Artificial electronic synapses or synaptic devices, which are capable of mimicking the functions of biological synapses in the human brain, are considered the basic building blocks for brain-inspired computing. Therefore, we investigated the emulation of synaptic functions in a simple Au nanogap. The synaptic functionality of neuromorphic hardware originates from a gradually modulated resistance. Previously, we investigated simple electromigration-based methods for controlling the tunnel resistance of nanogaps, called activation. In this study, a new type of artificial synaptic device based on planar Au nanogaps is demonstrated using a newly investigated activation procedure with voltage pulses. In the activation method with specific voltage pulses, the change in tunnel resistance of the Au nanogaps can be gradually controlled depending on the interval and amplitude of input voltage pulses. Moreover, Au inorganic synapses can emulate the synaptic functions of both short-term plasticity (STP) and long-term plasticity (LTP) characteristics. After the applied pulse is removed, the current decays rapidly at the beginning, followed by a gradual fade to a stable level. In addition, with repeated stimulations, the forgetting rate becomes decreases and the memory retention increases. Therefore, we observe an effect analogous to a memory transition from STP to LTP in biological systems. Our results may contribute to the development of highly functional artificial synapses and the further construction of neuromorphic computing architecture.
The basic building blocks for brain-inspired computing are neurons and their inter-cellular connections, called synapses. In this paper, we report artificial synapses composed of simple gold (Au) nanogaps that function using an electromigration-based method called "activation." In the activation technique, metal-atom transport is induced by a field emission current, resulting in a change in gap separation. Synaptic functionalities, including long-term potentiation, long-term depression, and spike-timing-dependent plasticity, were successfully implemented in the electromigrated Au nanogaps using activation. Furthermore, the integration of four artificial synapses in a 2 × 2 array and image memorization were achieved with the Au nanogap-based artificial synapses. These results illustrate that electromigrated Au nanogaps hold promise as synaptic devices for bio-inspired computational systems.
Feedback-controlled electromigration (FCE) is employed to control metal nanowires with quantized conductance and create nanogaps and atomic junctions. In the FCE method, the experimental parameters are commonly selected based on experience. However, optimization of the parameters by way of tuning is intractable because of the impossibility of attempting all different combinations systematically. Therefore, we propose the use of the Ising spin model to optimize the FCE parameters, because this approach can search for a global optimum in a multidimensional solution space within a short calculation time. The FCE parameters were determined by using the energy convergence properties of the Ising spin model. We tested these parameters in actual FCE experiments, and we demonstrated that the Ising spin model could improve the controllability of the quantized conductance in atomic junctions. This result implies that the proposed method is an effective tool for the optimization of the FCE process in which an intelligent machine can conduct the research instead of humans.
We report a simple method for the control of electrical characteristics of planar-type metal-based single-electron transistors (SETs) using field-emission-induced electromigration. The advantages of this method are as follows: (1) the fabrication of SETs is achieved by only passing a field emission current through a nanogap and (2) the charging energy of SETs can be controlled by adjusting the magnitude of the applied current during the procedure. In order to better control the electrical properties of the SETs, we investigate the relation between control parameters of the method and electrical characteristics of the SETs. When the field-emission-induced electromigration with the preset current of 500 nA was applied to the nanogaps, current-voltage characteristics of the nanogaps displayed the suppression of electrical current at low-bias voltages known as Coulomb blockade at 16 K. In addition, Coulomb blockade voltage was clearly modulated by the gate voltage periodically at 16 K, resulting in the formation of single island in the SETs by the field-emission-induced electromigration. Furthermore, as the preset current was increased, the charging energy of the SETs was decreased with decreasing the initial gap separation of the nanogaps. These results imply that the electrical characteristics of the SETs are controllable by the preset current of the method and the initial gap separation of the nanogaps. Field-emission-induced electromigration procedure allows us to simply control electrical characteristics of planar-type metal-based SETs.
We developed a simple and controllable nanogap fabrication method called “activation.” In the activation technique, electromigration is induced by a field emission current passing through the nanogaps. Activation enables the electrical properties of Ni nanogaps in a vacuum to be controlled and is expected to be applicable to Au nanogaps even in ambient air. In this study, we investigated the activation properties of Au nanogaps in ambient air from a practical point of view. When activation was performed in ambient air, the tunnel resistance of the Au nanogaps decreased from over 100 TΩ to 3.7 MΩ as the preset current increased from 1 nA to 1.5 μA. Moreover, after activation in ambient air with a preset current of 500 nA, the barrier widths and heights of the Au nanogaps were estimated using the Simmons model to be approximately 0.5 nm and 3.3 eV, respectively. The extracted barrier height is smaller than that of 4.6 eV resulting from activation in a vacuum and much lower than the work function of bulk Au. This difference implies the presence of atmospherically derived moisture or contamination adsorbed on the nanogaps. These results suggest that activation can be utilized for Au nanogap fabrication even in ambient air.
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