Chemical Treatment-Induced Indirect-to-Direct Bandgap Transition in MoS₂: Impact on Excitonic Emission

The unique electrical and optical properties of emerging two-dimensional transition metal dichalcogenides (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 constrained by high contact resistance. This limitation arises from their low intrinsic carrier concentrations 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-established electrical improvements post-DCE treatment, its effects on optical properties, specifically the retention of optical characteristics and excitonic behaviour, are not yet clearly understood. 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 in-gap states that facilitate non-radiative recombination, explaining the observed indirect PL suppression. This work demonstrates DCE's utility beyond n-type doping, showcasing its ability to engineer the optical band structure in MoS₂ by selectively suppressing indirect transitions. This capability directly paves the way for enhanced efficiency in 2D optoelectronic devices.