Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals

To explore new constituents in two-dimensional (2D) materials and to combine their best in van der Waals heterostructures is in great demand as being a unique platform to discover new physical phenomena and to design novel functionalities in interface-based devices. Herein, PbI₂ crystals as thin as a few layers are synthesized, particularly through a facile low-temperature solution approach with crystals of large size, regular shape, different thicknesses, and high yields. As a prototypical demonstration of band engineering of PbI₂ -based interfacial semiconductors, PbI₂ crystals are assembled with several transition metal dichalcogenide monolayers. The photoluminescence of MoS₂ is enhanced in MoS₂ /PbI₂ stacks, while a dramatic photoluminescence quenching of  WS₂ and WSe₂ is revealed in WS₂ /PbI₂ and WSe₂ /PbI₂ stacks. This is attributed to the effective heterojunction formation between PbI₂ and these monolayers; type I band alignment in MoS₂ /PbI₂ stacks, where fast-transferred charge carriers accumulate in MoS₂ with high emission efficiency, results in photoluminescence enhancement, and type II in WS₂ /PbI₂ and WSe₂ /PbI₂ stacks, with separated electrons and holes suitable for light harvesting, results in photoluminescence quenching. The results demonstrate that MoS₂ , WS₂ , and WSe₂ monolayers with similar electronic structures show completely distinct light-matter interactions when interfacing with PbI₂ , providing unprecedented capabilities to engineer the device performance of 2D heterostructures.