Coexistence of synaptic behaviour and negative differential resistance at room temperature in the resistive switching device based on natural indigo molecules

Abstract

The memory storage and processing capability of human brain is throwing excitement and challenges to create a low power consuming “brain-like functioning device” called neuromorphic device which is the basis of automation and robotics. We demonstrate a robust and highly efficient two-terminal device (Al/Indigo/Al) based on natural indigo molecules to emulate synaptic functionalities for use in neuromorphic applications. The indigo molecules are sourced from plants and are stable at room temperature. Molecules exhibit an optical energy gap of ∼ 1.9 eV evidenced by UV–vis spectroscopy. A detailed analysis of conduction mechanism is performed via dc-electrical characteristics. Consistent negative differential resistance (NDR) is observed, which we attribute to the deep trap states. The analysis shows that conduction is due to the trap-assisted Poole-Frenkel effec. The device maintains an extremely stable high-resistance state (HRS) and low resistance state (LRS) up to 9000 cycles of read – write - erase operation. Further, the essential characteristics such as paired pulse facilitation (PPF) and paired pulse depression (PPD) have been measured on the device. Potentiation (learning) and depression (forgetting) curves follow double exponential with more than 85% of the potentiation/depression occurring in the initial few pulses. Non-linearity (NL) calculations suggest that the device performance can be tuned by carefully choosing the pulse voltage and pulse width such that the nonlinearity factor is reduced. This work clearly indicates an easy approach for the fabrication of natural organic molecule-based artificial synapse that works efficiently at room temperature, as well as the method for selecting measurement parameters to obtain low-power consuming artificial synapse.