First-Principles Study on Strain-Tunable Electronic and Optical Properties of Janus Al₂SO Monolayer

Janus 2D materials have recently attracted significant attention for their unique characteristics resulting from broken out-of-plane symmetry. In this study, first-principles calculations within the framework of Density Functional Theory (DFT) have been carried out to explore the strain-dependent electronic and optical characteristics of the Janus Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>SO monolayer. The material exhibits semiconducting nature with a direct bandgap of 1.511 eV. Because of its asymmetric structure, the electronic and optical responses to strain are highly sensitive and anisotropic. Under the application of uniaxial and biaxial strain, Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>SO can transition from a direct bandgap to an indirect bandgap material and even a metallic state, with its bandgap tunable across a range from 0 eV to 1.975 eV. While uniaxial strain has a limited impact on carrier effective masses, biaxial strain causes more significant deviations. The electron and hole effective masses vary in the range of $0.34 \mathrm{~m}_{0}$ to $0.66 \mathrm{~m}_{0}$ and $0.23 \mathrm{~m}_{0}$ to $1.75 \mathrm{~m}_{0}$, respectively, under strain. Additionally, the absorption coefficient is prominently located in the ultraviolet (UV) region of the electromagnetic spectrum, indicating strong potential for $\mathbf{A l}_{\mathbf{2}} \mathbf{S O}$ monolayers in applications such as optoelectronics and strain-tunable switching devices.