Tunable electronic and optical properties of Ru₂S₂X (X = Si, Ge, Sn, Pb) monolayers

Two-dimensional (2D) semiconductors with tunable electronic bandgaps and strong excitonic effects are at the forefront of next-generation optoelectronic technologies. In this study, we introduce a novel family of 2D materials, Ru₂S₂X (X = Si, Ge, Sn, Pb), constructed by bridging two Ru-S monolayers with group-IV elements. Using state-of-the-art density functional theory (DFT), many-body GW corrections, and Bethe-Saltpeter equation (BSE) formalism, we systematically investigate their structural, electronic, optical, and mechanical properties. Our results reveal that Ru₂S₂Si and Ru₂S₂Ge exhibit indirect bandgaps close to 1.0 eV, while Ru₂S₂Sn and Ru₂S₂Pb display direct bandgaps down to 0.4 eV, enabling broadband absorption from visible to near-infrared. Phonon dispersion and ab initio molecular dynamics confirm their dynamic and thermal stability, while elastic analysis shows mechanical compliance suitable for flexible devices. Strong excitonic effects with binding energies between 0.17 and 0.30 eV are observed, indicating efficient light-matter interaction balanced with exciton dissociation. Orbital-resolved analyses demonstrate that the nature of the X-site atom governs the p-d hybridization strength, driving the indirect-to-direct bandgap crossover. These findings position Ru₂S₂X monolayers as a promising platform for tunable, stable, and efficient 2D optoelectronics, including photodetectors, solar absorbers, and flexible Nano devices.