The Study On Lattice Vibration Anisotropy In Two-dimensional Semiconductors:Rhenium Disulfide

In anisotropic two-dimensional(2D)semiconductor materials,the properties along different crystal orientations are different,originated from the different atomic arrangements along different crystal orientations.This anisotropy in 2D materials holds great promise for novel device applications,such as anisotropic electronic and optoelectronic devices.Rhenium disulfide(ReS₂)is a representative anisotropic 2D material,belonging to the transition metal dichalcogenides(TMDC)family.It shows rich and unique properties,including reduced crystal symmetry,optical and electric anisotropy,tunable interlayer coupling,as well as high carrier mobility.Investigations of ReS₂ crystal on its structure,physical properties and device performance have been attracting tremendous attention from worldwide researchers.In literature,there are extensive studies on in-plane anisotropy of 2D materials,whereas very limited investigation has been conducted on the out-of-plane anisotropy in these materials.This lack of out-of-plane anisotropy study serves as the motivation of this thesis.To address this issue,a home-built experimental system has been designed and successfully demonstrated,which can continuously rotate ReS₂ crystal from in-plane to out-of-plane while performing optical measurements.It can achieve high precision control of sample position and orientation in laboratory coordinates.Combined with Raman spectroscopy,this rotational system is further used to investigate the lattice vibration anisotropy of thin and thick ReS₂ crystals.This thesis systematically investigates the in-plane-to-out-of-plane anisotropy of ReS₂ crystal,using angle-resolved Raman spectroscopy combined with this home-built rotation system.The research findings mainly include:1)the strong polarization dependence of the interlayer Raman vibration mode in thin ReS₂ flakes;2)the strong in-plane-to-out-of-plane anisotropy and their polarized Raman features in thick ReS₂crystals.In addition,through analyzing the peak intensity of angle-resolved Raman spectra under four representative polarization configurations,we successfully discern the A_g-like and E_g-like nature of all the 18 Raman modes in ReS₂ crystal.This provides clear experimental evidence on the existence of A_g-like and E_g-like nature of Raman modes in ReS₂ crystal,which complements prior assignments which entirely rely on first-principles calculations.The in-plane-to-out-of-plane-angle-resolved polarized Raman characterization method proposed in this thesis can be well extended to other 2D anisotropic materials beyond ReS₂ crystal.This also provides experimental and theoretical foundations for further explorations on rich properties and diverse applications in 2D materials.