Spatially Resolved Persistent Photoconductivity in MoS$_2$-WS$_2$ Lateral Heterostructures

TL;DR

This study uses near-field microscopy to visualize persistent photoconductivity in MoS$_2$-WS$_2$ heterostructures, revealing long-lived free carriers and charge transfer effects that influence local optoelectronic properties.

Contribution

It provides the first spatially resolved imaging of persistent photoconductivity in 2D heterostructures, linking optical absorption, charge transfer, and long-lived carriers.

Findings

Photoconductivity onset matches optical absorption in domains.

Persistent carriers can last for days and increase carrier density by 200 times.

Charge transfer across heterointerface influences overall photoconductivity.

Abstract

The optical and electronic properties of 2D semiconductors are intrinsically linked via the strong interactions between optically excited bound species and free carriers. Here we use near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS$_2$--WS$_2$ lateral multijunction heterostructures using photon energy-resolved narrowband illumination. We find that the onset of photoconductivity in individual domains corresponds to the optical absorption onset, confirming that the tightly bound excitons in transition metal dichalcogenides can nonetheless dissociate into free carriers. These photogenerated carriers are most likely n-type and are seen to persist for up to days, and informed by finite element modeling we reveal that they can increase the carrier density by up to 200 times. This persistent photoconductivity appears to be dominated by contributions from the multilayer MoS$_2$ domains, and we attribute the flake-wide response in part to charge transfer across the heterointerface. Spatial correlation of our SMM imaging with photoluminescence (PL) mapping confirms the strong link between PL peak emission photon energy, PL intensity, and the local accumulated charge. This work reveals the spatially and temporally complex optoelectronic response of these systems and cautions that properties measured during or after illumination may not reflect the true dark state of these materials but rather a metastable charged state.