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.
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