Charge and photoionization properties of single semiconductor nanocrystals

The electrostatic charge and photoionization characteristics of 5-nm CdSe n anocrystals were directly observed with electrostatic force microscopy (EFM ) in dry air at room temperature. Measurements were made on individual nano crystals, as well as on those in self-assembled rafts, Nanocrystals are ini tially charge neutral if protected from sources of light. However, over a f ew weeks some nanocrystals develop a single positive charge if exposed to a mbient light. The determination of the charge magnitude per nanocrystal wit hin the framework of EFM theory is described. EFM measurements with simulta neous above band gap laser photoexcitation provide direct evidence of nanoc rystal photoionization. A small percentage of photoionized nanocrystals exh ibit a blinking behavior in their charge. The linear dependence of nanocrys tal photoionization rates on excitation intensity indicates that the ioniza tion process occurs via a single photon. EFM measurements of core/shell CdS e nanocrystals show that photoionization is slower in the presence of an el ectron barrier at the nanocrystal surface. Photoionization and subsequent n eutralization are quantitatively modeled with a two-level system.