Because of the strong Coulomb interaction and quantum confinement effect,2-dimensional transitionmetal dichalcogenides possess a stable excitonic population.To realize excitonic device applications,such as excitonic c...Because of the strong Coulomb interaction and quantum confinement effect,2-dimensional transitionmetal dichalcogenides possess a stable excitonic population.To realize excitonic device applications,such as excitonic circuits,switches,and transistors,it is of paramount importance for understanding the optical properties of transition metal dichalcogenides.Furthermore,the strong quantum confinement in 2-dimensional space introduces exotic properties,such as enhanced phonon bottlenecking effect,many-body interaction of excitons,and ultrafast nonequilibrium exciton-exciton annihilation.Exciton diffusion is the primary energy dissipation process and a working horse in excitonic devices.In this work,we investigated time-resolved exciton propagation in monolayer semiconductors of wSe_(2),MowSe_(2),and MoSe_(2),with a home-built femtosecond pump-probe microscope.We observed ultrafast exciton expansion behavior with an equivalent diffusivity of up to 502 cm^(2)s^(-1)at the initial delay time,followed by a slow linear dffusive regime(20.9 cm^(2)s^(-1))in the monolayer WSe_(2).The fast expansion behavior is attributed to energetic carrier-dominated superdiffusive behavior.We found that in the monolayers MowSe_(2)and MoSe_(2),the energetic carrier-induced exciton expansion is much more effective,with diffusivity up to 668 and 2295 cm^(2)s^(-1),respectively.However,the"cold"exciton transport is trap limited in MowSe_(2)and MoSe_(2),leading to negative diffusion behavior at later time.Our findings are helpful to better understand the ultrafast nonlinear diffusive behavior in strongly quantum-confined systems.It may be harnessed to break the limit of conventional slow diffusion of excitons for advancing more efficient and ultrafast optoelectronicdevices.展开更多
基金financially supported by the National Natural Science Foundation of China(no.62075115)Tsinghua University Initiative Scientific Research Program.
文摘Because of the strong Coulomb interaction and quantum confinement effect,2-dimensional transitionmetal dichalcogenides possess a stable excitonic population.To realize excitonic device applications,such as excitonic circuits,switches,and transistors,it is of paramount importance for understanding the optical properties of transition metal dichalcogenides.Furthermore,the strong quantum confinement in 2-dimensional space introduces exotic properties,such as enhanced phonon bottlenecking effect,many-body interaction of excitons,and ultrafast nonequilibrium exciton-exciton annihilation.Exciton diffusion is the primary energy dissipation process and a working horse in excitonic devices.In this work,we investigated time-resolved exciton propagation in monolayer semiconductors of wSe_(2),MowSe_(2),and MoSe_(2),with a home-built femtosecond pump-probe microscope.We observed ultrafast exciton expansion behavior with an equivalent diffusivity of up to 502 cm^(2)s^(-1)at the initial delay time,followed by a slow linear dffusive regime(20.9 cm^(2)s^(-1))in the monolayer WSe_(2).The fast expansion behavior is attributed to energetic carrier-dominated superdiffusive behavior.We found that in the monolayers MowSe_(2)and MoSe_(2),the energetic carrier-induced exciton expansion is much more effective,with diffusivity up to 668 and 2295 cm^(2)s^(-1),respectively.However,the"cold"exciton transport is trap limited in MowSe_(2)and MoSe_(2),leading to negative diffusion behavior at later time.Our findings are helpful to better understand the ultrafast nonlinear diffusive behavior in strongly quantum-confined systems.It may be harnessed to break the limit of conventional slow diffusion of excitons for advancing more efficient and ultrafast optoelectronicdevices.
