# In Situ Imaging Reveals Efficient Charge Separation in Monolayer MoS2–WS2 Type-II Heterojunctions

## TL;DR

This study uses imaging techniques to show how charge separates efficiently in a specific 2D material junction, which could improve solar energy and environmental applications.

## Contribution

The paper provides direct experimental evidence of type-II charge separation in in-plane MoS2–WS2 heterojunctions.

## Key findings

- Electrons accumulate in MoS2 while holes accumulate in WS2, showing asymmetric interfacial kinetics.
- In-plane heterojunctions show the largest photovoltage contrast compared to vertical and monolayer structures.
- The interface acts as a recombination center, limiting efficient carrier extraction.

## Abstract

Covalently bonded
in-plane two-dimensional (2D) transition metal
dichalcogenide (TMD) heterojunctions with atomically sharp interfaces
hold great promise for photocatalytic applications in solar energy
conversion and environmental remediation; however, their spatially
resolved charge distribution and transport, particularly under operando
conditions, remain poorly understood. Here, we employ photoscanning
electrochemical microscopy (photo-SECM) to directly visualize photoinduced
charge separation in monolayer MoS2–WS2 in-plane heterojunctions. Spatial separation of photogenerated carriers
is observed, with electrons accumulating in MoS2 and holes
in WS2, leading to strongly asymmetric interfacial kinetics:
Fc+ reduction proceeds rapidly on MoS2 (0.6
cm s–1), whereas Fc oxidation on WS2 is
significantly slower (0.008 cm s–1). High-resolution
surface photovoltage microscopy (SPVM) enables a quantitative comparison
of charge-separation capacity across architectures. The in-plane MoS2–WS2 heterojunction shows the largest photovoltage
contrast (−35 mV in MoS2, 20 mV in WS2), exceeding the vertical heterojunction (−18 mV in MoS2, 11 mV in WS2) and the individual monolayers (−12
mV for MoS2, – 1 mV for WS2), establishing
the following trend: in-plane > vertical > monolayers. Ultraviolet
photoelectron spectroscopy (UPS) indicates that this directional charge
separation is driven by intrinsic type-II band alignment, while photoluminescence
(PL) imaging shows that the interface acts as a recombination center
that limits efficient carrier extraction. These results provide direct
experimental evidence of type-II-driven charge separation in in-plane
heterojunctions and offer critical insights for interface design in
high-efficiency photocatalytic and optoelectronic systems.