  Van der Waals heterostructures give access to a wide variety of new phenomena
that emerge thanks to the combination of properties brought in by the
constituent layered materials. We show here that owing to an enhanced
interaction cross section with electrons in a type I van der Waals
heterostructure, made of single layer molybdenum disulphide and thin boron
nitride films, electrons and holes created in boron nitride can be transferred
to the dichalcogenide where they form electron-hole pairs yielding
luminescence. This cathodoluminescence can be mapped with a spatial resolution
far exceeding what can be achieved in a typical photoluminescence experiment,
and is highly valuable to understand the optoelectronic properties at the
nanometer scale. We find that in heterostructures prepared following the
mainstream dry transfer technique, cathodoluminescence is locally extinguished,
and we show that this extinction is associated with the formation of defects,
that are detected in Raman spectroscopy and photoluminescence. We establish
that to avoid defect formation induced by low-energy electron beams and to
ensure efficient transfer of electrons and holes at the interface between the
layers, flat and uniform interlayer interfaces are needed, that are free of
trapped species, airborne ones or contaminants associated with sample
preparation. We show that heterostructure fabrication using a pick-up technique
leads to superior, intimate interlayer contacts associated with significantly
more homogeneous cathodoluminescence.
