Silver-Nanoclusters and Vacancies Influence the Optical Properties of Spherical Cs₂AgBiCl₆ Nanocrystals

Halide double perovskites (DPs), such as Cs₂AgBiX₆ (X = Br/Cl), are emerging semiconductors for optoelectronic applications, offering less toxic alternatives compared to lead halide perovskites. Although exhibiting an indirect bandgap, Cs₂AgBiCl₆ DP nanocrystals (NCs) show bright photoluminescence spectra characterized by a spectrally broad red emission centered ≈650 nm and a narrower blue emission band centered at 425 nm. However, the origin of both emission bands is still under debate. In this paper, it is shown that silver (Ag) plays a crucial role in the explanation of both emission bands observed in these DP NCs. The trapping of holes in Ag vacancies leads to a spatial localization of the hole wave function on the scale of the lattice constant. This provides k-values for the hole wave function at all boundaries of the Brillouin zone and thus favors the recombination with electrons at the L-point. Accordingly, a thermally activated behavior of the red photoluminescence with an activation energy of 56 meV is observed. It is further shown that the high-energetic blue emission band originates from lecithin ligands attached to the NC surface. Surprisingly, their emission spectrum coincides exactly with the plasmon resonance of Ag nanoclusters, located on the surface of the DP NCs.