Ultrafine Nickel Sulfide-Based Bipolar Resistive Switching Device as Artificial Synapses for Neuromorphic Application

Abstract

Biologically inspired systems, particularly those that mimic the nervous system of living beings, are becoming more demanded due to their ability to solve illposed problems such as pattern recognition or communication with the external environment. Memristors are essential components to replicate high-density networks of biological synapses that control the effectiveness of communication among neurons and implement learning capability because of their tunable conductance. In this study, an organic−inorganic hybrid system of hexamethylenediamine-stabilized ultrafine nickel sulfide particles was synthesized by employing a complexation-mediated route.Here, we propose a nickel sulfide-based memristor as an artificial synapse for neuromorphic application. The current−voltage behavior of the device exhibited bipolar resistive switching with a stable ON and OFF state with an ON/OFF ratio value of 2.5 × 10 1 . The high-conductance state of the device showed the Ohmic conduction mechanism, and the low-conductance state of the device exhibited the Fowler−Nordheim tunneling mechanism. Using identical and nonidentical pulses, the synaptic plasticity behavior of the device was investigated, which revealed the inverse-symmetric and mirror-symmetric patterns, respectively. The device mimicked the spiketime-dependent plasticity properties for Hebbian learning with a conductance value change from −86 to 91%. We also designed the spiking neural network, consisting of 18 synapses and 11 integrated firing neurons, based on the winner-take-all strategy for unsupervised feature learning.

Evaluation of Nanostructured NiS2 Thin Films from a Single-Source Precursor for Flexible Memristive Devices

“…In the case of HRS, the Ohmic conduction is found to be dominated at a low voltage region ( I ∝ V ) (region b). This can be attributed to the thermally generated charge carriers taking part in the conduction at a low-applied field . At the mid- and high-voltage regions, the current shows voltage square dependence, hence following Child’s square law ( I ∝ V 2 ) (regions c and d), thus indicating the governance of the space-charge-limited-current (SCLC) conduction model, which is widely observed in most of the low-dimensional materials …”

“…This can be attributed to the thermally generated charge carriers taking part in the conduction at a low-applied field. 37 At the mid- and high-voltage regions, the current shows voltage square dependence, hence following Child’s square law ( I ∝ V 2 ) (regions c and d), thus indicating the governance of the space-charge-limited-current (SCLC) conduction model, which is widely observed in most of the low-dimensional materials. 38 …”

“…Interestingly, the device exhibited reliable performance under bending conditions with a shift in the ON current toward a higher level. This can be attributed to the polysulfide filament formation maintaining the flexibility within the thin NiS 2 layer . Moreover, the effect is reversible, and the ON current returns to its original position after bending (Figure S7).…”

“…This can be attributed to the polysulfide filament formation maintaining the flexibility within the thin NiS 2 layer. 37 Moreover, the effect is reversible, and the ON current returns to its original position after bending ( Figure S7 ). The retention test of the flexible device was studied to evaluate the stability and reliability of the device under strain.…”