2D vertical van der Waals (vdW) heterostructures with atomically sharp interfaces have attracted tremendous interest in 2D photonic and optoelectronic applications. Band alignment engineering in 2D heterostructures provides a perfect platform for tailoring interfacial charge transfer behaviors, from which desired optical and optoelectronic features can be realized. Here, by developing a two-step chemical vapor deposition strategy, direct vapor growth of monolayer PbI2 on monolayer transition metal dichalcogenides (TMDCs) (WS2, WSe2, or alloying WS2(1- x )Se2 x ), forming bilayer vertical heterostructures, is demonstrated. Based on the calculated electron band structures, the interfacial band alignments of the obtained heterostructures can be gradually tuned from type-I (PbI2/WS2) to type-II (PbI2/WSe2). Steady-state photoluminescence (PL) and time-resolved PL measurements reveal that the PL emissions from the bottom TMDC layers can be modulated from apparently enhanced (for WS2) to greatly quenched (for WSe2) compared to their monolayer counterparts, which can be attributed to the band alignment-induced distinct interfacial charge transfer behaviors. The band alignment nature of the heterostructures is further demonstrated by the PL excitation spectroscopy and interlayer exciton investigation. The realization of 2D vertical heterostructures with tunable band alignments will provide a new material platform for designing and constructing multifunctional optoelectronic devices.
Keywords: band alignments; charge transfers; photoluminescence enhancement; photoluminescence quenching; van der Waals heterostructures.