Electric Fields on Oxidized Silicon Surfaces: Static Polarization of PbSe Nanocrystals†

By use of electrostatic force microscopy, we measure the charge and polarizability of 12-nm PbSe nanocrystals on n- and p-type silicon with a 2-nm thermal oxide layer. Individual nanocrystals show a dielectric constant of >100. In ambient light, the nanocrystals generate static electric fields of magnitudes too weak to be caused by a full elementary charge. These nanocrystals are statically polarized by surface electric fields generated by fixed charges in the oxide substrate. We model this effect quantitatively and assign charge locations in the oxide. Upon 442-nm photoexcitation, we observe some of the nanocrystals (∼35%) photoionize and slowly relax overnight back to their initial states. Just above a charge in the oxide, the surface electric field can approach 108 V/cm.