Surface Science and Colloidal Stability of Double-Perovskite Cs₂AgBiBr₆ Nanocrystals and Their Superlattices

Capping ligand bonding and the thermal and colloidal stability of Cs2AgBiBr6 nanocrystals were studied. Oleylamine and oleic acid bonding to Cs2AgBiBr6 nanocrystals was studied with 1H nuclear magnetic resonance and nuclear Overhauser effect spectroscopy. Both molecules are present in the ionic metathesis synthesis reaction, but only oleylamine remains bound to the nanocrystals after purification. Oleic acid is not a capping ligand; however, the synthesis requires it, and its concentration determines the yield of the reaction while oleylamine primarily affects the uniformity of the sample. Substitution of oleic acid in the reaction with diisooctylphosphinic acid still yielded nanocrystals with similar size, cuboidal shape, uniformity, cubic double-perovskite crystal structure, and optical properties. Nanocrystals were assembled into superlattices and heated in air. Grazing incidence small-angle and wide-angle X-ray scattering showed that the nanocrystals sinter at 250 °C, but the crystal structure and preferred crystal orientation on the substrate does not change. When nanocrystals were dispersed in hexane and exposed to light, they precipitated within 24 h. Scanning electron microscopy showed that the aggregated nanocrystals still retained their initial size and shape and had not coalesced.