  We present the electrostatic control of photoluminescence of monolayer
MoS$_2$ at room temperature via integration of free-standing BaTiO$_3$ (BTO), a
ferroelectric perovskite oxide, layers. We show that the use of BTO leads to
highly tunable exciton emission of MoS$_2$ in a minimal range of gate voltages,
effectively controlling the neutral excitons to charged excitons (trions)
conversion. Due to BTO's ferroelectric polarization-induced doping we observe
large peak emission shifts as well as a large and tunable A trion binding
energy in the range of 40-100 meV. To further investigate the efficacy of
electrostatic control, we compared our measurements with those carried out when
the BTO is replaced by a hexagonal boron nitride (hBN) dielectric layer of
comparable thickness, confirming BTO's superior gating properties and thus
lower power consumption. Additionally, we take advantage of the ferroelectric
switching of BTO by fabricating devices where the BTO layer is decoupled from
the gate electrode with a SiO$_2$ layer. Choosing to isolate the BTO allows us
to induce large remanent behavior of MoS$_2$'s excitonic features, observing
hysteretic behavior in the peak energy ratio between A exciton and its trion,
as well as hysteretic behavior in the doping-related trion energy shift. This
study illustrates the rich physics involved in combining free-standing complex
oxide layers with two-dimensional materials.
