Strain Control Of Ultrafast Dynamics In Two-dimensional Metal Sulfides

Two-dimensional(2D)materials were selected as one of the top ten emerging technologies by Scientific American in 2016.Each one in this material system has atomic-scale thickness and exotic optoelectronic properties.They system mainly include graphene,hexagonal boron nitride,transition metal chalcogenides,and black phosphorus.Among them,the single-layer semiconductor transition metal chalcogenides(TMDs)are usually materials with direct band gaps,which locate mostly in the visible to near-infrared light region,and have strong interactions with light.Van der Waals(vd W)heterostructures constructed by these atomically thick 2D materials via vd W force have attracted even more attentions in recent years,because they are particularly suitable for the applications of nanoscale flexible electronic and optoelectronic devices,and are expected to change the landscape of semiconductor technology.In the practical applications of these flexible materials,it is inevitable to introduce the stresses applied to them because of bending.The research on the dynamic process of 2D-TMDs and their vd W heterostructures under different stresses has attracted the attention of researchers.It has been shown by reported studies that mechanical strains,including uniaxial and biaxial strains,applied to 2D-TMDs can change the lattice constant of the material,resulting in a decrease in lattice symmetry and significant changes in the electronic band structure.Therefore,strain engineering becomes a powerful means to tune the electronic and optical properties of 2D-TMDs and their vd W heterostructures.In this thesis,ultrafast spectroscopy techniques are employed to invistigate the ultrafast carrier dynamics in monolayer WSe₂ film and WSe₂/MoS₂ heterostructures on polyethylene terephthalate(PET)substrates at different tensile strains.The materials are grown by chemical vapor synthesis(CVD)method.The main contents of the thesis are as follows:(1)The evolution of carrier dynamics with tensile strains applied in monolayer WSe₂ is studied.In addition to discussing the exciton carrier behavior,we probe the dynamic behavior of pure electrons and also find evidence for indirect bandgap dynamics in the carrier dynamics of monolayer WSe₂.In the dynamical behavior of A excitons,we detect a distinct dynamical signal induced by an increase in the carrier population,and its time constant is close to that of thermal excitons cooling and relaxation in the K-valley.From the analysis of the exciton dynamics process,we believe that the increase in carrier population arises from the accompanying charge transfer from the K_C valley to the T_Cvalley during the thermal electron cooling relaxation in the K_C valley.The subsequent thermal exciton relaxation dynamics behaviors exhibit differences in coupling with optical phonons and acoustic phonons.The detection of the dynamical behavior of the corresponding B exciton reflects the cooling relaxation process of pure hot electrons in the K_C valley.The dependence of the relaxation lifetime on pump power suggests that the interaction of hot electrons with acoustic phonons may dominate the relaxation process.However,the time constant of its kinetic process shows only a slight increasing trend with the increase of tensile stress.The time constant of its kinetic process becomes smaller with the increase of tensile stress,which may indicate that the relaxation rate of electrons is accelerated.(2)The effect of specific tensile stress on the carrier dynamics of WSe₂/MoS₂heterojunction was investigated.We use narrow-band pump light to selectively excite WSe₂ A exciton transitions and ultra-broadband light to detect MoS₂ A exciton transitions.The electron transfer signal from WSe₂ to MoS₂ in WSe₂/MoS₂heterojunction is obtained.By extracting the MoS₂ A exciton relaxation curve for fitting analysis,three dynamic processes can be obtained.Corresponding to interlayer electron transfer(around 70 fs),sub-picosecond(0.5 ps)hot electron rapid cooling(or formation of interlayer excitons),and non-radiative electron-hole(e-h)recombination process(about 100 ps).It was found that the time for the electron transfer process and the non-radiative electron-hole recombination process was prolonged with the increase of the specific tensile stress applied.Studies have shown that tensile strain reduces the electron-phonon coupling strength,thereby suppressing electron transfer and nonradiative electron-hole recombination kinetics.It is also found that the thermal electron cooling as well as the tightly bound interlayer exciton formation lifetime are insensitive to the increase in tensile strain,which may be related to the change in the electro-phonon coupling caused by the applied tensile stress.