Preparation of cadmium sulfide quantum dots via electroporation of unilamellar vesicles, and, Theoretical studies of cadmium sulfide and lead sulfide clusters

Ultra small CdS quantum dots (8 to 22 Å diameter, according to TEM) were prepared via electroporation of synthetic, unilamellar DOPC vesicles (<D_h> = 175 nm). The slow self-aggregation (occurring over several hours) of the CdS monomers adsorbed on the exterior surface of the vesicles allows for convenient monitoring of the formation of (CdS)_n clusters, and their transformation to semiconductor quantum dots upon further cluster growth. Growth in the molecular size range is associated with a blue-shift of the UV absorption band, followed by a general red-shift in the quantum dot size regime. The emergence of the electronic band structure starts at the turning point of the spectral band-shift (at n = 5). Results of HOMO-LUMO gap calculations through DFT at B3LYP/6–31+G* level with full geometry optimization for n = 1–6 are in agreement with the trends in the spectral band-shift observed.; The structures of (PbS)_n clusters (n = 1–9) and (Pb ₁₃S₁₄)2− are investigated at the B3LYP level with different pseudo-potential basis sets on lead and 6–31+G* on sulfur. Full geometry optimization and extensive searches of the potential energy surface are carried out for clusters with n = 1–6. Cubic structure is formed at tetramer and the geometries of clusters with n > 4 develop toward the rock-salt crystal of galena; the (Pb₁₃S₁₄) 2− cluster has the same structure as the galena crystal. Even small PbS clusters (n > 2) start to take on the characteristics structure of galena crystal. The origin of this rock-salt crystal is analyzed through the covalent effect of the Pb-S bond and the orbital correlation diagram of PbS trimer. The HOMO-LUMO gaps of PbS clusters oscillate, resulting in an expected alternating red-shift and blue-shift of the absorption band with size growth. This tendency has been confirmed partly by our experimental observation.