Three-Stage Diffusion Process of Carriers in Thin InSe

Carriers play a critical role in the transport behavior and performance of 2D materials and devices, highlighting the importance of elaborating the diffusion dynamics in greater depth. This work presents a direct visualization of time-resolved diffusion process of carriers in thin indium selenide (InSe) using ultrafast transient absorption microscopy. Three distinct diffusion processes, including ultrafast hot carrier expansion, negative diffusion, and exponentially decaying slow diffusion, are found. This unexpected diffusion dynamics involves the initial giant carrier density gradient, different diffusion and cooling rate of hot carriers and of cold electrons, as well as carrier-phonon scattering. This study provides new findings on the dynamics of carrier transport in high-mobility 2D materials, and emphasize the role of carrier-phonon scattering in limiting carrier diffusion.