Large Resonant Stokes Shift in CdS Nanocrystals

The electronic spectrum of CdS colloidal quantum dots (QDs) with a radius of 1.0−2.3 nm is studied by low-temperature photoluminescence excitation spectroscopy. CdS QDs are found to exhibit a resonant Stokes shift of ∼20−70 meV, which is ∼4 times larger than similarly sized CdSe QDs. This effect can be reproduced by an effective-mass theoretical calculation, which reveals that the hole ground state (or highest occupied molecular orbital, HOMO) in CdS QDs is a P state and the ground-state exciton is an optically passive state. Compared to CdSe, in CdS, the smaller spin−orbit splitting causes the orbital-symmetry-forbidden dark exciton in a QD and its larger resonant Stokes shift. The band-edge photoluminescence in CdS QDs exhibits a lifetime of ∼200 ns at 10 K, which is consistent with the dark exciton mechanism.