Midway evaluation in Chemistry - Kusse Sukuta Bersha

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Title: Systematic analysis of AC-STEM images to determine the atomic structure of nanoclusters

Doctoral candidate: Kusse Sukuta Bersha

Doctoral committee:
Hannes Jónsson, Professor at the Faculty of Physical Sciences, University of Iceland (Advisor)
Egill Skúlason, Professor at the Faculty of Industrial Engineering, Mechanical Engineering and Computer Science, University of Iceland
Elvar Jónsson, Researcher at the Science Institute, University of Iceland

 Abstract:

Aberration-corrected scanning electron microscopy (AC-STEM) provides valuable information on the atomic scale structure of nanoclusters, an essential input for understanding their physical and chemical properties. A systematic method is developed for the extraction of three-dimensional atomic structure from AC-STEM images that is general enough to be applicable to irregular ordering of the atoms. Two-dimensional information from the AC-STEM image is complemented by an approximate description of the atomic interactions and at a final stage the structure is refined using electron density functional theory (DFT).

The method is applied to a published AC-STEM image of a Au55 nanocluster. Common neighbor analysis of the local structure shows that the Au55 cluster can be described as a combination of a part with icosahedral structure elements and a part with local atomic ordering characteristic of crystal packing, including a segment of a flat surface facet.

The energy surface of the cluster is explored by calculating the minimum energy path between the optimal fit structure and other close candidates generated. Low energy barriers are found for multi-atom, concerted rearrangements of the atoms, showing that such transitions can occur even at room temperature. The calculated paths reveal, furthermore, additional cluster configurations, some with lower DFT energy while providing nearly as good agreement with the AC-STEM image as the optimal fit.