  Non-volatile memory devices have been limited to flash architectures that are
complex devices. Here, we present a unique photomemory effect in MoS$_2$
transistors. The photomemory is based on a photodoping effect - a controlled
way of manipulating the density of free charges in monolayer MoS$_2$ using a
combination of laser exposure and gate voltage application. The photodoping
promotes changes on the conductance of MoS$_2$ leading to photomemory states
with high memory on/off ratio. Such memory states are non-volatile with an
expectation of retaining up to 50 % of the information for tens of years.
Furthermore, we show that the photodoping is gate-tunable, enabling control of
the recorded memory states. Finally, we propose a model to explain the
photodoping, and we provide experimental evidence supporting such a phenomenon.
In summary, our work includes the MoS$_2$ phototransistors in the non-volatile
memory devices and expands the possibilities of memory application beyond
conventional memory architectures.
