| Summary: | 碩士 === 國立臺南大學 === 材料科學系碩士班 === 107 === Atomically layered two-dimensional （2D） transition metal dichalcogenides （TMDs） possess fascinating properties like strong many-body Coulomb-mediated interactions. The formation of multi-particle （e.g., two-particle excitons, three-particle trions and four-particle biexcitons） is the manifestation of the interactions and has been studied by numerous researchers. But the phenomenon is generally observed under ultra-low temperature and is time-consuming for mapping the whole spatial distribution of excitons by photoluminescence （PL） spectrometer. Herein, the existence of exciton states in WS2 and WSe2 monolayers are demonstrated using second-harmonic generation （SHG）. This technique provides fast and straightforward optical methods for determining the crystal orientation and crystal morphology by large-area mapping and for efficiently detecting particular structural features of TMDs. The process of growing of WS2 and WSe2 monolayers directly on c-plane sapphire substrates was performed in a chemical vapor deposition （CVD） system, which provides a better platform for studying special characteristics than that of mechanically exfoliating from the bulk in most papers. In this research, we investigate the SHG of exciton, trion and biexciton energies in WS2 and WSe2 monolayers with varying excitation wavelength and the SHG enhancement changed with different location. This precise and practical technique establishes a convenient route to characterize and optimize the potential for TMDs.
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