Ultrafast Dynamics And First Principles Study On Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides has a unique sandwich atomic layer structure and shows many novel physical phenomena in photoelectric properties,including exciton effect,many-body effect and topological quantum spin Hall effect,and so on.The future nano field effect transistors,nano photoelectric sensors and non-dissipative conductive devices will depend on these unique properties.At present,the carrier dynamics and many-body effect of these materials have been widely and deeply studied by ultrafast spectroscopy,which builds a foundation for their further application in optoelectronic devices.At the same time,the first principle predicts the essence of many physical phenomena in this kind of materials,among which the discovery of Z₂ topological properties provides a new member for the topological insulator family.Their non-dissipative topological edge state is of great significance in both the current basic physics research and future energy-saving applications.The current research shows that the early exciton formation process of 1H phase transition metal dichalcogenides is still controversial.The study of their carrier process is helpful to reveal the physical response essence of optoelectronic devices.In addition,there is a lack of detailed discussion on the edge states of two-dimensional topological materials,which affects the observation and application of topological states.In this work,we discuss the carrier dynamics of 1H phase WS₂ and the edge state of 1T'-MoS₂ to further clarify their internal physical image.The details are as follows:(1)We applied femtosecond time-resolved transient absorption spectroscopy to reveal the nonequilibrium dynamic process of initial photogenerated electron and hole separation,during the formation of the lowest energy exciton in the K valley of the energy band of 1H phase manylayer WS₂,so as to distinguish the thermal equilibrium and cooling process of charged carriers,and the influence of the accompanying many-body effect on the exciton resonance peak.Firstly,the thermal equilibrium relaxation of holes is faster than that of electrons,and the observed dependence of thermal equilibrium time on pump fluense can prove that they are mainly carried out by carrier-carrier scattering.At the same time,the different dependence of the decay time constant of electrons and holes on the pump fluence indicates that the cooling process of electrons after thermal equilibrium may be dominated by acoustic phonons.While for holes,it is mainly LO phonons.Under the excitation of a exciton resonance,we observed that the exciton resonance peak reverse from red shift to blue shift at first,and then transits to slow blue shift.This rapid red shift process may be due to the strong screen caused by the Coulomb interaction between quasi-free charged carriers in electron hole plasma.The subsequent slow blue shift is the comprehensive result of many-body effect competition in the process of thermal exciton cooling.Our findings clarify the ultrafast dynamics of selective carriers and their many-body effects,which provides a new possibility for the development of optoelectronic devices,based on the transmission characteristics of single type carriers.(2)Then we also discuss the edge state of 1T'-MoS₂ in the framework of first principles,which is a well-known QSH insulator with large band gap.Based on the fact that the zigzag edge has higher device utilization in representing the topological state than the armchair edge,we used the zigzag nanoribbons to study the edge state of 1T'-MoS₂.Combined with the analysis of time inversion symmetry,we observed two types of conductive edge states on the energy band of the optimized nanoribbons,including topological edge state and dangling-bond edge state.The pure topological surface state of the energy bands on nanoribbons without structural optimization shows that the edge state of the dangling-bonds needs the reconstruction of the edge structure.Through atomic orbital projection analysis,we find that the dangling-bonds originates from the metal atom at the edge.In order to obtain a pure topological surface state at the real boundary,we use hydrogen passivation with specific configuration for metal atoms,and finally make the trivial conductive edge state disappear.At the same time,the spin polarization of the real boundary is discussed,which further reveals the relationship between topological spiral state and topological dispersion,and there are differences in the dependence on the width of bulk.