Size-dependent optical spectroscopy of a homologous series of CdSe cluster molecules

The optical properties and electronic structure of a homologous series of CdSe cluster molecules covering a size range between 0.7 and 2 nm are investigated. CdSe cluster molecules with 4, 8 10, 17, and 32 Cd atoms, capped by selenophenol ligands, were crystallized from solution and their structures determined by single-crystal X-ray diffraction. The cluster molecules are composed of a combination of adamanthane and barylene-like cages, the building blocks of the zinc blende and the wurtzite structures of the bulk CdSe. The onset of the room temperature absorption and low-temperature photoluminescence excitation spectra exhibit a systematic blue shift with reduced cluster size manifesting the quantum confinement effect down to the molecular limit of the bulk semiconductor. Blue-green emission, shifted substantially to lower energy from the absorption onset, is observed only at low temperature and its position is nearly independent of cluster size. The wavelength dependence of both photoluminescence and photoluminescence excitation was measured. The emission is assigned to forbidden transitions involving the cluster-molecule surface-capping ligands. This assignment is supported by the emission decay which exhibits distributed kinetics with microsecond time scale. The temperature dependence of the emission intensity is quantitatively explained by multiphonon-induced nonradiative relaxation mediated by low-frequency vibrations of the selenophenol capping ligands. Upon irradiation, the emission of all cluster molecules is quenched. Warming up and recooling leads to recovery of the emission (partial or complete) for all but the cluster molecule with 10 Cd atoms. This temporary darkening is assigned to the photoinduced charging of the cluster-molecule surface ligands, resembling the reversible on-off blinking of the emission observed for larger CdSe nanocrystals.