Tunable optical and electronic properties of monolayer MoS2 via substitutional doping

We present a comprehensive first-principles investigation of the electronic and optical properties of monolayer MoS${}_2$ doped with p-block elements (B, C, N, O, Al, Si, P, Ga, Ge, As, and Se) at the sulfur site. Our calculations demonstrate that substitutional doping profoundly alters the band structure, introducing localized or hybridized impurity states that can reduce, close, or maintain the band gap, depending on the dopant. Notably, B, N, Al, and Ga induce metallic-like behavior, whereas O, C, Se, and Si preserve semiconducting characteristics. Partial density of states analysis reveals that states near the Fermi level are dominated by Mo and S orbitals, with dopants playing a critical secondary role in modulating the host electronic structure. Optical property calculations show dopant-dependent tunability of absorption and transparency across UV, visible, and infrared regions. For example, Al doping enhances UV absorption, while P doping modifies the infrared response. Remarkably, all doped systems retain high visible transparency ($>$75%) despite structural and electronic perturbations, underscoring their potential for optoelectronic and transparent electronics applications. This work establishes substitutional doping as a powerful strategy for tailoring the electronic and optical properties of monolayer MoS${}_2$ for next-generation device engineering.