Study On High Pressure Modulation Of Excitons And Lattice Vibrational Behavior In MoSe₂ And Heterostructure

Transition metal chalcogenides（TMDs）with graphene-like structure have excellent optical and electrical properties,which hold broad application prospects in the field of optoelectronics and electricity.They have attracted widespread concern of the academic community.Due to the effect of the interlayer coupling,when thinning down bulks to monolayers,the energy band structure of TMDs transform from an indirect band gap to a direct band gap.The fundamental optical excitation of such monolayer TMDs exhibit obvious excitonic characteristics.Due to the large carrier effective masses,stronger quantum confinement,and reduced dielectric screening,the binding energies of excitons and trions in monolayer TMDs can be as high as a few hundred meV,an order of magnitude larger than conventional semiconductor quantum wells.Monolayer TMDs serving as quasi two-dimensional（2D）systems have been applied to explore excitonic physics including electrical and strained control of excitons,interexcitonic scattering,exciton-carrier broadening,biexciton formation,and exciton valley relaxation dynamics.In addition,the van der Waals heterostructures（HSs）formed by vertically stacking two different monolayer TMDs exhibit strong exciton relaxation effect and three-dimensional（3D）interlayer exciton states,which holds potential for P-N junction,photodetector,photovoltaic cell,transistor device applications.The modulation of quasi 2D systems under external physical conditions is an extremely important part for discovering new exciton characteristics,designing and implementing a new generation of exciton devices.Application of hydrostatic pressure can effectively shorten the atomic spacing and change change the physical properties of materials.TMDs materials may produce different compressive strains in in-plane and out-of-plane direction,which can effectively modulate the properties of excitons.In this dissertation,monolayer,bilayer,bulk MoSe₂ and MoSe₂-WSe₂ HS are served as the main research objects.Through high-pressure micro-region PL and Raman spectroscopy measurements,pressure-modulated evolutionary process of exciton and lattice vibration in MoSe₂ and its heterojunction were explored,the innovative results are following:Because samples are mainly attached to substrates such as Si/SiO2,sapphire,or quartz in the current high-pressure research on few-layers TMDs,the deformation of substrate materials under high pressure will affect the properties of few-layers TMDs,and sample preparation,transfer,and filling are more difficult.In order to reduce the influence of the substrate on the intrinsic properties of the sample,it is an ideal choice to transfer few-layers TMDs on the diamond anvil surface,and directly use diamond with properties of extremely high hardness and minimal deformation as the substrate material.However,due to the small size of the diamond anvil,a new sample preparation and transfer method must be designed.Therefore,we have invented an apparatus and method of transferring 2D material to a diamond anvil cell.Using this technique,we have completed the transfer of monolayer,multilayer and bulk TMDs to piston-cylinder DAC,and realized the preparation of monolayer MoSe₂-WSe₂ HS on diamond substrate,which overcomes the drawbacks of the conventional substrate and breaks the technical barriers in measuring electrical properties of few-layers TMDs at high pressure.A high-quality monolayer MoSe₂ sample based on a diamond substrate was prepared,and high pressure study was conducted using standard mixture of（4:1）methanol/ethanol or argon as the pressure-transmitting medium（PTM）.The pressure-induced charging effect was found,and high-pressure modulation of neutral and negative excitons in monolayer MoSe₂ was achieved.In standard mixture of（4:1）methanol/ethanol PTM experiment,negative exciton undergo blue shift with pressure increasing,and blue-shift rate decreases after 3.7 GPa.In argon experiment,the neutral exciton of monolayer MoSe₂ undergo a blue shift with pressure increasing,and then split into two parts after 3.7 GPa.Combining first-principles calculations,this phenomenon is indicated as a pressure-induced conduction band transition from direct to indirect band gap,that is,conduction band minima（CBM）shifts from K to（43）point,neutral exciton transition change from K-K points to a coexistence state of K-K and（43）-K points.At the same time,negative exciton transition change from K-K points to（43）-K points after 3.7 GPa,blue-shift rate of emission decreases conspicuously.It is revealed that the pressure-induced charging effect is caused by the interaction between the H atom of alcohol mixture PTMs and Se atoms of monolayer MoSe₂.This result can be extended to other 2D TMDs,which provides a new pathway for developing strained and optical sensing devices.A monolayer MoSe₂-WSe₂ HS based on a diamond substrate was prepared,and high pressure study was conducted.The renormalization of excitonic and vibrational systems was found for the first time,and modulation transformation of the excitonic and vibrational system dimension was achieved.The MoSe₂-WSe₂ HS first undergoes a transition from spotty-coupled status to true 3D heterobilayer,and 3D interlayer exciton appeared.Upon application of high pressure,the energy band structure of MoSe₂-WSe₂ HS occured conduction band K-Λcrossover,and negative exciton of MoSe₂ in HS appeared.In aspect of lattice vibration,the 3D A₂’’modes of MoSe₂-WSe₂ HS exhibits a blue shift with the pressure increasing,out-of-plane A₁’vibrations of WSe₂ and MoSe₂ in HS got into coherent,appeared in-phase and out-of-phase modes under pressure,which corresponding to different frequency evolution trends of vibration enhancement and vibration attenuation,respectively.The result of the renormalization of the exciton and vibration can provides basis and guidance for the application of flexible excitonic devices and the quantitative measurement of interlayer coupling strength.Bilayer and bulk MoSe₂ based on a diamond substrate was prepared,and high pressure study of their exciton and lattice vibration behavior was conducted.The high-pressure PL spectra of bilayer MoSe₂ were obtained.It was found that the two characteristic peaks X_K and X_Λcorresponding to the K-K andΛ-K exciton transitions exhibited blue shift and red shift evolution under pressure,respectively.The high-pressure Raman spectra of bilayer MoSe₂ were obtained.It was found that the intralayer vibration mode E¹_2g emerges a splitting phenomenon at 12.7 GPa,revealing the pressure-induced intralayer structural distortion of bilayer MoSe₂.The high-pressure XRD diffraction and high-pressure Raman spectra measurements were performed on bulk MoSe₂,and it was found that bulk MoSe₂ possesses good structural stability.This result helps to further understand the inherent structural properties of TMDs and design optoelectronic devices with controlled structure and vibrational system.