Vertical MoS2 double layer memristor with electrochemical metallization as an atomic-scale synapse with switching thresholds approaching 100 mV
Journal
NANO LETT
Date
2019.03.21
Abstract
Atomically thin two-dimensional (2D) materials？？such as transition metal dichalcogenide
(TMD) monolayers and hexagonal boron nitride (hBN)？？and their van der Waals layered
preparations, have been actively researched to build electronic devices such as field effect transistors,
junction diodes, tunneling devices, and more recently, memristors. 2D material memristors
built in lateral form, with horizontal placement of electrodes and the 2D material layers, have provided
an intriguing window into the motions of ions along the atomically thin layers. On the other
hand, 2D material memristors built in vertical form with top and bottom electrodes sandwiching
2D material layers may provide opportunities to explore the extreme of the memristive performance
with the atomic-scale inter-electrode distance. In particular, they may help push the switching voltages
to a lower limit, which is an important pursuit in the memristors research in general, given
their roles in neuromorphic computing. In fact, recently Akinwande et al. performed a pioneering
work to demonstrate a vertical memristor that sandwiches a single MoS2 monolayer between two
inert Au electrodes, but it could neither attain switching voltages below 1 V nor control the switching
polarity, obtaining both unipolar and bipolar switching devices. Here we report a vertical
memristor that sandwiches two MoS2 monolayers between an active Cu top electrode and an inert
Au bottom electrode. Cu ions diffuse through the MoS2 double layers to form atomic-scale filaments.
The atomic-scale thickness, combined with the electrochemical metallization, lowers
switching voltages down to 0.1 ~ 0.2 V, on a par with the state of the art. Furthermore, our memristor
achieves consistent bipolar and analog switching, and thus exhibits the synapse-like learning
behavior such as the spike-timing dependent plasticity (STDP), the very first STDP demonstration
among all 2D material based vertical memristors. The demonstrated STDP with low switching
voltages is promising not only for low power neuromorphic computing, but also from the point of
view that the voltage range approaches the biological action potentials, opening up a possibility for
direct interfacing with the mammalian neuronal networks.