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.