Synthesis and bandgap variation of molecular-size CdSe clusters via electroporation ofvesicles

Abstract

Molecular-size uncapped CdSe clusters are synthesized via the electroporationof synthetic dioleoylphosphatidylcholine vesicles of diameter 178 nm.Cd2+ ions, originally entrapped in the vesicles, are ejected through the transientpores of the vesicle membrane into the bulk where they can react withSeSO32− ions in basic condition to form CdSe clusters at room temperature. Growth andself-aggregation of CdSe clusters is slowed down to timescales of days by their adsorption atthe exterior surface of the vesicle, which permits spectral monitoring of the continuousgrowth process over a long period of time. Growth in the molecular size regimeis found to entail novel blue-shifts of the characteristic absorption band. Theblue-shift of the transition energy is in agreement with an analogous oscillation ofthe HOMO–LUMO gap calculated by the use of density functional theory for(CdSe)n clusters. The reason for the blue-shift absorption spectra of molecular-size CdSe clusters isinterpreted as the higher stability and bigger gaps of magic number. On the basis of thesimilarity between the experimental and theoretical trends, the characteristic absorptionpeaks of 256 nm, 215 nm can be assigned to the monomer or dimer, trimer clusters ofCdSe, respectively. Any further growth beyond trimers is associated with thecustomary monotonic red-shift of the absorption band, due to quantum size effects.