Memristors have shown an extraordinary potential to emulate the
plastic and dynamic electrical behaviors of biological synapses and
have been already used to construct neuromorphic systems with in-memory
computing and unsupervised learning capabilities; moreover, the small
size and simple fabrication process of memristors make them ideal
candidates for ultradense configurations. So far, the properties of
memristive electronic synapses (i.e., potentiation/depression, relaxation,
linearity) have been extensively analyzed by several groups. However,
the dynamics of electroforming in memristive devices, which defines
the position, size, shape, and chemical composition of the conductive
nanofilaments across the device, has not been analyzed in depth. By
applying ramped voltage stress (RVS), constant voltage stress (CVS),
and pulsed voltage stress (PVS), we found that electroforming is highly
affected by the biasing methods applied. We also found that the technique
used to deposit the oxide, the chemical composition of the adjacent
metal electrodes, and the polarity of the electrical stimuli applied
have important effects on the dynamics of the electroforming process
and in subsequent post-electroforming bipolar resistive switching.
This work should be of interest to designers of memristive neuromorphic
systems and could open the door for the implementation of new bioinspired
functionalities into memristive neuromorphic systems.
Oxidation of van der Waals-bonded layered semiconductors plays a key role in deterioration of their superior optical and electronic properties. The oxidation mechanism of these materials is, however, different from non-layered semiconductors in many aspects. Here, we show a rather unusual oxidation of tungsten disulfide (WS2) nanotubes and platelets in a high vacuum chamber at a presence of water vapor and at elevated temperatures. The process results in formation of small tungsten oxide nanowires on the surface of WS2. Utilizing real-time scanning electron microscopy we are able to unravel the oxidation mechanism, which proceeds via reduction of initially formed WO3 phase into W18O49 nanowires. Moreover, we show that the oxidation reaction can be localized and enhanced by an electron beam irradiation, which allows for on-demand growth of tungsten oxide nanowires.
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