Type-I van der Waals heterostructures, in which both the conduction and valence band edges of one material lie within the bandgap of an adjacent layer, offer unique opportunities for engineering charge carrier confinement and enhancing light emission in two-dimensional systems. In our original work (M. Z. Bellus, M. Li, S. D. Lane, F. Ceballos, Q. Cui, X. C. Zeng and H. Zhao, Nanoscale Horiz., 2017, 2, 31–36, we demonstrated a type-I heterostructure formed by monolayer MoS2 and ReS2, verified through both first-principles calculations and time-resolved spectroscopy. Since then, growing interest in type-I band alignment has led to the discovery of a broad range of new type-I systems through theoretical predictions and experimental methods. Moreover, dynamic tuning of band alignment via vertical electric fields or strain has enabled reversible transitions between type-II and type-I configurations. Applications of type-I heterostructures in light-emitting and photodetection devices have also been experimentally explored. Looking ahead, we anticipate continued development of type-I heterostructures with enhanced light-emitting performance and their integration into complex multilayer stacks with mixed band alignments to realize novel optoelectronic and quantum devices.
