Charge, polarizability, and photoionization of single semiconductor nanocrystals

The electrostatic properties of individual CdSe nanocrystals are directly m easured using electrostatic force microscopy (EFM) in dry air at room tempe rature. We determine that the static dielectric constant of CdSe nanocrysta ls with diameters similar to 5 nm is uniform. However, the charge per nanoc rystal is nonuniform, with some nanocrystals possessing a positive charge. Furthermore, a small fraction of the nanocrystals exhibit a blinking behavi or in their charge. This is entirely unexpected for a dielectric particle w ith no additional charge carriers. In addition, EFM measurements with simul taneous photoexcitation provide direct evidence of nanocrystal photoionizat ion and increased blinking behavior.