An atomic switch is a nanoionic device that controls the diffusion of metal ions/atoms and their reduction/oxidation processes in the switching operation to form/annihilate a conductive path. Since ...metal atoms can provide a highly conductive channel even if their cluster size is in the nanometer scale, atomic switches may enable downscaling to smaller than the 11 nm technology node, which is a great challenge for semiconductor devices. Atomic switches also possess novel characteristics, such as high on/off ratios, very low power consumption and non‐volatility. The unique operating mechanisms of these devices have enabled the development of various types of atomic switch, such as gap‐type and gapless‐type two‐terminal atomic switches and three‐terminal atomic switches. Novel functions, such as selective volatile/nonvolatile, synaptic, memristive, and photo‐assisted operations have been demonstrated. Such atomic switch characteristics can not only improve the performance of present‐day electronic systems, but also enable development of new types of electronic systems, such as beyond von‐ Neumann computers.
An atomic switch is a nanoionic‐device that controls the diffusion of metal ions/atoms and their reduction/oxidation processes in the switching operation to form/annihilate a conductive path. We review various atomic switches, such as the gap‐type and the gapless‐type two‐terminal atomic switches and three‐terminal atomic switches, as well as the novel functions of these devices, such as selective volatile/nonvolatile, synaptic, memristive, and photo‐assisted operations.
Quantized conductance was observed in a cation-migration-based resistive switching memory cell with a simple metal-insulator-metal (MIM) structure using a thin Ta(2)O(5) layer. The observed ...conductance changes are attributed to the formation and dissolution of a metal filament with an atomic point contact of different integer multiples in the Ta(2)O(5) layer. The results demonstrate that atomic point contacts can be realized in an oxide-based MIM structure that functions as a nanogap-based atomic switch (Terabe et al 2005 Nature 433 47). By applying consecutive voltage pulses at periodic intervals of different times, we also observed an effect analogous to the long-term potentiation of biological synapses, which shows that the oxide-based atomic switch has potential for use as an essential building block of neural computing systems.
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 1-4). 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 Ag(2)S 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 Ag(2)S 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 Ag(2)S element indicates a breakthrough in mimicking synaptic behaviour essential for the further creation of artificial neural systems that emulate characteristics of human memory.
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.
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.
The role of moisture in the switching characteristics of oxide‐based, gapless‐type atomic switches is clarified. A certain amount of residual water exists in the oxide layer, forming a hydrogen‐bond network at grain boundaries. Its stability has an impact on the ionization of Cu at the anode interfaces and the migration of Cu ions in the oxide layer, which plays a major role in determining the operation – important findings for the microscopic understanding of the switching behavior of oxide‐based atomic switches.
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.
Redox reactions at the Ag,Cu/Ta2O5 interfaces and the impacts of Ta2O5 film density on the electroforming process of Ag,Cu/Ta2O5/Pt structures are clarified. When the density of the Ta2O5 film is decreased, the redox current is enhanced and the forming voltage is reduced. This finding contributes to a detailed understanding and control of the resistive switching behavior of oxide‐based atomic switches.
Controlling movements of electrons and holes is the key task in developing today's highly sophisticated information society. As transistors reach their physical limits, the semiconductor industry is ...seeking the next alternative to sustain its economy and to unfold a new era of human civilization. In this context, a completely new information token, i.e., ions instead of electrons, is promising. The current trend in solid‐state nanoionics for applications in energy storage, sensing, and brain‐type information processing, requires the ability to control the properties of matter at the ultimate atomic scale. Here, a conceptually novel nanoarchitectonic strategy is proposed for controlling the number of dopant atoms in a solid electrolyte to obtain discrete electrical properties. Using α‐Ag2+δS nanodots with a finite number of nonstoichiometry excess dopants as a model system, a theory matched with experiments is presented that reveals the role of physical parameters, namely, the separation between electrochemical energy levels and the cohesive energy, underlying atomic‐scale manipulation of dopants in nanodots. This strategy can be applied to different nanoscale materials as their properties strongly depend on the number of doping atoms/ions, and has the potential to create a new paradigm based on controlled single atom/ion transfer.
A novel nanoarchitectonic strategy for controlling the number of dopant atoms in a solid electrolyte to enable discrete electrical properties at the atomic scale is proposed. Theoretical and experimental studies on α‐Ag2+δS nanodots reveal the role of electrochemical energy levels and cohesive energy for single ion/atom transfer processes desired in nanoionic devices for the next technological frontier.
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 RbAg(4)I(5) 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.
