Two-dimensional transitional metal dichalcogenide materials (2D TMD) provide new paradigm for the construction of novel devices based on heterostructures. The weak Van de Waals force between layers allows much easier growth and integration different 2D materials together to form devices with novel functionalities and applications. 2D TMD materials have attracted intense study in the past few years. Nonetheless, the optical and electronic structures of 2D materials often show strong stacking-dependent properties. For example, stacking order in MoS2 strongly affects the spin-orbital coupling which in turn determines the polarization of the light emitted. Detailed understanding of the inter-layer interaction will help greatly in tailoring the properties of 2D materials for applications. We have extensively used Raman/PL spectroscopy and imaging in the study of nano-materials and nano-devices, which provide critical information such as electronic structure, optical property, phonon structure, as well as defects, doping and stacking sequence. In this talk, we use Raman and PL techniques to study few-layer MoS2 samples. They show clear correlation with layer-thickness and stacking order. Our ab initio calculations reveal that difference in the electronic structures mainly arises from competition between spin-orbit coupling and interlayer coupling in different structural configurations.
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