Mixed-Chalcogen 2D Silver Phenylchalcogenides (AgE_1–<i>x</i>E_<i>x</i>Ph; E = S, Se, Te)

Alloying is a powerful strategy for tuning the electronic band structure and optical properties of semiconductors. Here, we investigate the thermodynamic stability and excitonic properties of mixed-chalcogen alloys of two-dimensional (2D) hybrid organic-inorganic silver phenylchalcogenides (AgEPh; E = S, Se, Te). Using a variety of structural and optical characterization techniques, we demonstrate that the AgSePh-AgTePh system forms homogeneous alloys (AgSe_1-<i>x</i>Te_<i>x</i>Ph, 0 ≤ <i>x</i> ≤ 1) across all compositions, whereas the AgSPh-AgSePh and AgSPh-AgTePh systems exhibit distinct miscibility gaps. Density functional theory calculations reveal that chalcogen mixing is energetically unfavorable in all cases but comparable in magnitude to the ideal entropy of mixing at room temperature. Because AgSePh and AgTePh have the same crystal structure (which is different from AgSPh), alloying is predicted to be thermodynamically preferred over phase separation in the case of AgSePh-AgTePh, whereas phase separation is predicted to be more favorable than alloying for both the AgSPh-AgSePh and AgSPh-AgTePh systems, in agreement with experimental observations. Homogeneous AgSe_1-<i>x</i>Te_<i>x</i>Ph alloys exhibit continuously tunable excitonic absorption resonances in the ultraviolet-visible range, while the emission spectrum reveals competition between exciton delocalization (characteristic of AgSePh) and localization behavior (characteristic of AgTePh). Overall, these observations provide insight into the thermodynamics of 2D silver phenylchalcogenides and the effect of lattice composition on electron-phonon interactions in 2D hybrid organic-inorganic semiconductors.