Tunable direct bandgap and optical response in \ch{Mo_{1-x}W_xS2} monolayer alloys: A first-principles investigation

Abstract

This study presents a comprehensive first-principles investigation of the structural, electronic and optical properties of monolayer \ch{Mo_{1-x}W_xS2} alloys, systematically exploring the full compositional range ($x=0$ to $1$) using density functional theory (DFT). We establish that these alloys are thermodynamically stable and maintain the characteristic 2H crystal structure with minimal structural perturbation upon alloying. A key finding is the preservation of a direct bandgap at the $K$-point across all compositions. This gap exhibits continuous tunability, increasing near-monotonically from \SI{1.696}{\electronvolt} (\ch{MoS2}) to \SI{1.858}{\electronvolt} (\ch{WS2}), a critical feature for tailoring optoelectronic devices. Electronic structure analysis reveals the systematic evolution of the orbital contributions of transition metal $d$ and sulfur $p$ at the edges of the band with composition. Consequently, the optical spectra, evaluated up to \SI{8}{\electronvolt}, show a progressive blueshift in the main features of the interband transition with increasing \ch{W} content, accompanied by predictable changes in key optical constants. Our comprehensive results validate the monolayer \ch{Mo_{1-x} W_xS2} as an electronically versatile platform that offers fine control over electronic and optical properties via alloying, making these tunable direct-gap semiconductors highly promising for next-generation photodetectors, light emitters, and potentially flexible optoelectronic applications exploiting their 2D nature.