  Trions, quasi-particles consisting of two electrons combined with one hole or
of two holes with one electron, have recently been observed in transition metal
dichalcogenides (TMDCs) and drawn increasing attention due to potential
applications of these materials in light-emitting diodes, valleytronic devices
as well as for being a testbed for understanding many-body phenomena.
Therefore, it is important to enhance the trion emission and its stability. In
this study, we construct a MoSe2/FePS3 van der Waals heterostructure (vdWH)
with type-I band alignment, which allows for carriers injection from FePS3 to
MoSe2. At low temperatures, the neutral exciton (X0) emission in this vdWH is
almost completely suppressed. The ITrion/Ix0 intensity ratio increases from
0.44 in a single MoSe2 monolayer to 20 in this heterostructure with the trion
charging state changing from negative in the monolayer to positive in the
heterostructure. The optical pumping with circularly polarized light shows a
14% polarization for the trion emission in MoSe2/FePS3. Moreover, forming such
type-I vdWH also gives rise to a 20-fold enhancement of the room temperature
photoluminescence from monolayer MoSe2. Our results demonstrate a novel
approach to convert excitons to trions in monolayer 2D TMDCs via interlayer
doping effect using type-I band alignment in vdWH.
