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Solid‐state batteries (SSBs) that use solid electrolytes instead of flammable liquid electrolytes have the potential to generate higher specific capacity and offer better safety. Magnesium (Mg) based SSBs with Mg metal anodes are considered to be one of the most promising energy storage candidates, because it gives high theoretical volumetric capacities of 3830 mAh cm−3. Here, we demonstrate an atomic layer deposition (ALD) process with a double nitrogen plasma process that successfully produces nitrogen‐incorporated magnesium phosphorus oxynitride (MgPON) solid‐state electrolyte (SSE) thin films at a low deposition temperature of 125 °C. The ALD MgPON SSEs exhibit an ionic conductivity of 0.36 and 1.2 μS cm−1 at 450 and 500 °C, respectively. The proposed ALD strategy shows the ability of conformal deposition nitrogen‐doped SSEs on pattered substrates and is attractive for using nitride ion‐conducing films as protective or wetting interlayers in solid‐state Mg and Li batteries. [ABSTRACT FROM AUTHOR]

The gap-type atomic switch is a novel neuromorphic device that possesses functions such as analog changes in resistance and short-term/long-term memory-based learning. However, it is difficult to integrate conventional gap-type atomic switches that use a vacuum gap and Ag2+δS, which has restricted their practical use. In this study, we developed a new, easy to fabricate gap-type atomic switch that incorporates a molecular layer as a gap and Ta2O5 as an ionic transfer material. This molecular gap-type atomic switch operates in a manner that is similar to conventional vacuum gap-type atomic switches. We also demonstrate stochastic operations using the aforementioned molecular gap-type atomic switches. These results indicate a higher potential for the practical use of gap-type atomic switches. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A novel triptycene-based azo polymer (TBAP) was explored as a switching material in an atomic switch showing resistive change under voltage sweep and pulse. Current atomic force microscopy (C-AFM) measurements on a TBAP film on a Ag electrode revealed that a TBAP atomic switch shows volatile and nonvolatile switching, depending on the amplitude of the bias voltage and sweep cycles, in which low-resistance states are attributed to quantized conductance with integer multiples of a single atomic point contact in a Ag filament formed between a C-AFM tip and the Ag electrode. The switch also exhibited a longer retention time with increased conductance states. Switching time from high-resistance to low-resistance states was found to decrease exponentially with an increase in the amplitude of voltage pulses, implying that the nucleation of metal atoms is likely to be the rate-limiting process. Repeated voltage sweeping with varying interval times resulted in a transition from volatile to nonvolatile switching behaviours, mimicking the learning process of the human brain. These results indicate that the TBAP atomic switch has great potential for organic neuromorphic electronics. [ABSTRACT FROM AUTHOR]

To realize the continuous development of information and communication equipment, it is also important to develop devices with new operating principles that overcome the functional and performance limitations of conventional electronics devices, such as transistors made of semiconductor materials. One group of devices that hold this promise is nanoionics devices that operate by using local ion transport and electrochemical reaction in solids. A number of nanoionics devices that control nanoelectrochemical phenomena through the use of ionic nanoarchitectonics have been created. Here the use of ionic nanoarchitectonics to locally control ion transport and electrochemical reactions through the use of ion conductors or mixed conductors as device materials is described. Further, the structures of nanoionics devices created by using the nanoarchitectonics, as well as their various electrical, magnetic and optical functionalities are introduced. It is expected that such nanoionics devices will complement conventional electronics devices, and make possible the further development of information and communication technologies. [ABSTRACT FROM AUTHOR]

\nImpact of StatementAs miniaturization of semiconductor memory devices is reaching its physical and technological limits, there is a demand for memory technologies that operate on new principles. Atomic switches are nanoionic devices that show repeatable resistive switching between high-resistance and low-resistance states under bias voltage applications, based on the transport of metal ions and redox reactions in solids. Their essential structure consists of an ion conductor sandwiched between electrochemically active and inert electrodes. This review focuses on the resistive switching mechanism of atomic switches that utilize a solid polymer electrolyte (SPE) as the ion conductor. Owing to the superior properties of polymer materials such as mechanical flexibility, compatibility with various substrates, and low fabrication costs, SPE-based atomic switches are a promising candidate for the next-generation of volatile and nonvolatile memories. Herein, we describe their operating mechanisms and key factors for controlling the device performance with different polymer matrices. In particular, the effects of moisture absorption in the polymer matrix on the resistive switching behavior are addressed in detail. As potential applications, atomic switches with inkjet-printed SPE and quantum conductance behavior are described. SPE-based atomic switches also have great potential in use for neuromorphic devices. The development of these devices will be enhanced using nanoarchitectonics concepts, which integrates functional materials and devices.This article reviews a series of works starting with the author’s 2011 paper on solid polymer electrolyte-based atomic switches, and describes the current status and future prospects for this technology. [ABSTRACT FROM AUTHOR]

We investigated the effects of relative humidity (RH) in the ambient atmosphere on the resistive switching behavior of a polyethylene oxide (PEO)-based atomic switch. Current–voltage, cyclic voltammetry, and electrochemical impedance measurements were carried out on a Ag/Ag salt-included PEO/Pt device at various RH levels. It was found that the operation voltages decreased in magnitude as the RH levels increased. This behavior was explained by enhanced Ag oxidation rates at the anode, which are facilitated by counter reactions of absorbed water molecules at the cathode. Increased Ag+ ion conductivity also contributes to the decreased operation voltages at higher RH levels, which may originate from a lowering of water molecule interactions with ether groups in the polymer chains. The results show that water molecules play a crucial role in determining the switching behavior and operation voltages of polymer-based atomic switches. [ABSTRACT FROM AUTHOR]

Changes in the resistance of a Ag2+δS nano-island, in which non-stoichiometric δ-Ag atoms work as a dopant in an n-type semiconductor, was induced by electrochemical removal of the dopant δ-Ag atoms using conductive-atomic force microscopy. The removed Ag atoms grew a Ag nanowire on a nano-island, the height of which corresponded to the measured resistance. Conductance (1/resistance) linearly decreased as the height of a Ag nanowire increased, in accordance with the theory of semiconductor conductivity. The technique has the potential for the dynamic control of conductance in nanostructures post-fabrication. [ABSTRACT FROM AUTHOR]

We measured, in vacuum, the temperature dependence of the Forming time in a Ag/Ta2O5/Pt gapless-type atomic switch, the results of which clearly suggest that the rate limiting process is the diffusion of Ag+ cations in the Ta2O5 layer. It is known that water molecules adsorbed in a Ta2O5 matrix enhance the diffusion of Ag+ cations, and this study further shows that desorbing water molecules by annealing at 623 K, which is known to be the desorption temperature of water molecules from Ta2O5 matrixes, increases the diffusion barrier height from 0.38 to 1.1 eV. We found that annealing at much lower temperatures, e.g. 343 K, for a longer period of time can also desorb the said water molecules. Accordingly, we measured the Forming time in air, which resulted in a completely different temperature dependence. The rate limiting process of Forming changes from ionic diffusion to nucleation when the process is conducted in air. [ABSTRACT FROM AUTHOR]

We successfully fabricated a magnesium phosphate film as a solid-state electrolyte (SSE) using plasma-assisted atomic layer deposition (ALD). By developing a ternary process consisting of alternative Mg–O and P–O sub-cycles, very uniform amorphous films were formed at relatively low deposition temperatures ranging from 125 °C to 300 °C. It was found that lower temperature deposition induces multi-bonding states in the phosphate matrix, which forms a mixture of pyrophosphates and metaphosphates. This results in increased hopping conduction of Mg ions in the disordered matrix. The film deposited at 125 °C exhibited higher ionic conductivities with a smaller activation energy than a sputtered magnesium phosphate film, which indicates the usefulness of ALD in fabricating Mg ion-conducting SSE films. The ALD film fabricated also showed reproducible conductivity and structural stability up to 400 °C. [ABSTRACT FROM AUTHOR]

Doping impurity atoms into semiconductor materials changes the resistance of the material. Selecting the atomic species of a dopant and the precise control of the number of dopant atoms in a unit volume can control the resistance to a desired value. The number of dopant atoms is usually controlled when the materials are synthesized. It can also be controlled after synthesizing by injecting dopant atoms using an ion implantation technique. This physical method has now enabled atom by atom implantation at the desired position. Here, we propose an additional technique, based on the electrochemical potential of dopant atoms in a material. The technique enables the dynamic control of the number of dopant atoms through the application of bias to the material. We demonstrate the controlled removal of dopant atoms using Ag2+δS and Ag-doped Ta2O5 as model materials. The change in resistance accompanying the removal of dopant atoms is also observed. [ABSTRACT FROM AUTHOR]

We demonstrate thermally stable resistive switching of a solid polymer electrolyte (SPE)-based atomic switch device, in which a Ag salt incorporated polyvinyl alcohol (PVA) film is sandwiched between Ag and Pt electrodes. The observed switching is attributed to the formation and dissolution of a Ag filament in the PVA matrix. The device exhibits good endurance characteristics, with large ON/OFF ratios (＞105) in the temperature range between 25 and 70 °C. This temperature stability can be explained by competition between increased segmental mobility and water evaporation at higher temperatures. The results indicate that the thermal behavior of the polymer matrix plays a crucial role in determining the temperature window for stable operation of SPE-based atomic switches. [ABSTRACT FROM AUTHOR]

A detailed understanding of the conductance quantization and resistive switching phenomena in redox-based memories is crucial for realizing atomic-scale memory devices and for finding the adequate design principles on which they can be based. Here, the emergence of quantized conductance states and their correlation with resistive switching characteristics in polymer-based atomic switches are investigated using combinations of current-voltage measurements and first-principles density functional theory (DFT) simulations. Various conductance states, including integer and half-integer multiples of a single atomic point contact and fractional conductance variations, are observed in an Ag/polyethylene oxide/Pt device under sweeping of bias voltage. Moreover, highly controllable and reproducible quantized conductance behaviors by tuning the voltage sweep rate and the sweep voltage range, suggesting well-controlled formation of the atomic point contact, are demonstrated. The device also exhibits longer retention times for higher conductance states. The DFT simulations reveal the transmission eigenstate of geometrically optimized atomic point contact structures and the impact of the atomic configurations and structural stability on the conductance state, which also explains their resistive switching behaviors. The well-defined, multiple quantized conductance states observed in these polymer-based atomic switches show promise for the development of new multilevel memory devices. [ABSTRACT FROM AUTHOR]

The morphology and composition of tantalum oxide (Ta 2 O 5 ) thin films prepared by electron-beam (EB) evaporation and radio-frequency sputtering (SP) were investigated by grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), atomic force microscopy, Fourier transformed infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). GIXRD revealed an amorphous nature for both films, and XRR showed that the density of the Ta 2 O 5 -EB films was lower than that of the Ta 2 O 5 -SP films; both films have lower density than the bulk value. A larger amount of molecular water and peroxo species were detected for the Ta 2 O 5 -EB films by FTIR performed in ambient atmosphere. XPS analyses performed in vacuum confirmed the presence of hydroxyl groups, but no trace of chemisorbed molecular water was detected. In addition, a higher oxygen nonstoichiometry (higher O/Ta ratio) was found for the EB films. From these results, we conclude that the oxygen nonstoichiometry of the EB film accounted for its lower density and higher amount of absorbed molecular water. The results also suggest the importance of understanding the dependence of the structural and chemical properties of thin amorphous oxide films on the deposition process. [ABSTRACT FROM AUTHOR]

Cu and Ag redox reactions at the interfaces with Ta2O5 and the impact of Ta2O5 film density on the forming process of Cu,Ag/Ta2O5/Pt atomic switch structures are investigated. Cyclic voltammetry measurements revealed that under positive bias to the Cu (Ag) electrode, Cu is preferentially oxidized to Cu2+, while Ag is oxidized to Ag+ ions. Subsequent negative bias causes a reduction of oxidized Cu (Ag) ions at the interfaces. The diffusion coefficient of the Cu and Ag ions in the Ta2O5 film is estimated from the results from different bias voltage sweep rates. It is also found that the redox current is enhanced and the forming voltage of the Cu/Ta2O5/Pt cell is reduced when the density of the Ta2O5 film is decreased. This result indicates the importance of the structural properties of the matrix oxide film in understanding and controlling resistive switching behavior. [ABSTRACT FROM AUTHOR]

Current–voltage measurements demonstrated the effects of temperature on the resistive switching behaviour of a gapless-type atomic switch based on a silver-ion-conductive solid polymer electrolyte (SPE) consisting of a mixture of polyethylene oxide (PEO) and AgClO4. The operation voltages decreased in magnitude with increased ambient temperature. The reduction of the operation voltages can be explained by the increased conductivity of silver ions in the amorphous PEO–salt complex region of the SPE film. This situation is completely different from a cell with a pure PEO film, in which the increased crystallinity of the PEO film may hinder the ionic conductivity, although similar switching behaviour was observed. It was also found that cells based on SPE and PEO show different behaviours under air and vacuum conditions. This is probably associated with different water uptakes from the ambient surroundings by the SPE and PEO films. The results suggest the importance of the crystallinity and water uptake ability of the matrix polymer film on the resistive switching characteristics. [ABSTRACT FROM AUTHOR]

ReRAMs using oxygen vacancy drift in their resistive switching are promising candidates as next generation memory devices. One remaining issue is degradation of the on/off ratio down to 102 or less with an increased number of switching cycles. Such degradation is caused by a local hard breakdown in a set process due to a very high electric field formed just before the completion of a conductive filament formation. We found that introducing an ultra-thin SiO2 layer prevents the hard breakdown by dynamical moderation of the electric field formed in the TaO x matrix, resulting in repeated switching while retaining a higher on/off ratio of about 105. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A compact neuromorphic nanodevice with inherent learning and memory properties emulating those of biological synapses is the key to developing artificial neural networks rivaling their biological counterparts. Experimental results showed that memorization with a wide time scale from volatile to permanent can be achieved in a WO3−x-based nanoionics device and can be precisely and cumulatively controlled by adjusting the device’s resistance state and input pulse parameters such as the amplitude, interval, and number. This control is analogous to biological synaptic plasticity including short-term plasticity, long-term potentiation, transition from short-term memory to long-term memory, forgetting processes for short- and long-term memory, learning speed, and learning history. A compact WO3−x-based nanoionics device with a simple stacked layer structure should thus be a promising candidate for use as an inorganic synapse in artificial neural networks due to its striking resemblance to the biological synapse. [ABSTRACT FROM AUTHOR]

Nonvolatile three-terminal operation is demonstrated using a Pt/Ta2O5-x/Pt, Pt structure, by controlling oxygen vacancy drift to make/annihilate a conductive channel between a source and a drain. The as-fabricated device is in an off-state. Application of a positive gate bias moves oxygen vacancies in a Ta2O5-x layer towards a channel region, making the channel region conductive. The conductive channel remains even after unloading the gate bias. Application of a negative gate bias, which moves the oxygen vacancies back towards the gate electrode, is required to turn off the device. The device shows a high ON/OFF ratio of up to 106. [ABSTRACT FROM AUTHOR]

Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg4I5 superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems. [ABSTRACT FROM AUTHOR]

Memory is believed to occur in the human brain as a result of two types of synaptic plasticity: short-term plasticity (STP) and long-term potentiation (LTP; refs , , , ). In neuromorphic engineering, emulation of known neural behaviour has proven to be difficult to implement in software because of the highly complex interconnected nature of thought processes. Here we report the discovery of a Ag2S inorganic synapse, which emulates the synaptic functions of both STP and LTP characteristics through the use of input pulse repetition time. The structure known as an atomic switch, operating at critical voltages, stores information as STP with a spontaneous decay of conductance level in response to intermittent input stimuli, whereas frequent stimulation results in a transition to LTP. The Ag2S inorganic synapse has interesting characteristics with analogies to an individual biological synapse, and achieves dynamic memorization in a single device without the need of external preprogramming. A psychological model related to the process of memorizing and forgetting is also demonstrated using the inorganic synapses. Our Ag2S element indicates a breakthrough in mimicking synaptic behaviour essential for the further creation of artificial neural systems that emulate characteristics of human memory. [ABSTRACT FROM AUTHOR]

Atomic switches are nanoionic devices that control the diffusion of metal cations and their reduction/oxidation processes in the switching operation to form/annihilate a metal atomic bridge, which is a conductive path between two electrodes in the on-state. In contrast to conventional semiconductor devices, atomic switches can provide a highly conductive channel even if their size is of nanometer order. In addition to their small size and low on-resistance, their nonvolatility has enabled the development of new types of programmable devices, which may achieve all the required functions on a single chip. Three-terminal atomic switches have also been developed, in which the formation and annihilation of a metal atomic bridge between a source electrode and a drain electrode are controlled by a third (gate) electrode. Three-terminal atomic switches are expected to enhance the development of new types of logic circuits, such as nonvolatile logic. The recent development of atomic switches that use a metal oxide as the ionic conductive material has enabled the integration of atomic switches with complementary metal-oxide-semiconductor (CMOS) devices, which will facilitate the commercialization of atomic switches. The novel characteristics of atomic switches, such as their learning and photosensing abilities, are also introduced in the latter part of this review. [ABSTRACT FROM AUTHOR]