  Vertically stacked atomic layers from different layered crystals can be held
together by van der Waals forces, which can be used for building novel
heterostructures, offering a platform for developing a new generation of
atomically thin, transparent and flexible devices. The performance of these
devices is critically dependent on the layer thickness and the interlayer
electronic coupling, influencing the hybridisation of the electronic states as
well as charge and energy transfer between the layers. The electronic coupling
is affected by the relative orientation of the layers as well as by the
cleanliness of their interfaces. Here, we demonstrate an efficient method for
monitoring interlayer coupling in heterostructures made from transition metal
dichalcogenides using photoluminescence imaging in a bright-field optical
microscope. The colour and brightness in such images are used here to identify
mono- and few-layer crystals, and to track changes in the interlayer coupling
and the emergence of interlayer excitons after thermal annealing in
mechanically exfoliated flakes as well as a function of the twist angle in
atomic layers grown by chemical vapour deposition. Material and crystal
thickness sensitivity of the presented imaging technique makes it a powerful
tool for characterisation of van der Waals heterostructures assembled by a wide
variety of methods, using combinations of materials obtained through mechanical
or chemical exfoliation and crystal growth.
