Chemical Vapor Transport Synthesis Of Layered Materials And Raman Scattering Study

Since the birth of graphene,two-dimensional?2D?ayered materials have triggered tremendous investigations among physicist,chemist and material scientist due to the quantum confinement in atomic thickness,whicl.brought about unique electronic and optical properties.On the other side,Raman spectroscopy technique,as a nondestructive and semi-qualitative testing tool,has been v idely used to study the lattice vibrational behavior as well as the excitations of electren and hole in solid state materials.Therefore,in this doctoral dissertation,we used Raman spectroscopy to systematically explore the various interplay between the e ectronic and structural subsystems in four 2D layered materials synthesized by chemical vapor transport methocd?CVT?.The main research contents and results are li ted as following:Polarized Raman Scattering of 2D Layered?NH₄?₂V₃O₈ and NbSe₃How to distinguish the vibrational modes for different Raman beaks is a basic and key topic for identification of new materials.The polarized Ra nan scattering need to adjust the polarization direction of the incident laser and detect the intensity of scatter???d light which has certain polarization direction relat???e to the incident laser through the polarizer,then according to intensity variation o the scattered light,the corresponding relationship between vibrational modes and Raman peaks can be finally determined.Herein,as to two CVT-grown?NH₄?₂V Os and NbSe₃ layered crystal,we made the single crystal rotating around the prop???gation direction of the incider t laser so as to change the angle between the polarizati???n direction and the axis of crystal,in the process the intensity evolution of scattered light was recorded.Combined group theory with selection rules of Raman scatt???ring,we finally clearly identified the vibrational modes for the two layered crystals vithout the information of the space orientation of crystal axis.Excitonic Fano Effect in the Raman scattering of 2D Laye???ed Ta₂NiSsThe well-known phenomenon in atom physics that the scat ering cross section for forward inelastic scattering of electrons by He atoms presents an asymmetry on either side of the main peak was explained for the first time by Far o based on interference effect in 1961.Then Fano-Anderson model was generalized and applied to various fields of physics,including condensed matter physics,nuclear physics and so on.In chapter three,through temperature-dependent Raman sca tering experiment,we discovered that the Raman peak located at 126cm^-1 in layered Ta₂NiS₅ was asymmetry.Moreover,the degree of asymmetric of the peak aggravated w th temperature decrease,which was ascribed to the excitonic Fano effect on the foundation of previous report.We fited the Raman spectra of different temperatures using the Fano line-shape function,and obtained the coupling strength coefficient of Fano interference.Polarized Raman spectra further showed that the renormalized phonon frequency at 126cm^-1 was angle-dependent,the theory analysis about Fan???effect determined that the renormalized phonon frequency was proportional to the square of the interaction matrix element between the discrete excitation and the continuum,which subsequently confirmed the anisotropic behavior of electron-phonon coupling in 2D layered Ta₂NiSs.Temperature-dependent Raman Scattering of 2D Layered Ta₂NiSsWhen the crystal's potential energy is expanded in powers of the displacement of the atoms from their equilibrium positions to cubic or quartic,the diagonalization of Hamiltonian for anharmonic crystal can lead to strong coupling between harmonic modes,which originate from the anharmonic terms in the potential energy.In the scattering process,the effects of the anharmonic terms can be summarized as follows,the harmonic normal mode frequencies ???j?suffers a complex shift ????j?+i?????j?,the real part of which gives the change in the value of the frequency,and the imaginary part of which is the reciprocal of the lifetime of the single-particle amplitude for the phonon state described by?kj?,both parts are temperature dependent.In the experiment,the change in the center of the peak is characterized by the real part of the complex shit,and the broadening of the peaks is characterized by imaginary part of the shift.In chapter four,we obtain the expression of complex shift through the process of the damping of three and four phonons.The fitting of the central peak positon as a function of T in layered Ta₂NiS₅ by the obtained expression gives the coefficients A and B.Resonant Raman scattering of 2D Layered Ta₂NiS₅When the frequency of incident or scattered light is close to the fundamental electronic transition energy,the Raman scattering cross section can be extremely enhanced.In chapter five,according to the characteristics of intermediate and final states in the light scattering process,we distinguished three different resonant cases.Then the Raman scattering cross section is derived incorporating isotropic electron-hole correlations,which are essential for intraband Frohlich scattering.In order to further obtain the actual enhancement factors,we use Green's-function formulation to treat the internal motion of the electron-hole pair.Finally,we study the triply resonant Raman scattering in layered MoS₂ with 638nm excitation.