| Classical theories of colloidal stability assume charged spheres with a homogeneous surface charge density, surrounded by a continuous cloud of ions. However, recent experiments indicate that for semiconductor nanoparticles this continuity approach breaks down. Independent experiments have demonstrated that semiconductor nanocrystals have a permanent electric dipole moment and, depending on the organic capping ligand, a net electric charge of just one or two elementary charges. Qualitatively, the dipole moment is revealed by the oriented attachment of neighboring nanoparticles into linear structures. Moreover, opposite net surface charges on nanoparticles appear to explain the growth of binary colloidal crystals that resemble ionic salts. Quantitatively, however, different techniques yield model-dependent values of the dipole moment that differ by an order of magnitude, and the dipole moment has not been measured for the same particles under the same conditions where a net charge was found. For fluorescent biolabels, light-emitting devices, or other applications of quantum dots, net charge and dipole moment respectively lower the quantum efficiency and shift the wavelength of photoluminescence and therefore are crucial to be investigated. Our hypothesis is that whenever a net charge is present on a semiconductor nanoparticle, that charge will dominate the anisotropic charge distribution and obscure intrinsic contributions of the nanocrystalline material and its polar facets. The aim of our project is to elucidate the effect of net charges on the anisotropic charge distribution of colloidal quantum dots. Therefore, we propose dielectric spectroscopy measurements on colloidal dispersions of nanoparticles with or without a net charge. The charge will be determined not only by laser Doppler electrophoresis, the optical technique that was used until now and that may lead to photoionization of the particles, but also by acoustic electrophoresis with or without light, the latter to examine the particles in their ground state. Dielectric spectroscopy will also be performed with or without light, at high concentrations where dipolar chains of particles may be present (an issue that is crucial for the interpretation but that was neglected in the past), and at low concentrations where dipolar chains should be absent. The presence or absence of dipolar chains in three dimensions will be deduced indirectly from the total pair contact interaction energy measured in two dimensions by quantitative cryogenic electron microscopy, a novel technique pioneered by us in the study of nanoparticle interactions, and it will be confirmed directly by sedimentation velocities measured by analytical centrifugation. With the latter technique, sedimentation-diffusion equilibrium profiles will provide independent information about the net charge, as we recently showed for charged silica nanoparticles. In conclusion, the project will use novel experimental approaches to gain new insights into the charge distribution of quantum dots, which is of interest both from an applied and a fundamental point of view. |
