Heteronanocrystal Transformations through In-situ Transmission Electron Microscopy

Abstract

Synthesis of nanocrystals (NCs) and heteronanocrystals (HNCs) has developed very fast particularly in the last decade. Nowadays, various structures and different morphologies of NCs and HNCs can be prepared with a very narrow size distribution. The nanoscale dimensions of these crystals lead to interesting phenomena (e.g. electronic properties) that are not found in their bulk counterparts. Therefore, NC and HNC research is now a hot topic in materials science and applied physics. Despite having interesting properties, the stability of NCs and HNCs is an issue outside the solution in which they are suspended. This raises a concern for their potential applications. Regarding their thermal stability, a number of studies have been reported showing morphological as well as chemical transformations with changing temperature. In this thesis, the focus is on in-situ heat induced transformations of HNCs and detailed characterization of these transformations through advanced transmission electron microscopy (TEM) techniques. Two processes were observed through in-situ heat treatment of the CdSe–CdS–ZnS core–multishell system: diffusion and simultaneous evaporation of different species. The structure evolves into a CdxZn1?xSe–CdyZn1?yS core–shell system through Zn inner diffusion and simultaneous partial evaporation of Cd and S. A more complex transformation is a temperature induced reconfiguration in the FexO/CoFe2O4 core/shell system. In this case, the FexO diffuses out of the core location and segregates at the outside of the CoFe2O4 shell. At the same time, the CoFe2O4 shell shrinks and fills the core volume. Co diffusion into the FexO structure is also observed with heating. Furthermore, in-situ heating studies of the CdSe-PbSe dumbbell system (a CdSe nanorod with PbSe NC tips) have shown a novel epitaxial nanowire growth process. Upon heating, PbSe starts to grow into CdSe with simultaneous Cd evaporation. This process proceeds as a cation exchange mechanism, whereby Pb is replacing Cd in CdSe. The crystal structure changes epitaxially at the same time from hexagonal wurtzite (CdSe) to cubic rock-salt (PbSe). Considering that the initial phases of CdSe and PbSe domains are solid and the final evaporation of Cd into the vapor phase, this novel epitaxial growth process is named solid–solid–vapor (SSV) growth. Observations reported in this thesis clearly show that transformations of HNCs upon heat treatment can differ significantly from one another. These studies can be regarded as first reference points when these HNC systems are considered to be implemented in possible future applications and for future in-situ heat treatment studies of HNCs.