Moiré exciton dynamics in twisted MoS2/WS2 bilayers

When talking about two-dimensional (2D) materials in material science, it refers to an extremely thin film consisting of only one atomic layer. The most famous example of this is graphene, which is a planar carbon compound obtained by exfoliating three-dimensional (3D) graphite. In this process, a tape is carefully peeled off from the 3D material, resulting in a single layer of carbon compounds on the tape with extraordinary mechanical and electronic properties, such as a strength that can be up to a hundred times stronger than steel and a conductivity that is 1,000,000 times higher than copper. The discovery and characterization of graphene has led to the Nobel prize in 2010. But there are other materials like graphene that can be only one layer thick, with special properties compared to their thicker forms. In 2010, exfoliation was successfully applied for the first time to a class of materials called transition metal dichalcogenides (TMDs). In this project, we are studying a specific type of this 2D material called molybdenum disulfide (MoS2) and Tungsten disulfide (WS2). TMDs are semiconductors whose optical, thermal, and electronic properties change drastically when transitioning from 3D to 2D materials. They change how they conduct electricity when you shine light on them, or absorb different colors of light. Electron transport and absorption behavior can also be manipulated by stacking multiple layers of different TMDs on top of each other and twist the respective thin layers relative to each other. It can be understood as twisting two pieces of paper on top of each other, resulting in different patterns depending on how you twist them. Various 2D TMDs can be used as optoelectronic sensors in photodetectors, in photovoltaic cells, as catalysts in water electrolysis, or as transistors. Understanding the process of charge transfer between neighboring atomic layers is crucial for all these applications. In the submitted project, we aim to tackle this long-standing question and investigate the process of charge transfer between neighboring atomic layers in dependence of their twist angle. Different thin layers of MoS2/WS2 are fabricated, with the orientation of the MoS2 and WS2 layers twisted relative to each other to control electron transport between the layers. Subsequently, the samples are excited with a laser, and the current flow between the neighboring atomic layers is detected over time.