Study On Raman Scattering And Optical Absorption Ehancement Of Two-dimensional Materials

The micromechanical exfoliated graphene opened the research field of two-dimensional?2D?materials.Compared with the conventional electronic and optical materials,2D materials show many unique properties such as record-high charge carrier mobility,strong light-matter interaction,and superior mechanical properties.What's more,because that there is no hanging bond on its surface,2D materials can directly form heterojunctions with silicon substrate via van der Waals epitaxy without worrying about the lattice mismatch of the traditional heterojunctions,which implies promising applications for novel devices.Raman spectroscopy is an important means to characterize the properties of two-dimensional materials.The optical absorption efficiency of optoelectronic devices directly affects their working efficiency.However,because of the ultra-thin properties of single-layer two-dimensional materials,most of the incident light is transmitted to the sample,and only a small part of the light is scattered and absorpted.The absorptivity of suspended monolayer graphene in visible and near infrared bands is only 2.3%.As for two-dimensional TMDCs such as MoS₂,its absorptivity is also only about 10%.Weak Raman signal and low optical absorption limit the further application of two-dimensional materials.In this paper,the Raman scattering and light absorption enhancement techniques of graphene and 2D TMDCs are studied.The main content of the thesis is as follows.The properties of two-dimensional materials are closely related to their thickness.Optical characterization methods have the advantages of nondestructive and efficient.Firstly,we reviewed the optical characterization methods of two-dimensional materials and introduced the basic principles of the optical characterization methods such as optical contrast,Raman spectroscopy,photoluminescence,multiphoton imaging and hyperspectral imaging.Secondly,the transfer matrix method is used to deduce the formula for calculating the Raman scattering intensity of two-dimensional materials on multi-layered dielectric substrates.The optimum thickness of Al₂O₃/Ag dielectric substrates for Raman enhancement is determined by theoretical calculation with MATLAB.The results show that the Raman signal of monolayer graphene on a specific Al₂O₃/Ag dielectric substrate is about 6 times higher than that on the 300 nm SiO₂/Si substrate.Importantly,there is not much difference between the experiemtal and the theoretical result,which shows that the theoretical calculation and experimental measurements are consistent.In addition,compared with the commonly used 300 nmSiO₂/Si substrates,the optical contrast of graphene on the Al₂O₃/Ag dielectric substrate is larger.Thirdly,combine the metal micro-nano arrays with monolayer MoS₂.By exciting the plasma of the metal surface,the surface electric field of MoS₂ is enhanced,and the optical absorption of MoS₂ in the visible range is enhanced.Furthmore,we investigated the effects of geometric parameters p,h and g of micro-nano arrays on the optical absorptivity of monolayer MoS₂ and proposed a model to enhance the optical absorption of MoS₂ in a wider spectrum range.Finally,based on the magnetic resonances,we propose a peridodic metal-dielectric-metal nanostructure.This structure can enhance the absorption of graphene in the near infrared band by increasing the local field.By making use of the software of COMSOL,we found the absorptivity in 1550 nm was up to 68%.In addition,we systematically studied the effect of structural parameters of the nanostructure on the absorption characteristics of monolayer graphene.It was found out that the absorption spectra of the graphene could be adjusted by changing the structural parameters of the nanostructure.With the help of inductor-capacitor?LC?equivalent circuit model,the mechanism towards the change of the absorption spectra was successfully explained.