Band Structure,Raman Spectroscopy And Optoelectronic Devices Based On Two-dimensional Materials

Two-dimensional materials(2DMs)have attracted great research interest due to their unique properties of mechanics,thermotics,optics,electronics and electromagnetism.These excellent properties are due to their unique two-dimensional characteristics and novel interlayer interactions.Exploring the elementary excitations in 2DMs,including electrons,phonons,excitons,plasmons,and their interactions,are the basis to understand new physics and engineer 2DMs based optoelectronic devices.In this paper,we employ first-principles calculations,Raman spectroscopy and micro-nano fabrication technology to systematically study a series of promising 2DMs as well as their heterostructures.The influences of electrons,excitons,phonons,etc.in 2DMs regarding to the properties of optoelectronic devices have been discussed.The content of the thesis is mainly divided into the following five parts:(1)Strong coupling in graphene/black phosphorus heterostructure.Based on first-principles calculations of density functional theory,the electronic bandgap properties of graphene/black phosphorus heterostructures are studied.The results show that charges are transferred from graphene to black phosphorus with the formation of the heterostructure,and the heterostructure forms some band gaps in the band structure,indicating the existence of strong coupling effect in the van der Waals heterostructure.Then,we establish a tightly bound Hamiltonian model with combination of the charge density symmetry of the Bloch wave function.The calculation results find the Bloch wave resonance at a specific K point between the graphene and black phosphorus layers,which explains the strong electronic coupling in van der Waals heterostructure.This work improves the understanding of the interaction and composition mechanism of the 2D van der Waals heterostructure,and offers a way to further optimize their optoelectronic applications.(2)Study on the lattice dynamics of indium-selenide-bromide(In Se Br).The strong exciton-phonon coupling has been found in the new two-dimensional ternary layered semiconductor In Se Br with Raman spectroscopy.By employing the polarization-resolved Raman spectroscopy,three Raman modes in In Se Br including two A_lg modes and one E_g mode have been observed.Then,the strong exciton-phonon coupling has been obtained by combining temperature-dependent Raman spectroscopy and density functional theory based first-principles calculations.The results provide a way for the development and engineering of new optoelectronic devices based on two-dimensional ternary layered semiconductor materials.(3)Study on anisotropic Raman spectroscopy of red phosphorus(RP).We have experimentally found giant optical anisotropic quantum efficiency in RP.The anisotropic optical properties of RP with polarization-resolved Raman spectroscopy,photoluminescence spectroscopy and third harmonic measurement have been experimentally studied the exciton quantum efficiency and third-order nonlinear effects of RP are much high with 40 times and100 times,respectively,comparing with monolayer Mo S₂.The coexistence of exceptionally large optical anisotropy and huge quantum efficiency in red phosphorus will make a huge contribution to new technological innovation.(4)Study on surface enhanced Raman scattering based on two-dimensional materials.The enhancement effects of Raman scattering in organic dye molecule Rhodamine 6G(R6G)have been found with assistance of tungsten disulfide(WS₂)and graphene.First,the Raman peak of R6G on the surface of WS₂ is significantly enhanced.the Raman enhancement effect of R6G is characterized by comparing different parameters such as excitation wavelengths,layer thickness and concentration of R6G.The results show that when the excitation light wavelength is 532 nm(B exciton of a single-layer WS₂),the enhancement amplitude is the highest,indicating that the exciton-phonon coupling can significantly enhance R6G Raman scattering.Secondly,the Raman scatterings of R6G on the graphene are characterized under different excitation light wavelengths,thickness,and R6G concentrations.The results show that single-layer graphene allows a significant enhancement to the Raman spectrum of R6G,while the enhancement is independent on the excitation light wavelength,indicating that the charge transfer induced surface enhanced Raman scattering of R6G on the graphene surface.(5)Two-dimensional material based optoelectronic devices and characterizations.First,graphene-based field effect transistors and modulators are fabricated and employed to realize the Raman modulation.When the gate voltage is changed from 0 V to 60 V,the peak position of the G peak(1580 cm^-1)enables red shifts with?5 cm^-1,while the gate voltage changes from 0 V to-60 V,it allows red shifts with 10 cm^-1.On the other hand,the 2D peak(2700cm^-1)barely shifts when the gate voltage changes from-60 V to 60 V.Second,a 1T-phase tantalum disulfide(Ta S₂)field effect transistor is fabricated,and the charge density wave phase transition of 1T-phase Ta S₂ was observed from both optical reflectance and resistance.