Ultrafast Optical Control of Exciton Diffusion in WSe$_2$/Graphene Heterostructures Revealed by Heterodyne Transient Grating Spectroscopy

TL;DR

This study demonstrates ultrafast optical control of exciton diffusion in WSe$_2$/graphene heterostructures, revealing a significant increase in diffusion constant and dynamic screening effects on picosecond timescales, with implications for ultrafast optoelectronics.

Contribution

It introduces a method to optically tune exciton diffusion in 2D heterostructures on ultrafast timescales, highlighting the role of charge transfer and screening effects.

Findings

Diffusion constant increases from 2 to 40 cm$^2$/s under optical control.

Diffusion dynamics can be tuned within a few picoseconds.

Interlayer interactions influence exciton diffusion behavior.

Abstract

Using heterodyne transient grating spectroscopy, we observe a significant enhancement of exciton diffusion within a monolayer WSe$_2$ stacked on top of graphene. We further demonstrate that the diffusion dynamics can be optically tuned on the ultrafast time scale (i.e. a few picoseconds) by altering the photoexcited charge carrier density in graphene. The results reveal that, on a time scale of a few picoseconds, the effective diffusion constant in the WSe$_2$/graphene heterostructure is approximately 40 cm$^2 $/s, representing a substantial improvement over the 2 cm$^2 $/s typical for an isolated monolayer of WSe$_2$. The enhanced diffusion can be understood in terms of a transient screening of impurities, charge traps, and defect states in WSe$_2$ by photoexcited charge carriers in graphene. Furthermore, we observe that the diffusion within WSe$_2$ is affected by interlayer interactions, like charge transfer, exhibiting different dynamic states depending on the incident excitation fluence. These findings underscore the dynamic nature of screening and diffusion processes in heterostructures of 2D semiconductors and graphene and provide valuable insights for future applications of these systems in ultrafast optoelectronic devices.