Colloidal Interactions of Quantum Dots in Apolar Liquids

Abstract

In this thesis, the main topic is the interactions of nanoparticles in apolar liquids. These includes both the colloidal interactions between nanoparticles and the interaction of the nanoparticles with an external potential from a liquid/air interface or a magnetic field. The understanding of these interactions would allow for better understanding of their colloidal properties, for instance the formation of self-assembled superlattices. In the first part of this thesis the use of cryogenic electron microscopy to investigate the interactions between quantum dots is discussed. First the interactions between the quantum dots are studied using two dimensional snapshots of the distributions of the quantum dots in the liquid. From these snapshots, acquired at different temperatures, the interaction free energy, enthalpy, and entropy are obtained for quantum dot clusters of different size and shap. Also the three dimensional distribution of the nanoparticles is related to the properties of the interfaces of the liquid. Finally, the results of this method are compared to other methods via the second virial coefficient of osmotic pressure a number that contains information about interactions and can be obtained with several independent methods. In chapter 4, a non-regularized inversion method is applied to the analysis of magnetization curves of ferrofluids. In this way the magnetic size distribution is obtained directly from the magnetization curves and is less model-dependent than when the shape of the distribution is assumed from the start. Knowledge of the interactions between nanoparticles is especially important for the self-assembly of these nanoparticles into superlattices. In chapter 6 this self-assembly is explored for a system containing magnetic cobalt nanoparticles and PbSe quantum dots, which form superlattices with a AlB2 type structure on an ethylene glycol interface. When these assemblies are prepared in the presence of a magnetic field, the cobalt nanoparticles align with the field, which results in the formation of anisotropic binary superlattices still with the AlB2 type internal structure. A minimalistic new method is presented for the synthesis of monodisperse cobalt nanoparticles, based on the well-known decomposition of dicobalt-octacarbonyl now performed in a glass test tube as reaction vessel. In chapter 7, infrared spectroscopy is used to study the adsorption of ligands on the surface of nanoparticles. The first part of this chapter deals with the correlation between ligand adsorption and the observed increase of the photoluminescence upon dilution of the dispersion. It is shown that the typical time scale of the increase in luminescence corresponds to the time scale on which the capping density of the nanoparticles changes after dilution. In the second part of the chapter, the adsorption isotherm of oleic acid on PbSe quantum dots is measured. To compare our results with previous methods, the adsorption isotherm of oleic acid on magnetite is taken as a reference. It is shown that oleic acid binds much more strongly to magnetite than to PbSe; while, the maximum adsorption density is similar for both nanoparticles