| Summary: | 碩士 === 國立交通大學 === 電子物理系所 === 107 === Vertically stacked heterostructures made of atomically thin two-dimensional (2D) layered materials, such as graphene, boron nitride, and transition metal dichalcogenides (TMDs), have recently attracted much attention due to their versatility in creating functional artificial materials. Replacing the constituent materials and controlling the structure pave the way for future designing of heterostructure devices. 2D PdSe2 crystal is a member of 2D TMDs, featuring a unique puckered pentagonal structure with in-plane anisotropy, and is theoretically predicted to exhibit a widely tunable bandgap. In this thesis, we demonstrate that the layer number and crystal anisotropy of 2D PdSe2 can be identified by polarization-resolved Raman spectroscopy and second harmonic generation (SHG). The strong interlayer interaction in 2D PdSe2 is revealed by measuring the interlayer breathing mode in low-frequency Raman spectroscopy, which can be well predicted by the conventional linear chain model corrected by the minor interlayer restoring forces. The layer number is further confirmed by SHG, where a strong SHG signal can be observed in the even-layer PdSe2 non-centrosymmetric, and a nearly vanished SHG in odd-layer PdSe2. Polarization-resolved Raman and SHG measurements also reveal in-plane anisotropy, which is consistent with the crystallographic axes determined by transmission electron microscopy. On the other hand, we also demonstrate that the low-frequency Raman spectroscopy can be used to identify the stacking configuration of WS2/MoS2 heterobilayers. WS2/MoS2 heterobilayer formed by vertical CVD grown of WS2 on MoS2. Since its electronic and optical properties strongly depend on the stacking configuration due to the symmetry-dependent interlayer coupling, we found that the stacking configuration of TMD heterobilayers can be characterized by low-frequency (< 60 cm-1) shear and breathing Raman modes, corresponding to in-plane and out-of-plane layer vibrations, respectively.
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