Separating electrons and holes by monolayer increments in van der Waals heterostructures

Since the discovery of graphene and its outstanding chemical, optical, and mechanical properties, other layered materials have been fiercely hunted for throughout various techniques. Thanks to their van der Waals interaction, acting as weak glue, different types of layered materials with mismatched lattices can be stacked with high quality interfaces. The properties of the resulting multilayer structures can be tuned by choice of the materials, layer thicknesses, and sequence in which they are arranged. This opens the possibility for a large array of applications across many different fields. Here we present a systematic study with two-dimensional stacked layered materials, where their properties are tailored monolayer by monolayer. By arranging WSe2, MoSe2, WS2, and MoS2 monolayers in predetermined sequences, that are predicted to have a ladder band alignment in both the conduction and valence bands, we separate electrons and holes between the two utmost layers by monolayer increments. The samples studied are a WSe2 monolayer, a WSe2-MoSe2 bilayer, a WSe2-MoSe2-WS2 trilayer, and a WSe2-MoSe2-WS2-MoS2 four-layer. We observe an increase in absorbance, a decrease in photoluminescence, a variation in interlayer charge transfer, and photocarrier lifetimes that are extended up to a few nanoseconds as additional layers were added. With these results, we demonstrate that van der Waals stacked two-dimensional materials can form effective complex stacks and are promising platforms for fabricating ultrathin and flexible optoelectronics.