Mixed-Chalcogen\n2D Silver Phenylchalcogenides\n(AgE<sub>1–<i>x</i></sub>E<sub><i>x</i></sub>Ph; E = S, Se, Te)

Alloying is a powerful strategy for tuning the electronic\nband\nstructure and optical properties of semiconductors. Here, we investigate\nthe thermodynamic stability and excitonic properties of mixed-chalcogen\nalloys of two-dimensional (2D) hybrid organic–inorganic silver\nphenylchalcogenides (AgEPh; E = S, Se, Te). Using a variety of structural\nand optical characterization techniques, we demonstrate that the AgSePh-AgTePh\nsystem forms homogeneous alloys (AgSe<sub>1–<i>x</i></sub>Te<sub><i>x</i></sub>Ph, 0 ≤ <i>x</i> ≤ 1) across all compositions, whereas the AgSPh-AgSePh and\nAgSPh-AgTePh systems exhibit distinct miscibility gaps. Density functional\ntheory calculations reveal that chalcogen mixing is energetically\nunfavorable in all cases but comparable in magnitude to the ideal\nentropy of mixing at room temperature. Because AgSePh and AgTePh have\nthe same crystal structure (which is different from AgSPh), alloying\nis predicted to be thermodynamically preferred over phase separation\nin the case of AgSePh-AgTePh, whereas phase separation is predicted\nto be more favorable than alloying for both the AgSPh-AgSePh and AgSPh-AgTePh\nsystems, in agreement with experimental observations. Homogeneous\nAgSe<sub>1–<i>x</i></sub>Te<sub><i>x</i></sub>Ph alloys exhibit continuously tunable excitonic absorption\nresonances in the ultraviolet–visible range, while the emission\nspectrum reveals competition between exciton delocalization (characteristic\nof AgSePh) and localization behavior (characteristic of AgTePh). Overall,\nthese observations provide insight into the thermodynamics of 2D silver\nphenylchalcogenides and the effect of lattice composition on electron–phonon\ninteractions in 2D hybrid organic–inorganic semiconductors.