Study On Exciton Luminescence Properties Of Two Dimensional Transition Metal Dichalcogenides

Since graphene was successfully exfoliated into a single atomic layer,two-dimensional materials have attracted widespread attentions due to their novel optoelectronic properties.Among them,two-dimensional transition metal dichalcogenides have attracted great interest in scientific research in the past decade due to their large exciton binding energy,strong spin-orbit coupling,and direct band gap in the visible light region.In addition,the phenomenon of single-photon emission has been found in two-dimensional transition metal dichalcogenides,thus extending the single-photon source to two-dimensional systems,but the origin of these quantum emitters is still not very clear.Besides,two-dimensional transition metal dichalcogenides possess two degenerate but non-equivalent valleys due to the broken inversion symmetry.Its valley characteristics are one of the most attractive features of two-dimensional transition metal dichalcogenides.However,the valley polarization,one of the characteristics of valleys,is not very high,which limits the application of valleytronics.Based on the above-mentioned problems,our main research work is as follows:1.We observed defect-related single-photon emitters in single-layer WSe₂ through polarization-resolved magneto-optical spectroscopy,and proposed a theoretical model to explain the underlying physical images.In this experiment,three types of single-photon emitters were observed.They have different g-factors and fine structure splittings.The mechanisms are that these quantum emitters come from different defect energy levels and energy band transitions,resulting in different g factors.And the wave functions of electrons and holes originating from different transitions between different defect energy levels,conduction bands and valence bands have different spatial overlaps,resulting in different fine structure splittings.This work combined experiments explained the recombination mechanism and further clarified the origin of these quantum emitters in layered two-dimensional materials.2.In the twisted WSe₂/WSe₂ homostructures,we observed localized interlayer excitons with very narrow line widths of about 100 to 200μe V.By performing polarization-resolved photoluminescence spectroscopy measurements with angle dependent magnetic field configurations at low temperatures,we observed the anisotropy of the g-factor of these localized interlayer excitons.And based on the spin-correlated orbital current model,the spatial orientations of the electric dipole moments of these interlayer excitons are determined.We prepared three WSe₂/WSe₂homobilayers with different twist angles,and observed that the twist angles between bilayers will affect the spatial orientation of the electric dipole moment of the localized interlayer excitons.Our work not only proves the different spatial orientations of the dipole moments of the localized interlayer excitons in the twisted WSe₂/WSe₂homobilayers,but also expands the energy range and freedom of manipulating these quantum emitters in a two-dimensional system.3.We use LaMnO₃（LMO）ferromagnetic films and monolayer WS₂ to construct WS₂/LMO heterostructure.The valley polarization of WS₂ under non-resonant excitation can be enhanced to 80%at 4.2 K,which is much higher than that for monolayer WS₂ on Si O₂/Si substrate with a valley polarization of 15%.Furthermore,the greatly enhanced valley polarization can be maintained to a high temperature of about 160 K with a valley polarization of 53%.In addition,the temperature dependence of valley polarization is strongly correlated with the thermomagnetic curve of LMO,indicating an exciton-magnon coupling between WS₂ and LMO.We introduced a simple model to illustrate the underlying physical mechanism.Besides,we also observed two interlayer excitons with opposite valley polarizations in the WS₂/LMO heterostructure,which was caused by the spin-orbit coupling induced splitting in the conduction band of monolayer WS₂.Our research results prove that the valley properties of transition metal dichalcogenides can be manipulated by constructing a ferromagnetic van der Waals heterostructure,providing an effective and simple method for the practical valleytronic applications of two-dimensional transition metal dichalcogenides.