Fundamentals and Applications of Semiconductor Nanocrystals : A study on the synthesis, optical properties, and interactions of quantum dots

Abstract

This thesis focuses on both the fundamental aspects as well as applications of colloidal semiconductor nanocrystals, also called quantum dots (QDs). Due to the unique size-dependent optical and electronic properties of QDs, they hold great promise for a wide range of applications like solar cells, displays, lasers, or as contrast agent for bio-imaging. The work presented in this thesis can be divided in three sections. In the first part, the synthesis and self-assembly of PbSe nanocrystals of various shapes is described. It is found that a small contamination of acetate is responsible for the formation of star-shaped nanocrystals, whereas spherical and cubic nanocrystals can be obtained when the reaction mixture is absolutely free of acetate. This is followed by a detailed analysis of the size-dependent optical properties of spherical PbSe QDs, and our basic findings are supported by tight binding calculations. The second part shows how small aggregates of CdTe QDs that are stable in dispersion can be obtained using molecular cross-linkers. As a result of the formation of small aggregates, exciton energy transfer between neighboring quantum dots occurs, which is measured by (time-resolved) photoluminescence spectroscopy. Using two different models, the energy transfer rate can be determined, as well as the cross-link efficiency of the different dithiol molecules used. Finally, the influence of thiol ligands on the fluorescence of CdTe QDs is exploited to investigate the exchange-kinetics of native ligands by thiol ligands. By measuring the change in emission intensity upon thiol addition in situ, detailed information on the exchange rates, equilibrium constants and activation energies are obtained. Finally, the last three chapters of the thesis describe the usage of QDs as a basis for the development of multimodal contrast agents for bio-imaging. In the first example, the QDs are directly coated by paramagnetic lipids, making them detectable not only by fluorescence microscopy but also by magnetic resonance imaging (MRI). By also integrating pegylated and bio-functionalized lipids in the micellar coating, the particles are highly bioapplicable and able to specifically bind to e.g. tumors. In the second example, the QDs are first incorporated in small silica spheres of approximately 35 nm with a high degree of control, using a reverse microemulsion method. Next, the QD/silica particles are made hydrophobic and coated by a similar lipid coating. The advantage of this approach is that multiple functionalities can be integrated with a high payload; not only in the lipid coating, but also within the silica sphere. In addition, the size of the silica carrier particle can be exactly tuned over a wide range (25 nm to over a micrometer). An in vivo biodistribution test on mice shows that the lipid coated silica spheres have a ten-fold increase in blood circulation half-life as compared to uncoated silica particles. The versatility of the approach is further demonstrated by a trimodal particle based on this concept that can be simultaneously detected by MRI, CT, and fluorescence microscopy.