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ABSTRACTAs 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 integrate functional materials and devices.

Polyaniline-based atomic switches are material building blocks whose nanoscale structure and resultant neuromorphic character provide a new physical substrate for the development next-generation, nanoarchitectonic-enabled computing systems. Metal ion-doped devices consisting of a Ag/metal ion doped polyaniline/Pt sandwich structure were fabricated using an in situ wet process. The devices exhibited repeatable resistive switching between high (ON) and low (OFF) conductance states in both Ag+ and Cu2+ ion-doped devices. The threshold voltage for switching was>0.8 V and average ON/OFF conductance ratios (30 cycles for 3 samples) were 13 and 16 for Ag+ and Cu2+ devices, respectively. The ON state duration was determined by the decay to an OFF state after pulsed voltages of differing amplitude and frequency. The switching behaviour is analagous to short-term (STM) and long-term (LTM) memories of biological synapses. Memristive behaviour and evidence of quantized conductance were also observed and interpreted in terms of metal filament formation bridging the metal doped polymer layer. The successful realization of these properties within physical material systems indicate polyaniline frameworks as suitable neuromorphic substrates for in materia computing.

ABSTRACTPolyaniline-based atomic switches are material building blocks whose nanoscale structure and resultant neuromorphic character provide a new physical substrate for the development next-generation, nanoarchitectonic-enabled computing systems. Metal ion-doped devices consisting of a Ag/metal ion doped polyaniline/Pt sandwich structure were fabricated using an in situ wet process. The devices exhibited repeatable resistive switching between high (ON) and low (OFF) conductance states in both Ag+ and Cu2+ ion-doped devices. The threshold voltage for switching was>0.8 V and average ON/OFF conductance ratios (30 cycles for 3 samples) were 13 and 16 for Ag+ and Cu2+ devices, respectively. The ON state duration was determined by the decay to an OFF state after pulsed voltages of differing amplitude and frequency. The switching behaviour is analagous to short-term (STM) and long-term (LTM) memories of biological synapses. Memristive behaviour and evidence of quantized conductance were also observed and interpreted in terms of metal filament formation bridging the metal doped polymer layer. The successful realization of these properties within physical material systems indicate polyaniline frameworks as suitable neuromorphic substrates for in materia computing.

The atomic scale switch, which operates on the principle of solid-state ionics, is an ultrafine device that takes advantage of the fact that the properties of materials can be changed significantly by the transport and chemical reaction of a small number of ions in a solid. The switch (e.g. ‘atomic switch’) actually works by using an ion-conducting solid electrolyte or an ion-/electron-conducting mixed-conductor as the device material, and by applying an external voltage to control local ion transport and electrochemical reaction. With the application of an external voltage, a bridge is formed as a conductive filament in the solid electrolyte or the mixed conductor between electrodes. The atomic structure of the point contact in said filament can be reversibly changed by precise control of the applied voltage. By controlling the atomic structure of the point contact, interesting functions are obtained, such as fast on/off resistive switching, switching between each state of quantized conductance and neuromorphic properties. This atomic scale switch has the potential to overcome the functional and performance limitations of conventional integrated circuits because it can be used in conjunction with extant semiconductor devices.

For the successful implication of brain-inspired computing using neuromorphic hardware, it is important to understand the detailed mechanisms of the biological brain. Here, we compare the atomic switch device with the N-methyl-D-aspartate receptor-based (NMDAR) signaling pathway which is the key to cognition in the human brain. An Ag/Ag2S/Pt gapless-type atomic switch configuration is realized using an atomic force microscope. The current responses to single and multiple voltage pulses of varied amplitude, width, and interval are studied to identify and control the conductance levels. This enables us to distinguish the sensory, short-term, and long-term memories along with the passive forgetting behavior and multilevel memory storage properties. Drawing a one-to-one comparison of these aspects of the atomic switch with the learning in the NMDAR pathway, we argue that the atomic switch has the potential not only as an edge-intelligent device but also as the missing link between biology and technology. [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]

Unlike conventional synaptic transmission, biological field-mediated neural transmissions solely due to extracellular field effects are hardly emphasized in artificial synapse systems. Using an Ag 2 S gap-type atomic switch, we demonstrate the field-mediated transmission for low bias stimulations which is capable of generating measurable current due to pre-conditioning of the junction by a change in the electrochemical potential of ions prior to any precipitation. The observed current–voltage (I − V) characteristics for V < 100 mV are in good agreement with Simmon's tunneling model for asymmetric barrier (ϕ) in the range 0 < V < ϕ. For more than 100 consecutive voltage sweep cycles, an increase in hysteresis corresponding to an input energy of 10 − 1000 pJ was observed. The average barrier height was lowered from 1 eV to 0.125 eV and the effective mass of carrier electrons increased from 0.25 m e to 2.5 m e until the actual precipitation began. These results are of practical importance in developing artificial neural networks capable of mimicking field-mediated communications found in the cerebellum, heart, retina, and olfactory systems. [Display omitted] • Ag 2 S atomic switch exhibits field-mediated transmission as in biological ephapse. • Field-mediated communications are energy-efficient, faster with lower noise level. • Small current under subthreshold stimulation is due to tunneling across barrier. • Subthreshold voltages modulate electrochemical potential by redistribution of ions. [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]

In this article, we investigate dynamics of electrically driven abrupt and volatile resistive switching characteristics, which are called threshold switching (TS) characteristics, in insulator-to-metal transition NbOx switch, BTex-based ovonic threshold switch, and Ag-based atomic switch by adopting field-induced nucleation theory. To verify field-induced nucleation phenomena in the initial electroforming and electrically driven TS processes of the TS devices, we apply constant square voltage pulses with various voltage magnitudes and measure the delay time required for the electrical switching process. During both the forming and switching processes, it is observed that all the TS devices exhibit switching delay time exponential in 1/voltage with different nucleation barrier energies (W0). In addition, we find that the W0 for the first forming process appears to be larger than that for the subsequent switching process and the difference is observed to be greater if the formed device is crystallized and smaller if not. Moreover, we find that the effective barrier energy and the fastest speed that a device can have (τ0) are closely related to switching speed of the TS devices. Also, we reveal that the hold voltage characteristics are affected by the W0 and carrier concentration of the solid electrolyte. [ABSTRACT FROM AUTHOR]

Abstract Three-terminal structures have an advantage over two-terminal structures in logic applications and neuromorphic circuits, However, three-terminal operation based on Valence Change RAM still requires a larger gate bias to form/dissolve a conductive path between the source and the drain, especially for turning off. Here, reduction in gate bias and gate leakage current in nonvolatile operation of oxygen vacancy drift-controlled three-terminal ReRAM is demonstrated by W/Ti (gate)/TaO x (resistance switching layer)/Pt (source), Pt (drain) structure. Introduction of a Ti thin layer between W and TaO x layers prevents a conductive channel formation between gate and source/drain electrodes. Consequently, as-fabricated high resistance between gate and source/drain is kept, resulting in smaller gate leakage current. We also achieved interface engineering on a sidewall structure of Pt (source)/SiO 2 (insulator)/Pt (drain) multi-layer, reducing in an operating bias from 10 V to 4 V or less. Highlights • Oxygen vacancy drift controlled three-terminal ReRAM with a smaller gate bias (<4 V) • Oxygen vacancy drift controlled three-terminal ReRAM with a smaller gate leakage current (negligibly small) • An oxidized TiO x layer under bias prevents the complete formation of a conducting path between the gate and the source/drain. [ABSTRACT FROM AUTHOR]

We propose a simple model for an atomic switch-based decision maker (ASDM), and show that, as long as its total number of metal atoms is conserved when coupled with suitable operations, an atomic switch system provides a sophisticated ``decision-making'' capability that is known to be one of the most important intellectual abilities in human beings. We considered a popular decision-making problem studied in the context of reinforcement learning, the multi-armed bandit problem (MAB); the problem of finding, as accurately and quickly as possible, the most profitable option from a set of options that gives stochastic rewards. These decisions are made as dictated by each volume of precipitated metal atoms, which is moved in a manner similar to the fluctuations of a rigid body in a tug-of-war game. The ``tug-of-war (TOW) dynamics'' of the ASDM exhibits higher efficiency than conventional reinforcement-learning algorithms. We show analytical calculations that validate the statistical reasons for the ASDM to produce such high performance, despite its simplicity. Efficient MAB solvers are useful for many practical applications, because MAB abstracts a variety of decision-making problems in real-world situations where an efficient trial-and-error is required. The proposed scheme will open up a new direction in physics-based analog-computing paradigms, which will include such things as ``intelligent nanodevices'' based on self-judgment.

Abstract A theoretical model based on the single electron tunneling phenomenon is employed to calculate the time-dependent electrical resistance of an Ag–Ag 2 S–Pt atomic switch at different applied voltages. While a negative voltage is applied to Pt electrode, Ag atoms precipitate on the surface of Ag 2 S electrode where they form Ag clusters. The resistance of switch decreases as Ag clusters grow larger between two electrodes. Our model calculations imply the time required to decrease the resistance of switch below the resistance quantum (switching time) is mainly determined by the Coulomb blockade effect of Ag clusters. The switching time is found to decrease exponentially with increasing the applied voltage, which agrees very well with the experimental observations. Highlights • Ag nanowires are formed between Pt and Ag 2 S electrodes. • Coulomb blockade effect of Ag nanowires significantly affects the switching time. • Switching time of Ag–Ag 2 S–Pt switches decreases exponentially with applied voltages. [ABSTRACT FROM AUTHOR]

Atomic switches are well-known promising candidates for future application in non-volatile logic memory devices. The resistive switching characteristics of these devices depend on the formation of a conductive filament (CF) by active metal electrodes. However, the formation of a stable CF is still a challenge owing to filament overgrowth in the solid electrolyte. To achieve controlled CF growth, we have used a modified active electrode with different Cu x Se 1−x composition ratios (0.01 < x < 0.45) and a titanium (Ti) buffer layer. The optimum composition was determined to be Cu 0.11 Se 0.89 /Ti (2.5 nm) for which excellent resistive switching properties were observed, such as a high on/off ratio of 10 4 , low operating voltage, uniform resistance distribution, ten-year data retention at 85 °C, and uniform endurance (2000 cycles). The improvement is can be described from the high controllability of the oxidation-reduction reaction rate of the optimized Cu Se modified active electrode by the means of Ti buffer layer and the Ti O bond formation at the Ti/Al 2 O 3 interface. In addition, depth profiles of the conductive filament based on Cu 0.11 Se 0.89 with a Ti buffer layer were studied by performing current atomic force microscopy (I-AFM) to evaluate the enhancements in electrical performance and reliability resulting from the insertion of the Ti buffer layer. [ABSTRACT FROM AUTHOR]

Atomic switches are nanoionic devices that are operated by controlling redox reactions and the local migration of metal ions in solids. The essential mechanism is the growth and shrinkage of a metal filament formed between two electrodes, resulting in repeatable resistive switching between high-resistance and low-resistance states, which can be used for next-generation nonvolatile memories. This review focuses on the operating mechanism and resistive switching characteristics of two- and three-terminal atomic switches using a thin metal oxide layer as an ion-conducting matrix. First, we describe the operating mechanism of a two-terminal atomic switch based on nucleation theory and present the results of temperature dependence and switching speeds to determine the validity of our switching model. Then, we discuss the effects that moisture absorption in the oxide matrix has on the fundamental processes and switching behavior in order to elucidate the importance of the porosity of the oxide matrix. Finally, we demonstrate a three-terminal atomic switch and describe the impact of the anode material or metal-ion species. These findings will contribute to the development of next-generation logic circuits with low-voltage operation and low-power consumption. [ABSTRACT FROM AUTHOR]

We propose a simple model for an atomic switch-based decision maker (ASDM), and show that, as long as its total number of metal atoms is conserved when coupled with suitable operations, an atomic switch system provides a sophisticated "decision-making" capability that is known to be one of the most important intellectual abilities in human beings. We considered a popular decisionmaking problem studied in the context of reinforcement learning, the multi-armed bandit problem (MAB); the problem of finding, as accurately and quickly as possible, the most profitable option from a set of options that gives stochastic rewards. These decisions are made as dictated by each volume of precipitated metal atoms, which is moved in a manner similar to the fluctuations of a rigid body in a tug-of-war game. The "tug-of-war (TOW) dynamics" of the ASDM exhibits higher efficiency than conventional reinforcement-learning algorithms. We show analytical calculations that validate the statistical reasons for the ASDM to produce such high performance, despite its simplicity. Efficient MAB solvers are useful for many practical applications, because MAB abstracts a variety of decisionmaking problems in real-world situations where an efficient trial-and-error is required. The proposed scheme will open up a new direction in physics-based analog-computing paradigms, which will include such things as "intelligent nanodevices" based on self-judgment. [ABSTRACT FROM AUTHOR]

A silver sulfide (Ag2S) island as an ionic conductor in resistive switching memories was formed and a protrusion of silver from the Ag2S formed by an electrochemical reaction was observed using a scanning tunneling microscope.

Abstract: A hard X-ray photoelectron spectroscopy (HX-PES) has been employed in order to investigate electrochemical polarization at carbon/amorphous tantalum oxide thin film hetero-interface. In situ HX-PES observation of Ta 4f and O 1s spectra has revealed parallel chemical shift of the core levels within the redox window width of 2.4eV, indicating Fermi level shifts due to the redox state modulation mediated by protonation and deprotonation under applied dc bias voltage. In addition to protons, oxide ions are suggested to take part in the redox state variation especially under a larger applied electric field. The results have been discussed in framework of solid state electrochemistry and Pourbaix-type E-pO diagram to indicate electrochemical stability domains of solid oxide systems is proposed. [Copyright &y& Elsevier]

Polymer-based atomic switch memristors via the formation/dissolution of atomic-scale conductive filaments are considered as the leading candidate for next-generation nonvolatile memory. However, the instability of conductive filaments of incomplete bridge makes their switching performances unsatisfied. In this work, we report a flexible polymeric memristor using polyethylenimine incorporated with silver salt. The memristor device exhibited superior performances at room temperature with a favorable endurance, high ON/OFF ratio, good retention, and low operating voltage. These satisfactory performances are attributed to the pre-existing Ag ions in the polymer, guiding the formation of a robust Ag filament. In addition, the device shows stable bipolar switching behavior in bending conditions or after hundreds of bending cycles. In our work, we provide a simple and efficient method to construct robust filament-based memristors for flexible electronics.

In situ hard x-ray photoemission spectroscopy (HX-PES) and soft x-ray absorption spectroscopy (SX-XAS) have been employed to investigate a local redox reaction at the carbon/Gd-doped CeO2 (GDC) thin film heterointerface under applied dc bias. In HX-PES, Ce3d and O1s core levels show a parallel chemical shift as large as 3.2 eV, corresponding to the redox window where ionic conductivity is predominant. The window width is equal to the energy gap between donor and acceptor levels of the GDC electrolyte. The Ce M-edge SX-XAS spectra also show a considerable increase of Ce3+ satellite peak intensity, corresponding to electrochemical reduction by oxide ion migration. In addition to the reversible redox reaction, two distinct phenomena by the electrochemical transport of oxide ions are observed as an irreversible reduction of the entire oxide film by O2 evolution from the GDC film to the gas phase, as well as a vigorous precipitation of oxygen gas at the bottom electrode to lift off the GDC film. These in situ spectroscopic observations describe well the electrochemical polarization behavior of a metal/GDC/metal capacitor-like two-electrode cell at room temperature. [ABSTRACT FROM AUTHOR]

The impact of the electrochemical nucleation on the switching kinetics in many nanoscaled redox-based resistive switching memories is critically discussed. In the case of the atomic switch, the system is site invariant and the nucleation process is strictly localized below the STM tip. Using RbAgI solid electrolyte, nucleation was found to be rate limiting. The electrochemical metallization memory cells (gapless type atomic switch) operate at conditions closer to the conventional nucleation. They introduce additional difficulties for interpretation of the experimental results due to formation of hillocks, of surface oxide barrier films, induction of strain, and the influence of the high electric field. In valence change memories, the nucleation seems to be less important because of the higher applied voltages. The results are discussed in the context of the atomistic theory of electrochemical nucleation. We believe that re-analysis of the experimental data for many systems will reveal that the nucleation is limiting the switching time. [ABSTRACT FROM AUTHOR]

Abstract: The development of nanometer-scale devices operating under a new principle that could overcome the limitations of current semiconductor devices has attracted interest in recent years. We propose that nanoionic devices that operate by controlling the local transport of ions are promising in this regard. It is possible to control the local transport of ions using the solid electrochemical properties of ionic and electronic mixed conductors. As an example of this concept, here, we report a method of controlling the transport of silver ions of the mixed-conductor silver sulfide (Ag2S) crystal and basic research on nanoionic devices based on this mixed conductor. These devices show unique functions such as atom deposition, resistance switching, and quantum point contact switching. The switches operate through the formation and dissolution of an atomic bridge between the electrodes, and the behavior is realized by control of the local solid-state electrochemical reaction. Potential nanoionic devices utilizing the unique functions and characters that do not exist in conventional semiconductor devices are discussed. [Copyright &y& Elsevier]

Atomic switch-based selectors, which utilize the formation of conductive filaments by the migration of ions, are researched for cross-point array architecture due to their simple structure and high selectivity. However, the difficulty in controlling the formation of filaments causes uniformity and reliability issues. Here, a multilayer selector with Pt/Ag-doped ZnO/ZnO/Ag-doped ZnO/Pt structure by the sputtering process is presented. A multilayer structure enables control of the filament formation by preventing excessive influx of Ag ions. The multilayer selector device exhibits a high on-current density of 2 MA cm -2 , which can provide sufficient current for the operation with the memory device. Also, the device exhibits high selectivity of 10 10 and a low off-current of 10 -13 A. The threshold voltage of selector devices can be controlled by modulating the thickness of the ZnO layer. By connecting a multilayer selector device to a resistive switching memory, the leakage current of the memory device can be reduced. These results demonstrate that a multilayer structure can be used in a selector device to improve selectivity and reliability for use in high-density memory devices., (© 2021 Wiley-VCH GmbH.)

We investigated chemical vapor-deposited (CVD) two-dimensional (2D) niobium diselenide (NbSe 2 ) material for the resistive switching and synaptic characteristics. Three different atomic switch devices with Ag/HfO 2 /Pt, Ag/Ti/HfO 2 /Pt, and Ag/NbSe 2 /HfO 2 /Pt were studied as both memory and neuromorphic devices. Both the inserted Ti and NbSe 2 buffer layers effectively control the stochastic Ag-ion diffusion, leading to suppressed variation of switching characteristics, which is a critical issue in an atomic switch device. Especially, the device with the 2D NbSe 2 buffer layer strikingly enhanced the device reliability in both endurance and retention. In conjunction with scanning transmission electron microscopy (STEM) and energy-dispersive spectrometry (EDS) analysis of the control of the Ag-ion migration, it was understood that filament connection is interrelated with the SET and RESET processes. Besides resistive behaviors in the memory device, various synapse functions such as spike-rate-dependent plasticity (SRDP), forgetting curve, potentiation, and depression were demonstrated with an atomic switch with the 2D NbSe 2 buffer layer. Furthermore, the emulated long-term synaptic property was simulated using the MNIST 28 × 28 pixel database. Using adopting a CVD 2D NbSe 2 blocking layer, the stochastic Ag-ion diffusion behavior is well-controlled and therefore stable switching and synapse functions are attained.

The first report of a quantized conductance atomic threshold switch (QCATS) using an atomically-thin hexagonal boron nitride (hBN) layer is provided. This QCATS has applications in memory and logic devices. The QCATS device shows a stable and reproducible conductance quantization state at 1·G 0 by forming single-atom point contact through a monoatomic boron defect in an hBN layer. An atomistic switching mechanism in hBN-QCATS is confirmed by in situ visualization of mono-atomic conductive filaments. Atomic defects in hBN are the key factor that affects the switching characteristic. The hBN-QCATS has excellent switching characteristics such as low operation voltage of 0.3 V, low "off" current of 1 pA, fast switching of 50 ns, and high endurance > 10 7 cycles. The variability of switching characteristics, which are the major problems of switching device, can be solved by reducing the area and thickness of the switching region to form single-atom point contact. The switching layer thickness is scaled down to the single-atom (≈0.33 nm) h-BN layer, and the switching area is limited to single-atom defects. By implementing excellent switching characteristics using single-layer hBN, the possibility of implementing stable and uniform atomic-switching devices for future memory and logic applications is confirmed., (© 2021 Wiley-VCH GmbH.)

The atomic switches have attracted wide attention owing to their applications in nonvolatile electric devices. The atomic switch is operated by the formation and dissipation of a metallic filament inside a metal sulfide film, which is controlled by a solid electrochemical reaction. Although the metallic filament is considered to consist of metal atoms, the chemical species of the metallic filament are difficult to be identified due to challenges in observing the metallic filament inside the solid. In this study, we report the investigation on the metallic filament in the atomic switch with metal sulfide based on point-contact spectroscopy (PCS). By cooling the atomic switch, the switch voltage increased to 1 V, which allowed for the PCS measurement. The PCS revealed that the metallic filament was composed of Ag atoms in the case of the Pt/Ag 2 S/Ag atomic switch. We applied this technique to the Pt/Cu 2 S/Ag and Pt/Ag 2 S/Cu atomic switches to uncover the formation process of the metallic filament. In both atomic switches, the chemical species of the metallic filament were Ag. The metal atoms were supplied from both the metal electrode and the sulfide layer.

A silver sulfide (Ag 2 S) island as an ionic conductor in resistive switching memories was formed and a protrusion of silver from the Ag 2 S formed by an electrochemical reaction was observed using a scanning tunneling microscope. [ABSTRACT FROM AUTHOR]

The next-generation electronic society is dependent on the performance of nonvolatile memory devices, which has been continuously improving. In the last few years, many memory devices have been introduced. However, atomic switches are considered to be a simple and reliable basis for next-generation nonvolatile devices. In general, atomic switch-based resistive switching is controlled by electrochemical metallization. However, excess ion injection from the entire area of the active electrode into the switching layer causes device nonuniformity and degradation of reliability. Here, we propose the fabrication of a high-performance atomic switch based on Cu x -Se 1- x by inserting lanthanide (Ln) metal buffer layers such as neodymium (Nd), samarium (Sm), dysprosium (Dy), or lutetium (Lu) between the active metal layer and the electrolyte. Current-atomic force microscopy results confirm that Cu ions penetrate through the Ln-buffer layer and form thin conductive filaments inside the switching layer. Compared with the Pt/Cu x -Se 1- x /Al 2 O 3 /Pt device, the optimized Pt/Cu x -Se 1- x /Ln/Al 2 O 3 /Pt devices show improvement in the on/off resistance ratio (10 2 -10 7 ), retention (10 years/85 °C), endurance (∼10 000 cycles), and uniform resistance state distribution.

In this study, we demonstrate a high-performance solid polymer electrolyte (SPE) atomic switching device with low SET/RESET voltages (0.25 and -0.5 V, respectively), high on/off-current ratio (10 5 ), excellent cyclic endurance (>10 3 ), and long retention time (>10 4 s), where poly-4-vinylphenol (PVP)/poly(melamine-co-formaldehyde) (PMF) is used as an SPE layer. To accomplish these excellent device performance parameters, we reduce the off-current level of the PVP/PMF atomic switching device by improving the electrical insulating property of the PVP/PMF electrolyte through adjustment of the number of cross-linked chains. We then apply a titanium buffer layer to the PVP/PMF switching device for further improvement of bipolar switching behavior and device stability. In addition, we first implement SPE atomic switch-based logic AND and OR circuits with low operating voltages below 2 V by integrating 5 × 5 arrays of PVP/PMF switching devices on the flexible substrate. In particular, this low operating voltage of our logic circuits was much lower than that (>5 V) of the circuits configured by polymer resistive random access memory. This research successfully presents the feasibility of PVP/PMF atomic switches for flexible integrated circuits for next-generation electronic applications.

By means of scanning probe microscopy we demonstrate that Au(+) on NaCl films adsorbs in an embedded, slightly off-centered Cl-Cl bridge position and can be switched between two equivalent mirror-symmetric configurations using the attractive force exerted by a scanning probe tip. Density functional theory calculations demonstrate that the displacement of the Au atom from the centered position of the bridge configuration is accompanied by a large lifting of the closest Cl atom leading to significant changes in the local electrostatic field. Our findings suggest that Au(+) can be used to toggle the local electrostatic field.

We assemble bistable silicon quantum dots consisting of four buckled atoms (Si4-QD) using atom manipulation. We demonstrate two competing atom switching mechanisms, downward switching induced by tunneling current of scanning tunneling microscopy (STM) and opposite upward switching induced by atomic force of atomic force microscopy (AFM). Simultaneous application of competing current and force allows us to tune switching direction continuously. Assembly of the few-atom Si-QDs and controlling their states using versatile combined AFM/STM will contribute to further miniaturization of nanodevices.