Molecular limit of a bulk semiconductor: Size dependent optical spectroscopy study of CdSe cluster molecules

Steady state and time-resolved photoluminescence measurements of a homologo us series of CdSe cluster molecules were performed over a broad temperature range (T = 5-200 K). The absorption and low temperature PLE onset of the c lusters shifts systematically to the blue in smaller clusters, manifesting the quantum confinement effect. The emission in all cluster molecules is ob served only at low temperatures and is red-shifted significantly from the a bsorption onset. It is assigned to optically forbidden transitions involvin g surface states, as substantiated by the mus range of lifetimes and by the involvement of low frequency vibrations of capping selenophenol ligands in the nonradiative relaxation of excited cluster molecules.