Study On Ultrafast Dynamics Of Carriers In WS₂ And CsPbBr₃ Semiconductors

The generation,transport and recombination of photo-excited carriers can alter the properties of semiconductor photoelectric devices.It is central to exploring and understanding the dynamics of photo-excited carriers in semiconductors.Ultrafast spectroscopy is utilized as a useful experimental tool to probe the dynamics of photo-excited carriers.The transient absorption spectra measurements can provide the information of the generation,transport and recombination of photo-excited carriers in materials.This dissertation report on a systematic study on ultrafast dynamics of photo-excited carriers in monolayer TMD,van der Waals heterostructures and all-inorganic cesium lead halide perovskite via pump-probe transient absorption spectrum system.The finding are summarized as below:1.The transition metal dichalcogenides?TMDs?have received much attention,this class materials is characterized by large excition binding energy,strong Coulomb interactions and spin-valley coupling.Hence,TMDs offer excellent opportunities to probe properties of atomically thin semiconductors and their potential for applications in photoelectric.Within this context,it is important to explore the respond of TMDs after excitation.In my dissertation,the dynamics of photo-excited carriers in monolayer WS₂ were studied via pump-probe transient absorption spectrum system.We find that the excitons recombined in the form Auger type exciton-exciton annihilation,and the lifetime of excition is about 27 ps.By analyzing the photobleaching signals at 613 nm with different pump pulses?590 nm,580 nm and 570 nm,respectively?,we conform the formation of excitons in the sub-picosecond decay component.The transient absorption spectra were analyzed by tracking the line width broadening,bleaching and energy shift of A-exciton resonance as a function of time delay after the monolayer WS₂ was excited.Two main reasons for the pump-induced changes of the optical response were identified,which are the many-body interactions of the photo-excited carriers and the cooling process of material through the heat transfer to substrate by phonon.Our results revealing the dynamics of photo-excited carriers,and provide the basic information for applications of monolayer WS₂ in photo-electronic devices.2.The new 2D materials can be used to fabricate van der Waals heterostructures by simply stacking,which allow the fabrication of multilayer heterostructures without lattice matching requirement.Properties of atomically thin van der Waals heterostructures could be altered by the charge transfer across the interface.Thus gaining insights into the photo-excited carriers transfer across the interface is central to understanding and harnessing the properties of the heterostructures.In my dissertation,the graphene-WS₂ heterostructure and dual graphene-WS₂ quadrilayer heterostructures are used as examples to study the charge transfer across the interface via pump-probe transient absorption spectrum system.We found the photo-excited carriers in WS₂ can transfer to graphene efficiently,and the band-gap of WS₂ can be changed by the screening effect of adjacent layers.Our results show the transfer of photo-excited carriers from WS₂ to graphene take place within 83 fs and 114 fs for the graphene-WS₂ heterostructure and dual graphene-WS₂ quadrilayer heterostructures,and the Coulomb field of the charge can effectively affect the interlayer electron transfer,this effect indicates the charge transfer across the interface in heterostructures can be controlled by external electric field.The ultrafast transfer times support the charge transfer mechanisms via layer-coupled states and Coulomb field.Our findings also provide the basic information for understanding the photo-excited carriers transfer across the interface of van der Waals heterostructures.3.All inorganic perovskite lead halide nanocrystals have attracted tremendous attention,which have exceptional performance in light emitting devices and photoelectric conversion devices.The performance of these perovskite devices are strongly influenced by the dynamics of photo-induced carriers in perovskite.The dynamics of photo-excited carriers in CsPbBr₃ quantum dot thin films were studied via pump-probe transient absorption spectrum system,and the transient absorption spectra was analyzed by singular value decomposition and global fitting.Our results show the red-shift and blue-shift of the band edge can be attributed to the exciton-exciton interaction and Burstein-Moss effect.The results also reveal the dynamic curves and corresponding preexponential factors of five kinetics processes of photo-excited carriers.The relaxation time of hot carriers is 0.4 ps,the decay time on tens of picoseconds time scale and hundreds of picoseconds time scale can be attributed to the lifetime of biexcitons and charged excitons,and the decay of the pump-probe signal on the nanoseconds time scale can be attributed to the radiative recombination process of excitons.The experimental data and corresponding theoretical analysis can help understand the dynamics of photo-excited carriers in CsPbBr₃ quantum dot.In summary,the results and theoretical analysis of my dissertation provide solid foundation for understanding the optical and electronic properties of TMDs and perovskite lead halide semiconductors and their photoelectric applications.