Spectroscopic Investigation of Oxygen Sensitivity in CdTe and CdTe/CdS Nanocrystals

Excited-state dynamics of CdTe nanocrystals and CdTe/CdS core/shell nanocrystals have been studied on the picosecond time scale using pump probe transient absorption spectroscopy. The results show, that exposure of CdTe nanocrystals to air can produce profound changes in the ultrafast excited-state dynamics. These changes occur even after a brief air exposure, but they are not observed in CdTe nanocrystals that are protected by a thin CdS shell. The quenching is caused by reaction of molecular oxygen with Te atoms on the nanocrystal surface, which produces traps for one or both excited carriers. The trapping event occurs on the single picosecond time scale and is sufficiently fast to compete with hot carrier cooling. This phenomenon has important implications for interpretation of excited-state dynamics in CdTe-containing nanoheterostructures as well as ultrafast charge separation in photoexcited nanostructures.