  Neuromorphic computing has emphasized the need for memristors with
non-volatile, multiple conductance levels. This paper demonstrates the
potential of hexagonal boron nitride (hBN)/graphene heterostructures to act as
memristors with multiple resistance states that can be optically tuned using
visible light. The number of resistance levels in graphene can be controlled by
modulating doping levels, achieved by varying the electric field strength or
adjusting the duration of optical illumination. Our measurements show that this
photodoping of graphene results from the optical excitation of charge carriers
from the nitrogen-vacancy levels of hBN to its conduction band, with these
carriers then being transferred to graphene by the gate-induced electric field.
We develop a quantitative model to describe our observations. Additionally,
utilizing our device architecture, we propose a memristive crossbar array for
vector-matrix multiplications.
