Stability and Excitonic Properties of Ag ₂ S Single Layer from First-Principles Calculations

Low-dimensional materials exhibiting strong excitonic effects are promising candidates for optoelectronic and energy-harvesting technologies. In this work, using many-body perturbation theory, we explore the excitonic properties of a buckled silver sulfide (Ag2S) single layer. Starting from density functional theory, we apply quasiparticle corrections via the single-shot G0W0 approach and solve the Bethe-Salpeter equation to accurately capture excitonic effects in the optical absorption spectrum. Our calculations reveal a strongly bound bright exciton with a binding energy exceeding 800 meV, associated with a fundamental bandgap of 3.16 eV, and a majority of dark states in the spectrum. The excitonic wave functions are delocalized in real space, and while dark excitons exhibit similar spatial and momentum-space characteristics, bright excitons show significantly extended and distinct spatial distributions. Our results demonstrate the influence of the orbital projection of the band structure in the excitonic properties, and that buckled Ag2S is a compelling low-dimensional material for light-emitting technologies based on bright-exciton physics, as well as for quantum information storage using long-lived dark exciton states.