Chemical Treatment-Induced Indirect-to-Direct Bandgap Transition in MoS2: Impact on Optical Properties

TL;DR

This study shows that chemical DCE treatment of MoS2 selectively suppresses the indirect optical transition, enabling optical bandgap engineering while enhancing electrical conductivity, which could benefit 2D optoelectronic devices.

Contribution

It provides a systematic analysis of DCE's effects on MoS2's optical properties and reveals the mechanism behind indirect transition suppression through DFT simulations.

Findings

DCE treatment rapidly reduces indirect bandgap transition in MoS2

Chlorine atoms bind to sulphur vacancies creating mid-gap states

DCE enhances n-type doping and enables optical bandgap engineering

Abstract

The unique electrical and optical properties of emerging two-dimensional transition metal dichal-cogenides (TMDs) present compelling advantages over conventional semiconductors, including Si, Ge, and GaAs. Nevertheless, realising the full potential of TMDs in electronic and optoelectronic devices, such as transistors, light-emitting diodes (LEDs), and photodetectors, is con-strained by high contact resistance. This limitation arises from their low intrinsic carrier concen-trations and the current insufficiency of doping strategies for atomically thin materials. Notably, chemical treatment with 1,2-dichloroethane (DCE) has been demonstrated as an effective post-growth method to enhance the n-type electrical conductivity of TMDs. Despite the well-documented electrical improvements post-DCE treatment, its effects on optical properties, specifically the retention of optical characteristics and excitonic behaviour, are not yet clearly under-stood. Here, we systematically investigate the layer- and time-dependent optical effects of DCE on molybdenum disulfide (MoS2) using photoluminescence (PL) spectroscopy and Density Functional Theory (DFT) simulations. Our PL results reveal a rapid reduction in the indirect bandgap transition, with the direct transition remaining unaffected. DFT confirms that chlorine (Cl) atoms bind to sulphur vacancies, creating mid-gap states that facilitate non-radiative recom-bination, explaining the observed indirect PL suppression. This work demonstrates DCE's utility not only for n-type doping but also for optical band structure engineering in MoS2 by selec-tively suppressing indirect transitions, potentially opening new avenues for 2D optoelectronic device design.