Tunable excitonic properties in two-dimensional heterostructures based on solution-processed PbI₂ flakes

We investigate the manifestations of band structure engineering in few-layer PbI₂-based heterostructures by probing their tunable optical properties. First, we have successfully prepared atomically thin flakes from PbI₂ solution by two distinct approaches. A drop-casting of PbI₂ solution onto various substrates followed by a simple heating process yields abundant flakes with different thickness and regular shape. Mechanical exfoliation of PbI₂ bulk crystals, obtained from a low-temperature recrystallization process of PbI₂ solution, also gives ultrathin PbI₂ flakes of high quality. Moreover, these PbI₂ flakes are employed to construct various van de Waals heterostructures. A significant enhancement of photoluminescence in MoSe₂ interfaced with PbI₂ was observed at different laser excitation intensity, due to the forming of type-I band alignment. Type-I band alignment can also be investigated in MoS₂/PbI₂ heterostructure, while type-II band alignment is built-in WSe₂/PbI₂ heterostructure. These results demonstrate that the strong interfacial coupling between PbI₂ and other two-dimensional semiconductors can modulate their band alignment, and as a result, the exciton properties noticeably, which provides new insights of building a designer heterostructure device at the atomic level.