Chemical treatment-induced indirect-to-direct bandgap transition in MoS₂: impact on excitonic emission

Effective doping is crucial for overcoming performance limitations in two-dimensional (2D) transition metal dichalcogenide (TMD) devices. For light-emitting applications, however, doping must increase carrier injection without quenching excitonic emission. While chemical treatment with 1,2-dichloroethane (DCE) has been demonstrated as an effective post-growth n-doping method for 2D TMDs, its effects on optical properties, specifically the retention of optical characteristics and excitonic behaviour, remain unclear. Here, we investigate the layer- and time-dependent optical effects of DCE on molybdenum disulfide (MoS₂) using photoluminescence (PL) spectroscopy and Density Functional Theory (DFT). Our results show that DCE treatment rapidly reduces the indirect bandgap transition, while leaving the direct transition unaffected. DFT confirms that chlorine atoms bind to sulphur vacancies, creating in-gap states that facilitate non-radiative recombination and suppress the indirect PL. This work demonstrates DCE can selectively engineer the optical band structure in MoS₂, paving the way for more efficient 2D optoelectronic devices.