Au₆S₂ monolayer sheets: Metallic and semiconducting polymorphs

Gold-sulfur interfaces, including self-assembled monolayers of thiol molecules on gold surfaces, thiolate-protected gold nanoclusters, and gold sulfide complexes, have attracted intensive interest due to their promising applications in electrochemistry, bioengineering, and nanocatalysis. Herein, we predict two hitherto unreported two-dimensional (2D) Au₆S₂ monolayer polymorphs, named as G-Au₆S₂ and T-Au₆S₂. The global-minimum G-Au₆S₂ monolayer can be viewed as a series of [-S-Au-]_n and [-Au₄-]_n chains packed together in parallel. The metastable T-Au₆S₂ monolayer resembles the widely studied T-MoS₂ monolayer structure with each Mo atom substituted with an octahedral Au₆ cluster, while the S atom is bonded with three Au atoms in a μ₃ bridging mode. The G-Au₆S₂ monolayer is predicted to be metallic. The T-Au₆S₂ monolayer is predicted to be a semiconductor with a direct bandgap of 1.48 eV and high carrier mobility of 2721 cm² V^-1 s^-1, ∼10 times higher than that of semiconducting H-MoS₂. Moreover, the T-Au₆S₂ monolayer can absorb sunlight efficiently over almost the entire solar spectrum. These properties render the G- and T-Au₆S₂ monolayers promising materials for advanced applications in microelectronics and optoelectronics.