2D TMD and perovskite materials have attracted intensive research interests due to their unique electrooptical properties. Detailed understanding structural information and layer-layer interaction will help greatly in tailoring their properties for applications. We use Raman and PL spectroscopic/imaging techniques to study few-layer 2D TMD samples and perovskites. Our results show that layer-layer coupling and stacking sequence significantly affect spin-orbit coupling in 2D samples. High pressure study and ab initio calculations are used to elucidate the 4000 times PL enhancement in CH3NH3PbBr3. The proposed indirect to direct band gap transition is further confirmed using time resolved PL measurements.
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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