Theoretical Study On Raman Scattering In 2D Materials

In recent years,two-dimensional materials have been extensively studied due to their unique physical and chemical properties.Unlike bulk materials,electrons in two-dimensional materials can only move freely in the two-dimensional atomic planes,but are subject to quantum constraints in the third dimension,which makes two-dimensional materials exhibit many novel characteristics in terms of electrical,optical,thermal,and mechanical properties.Raman spectroscopy,as a fast,non-destructive,and highly sensitive analysis method,is widely used in the study of the structure and performance of two-dimensional materials.Non-resonant Raman spectroscopy is often used to determine the number of layers of two-dimensional materials.However,there are different theoretical explana-tions for the variation of the Raman frequency shift with the number of layers in the existing literature.Further research is needed to analyze which explanation is more general for two-dimensional materials.Resonance Raman has a complicated physical process,and most of the literature studies on it either adopt approximate models,with limite application scope,or qualitative analysis based on experimental Raman,lacking in-depth understanding of the mechanism.Therefore,it is necessary to start from the first principles and develop a universal and quantitative calculation method for reso-nance Raman simulation.Many literatures have reported the unconventional frequency red-shift of in-plane Raman mode E2g of transition metal dichalcogenides(TMDCs)system with increasing number of layers.There are two different theoretical explanations for this phenomenon:Davydov splitting and charge screen effect.Using Placzek’s polarizability theory,this paper studies the non-resonant Raman of two two-dimensional material systems of h-BN and InI.It is found that the Raman peaks corresponding to the in-plane vibration modes of these two types of materials also gradually redshift with the increase of the number of layers.Considering the above two theoretical models separately,it is found that Davydov splitting cannot explain the abnormal Raman redshift in h-BN,while the charge screen effect can perfectly explain the abnormal phenomenon in h-BN and InI materials.It is concluded that the charge screen effect is more general in explaining the abnormal redshift phenomenon of the in-plane Raman mode with increasing layer number for various two-dimensional materials.Under the framework of time-dependent perturbation quantum theory,through programming,we developed a code for simulating and analyzing resonance Raman spectrum and applied this code to the resonance Raman calculation of different TMDCs materials.In this paper,the circularly polarized single resonant Raman of monolayer MoS2 is calculated,and the E’ Raman peak that appears when the incident and scat-tered light have the same chirality under high laser energy is obtained.The Raman tensor calculated by the code is used to explaine this phenomenon.This paper calcu-lates the single resonance Raman of single-layer ReS2.By designing different physical processes in the code,it is found that the quantum interference effect in the Raman scattering processes leads to the appearance of chiral Raman in the achiral system.In this paper,the double resonance Raman of monolayer MoTe2 is also calculated,and the mode combination of 6 second-order Raman peaks is quantitatively calculated.This work provides a quantitative calculation method and a strong theoretical basis for the identification of the second-order Raman peak that has been controversial.Throughtout the paper,the results calculated by our code are all in good agreement with the relevant experimental data,which shows the reliability and universality of this code.