The morphology, composition, and distribution of elements within nanocrystals are critical parameters which dictate the material's properties and performance in a diverse array of emerging applications. The (scanning) transmission electron microscope ((S)TEM) represents a powerful tool for probing the structure and chemistry of materials on the nanoscale. Understanding of the mechanisms by which nanocrystals grow, transform, and degrade is vital if we are to develop rational synthesis routes and hence control the properties of the resulting materials. Electron microscopy represents a key tool in developing such an understanding. In situ techniques, where the material of interest is subjected to stimuli such as heat or a chemically reactive environment in the microscope, allow direct observation of dynamic transformations. Ex situ approaches, where multiple samples are prepared in the lab with the reaction parameters systematically altered, can also give important mechanistic insights. This thesis explores the use of both in situ and ex situ (S)TEM to gain insights into the growth and transformation of nanocrystals. Ex situ TEM is used to assess the structure of PbS nanocrystals in a polymer matrix, revealing new methods of morphological control through reaction temperature, precursor structures (appendix 4), and the processing of the polymer matrix (appendix 5). In situ techniques are used to observe the solution phase growth and shelling of nanocrystals (appendix 1) as well as the transformations of nanocrystals during heating in vacuum (appendices 2 and 3). The subjects of my in situ investigations are systems with heterogeneous distributions of elements. Historically, in situ electron microscope has been largely limited to imaging. However, to understand many dynamic transformations knowledge of changing elemental distributions is vital. For this reason, I have focused on the use of energy dispersive X-ray (EDX) spectroscopy to reveal changes in composition and elemental distributions during in situ experiments (appendices 1-3). This type of in situ elemental mapping is especially challenging for liquid-cell experiments, and my results represent the first report of EDX spectrum imaging for nanomaterials in liquid (appendix 1).
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:727864 |
| Date | January 2016 |
| Creators | Lewis, Edward |
| Contributors | Haigh, Sarah |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/advanced-electron-microscopy-techniques-for-mechanistic-studies-of-the-growth-and-transformation-of-nanocrystals(e937d821-4aa0-4dbc-a25d-8972876e96ec).html |
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