Synthesis, characterization and structural identification of multi-elemental chalcogenide polytypic nanocrystals

This thesis contributes to the development of novel synthesis protocols for polytypes and understanding of nanocrystal atomic structures in copper-based multi-elemental colloidal nanocrystals. The influence of ligands, precursors along with temperature as isolated factors in crystal phase and morphology is demonstrated. This allows a understanding of the intricate mechanisms behind the nucleation and sequential crystal growth. The thorough structural identification of sophisticated nanocrystal structures at an atomic level shines lights on elusive cationic ordering and disordering in multi-elemental colloidal nanocrystals and pushes the understanding of cationic precursor incorporation mechanisms beyond the current limitations. The synthesis of highly monodisperse CuZnSe2 (CZSe) colloidal nanocrystals is reported in Chapter 3. The crystal phase control of these nanocrystals with novel chemical compositions was achieved by controlling the presence or absence of phosphate-based ligands. Furthermore, the occurrence of polytypism between zinc blende and wurtzite was achieved by changing temperature and precursors. This understanding and control of crystal phase and polytypic occurrence in this system is of vital importance in applications such as thermoelectrics, photocatalysis and photovoltaics. Chapter 4 describes the dominating effects of precursor choice on the controlled occurrence of polytypism in the colloidal synthesis of CuαZnβSnγSeδ (CZTSe) nanocrystals. The synthesis of a linear polytype was simply triggered by the change of Sn precursor while the other metal, chalcogenide precursors along with temperature, solvents and surfactants remained the same. Three dimensional branched polytypic structures were synthesized at elevated temperature where in this case the choice of chalcogenides is the critical control factor. Chapter 5 fully deciphers the atomic structure in tetrapod CuαZnβSnγSeδ (CZTSe) nanocrystals with varied cationic compositions. This thorough structural identification study employs the chemical composition sensitive technique, scanning transmission electron microscope high-angle annular dark-field imaging (STEM-HAADF) to explore the atomic arrangement according to high-contrasted intensities scattered by nuclei of different masses. Real-space quaternary atomic models were built according to the analysis results, which allows insights of the cationic incorporation and cationic arrangement disruption in a continuous Bravais lattice. Chapter 6 summarizes the thesis and makes plans on further work based on the work that is already presented in this thesis.