Charge and photoionization properties of single semiconductor nanocrystals

The electrostatic charge and photoionization characteristics of 5-nm CdSe nanocrystals were directly observed with electrostatic force microscopy (EFM) in dry air at room temperature. Measurements were made on individual nanocrystals, as well as on those in self-assembled rafts, Nanocrystals are initially charge neutral if protected from sources of light. However, over a few weeks some nanocrystals develop a single positive charge if exposed to ambient light. The determination of the charge magnitude per nanocrystal within the framework of EFM theory is described. EFM measurements with simultaneous above band gap laser photoexcitation provide direct evidence of nanocrystal photoionization. A small percentage of photoionized nanocrystals exhibit a blinking behavior in their charge. The linear dependence of nanocrystal photoionization rates on excitation intensity indicates that the ionization process occurs via a single photon. EFM measurements of core/shell CdSe nanocrystals show that photoionization is slower in the presence of an electron barrier at the nanocrystal surface. Photoionization and subsequent neutralization are quantitatively modeled with a two-level system.