Quasi-type-I band alignment in WS2/MoS2 heterostructures produced by H2S-based CVD

In this work we report the synthesis and characterization of WS2/MoS2 heterostructures prepared by sequential chemical vapor deposition (CVD) using H2S and thermally evaporated metal as precursors. A thin WS2 covers (1-2 monolayers) are produced on pre-formed MoS2 nanowall films, resulting in nearly complete quenching of the MoS2 photoluminescence (PL) and emergence of a dominant WS2 PL peak at higher energy. Remarkably, electrical measurements reveal that the thermally activated conduction in the both layered materials have nearly the same activation energies, suggesting comparable donor activation energies and a possible near-alignment of the CBM positions within experimental uncertainty. This particular "quasi-type-I" alignment is in sharp contrast to the conventional staggered (type-II) alignment normally expected in WS2/MoS2 heterostructures. Transient absorption pump-probe spectroscopy shows signatures consistent with ultrafast interlayer charge transfer and possible interlayer exciton formation. We present a comprehensive study of the optical, structural, and transport properties of prepared samples, and discuss the carrier transfer mechanisms that give rise to the observed PL redistribution.