Ultrafast Spectroscopic Study On Photophysical Properties Of Two-dimensional WSe₂ Semiconductor

Two-dimensional(2D)transition metal dichalcogenides materials(TMDCs)have attracted extensive attentions because of their ultrathin thickness and unique optoelectronic properties.Due to the ultrathin nature and high carrier mobility of 2D TMDCs,they can be used to fabricate low power consumption,high performance and portable devices.As a member of TMDCs,monolayer WSe₂ has a high hole mobility,photoluminescence(PL)quantum yield,on-off ratio and good stability.Therefore,monolayer WSe₂ can be used to design various van der Waals heterojunctions and fabricate various stable optoelectronic devices.In addition,exciton-related processes play an important role in optoelectronic applications of 2D TMDCs.However,comparative studies on the electronic structure evolution and exciton energy relaxation processes of WSe₂ layers from the perspective of transient experiments are scarce.In this thesis,we systematically study the effects of internal and external factors on the dynamics of excitons in WSe₂ layers from the perspective of transient experiments through a femtosecond pump-probe system.The internal factor includes the increase of WSe₂ layers,the external factors include the chemical treatment and the form of heterojunctions with other 2D materials,etc.The main research contents of this thesis are listed as follows:1.Organic super acid treated two-dimensional WSe₂ has attracted much attention due to its large photoluminescence quantum yield,high mobility,and good stability.However,there are few reports on the effect of organic super acid treatment on the monolayer WSe₂ exciton dynamics.Using broadband transient absorption(TA)spectra obtained by a femtosecond pump-probe system,we determine the dynamics of previously controversial A'and C excitons in monolayer and bulk WSe₂.In addition,we also observe the hole relaxation process between the two spin-split states of the valence band maximum in the monolayer WSe₂ treated with organic super acid treatment.We find that the monolayer WSe₂ with organic super acid treatment does not change the peak positions of the excitonic states,but enhance the oscillator strength of excitonic states which leads to a significant increase in the intensity of these bleaching peaks,corresponding to stronger steady-state photoluminescence.This could be related to the strain release caused by the defect repair effect during the organic super acid treatment.Our work provides a useful way to study exciton dynamics in 2D layered semiconductors.2.The exciton states of WSe₂ show a dependence on the number of layers.We use a femtosecond pump-probe system to excite and probe monolayer,bilayer,tri-layer and bulk WSe₂,respectively,and study the electronic structure evolution,exciton energy relaxation of WSe₂ samples with different layers from the perspective of transient experiments,exciton energy shifting dynamics and power-dependent spectral properties.The TA spectra of the samples at different delay time were measured by broadband probe pulses from 400 to 800 nm,and the spectral evolution of the exciton ground state bleaching(GSB)signal was recorded.With the increase of layer thickness,the quantum confinement effect becomes weaker,but the Coulomb screening effect is enhanced.In addition,the interlayer interactions in odd and even layers of 2D TMDCs are also different.All of these will affect the exciton resonance states of WSe₂.As a result,the GSB peaks of A,A'and C excitons in WSe₂ sample show red-shift,but the GSB peaks of the B exciton are almost unchanged.Furthermore,we investigate the A exciton energy shift dynamics in WSe₂ samples from the hot-exciton perspective.Under high-energy excitation,the energy shift dynamics of A exciton can be divided into three processes:hot excitons formation,hot excitons cooling and excitons recombination.The excitons recombination processes include exciton-exciton annihilation and cooling excitons recombination.This provides an in-depth perspective for studying hot-excitons related processes in 2D TMDCs.3.In order to understand the commonalities and differences in spin dynamics between monolayer and few-layer TMDCs and realize novel spintronic devices based on TMDCs,it is necessary to perform a comprehensive study of the ultrafast dynamics of spin generation and relaxation in these materials.We study the depolarization dynamics of the exciton spin degrees of freedom in monolayer WSe₂ and WSe₂/graphene heterojunction by circularly-polarized ultrafast spectroscopy,respectively,and propose the corresponding valley depolarization mechanism to describe coherent/collision spin relaxation dynamics.Furthermore,the effect of graphene on the spin/valley polarization dynamics of the A exciton in monolayer WSe₂has been further investigated.We find that graphene prevents the formation of the charged states in WSe₂ by extracting the electrons in WSe₂,and further affects the valley depolarization of the A exciton in monolayer WSe₂/graphene heterojunction.Both A'and B excitons exhibit negative spin/valley polarization in monolayer WSe₂ and WSe₂/graphene heterojunction.This could be attributed to two physical mechanisms:1.Two-photon absorption and stimulated Boson scattering;2.Effective Dexter-like intervalley coupling between A and B/A'excitons.The former one plays a critical role.4.Since the bilayer WSe₂ provides an additional layer degree of freedom,we further investigate the effect of interlayer interaction on the depolarization dynamics of the WSe₂ and WSe₂/graphene heterojunction by probing the bilayer WSe₂ and bilayer WSe₂/graphene heterojunction.We find that the contribution of interlayer transfer to the A exciton depolarization is relatively small,so the bilayer can be regarded as two individual monolayers with weak interaction.However,the intervalley coupling is weakened due to the larger dielectric screening of the bilayer than that of the monolayer.These results have important implications for understanding the ultrafast spin accumulation and relaxation processes in layered TMDCs.