  Non-volatile resistive switching is demonstrated in memristors with
nanocrystalline molybdenum disulfide (MoS$_2$) as the active material. The
vertical heterostructures consist of silicon, vertically aligned MoS$_2$ and
chrome / gold metal electrodes. Electrical characterizations reveal a bipolar
and forming free switching process with stable retention for at least 2500
seconds. Controlled experiments carried out in ambient and vacuum conditions
suggest that the observed resistive switching is based on hydroxyl ions
(OH$^-$). These originate from catalytic splitting of adsorbed water molecules
by MoS$_2$. Experimental results in combination with analytical simulations
further suggest that electric field driven movement of the mobile OH$^-$ ions
along the vertical MoS$_2$ layers influences the energy barrier at the
Si/MoS$_2$ interface. The scalable and semiconductor production compatible
device fabrication process used in this work offers the opportunity to
integrate such memristors into existing silicon technology for future
neuromorphic applications. The observed ion-based plasticity may be exploited
in ionicelectronic devices based on TMDs and other 2D materials for memristive
applications.
