  Carrier transport in nanodevices plays a crucial role in determining their
functionality. In the post-Moore era, the behavior of carriers near surface or
interface domains the function of the whole devices. However, the femtosecond
dynamics and nanometer-scale movement of carriers pose challenges for imaging
their behavior. Techniques with high spatial-temporal resolution become
imperative for tracking their intricate dynamics. In this study, we employed
transient reflectivity microscopy to directly visualize the charge separation
in the atomic interface of WS2-WSe2 in-plane heterojunctions. The carriers'
drifting behavior was carefully tracked, enabling the extraction of drift
velocities of 30 nm/ps and 10.6 nm/ps for electrons and holes. Additionally,
the width of the depletion layer was determined to be 300 nm based on the
carriers' moving trajectory. This work provides essential parameters for the
potential effective utilization of these covalent in-plane heterojunctions,and
demonstrates the success of transient optical imaging in unraveling the
electrical behavior of nano devices, paving the way for a new avenue of
electro-optical analysis.
