  The existence of bound charge transfer (CT) excitons at the interface of
monolayer lateral heterojunctions has been debated in literature, but contrary
to the case of interlayer excitons in vertical heterostructure their
observation still has to be confirmed. Here, we present a microscopic study
investigating signatures of bound CT excitons in photoluminescence spectra at
the interface of hBN-encapsulated lateral MoSe$_2$-WSe$_2$ heterostructures.
Based on a fully microscopic and material-specific theory, we reveal the
many-particle processes behind the formation of CT excitons and how they can be
tuned via interface- and dielectric engineering. For junction widths smaller
than the Coulomb-induced Bohr radius we predict the appearance of a low-energy
CT exciton. The theoretical prediction is compared with experimental
low-temperature photoluminescence measurements showing emission in the bound CT
excitons energy range. Our joint theory-experiment study presents a significant
step towards a microscopic understanding of optical properties of
technologically promising 2D lateral heterostructures.
