Self-organization of mono- and bi-modal PbS nanocrystal populations in superlattices

Here the synthesis of distinct monomodal and bimodal PbS nanocrystal (NC) populations, with narrow size-distribution, is reported. The ability to achieve careful control of NC size and size distribution allowed the preparation, in one single synthetic step, of two distinct populations of PbS NCs, with tuneable size ratio. The NC growth was carefully studied in order to gain insight into the mechanism underlying the formation of the mono and bimodal PbS NC families. The synthesized PbS NCs were structurally and chemically characterized, and subsequently used as building blocks for fabricating solid crystal assemblies by solvent evaporation. In particular the role played by different parameters, namely NC size and concentration, dispersing solvent and substrate, on crystallinity, geometry and structure of the obtained solids was systematically investigated. Interestingly the assembly of bimodal PbS NC samples leads to the formation of diverse superlattice structures, with a final geometry dependent on the NC size and the size ratio in the bimodal population. The synthetic procedure was then ultimately responsible of the superlattice structures, through the control of the PbS NC size and size ratio in the bimodal population.