First-principles study of AlGaSi₂X₆ (X = S, Se, Te) monolayers: structural, electronic, transport and photocatalytic...

Density functional theory (DFT) calculations were employed to investigate quaternary AlGaSi₂X₆ (X = S, Se, Te) monolayers as two-dimensional semiconductors for photocatalytic and nanoelectronic applications. All three monolayers are predicted to be both dynamically and thermally stable and exhibit indirect band gaps that systematically decrease with increasing chalcogen atomic weight, from 2.79 eV for AlGaSi₂S₆ to 2.32 eV for AlGaSi₂Se₆ and 1.12 eV for AlGaSi₂Te₆. Vacuum-referenced band-edge alignments indicate that AlGaSi₂S₆ and AlGaSi₂Se₆ can thermodynamically drive overall water splitting under illumination, whereas AlGaSi₂Te₆ possesses an insufficient band gap to provide the required photovoltage. Gibbs free-energy profiles further support photoassisted hydrogen and oxygen evolution reactions (HER/OER) on the S- and Se-based monolayers. Moreover, AM1.5G solar spectrum estimates yield solar-to-hydrogen (STH) efficiencies of 3.90% for AlGaSi₂S₆ and 10.86% for AlGaSi₂Se₆. Deformation-potential analysis predicts electron-dominated transport with carrier mobilities reaching 1.7 × 10³ cm² V^-1 s^-1, identifying AlGaSi₂Se₆ as the most promising overall candidate among these monolayers.