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Kudera, Stefan (2007): Formation of Colloidal Semiconductor Nanocrystals: The Aspect of Nucleation. Dissertation, LMU München: Fakultät für Physik



The present work describes different techniques to control some ma jor parameters of colloidal nanocrystals. The individual techniques rely on the manipulation of the nucleation event. The sensitive control of the nanocrystals’ size and shape is discussed. Furthermore the formation of hybrid nanocrystals composed of different materials is presented. The synthesis technique for the production of the different samples involves organic solvents and surfactants and reactions at elevated temperatures. The presence of magic size clusters offers a possibility to control the size of the nanocrystals even at very small dimensions. The clusters produced comprise ca. 100 atoms. In the case of CdSe, nanocrystals of this size emit a blue fluorescence and therefore extend the routinely accessible spectrum for this material over the whole visible range. Samples fluorescing in the spectral range from green to red are produced with standard recipes. In this work a reaction scheme for magic size clusters is presented and a theoretical model to explain the particular behaviour of their growth dynamics is discussed. The samples are investigated by optical spectroscopy, transmission electron microscopy, X-ray diffraction and elemental analysis. Shape controlled nanocrystals might be of interest for a variety of applications. The size dependent properties of nanocrystals are dominated by their smallest dimension. Therefore anisotropically shaped nanocrystals exhibit similar optical and electronic properties as spherical nanocrystals with a compatible diameter. This makes nanorods and nanowires an appealing object for electronics. Another possible application for these materials is to incorporate them into synthetic materials to influence their mechanical stability. Here, a method to form branched nanocrystals is discussed. It turned out that the presence of small impurities in the reaction vessel triggers the formation of branching points. Furthermore this synthesis technique offers some insights into the architecture of the branching point. The branching point is analysed by high resolution transmission electron microscopy and proves for the occurrence of a multiple twinned structure are strengthened by simulation of the observed patterns. Incorporation of a second material into a nanocrystal adds different functionality to the entire ob ject. Ideally both materials contribute with their own functionality and they are not affected by the presence of the other material. Two different techniques to generate nanocrystals of this type are presented. The first relies on a seeded growth approach in which the nucleation of the second material is allowed only on defined sites of the seeds. Anisotropic nanorods show a reactivity that varies for the individual facets. Using such nanorods as seeds dumbbell structures are formed. The second technique uses the tips of pre-formed nano-dumbbells as sacrificial domains. The material on the tips is replaced by gold. In any of the processes a different aspect of the nucleation event or the earliest stage of the growth is of relevance. In the growth of the magic size clusters the nucleation event itself is slowed down to a pace at which the experimenter can follow any step. The occurrence of branching can be traced down to the emergence of defects in the crystalline structure in the earliest stage of the growth. Hybrid materials are formed by a seeded-growth mechanism. Pre-formed nanocrystals provide the nucleation sites for the second material.