Investigating Optical Properties Of Two Dimensional Materials By Utilizing Scattering-Type Scanning Near-Field Optical Microscope(s-SNOM)

Because of the low plasmon damping, remarkable tunability and high quantum efficiency for light-matter interactions, graphene has intriguing potential applications in a new area of two-dimensional（2D） plasmonics. Monolayer molybdenum disulphide（MoS₂） is probably the second most-studied 2D material after graphene, its optical properties of monolayer are radically different from the properties of the bulk crystal, single and multilayer MoS₂ with direct/indirect band-gap of ¹.2-1.8 eV, have potential applications in photovoltaics, photodetectors and light emitters. In this thesis, our goal was to investigate graphene plasmons and the optical property of MoS₂ by utilizing s-SNOM, our research mainly includes the following three parts:1. Firstly, we launched and detected the surface plasmons on graphene nanostructure fabricated by chemical vapour deposition（CVD） by using s-SNOM, and found the localized modes of plasmon on nanostructure strongly depended on the excited wavelength and geometry. We also observed a constructive interference in nanogap width 25 nm. The near-field characterization results of zigzag edge and armchair edge showed that greater plasmon broadening can be observed at the zigzag edge. Moreover, the near-field optical images revealed the grain boundary of polycrystalline graphene film, and we could use the infrared nano-imaging results qualitative analysis the localized carrier density and chemical composition.2. Secondly, the plasmons of graphene nanostructures fabricated by other growth methods were demonstrated, and found that more shallow and narrower the channel is, stronger the intensity of near-field amplitude will be. The plasmon of nanographene strongly depended on its size and excited wavelength, the localized mode of plasmon shifted to the center part from the edge of nanographene as the size increased, and the intensity of near-field amplitude became enhanced with increasing the wavelength of incident light.3. Finally, we investigated the near-field optical property by utilizing s-SNOM, and found that the grain boundary in monolayer MoS₂ could be observed in AFM image and PL mapping, but not visible in near-field optical image. The near-field signal increased as the tip swap from monolayer MoS₂ to few-layer MoS₂ film that meant an enhancement of the scattering light.