Defect-Driven Optical Modulation in Lead-Free Perovskite Nanocrystals (Cs2AgBiCl6): Interplay of Surface Silver Chloride and Silver Nanoclusters.

Lead-free double perovskite nanocrystals (NCs) are attractive alternatives to Pb-based halide perovskites; however, controlling their phase evolution, defect states, and emission properties remains a significant challenge. Here, we report a precursor-stoichiometry-driven structural evolution in Cs-Ag-Bi chloride NCs, revealing a direct correlation between composition, phase transformation, and photophysical behavior. At a Cs:Ag:Bi ratio of 2:1:1, cubic Cs2AgBiCl6 NCs embedded with Ag/AgCl nanodomains (CAB-1) are formed, whereas reducing the Cs content promotes partial conversion to Bi-rich Cs3Bi2Cl9 and the spontaneous formation of hollow Cs3Bi2Cl9/Cs2AgBiCl6 heterostructures (CAB-2). Structural characterization by PXRD, TEM, SAED, and XPS confirms the controlled evolution of phase composition and morphology, while spectroscopic analyses reveal the presence of plasmonically active Ag species in CAB-1. Both heterostructures exhibit dual-band luminescence comprising a broadband visible emission centered at ∼450-500 nm and an intense near-infrared (NIR) emission at ∼930-950 nm that can be excited below the optical bandgap. Excitation- and power-dependent photoluminescence measurements indicate that the NIR emission originates from deep localized defect states and/or self-trapped excitons, with Ag/AgCl nanodomains further influencing carrier relaxation pathways through interfacial electronic interactions. These results demonstrate that systematic variations in precursor stoichiometry can direct phase reconstruction, heterostructure formation, and visible-to-NIR emission in lead-free double perovskite NCs, providing new opportunities for defect engineering and broadband photonic materials.