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Bound electron-hole pairs, or excitons, are the workhorses in layered transition metal dichalcogenide semiconductors. Like the negative and positive charge carriers from which it forms, the exciton exhibits great mobility that ultrafast transient diffusion is required for ultrafast information processes.

Scientists at Tsinghua have been able to directly observe the ultrafast motion of nonequilibrium exciton in monolayers WSe₂, MoWSe₂, and MoSe₂, immediately after they are excited with a femtosecond laser with the home-built pump-probe microscope—and it was found that these excitons travel at least 200 nm within 1 ps, much faster than previously expected.

"Generally, exciton diffusion is a linear process driven by the population gradient, where excitons migrate from high concentration region to low concentration region," says Zhou at Tsinghua University. "We observe that exciton diffuses much faster than expected at the early time, we call it superdiffusion."

Superdiffusion occurs in very short time scales. The effective diffusivity coefficient of this process can reach up to 102—103 cm² s^−1. This super-diffusive behavior improves the spatial migration of excitons, which is expected to break the traditional limitation of photovoltaic efficiency, or could be used for ultrafast electronic devices due to its nature of ultrashort time scales.

This work is 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.

The research is published in the journal Ultrafast Science.