Charge, Polarizability, and Photoionization of Single Semiconductor Nanocrystals

The electrostatic properties of individual CdSe nanocrystals are directly measured using electrostatic force microscopy (EFM) in dry air at room temperature. We determine that the static dielectric constant of CdSe nanocrystals with diameters $\sim 5\mathrm{nm}$ is uniform. However, the charge per nanocrystal is nonuniform, with some nanocrystals possessing a positive charge. Furthermore, a small fraction of the nanocrystals exhibit a blinking behavior in their charge. This is entirely unexpected for a dielectric particle with no additional charge carriers. In addition, EFM measurements with simultaneous photoexcitation provide direct evidence of nanocrystal photoionization and increased blinking behavior.