results herein clearly suggest that the rate limiting process is the drift of Cu cations in the Ta2O5 layer. We determine that the activation energy is 0.4 eV.

Highly-controlled conductance quantization is achieved near a single-atom point contact in a redox-based atomic switch device, in which a poly(ethylene oxide) (PEO) film is sandwiched between Ag and Pt electrodes. Current-voltage measurements revealed reproducible quantized conductance of [?]1G 0 for more than 102 continuous voltage sweep cycles under a specific condition, indicating the formation of a well-defined single-atom point contact of Ag in the PEO matrix. The device exhibited a conductance state distribution centered at 1G 0, with distinct half-integer multiples of G 0 and small fractional variations. First-principles density functional theory simulations showed that the experimental observations could be explained by the existence of a tunneling gap and the structural rearrangement of an atomic point contact.

We proposed and fabricated a current-driven phase-change optical gate switch using a Ge2Sb2Te5(GST225) thin film, an indium-tin-oxide (ITO) heater, and a Si waveguide. Microfabrication technology compatible with CMOS fabrication was used for the fabrication of the Si waveguide. The repetitive phase changing of GST225 was obtained by injecting a current pulse into the ITO heater beneath the GST225 thin film. The switching operation was observed by injecting a 100-ns current pulse of 20 mA into the ITO heater. The average extinction ratio over the wavelength range of 1,525 to 1,625 nm was 1.2 dB.

The resistive switching behavior of Cu/Ta2O5/Pt atomic switches, in which the Ta2O5 film was deposited by electron-beam (EB) evaporation and radio-frequency sputtering (SP), was investigated under different relative humidity (RH) levels. Fourier-transformed infrared spectroscopy and X-ray photoelectron spectroscopy measurements revealed that both films possess the oxygen-rich composition and higher water absorption capability of EB films. The Cu/Ta2O5-SP/Pt cell showed a stable, nonvolatile switching behavior in the observed RH range, whereas the Cu/Ta2O5-EB/Pt cell exhibited a similar behavior up to 50% RH, but altered from nonvolatile to volatile switching at higher RH levels. The observed volatile switching behavior of the Cu/Ta2O5-EB/Pt cell can be explained by increased ion migration, assisted by absorbed water and/or proton conduction in hydrated environments. The results indicate that the water uptake ability of the matrix film plays a crucial role in determining the resistive switching behavior of oxide-based atomic switches.

Redox reactions at the Cu/Ta2O5 interface and subsequent Cu ion transport in a Ta2O5 film have been investigated by means of cyclic voltammetry (CV) measurements. Under positive bias to the Cu electrode, Cu is preferentially oxidized to Cu2+ and then to Cu+. Subsequent negative bias causes a reduction of the oxidized Cu ions at the interface. It was found that CV curves change drastically with varied relative humidity levels from 5 to 85%. At higher humidity levels, the ion concentrations and diffusion coefficients, estimated from the CV curves, suggest increased redox reaction rates and a significant contribution of proton conduction to the ionic transport. The results indicate that the redox reactions of moisture are rate-limiting and highlight the importance of water uptake by the matrix oxide film in understanding and controlling the resistive switching behavior of oxide-based atomic switches.

We directly observed anodic dissolution and subsequent filament growth behavior in a planar atomic switch structure with Ag salt incorporated polyethylene oxide (Ag-PEO) film using in situ optical microscopy and ex situ scanning electron microscopy. The high ionic conductivities of Ag-PEO films enable the investigation of filament formation under voltage bias, even in micrometer-scaled devices. It was found that the filament formation changes from unidirectional growth to dendritic growth, depending on its distance from the grounded electrode. Based on this understanding of filament growth dynamics in planar devices, highly stable resistive switching was achieved in an Ag/Ag-PEO/Pt stacked device with an Ag-PEO film thickness of 100 nm. The device showed repeated switching operations for more than 102 sweep cycles, with a high ON/OFF resistance ratio of 105.

Representing a synapse, which is the basic unit of neural communication in the human brain, by a single device is a challenging task. On page 3606, Alpana Nayak and co‐workers represent a synapse by a single solid‐state electrochemical nanodevice called a Cu2S gap‐type atomic switch. The switch exhibits the sensory, short‐ and long‐term memories and shows the potential for perceiving two important environmental factors, namely air and temperature.

Removal of a sulfur atom from the topmost layer of a MoS2surface forms electronic states in the band-gap of an inherently semiconducting material. Scanning tunneling spectroscopy measured at sulfur vacancies, which were made by sulfur atom removal using the high electrical field of a scanning tunneling microscope, shows stepwise increases in the current in a band-gap region, corresponding to the formation of electronic states. The periphery of sulfur vacancies also show linear current--voltage ($I/V$) characteristics, suggesting that electronic states in the periphery are modified due to the removal of sulfur atoms.

We demonstrate a conceptually new atom transistor operation by electric-field control of the nanoionic state. The new atom transistor possesses novel characteristics, such as dual functionality of selective volatile and nonvolatile operations, very small power consumption (pW), and a high ON/OFF ratio [106(volatile operation) to 108(nonvolatile operation)], in addition to complementary metal oxide semiconductor (CMOS) process compatibility enabling the development of future computing systems that fully utilize highly-integrated CMOS technology. Cyclic endurance of 104times switching was achieved with the prototype.

By injecting low-energy minority carriers from the tip of a scanning tunneling microscope (STM) and analyzing the light emitted from the tip-sample gap of the STM, it is possible to study the optical and electronic properties of individual semiconductor nanostructures with an extremely high spatial resolution close to the atomic scale. This technique has been applied to investigate the transport properties of hot electrons injected into AlGaAs/GaAs quantum well structures and the optical properties of single self-assembled InAs/AlGaAs quantum dots. The physical principles, usefulness and future expectations of this novel technique are discussed.