Stable versus Labile Ligand Passivation on Silver Chalcogenide Nanocrystals

We investigated site-specific ligand passivation in silver chalcogenide nanocrystals (NCs). The binding modes of ligands and the stability of their passivation (stable versus labile) exhibited distinct preferences depending on whether they interacted with surface sites (out-of-bounds atoms) or with sites embedded within the crystal lattice (crystal lattice atoms). Out-of-bounds Ag atoms at cation-rich surfaces were stably passivated by alkanethiolate ligands, which replaced labile initial ligands. In contrast, lattice Ag atoms achieved stable passivation through quaternary ammonium ion-pair ligands, which superseded neutral alkanethiols. This strategy effectively mitigated the structural instability of silver chalcogenide NCs, which are otherwise prone to fusion and ripening and often complicate cation exchange reactions or epitaxial overcoating with wide-bandgap semiconductor shells. Leveraging site- and ligand-specific interactions, cation-rich CdTe NCs and CdTe-CdS core-shell NCs were pretreated with alkanethiol ligands, enabling silver cation exchange to produce Ag₂Te NCs and Ag₂Te-Ag₂S core-shell NCs without unintended ripening, while preserving uniform size and sharp optical features. The resulting heteroepitaxial and concentric shelled Ag₂Te-Ag₂S core-shell NCs exhibited more than a 20-fold enhancement in photoluminescence at ∼1500 nm compared to Ag₂Te NCs, owing to type-I band alignment. This ligand passivation strategy is promising for environmentally friendly infrared optics and bioimaging applications of heavy-metal-free silver chalcogenide NCs.