Study On Ultrafast Carrier Heat Transport In Two-dimensional WS₂ Based On Femtosecond Pump-probe Technology

Tungsten disulfide（WS₂）is a kind of van der Waals layered transition metal chalcogenides（TMDCs）.Different from graphene and other two-dimensional materials,two-dimensional TMDCs have unique properties such as stable exciton at room temperature,thickness-dependent bandgap and spin-orbit coupling.They are widely used in the field of optoelectronics.Due to the influence of electronic structure and external environment,carriers will show different dynamics after electric or optical excitation TMDCs.The efficiency of charge collection in devices largely depends on the competition between different relaxation channels of photogenerated carriers.After non-resonant excitation of semiconductors,photogenerated high-energy hot carriers quickly dissipate the excess energy into the lattice,reduce the device’s efficiency and increase heat production.If hot carriers can be collected as much as possible before their energy loss,the device’s efficiency will be greatly improved.In this sense,this paper systematically focuses on the spatiotemporal dynamics of photogenerated hot carriers and excitons in WS₂;The in-plane transport modes of hot carriers and excitons in WS₂ are analyzed,and an ultrafast cascade process is found;The theoretical model of heat conduction in space-time domain is established;The relationship between macroscopic heat transport and microscopic hot carrier and exciton transport is revealed.Specific results are as follows:1.Based on femtosecond pump-probe technology,transient absorption spectra of WS₂ were measured.By analyzing the spectral characteristics and relaxation dynamics curves,the composition and relaxation process of hot carriers and excitons in WS₂irradiated by femtosecond laser are determined.The hot carrier cooling and band-edge exciton generation processes with sub-ps lifetime are found.2.Through spatiotemporal-resolved transient absorption microscopy imaging,it is observed that the hot carrier in WS₂ expands more than 500 nm in the initial 1.4 ps,and its average expansion velocity is close to 1000 cm² s^-1.In the subsequent period of 1.4-2ps,this ultrafast expansion process decays rapidly,and finally transitions to the linear exciton diffusion in the time window greater than 2 ps.Combined with the relaxation curves,it is found that hot carrier expansion and linear exciton diffusion correspond to fast（sub ps）and slow（hundred ps）relaxation processes,respectively.Further,through the power-varying experiment,it is found that the rate of initial hot carrier expansion is strongly related to the excitation power,while the linear exciton diffusion rate has little relationship with the excitation power.These indications indicate that the cascade diffusion of hot carriers and band-edge excitons occur in WS₂ under laser irradiation.Finally,the universality of this transport process in the system is verified by changing the WS₂ layer thickness and substrate.3.Through the heat conduction equation and thermo-optic conversion process based on the two-temperature model of carrier and lattice,the spatiotemporal-resolved optical response of WS₂ is simulated.The micro transport of photogenerated carriers is simulated through macro heat conduction process.The ultrafast expansion of hot carriers and the linear diffusion process of excitons are theoretically calculated.The calculated results are in good agreement with the measured results,which proves the phenomenon of the cascade process.Through the adjustment of thermodynamic parameters,the diffusion mode of photogenerated carriers can be well regulated and predicted,which reflects the applicability of the theoretical model.