  Neutral and charged excitons (trions) in atomically-thin materials offer
important capabilities for photonics, from ultrafast photodetectors to
highly-efficient light-emitting diodes and lasers. Recent studies of van der
Waals (vdW) heterostructures comprised of dissimilar monolayer materials have
uncovered a wealth of optical phenomena that are predominantly governed by
interlayer interactions. Here, we examine the optical properties in NbSe$_2$ -
MoSe$_2$ vdW heterostructures, which provide an important model system to study
metal-semiconductor interfaces, a common element in optoelectronics. Through
low-temperature photoluminescence (PL) microscopy we discover a sharp emission
feature, L1, that is localized at the NbSe$_2$-capped regions of MoSe$_2$. L1
is observed at energies below the commonly-studied MoSe$_2$ excitons and
trions, and exhibits temperature- and power-dependent PL consistent with
exciton localization in a confining potential. Remarkably, L1 is very robust
not just in different samples, but also under a variety of fabrication
processes. Using first-principles calculations we reveal that the confinement
potential required for exciton localization naturally arises from the in-plane
band bending due to the changes in the electron affinity between pristine
MoSe$_2$ and NbSe$_2$ - MoSe$_2$ heterostructure. We discuss the implications
of our studies for atomically-thin optoelectronics devices with
atomically-sharp interfaces and tunable electronic structures.
