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

TL;DR

This paper uses first-principles calculations to show that monolayer Mo-W-S2 alloys are thermodynamically stable, maintain a direct bandgap that can be continuously tuned, and exhibit predictable optical property changes across the compositional range.

Contribution

It provides a comprehensive first-principles analysis of the structural, electronic, and optical properties of Mo-W-S2 alloys, demonstrating their stability and tunable direct bandgap for optoelectronic applications.

Findings

Alloys are thermodynamically stable with minimal structural perturbation.

The direct bandgap is preserved and tunable from 1.696 eV to 1.858 eV.

Optical spectra show a progressive blueshift with increasing W content.

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.