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