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.

It is demonstrated that a Cu 2 S gap-type atomic switch, referred to as a Cu 2 S 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 Cu 2 S inorganic synapse is considered analogous to the change in strength of a biological synaptic connection known as the synaptic plasticity. The plasticity of the Cu 2 S inorganic synapse is controlled depending on the interval, amplitude, and width of an input voltage pulse stimulation. Interestingly, the plasticity is infl uenced 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 infl uence 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 Cu 2 S inorganic synapse justifi es its potential as an advanced synthetic synapse with air/temperature sensibility for the development of artifi cial neural networks.

The switching time of a Ag 2 S atomic switch, in which formation and annihilation of a Ag atomic bridge is controlled by a solid-electrochemical reaction in a nanogap between two electrodes, is investigated as a function of bias voltage and temperature. Increasing the bias voltage decreases the switching time exponentially, with a greater exponent for the lower range of bias than that for the higher range. Furthermore, the switching time shortens exponentially with raising temperature, following the Arrhenius relation with activation energy values of 0.58 and 1.32 eV for lower and higher bias ranges, respectively. These results indicate that there are two main processes which govern the rate of switching, first, the electrochemical reduction Ag þ þ e -fAg and, second, the diffusion of Ag þ ions. This investigation advances the fundamental understanding of the switching mechanism of the atomic switch, which is essential for its successful device application.SECTION Electron Transport, Optical and Electronic Devices, Hard Matter N anoionics-based resistive switching devices have been attracting much attention in recent years to overcome the physical and economical limitations of current semiconductor technology. 1 A lot of research has been aimed at finding a reliable switching mechanism that can permit ever smaller and more powerful electronics. 2 Recently, we have developed a conceptually new nanodevice called an atomic switch, in which formation and annihilation of a metal atomic bridge across a nanogap between a solid-electrolyte electrode and a counter metal electrode is controlled by a solid-electrochemical reaction. 3 The switching operation can be achieved by only changing the polarity of the bias voltage applied to either electrode. For instance, applying a positive bias voltage to the solid-electrolyte electrode, the metal ions in the electrode reduce to metal atoms, forming a conductive atomic bridge between the electrodes. This decreases the resistance between the two electrodes to a certain ON resistance, which means that the switch is turned ON. When the polarity of the applied voltage is reversed, the metal atoms in the conductive atomic bridge are oxidized and are incorporated back into the solid-electrolyte electrode. This annihilates the conductive bridge between the two electrodes, turning the switch OFF. Similar controlled formation and annihilation of an atomic bridge has also been achieved in an ionic conductive material sandwiched between two electrodes using the solid-electrochemical reaction. 4 The ease of operation and simple structure of the atomic switch make it suitable for configuring logic gates 3 and memory devices. 4 In addition, its unique features, namely, low ON resistance, scalability down to nanometer size, low power consumption, and operation at room temperature, enable the development of a novel programmable logic device 5 that can achieve all functions with a single chip.Because the operating mechanism of the atomic switch is very different from that of conventional semicond...

Memorization caused by the change in conductance in a Ag2S gap-type atomic switch was investigated as a function of the amplitude and width of input voltage pulses (Vin). The conductance changed little for the first few Vin, but the information of the input was stored as a redistribution of Ag-ions in the Ag2S, indicating the formation of sensory memory. After a certain number of Vin, the conductance increased abruptly followed by a gradual decrease, indicating the formation of short-term memory (STM). We found that the probability of STM formation depends strongly on the amplitude and width of Vin, which resembles the learning behavior of the human brain.

The switching time of a Cu(2)S-based gap-type atomic switch is investigated as a function of temperature, bias voltage, and initial off-resistance. The gap-type atomic switch is realized using a scanning tunneling microscope (STM), in which the formation and annihilation of a Cu-atom bridge in the vacuum gap between the Cu(2)S electrode and the Pt tip of the STM are controlled by a solid-electrochemical reaction. Increasing the temperature decreases the switching time exponentially with an activation energy of about 1.38 eV. Increasing the bias voltage also shortens the switching time exponentially, exhibiting a greater exponent for the lower bias than for the higher bias. Furthermore, faster switching has been achieved by decreasing the initial off-resistance between the Cu(2)S electrode and STM tip. On the basis of these results, we suggest that, in addition to the chemical reaction, the electric field in the vacuum gap plays a significant role in the operation of a gap-type atomic switch. This investigation advances our understanding of the operating mechanism of an atomic switch, which is a new concept for future electronic devices.