| The traditional silicon semiconductor devices based on the electron charge degree of freedom are the heart of modern electronic devices and play a key role in the development of the information era.However,with the decreased sizes of devices,the short channel effect becomes evident,so it is urgent to find a novel degree of freedom as a carrier for information coding,manipulation,and transport.At present,the valley degree of freedom（valley pseudospin）found in two-dimensional transition metal dichalcogenides and their heterostructures is a potential candidate,which possesses the advantages of high integration,non-volatile and fast computing speed,and has great application prospects.As an important carrier of valley information,excitons in two-dimensional transition metal dichalcogenides and their heterostructures can be selectively excited and read by circularly polarized light,which is the basis for the development of optoelectronic devices utilizing valley degree of freedom.Exploring the fundamental properties of excitons to pursue the longer lifetime of valley pseudospins is one of the key topics in this emergent research field.With the development of research,the basic characteristics of excitons in two-dimensional transition metal dichalcogenides and their heterostructures have gradually been discovered,but there are still some unsolved problems,such as the role of excitons in angular momentum transfer between phonons and photons,and the basic properties of interlayer excitons in multilayer heterostructures.In this thesis,we investigated the exciton properties in two-dimensional transition metal dichalcogenides and their heterostructures using helicity-resolved Raman spectrum,photoluminescence spectrum,two-color pump-probe laser spectroscopy,etc.The main contents of my research works are following:1.Study on the exciton-assisted transfer of phonon angular momentum in monolayer MoS2In this work,we investigate the transfer mechanism of phonon angular momentum in monolayer MoS2 using the home-made helicity-resolved Raman spectroscopy with in-plane propagation of light.It is found that the transfer of angular momentum in Raman scattering is an exciton-assisted process:Firstly,the angular momentum of the incident photon is transferred to the exciton;then the exciton transfers the angular momentum to phonon and low-energy exciton through the deformation potential;finally,the angular momentum of the low-energy exciton is transferred to the emitting photon（Raman signals）.Through the measurements of linearly polarization-resolved Raman spectra,it is found that the scattering of photo-excited valley excitons caused by nondegenerated A1′and doubly degenerated E′phonons will preserve and destroy the valley coherence of exciton ensembles,respectively.Moreover,we found a novel Raman scattering geometry to enhance the Raman signal of two-dimensional（2D）transition metal dichalcogenides,which provides a new method for high-efficiency detection of phonon information in 2D materials.2.Study on the trilayer exciton in WSe2/WS2/MoS2 heterostructuresIn this work,we successfully fabricated a WSe2/WS2/MoS2 heterotrilayer and observed the emission of trilayer excitons composed of electrons in the MoS2 and holes in the WSe2 spanning the interlayer WS2,which demonstrated the feasibility of constructing trilayer excitons in multilayer transition metal dichalcogenides heterostructures.It is found that the trilayer exciton has a lower exciton binding energy than that in the WSe2/MoS2 heterobilayer due to the insertion of monolayer WS2,which is manifested by the blueshift of this trilayer exciton peak relative to the bilayer exciton peak.Through the measurements of the exciton peak positions as a function of excitation power,it is found that the binding energy of trilayer exciton is～8 me V lower than that of bilayer exciton.Moreover,the trilayer exciton has a longer lifetime（～5 ns）than the bilayer exciton（～2 ns）resulting from the reduction of electron-hole wavefunction overlap. |
