Excited Hole Photochemistry of CdSe/CdS Quantum Dots

One-photon excitation of CdSe/CdS quantum dots (QDs) in room-temperature chloroform results in a delayed and reversible photodarkening of the sample. There is little or no prompt loss of photoluminescence intensity and the subsequent photodarkening takes place on the tens of minutes time scale. The photodarkening kinetics have been studied as a function of the solvent, particle ligation, concentration, and shell thickness. The results indicate that the photochemistry is the result of a hot hole being transferred to a surface ligand, followed by dissociation to a QD(-)/L+ ion pair. The hot hole is generated by a trion Auger process through a surface charging mechanism. In this mechanism, the presence of unpassivated chalcogenide surface atoms in II-VI semiconductor QDs results in electrons that are in an equilibrium between the valence band and chalcogenide surface orbitals, that is, a surface cadmium vacancy has resulted in a p-type QD. The latter state corresponds to a surface-charged QD. Photoexcitation of a surface-charged QD produces a trion, which can undergo Auger recombination to produce an unrelaxed hole. Following QD to ligand hole transfer and ligand dissociation, the positively charged ligand can react with another QD in solution causing photodarkening. Charge recombination occurs on a slower time scale and reverses the loss of photoluminescence intensity.