Interlayer Charge Transfer in van der Waals Heterostructures Formed by Transition Metal Dichalcogenide Monolayers

Abstract

Since the discovery of graphene and its outstanding chemical, optical, and mechanical properties, other layered materials have been fiercely hunted for through physical and chemical means. Thanks to their van der Waals interaction, acting as weak glue, different types of layered materials can be stacked without considering their lattice mismatch. The properties of the resulting multilayer structures can be tuned by choice of the materials, layer thicknesses, sequence in which they are arranged, the relative orientation between the layers, and by external electrical, mechanical, and optical controls. This opens the possibility for a large array of applications across many different fields. With the vision to obtain the ability to precisely engineer materials with desired properties, using two-dimensional materials as Lego-blocks, early studies have shown that research in van der Waals stacked two-dimensional materials to be rich in discoveries and still on its early stages due to their abundant diversity. In order to characterize, understand, and improve the properties of van der Waals stacked two-dimensional materials, we first introduce and discuss the noninvasive laser spectroscopy techniques utilized to study them. To correctly interpret the data and to understand the limits of our ultrafast laser spectroscopy system, the dynamics the photocarriers undergo after the pump photoexcitation is examined. Next, the van der Waals stacked two-dimensional materials are introduced by order complexity. First, we discuss the results obtained from bilayers of MoS2–MoSe2 and MoSe2–WS2, which set the groundwork needed to understand more complex structures. We then move on to discuss trilayer MoS2–WS2–MoSe2 and are able to time resolve the electron transfer process as electrons relocate from the MoSe2 into the MoS2 layer. Finally, in the spirit of trying to engineer a new ultrathin material with a high absorption of light in the visible regime as well as extended the photocarrier lifetimes, we fabricated a set of samples that grew in complexity as additional layers were added. It was discovered that in our more complex multilayer structure WSe2–MoSe2–WS2–MoS2, that the absorbance peaked at 50% with about just 2.5 nm of material. Moreover, the photocarrier lifetimes were extended up to a few nanoseconds. With all these we show that van der Waals stacked two-dimensional materials can be engineered layer by layer with the resulting stack having desired properties.