Optical Properties of Quantized Semiconductor Particles

CdS nanoclusters ranging in diameters between 1 and 4 nm were prepared in aqueous solution using aliphatic mercaptoalcohols as ligands. The photon energies of the lsls absorption and the respective oscillator strengths are in accordance with size quantization theory. Nonlinear optical properties are discussed in terms of the influence of trapped charge carriers on the excitonic transition. Onion-shell-like composite particles from CdS and HgS were prepared by successive substitution and reprecipitation processes. Particles with a core radius of 2 nm, a shell of up to 1 nm HgS followed by a final shell of up to 1.5 nm CdS were obtained. Electrons and holes were localized in the HgS shell, giving rise to excitonic fluorescence. Dense, yet randomly packed, nanoparticulate ZnO films were found to show reversible optical changes upon cathodic polarization. Some cluster species crystallize in the form of macroscopic threedimensional superlattices, which were investigated by single-crystal X-ray analysis. The neutral Cd17S4(RS)26 clusters are covalently linked in the superlattice, the structure of which exhibits self-similarity to the interior structure of the clusters.