Transient Superdiffusion of Energetic Carriers in Transition Metal Dichalcogenides Visualized by Ultrafast Pump-Probe Microscopy

Abstract

Because of the strong Coulomb interaction and quantum confinement effect, 2-dimensional transition metal 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 diffusive 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 optoelectronic devices.

Coexistence of Incoherent and Ultrafast Coherent Exciton Transport in a Two-Dimensional Superatomic Semiconductor

“…In transport regime iii, the population appears to contract on itself over 500 ps, indicating negative effective diffusion. This intriguing effect has been observed and predicted in a variety of systems including 2D materials − and molecular crystals and is explained by competing spatio-kinetic effects when multiple photoexcited species are present. Transient reflectance spectra of CsRe 6 Se 8 I 3 (Figure f), which displays a dominant response due to photoinduced bleaching of the lowest optical transition, support the notion that several species or competing processes are present: three components with subtly different spectral contributions and lifetimes of ∼0.3, 11, and 440 ps are required to model the spectral evolution via global analysis (Figure S5).…”

Application and prospect of 2D materials in photodetectors

“…In addition, the room-temperature carrier mobility of TMDCs is up to 1000 cm 2 V −1 s −1 [12]. Zhou et al directly observed the ultrafast transport of non-equilibrium excitons in monolayer WSe 2 , MoWSe 2 and MoSe 2 , which is conducive to the realization of ultra-fast optoelectronic devices [78].…”

Unveiling Spatiotemporal Diffusion of Hot Carriers Influenced by Spatial Nonuniform Hot Phonon Bottleneck Effect in Monolayer MoS2

“…26 On the other hand, the slow negative diffusion observed in monolayer Mo 1−x W x Se 2 and MoSe 2 is explained by the spatial variation of a defect-filling effect. 29 Nevertheless, direct experimental evidence supporting these interpretations is currently lacking. In this study, the carrier transport in monolayer MoS 2 (1L-MoS 2 , Sample preparation is shown in Supporting Information Section 1) was investigated through the spatiotemporal mapping of carrier density using TAM.…”

“…Our TAM results demonstrate the cascaded carrier transport dynamics that are consistent with the previous findings in multilayer WS 2 and monolayer WSe 2 . 26,29 The diffusion dynamics of hot carriers and band edge C-excitons were separately measured, providing novel insight into the negative diffusion process.…”

“…The carrier diffusion processes can be attributed to a sequential cascade of carrier transport involving the diffusion processes of hot carriers and band edge C-excitons which are consistent with the previous findings. 26,29 During the initial 0.9 ps, the primary expansion is characterized by the hot carriers, caused by a pronounced temperature gradient and consequently an enhanced heat transfer capacity. 39 After 0.9 ps, the second stage manifested as carrier spatial negative diffusion, following the predominant bang edge C-exciton diffusion.…”