Excitonic devices with van der Waals heterostructures: valleytronics meets twistronics

2D semiconducting transition metal dichalcogenides comprise an emerging class of materials with distinct properties, including large exciton binding energies that reach hundreds of millielectronvolts and valley-contrasting physics. Thanks to the van der Waals interaction, individual monolayers can be assembled to produce synthetic crystals with tailored properties not found in the constituent materials. The interlayer excitons in these structures provide an optically addressable spin and valley degrees of freedom with long lifetime. These quasi-particles can be controlled in solid-state devices to implement (pseudo)spin-based computation schemes or to study fundamental bosonic interactions. In this Review, we discuss recent progress in the field, focusing on device architectures and engineering techniques that allow the properties of interlayer excitons to be tailored, such as electrostatic modulation, relative twist angle, strain and substrate engineering. Because the excitonic response is highly sensitive to the local environment and layer morphology, we also discuss the advances and challenges in the fabrication of excitonic devices. This examination allows us to highlight critical points that require further investigation, as well as to comment on future research perspectives.