Cluster precursors of uncapped CdS quantum dots via electroporation of synthetic liposomes. Experiments and theory.

Subnanometer size cluster precursors of uncapped CdS quantum dots were produced via the electroporation of synthetic dioleoylphosphatidylcholine (DOPC) unilamellar bilayer vesicles of mean hydrodynamic diameter Dh = 175 nm. During electroporation, Cd2+ ions are ejected from the interior compartments of the vesicles into the bulk solution where they react with S(2-) ions to form CdS monomers. The monomers adsorb on the exterior surface of the vesicles, where their spontaneous self-aggregation to (CdS)n clusters occurs on the hour and day time scale. The stepwise growth of the clusters was monitored through the time evolution of the UV absorption spectrum of the solution. The process is characterized by initial stepwise blue shifts of the absorption maxima: 285 nm --> 269 nm --> 245/275 nm --> 240 nm --> 236 nm, followed by a red shift to 494 nm. Nonlocal density functional theory (DFT) calculations of the optimized geometry and HOMO-LUMO gap of (CdS)n particles with n = 1-6 were carried out. The optimized structures are characterized by strong Cd-Cd bonds, with the S atoms bridging those bonds or capping the faces of the Cd polyhedra. The structure of such clusters bears no resemblance to fragments of the bulk crystal. The trend of the calculated HOMO-LUMO gaps facilitates the attribution of aggregation numbers (n) to particular clusters responsible for the observed absorption bands: n = 1 (285 nm), n = 2 (269 nm), n = 4 (245/275 nm --> 240 nm), n = 5 (236 nm), and larger quantum dots absorbing around 494 nm. The multiple bands assigned to the tetramer reflect the existence of its two distinct structures with similar stability.