High Pressure Study On Structure And Anisotropy Of Rhenium Disulfide

In recent years,two-dimensional transition metal dichalcogenides（TMDCs）with layered structures have attracted considerable attention due to their distinctive structural,electrical and optical properties.Rhenium disulfide（ReS₂）is a particularly noteworthy member of TMDCs,which shows unique structural and physical properties such as reduced crystal symmetry,photoelectric anisotropy and highly adjustable interlayer coupling.It is helpful to improve the understanding of ReS₂-based novel optoelectronic devices by effectively modifying the structural and physical properties.Pressure engineering is known as an efficient,continuous and reversible technique capable of tuning materials structure,as well as their electrical,optical and other physical properties.Therefore,the crystal structure,electronic structure and anisotropic properties of ReS₂can be effectively modified by pressure.The current high-pressure studies of ReS₂ have been limited to a single in-plane direction,and the anisotropic response under high pressure is rarely reported.Here,this thesis systematically investigates the structure and anisotropy of ReS₂ through the combination of in-situ pressure engineering,polarized Raman spectroscopy and polarized photoluminescence（PL）spectroscopy.Specifically,the results are as follows:Firstly,by combining theoretical calculation and polarized Raman spectroscopy,the low symmetry,anisotropy and crystal axis direction of thin layer and bulk ReS₂ under atmospheric pressure are systematically studied.The electronic structure of bulk ReS₂ is explored by polarized PL spectroscopy,which confirms that bulk ReS₂ is an indirect bandgap semiconductor.Furthermore,the PL spectra at room temperature and atmospheric pressure do not show any polarization angle-dependent characteristics.Secondly,by combining in-situ pressure engineering and polarized Raman spectroscopy,pressure-induced evolution of 18 Raman-active modes in bulk ReS₂ crystal is systematically studied.This thesis finds that ReS₂ undergoes a structural transformation from 1T’to a distorted-1T’phase at 3.04 GPa,followed by an intralayer deformation of Re₄ clusters occurring at 14.24 GPa.Interlayer transitions from disordered to ordered stacking in different in-plane directions are observed at 22.08 GPa and 25.76 GPa when the laser is polarized in different directions,which reflects the pressure-enhanced in-plane anisotropy,i.e.,the anisotropy of ReS₂ crystal becomes more prominent under high pressure.Finally,the transformation in ReS₂ from an indirect to a direct bandgap at 3.04 GPa is discovered by using in-situ pressure engineering and polarized PL spectroscopy.Through fitting with Voigt function,it is found that the high-pressure PL spectra of ReS₂are similar to that at atmospheric pressure under two orthogonal laser polarization conditions,that is,no anisotropic behavior is observed in either case.These findings demonstrate the effectiveness of pressure in tuning materials properties,and shed light on potential application of ReS₂ crystals in anisotropic optical and optoelectronic devices.