Electrostatic force microscopy study of the charge and polarizability of semiconducting and metallic nanocrystals.

摘要

This thesis addresses the fundamental charge and polarizability properties of individual nanocrystals. The first part focuses on the theory of the electrostatic force microscope and how this method is used to understand the properties of these small particles. The microscope scans a sample using a conductive tip and both the surface topography and the electrostatic force gradients on the surface are mapped out simultaneously. This data is then interpreted using appropriate electrostatic models which are developed in order to account for the magnitude and nature of the measured fields. The tip is modeled as a cone with a sphere at the end to correctly describe the tip-surface capacitance, and each probe is independently calibrated to determine its specific geometric parameters. To determine the nanocrystal charge, the total force experienced by the tip is modeled as the force due to a charge within a polarizable sphere, plus its image acting on the sphere and the cone. Nanocrystal polarizability, due to the AC polarization of the particle, is modeled using an expression for the potential due to a polarized dielectric sphere in the field of an external point charge located at some distance from the sphere center.;Based on our understanding of the electrostatic fields measured with the EFM, in the next part 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.;We next explore the effect of surface ligands on nanocrystals charge using a variety of semiconducting and metallic nanocrystals. This is done to address recent reports in the literature that provide evidence of quantized metal and semiconducting nanocrystalline charge in solution. We measure the charge of a variety of nanocrystals on a highly oriented pyrolytic graphite (HOPG) surface and use ligand exchange chemistry to vary this charge. We find that nanocrystal charge is influenced by the passivating layer in addition to charge transfer effects with the HOPG substrate.;In the final chapter, we investigate the EFM charge signal of bare nanoparticles on HOPG. When two different metals are brought into contact, there is a net charge flow to compensate for the difference in work functions between the two materials. Using an electrochemical technique, we deposit surfactant-free, nano-sized metal particles (Au, Ag, Pd, Pt and In) of varying work functions on HOPG and measure the EFM signal. We observe a negative static field signal for all metal particles on HOPG. This was attributed to charge transfer effects, where the relative magnitude of the static field signal implied a higher work function of the metals compared to HOPG and shows a large bias in the reported literature values of the bulk metal work function.