Optical Anisotropy And High-pressure Response Of Two-dimensional Transition Metal Dichalcogenides ReS₂

Anisotropic 2D materials have great potential in realizing polarized optoelectronic devices.In particular,in the rapidly developing 2D materials family,Rhenium disulfide（ReS₂）crystals have captured significant attention due to their low symmetry triclinic crystal structures.Classified under Group VII transition metal dichalcogenides（TMDCs）,ReS₂ boasts attractive properties such as an extremely high refractive index,significant oscillator strength,and good chemical stability.Its inherent low symmetry and in-plane birefringence generate prominent optical anisotropy and complicated Raman vibration modes.In contrast to many other TMDCs,ReS₂ features weak interlayer coupling,allowing its unique photoelectric properties to persist from bulk to monolayer forms,making it promising for realizing polarization-sensitive optical devices.However,the current methods used to detect the anisotropy of 2D materials are still limited in some aspects.Hence,a simple,non-destructive,and low-cost measurement method is desirable for promoting the rapid development of 2D materials.Moreover,although the Raman spectra of ReS₂ exhibit rich features（18 peaks）,vibration nature of the Raman modes is still not well-understood,and theoretical disputes remain unresolved.Recent studies have shown that the Raman peaks of in-plane and out-of-plane vibration modes exhibit distinct response behaviors under high pressure.Accordingly,this dissertation explores the optical anisotropy（in-plane and out-of-plane）of 2D TMDCs（ReS₂）materials,and their response to high pressure.The primary focus of this dissertation includes the following:1.The development of a novel technique for measuring the anisotropy of 2D materials known as dichromatic polarized reflectance.To overcome the current limitations of measuring the anisotropy of 2D materials,this study proposes the use of dichromatic polarized reflectivity to rapidly identify the crystal axis orientation of 2D materials.By using this method,researchers are able to identify the crystal orientation（Re-Re chain）of 2D ReS₂ with different thickness rapidly and accurately.Moreover,this technique can be extended to multi-chromatic schemes for enhanced measurement and can be applied to a range of 2D materials.This dissertation offers a simple and effective new method for examining the anisotropy of 2D materials.2.To enhance the existing research on the out-of-plane optical anisotropy of 2D TMDCs materials,this dissertation presents a method on analyzing the in-plane and out-of-plane optical anisotropy in ReS₂ crystals through solid-angle resolved Raman spectroscopy.The dissertation details the utilization of this technique in discerning the vibrational nature of Raman modes in ReS₂ crystals,allowing us to tap into the full 3x3Raman tensor and accurately characterize the A_g-like and E_g-like nature of all 18 Raman modes.3.This dissertation presents a report on the orientation-polarization dependence of pressure-induced Raman anomalies in anisotropic 2D ReS₂.This study conducted an in-situ high-pressure（0-30.24 GPa）optical analysis（Raman）of the 2D ReS₂ crystal,exploring four specific configurations of sample orientation:horizontal atomic plane placement（H）and vertical atomic plane placement（V）,and laser polarization（parallel or perpendicular to the crystal b axis）.Interestingly,experiment observed an unusual disappearance behavior of Raman modes in the vertical structure（a departure from the widely used horizontal structure）.Experiment conducted a thorough analysis of the pressure responses of the Raman peaks under different configurations,and combined it with the calculation results of the Raman tensor.Finally,using this approach,researchers are able to determine the influence of the pressure on the different components of the Raman tensor of the ReS₂ crystal.In conclusion,this dissertation presents a simple,efficient,clean,nondestructive,and affordable measurement method that is expected to be widely used in the field of anisotropy research.The in-plane and out-of-plane Raman optical anisotropy of ReS₂ is deeply explored,providing clear experimental evidence for identifying vibrational nature of the Raman modes of ReS₂ and offering important insights for studying the vibration properties of Raman modes in layered anisotropic crystals.Additionally,the evolution process of in-plane and out-of-plane Raman spectra of ReS₂ under high pressure is studied,presenting new evidence for the significant tunability of crystal structures induced by pressure engineering.This experimental method also provides additional freedom for the study of pressure engineering of 2D anisotropic materials.