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

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.