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

Steady state and time-resolved photoluminescence measurements of a homologous 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 clusters shifts systematically to the blue in smaller clusters, manifesting the quantum confinement effect. The emission in all cluster molecules is observed only at low temperatures and is red-shifted significantly from the absorption onset. It is assigned to optically forbidden transitions involving surface states, as substantiated by the μs range of lifetimes and by the involvement of low frequency vibrations of capping selenophenol ligands in the nonradiative relaxation of excited cluster molecules.