Quantitative scanning transmission electron microscopy characterisation of size-selected gold clusters

[摘要] This thesis presents a systematic study on the three-dimensional atomic structures of the size-selected gold (Au) using aberration-corrected scanning transmission electron microscopy (STEM), fitted with a high angle annular dark field (HAADF) detector. The work involves experimental quantification of the microscope, HAADF-STEM image modeling and cluster structure identification. The study explores the cluster structure distribution as a function of experimental conditions and investigates their structural stability by electron beam irradiation. The Au clusters studied were produced by inert gas aggregation, size-selected by a lateral time-of-flight filter and soft-landed on thin amorphous carbon films. A quantitative image analysis methodology has been developed for rapid cluster structure identification, which involves direct comparison of atomic-resolved experimental images with simulated images. Applying the method to over two thousand individually imaged Au887 clusters (each containing nominal 887 gold atoms) revealed a distribution of structural isomers including icosahedron, decahedron and FCC structures. It was found that the isomer distribution depended critically on the cluster formation conditions. A quick condensation condition led clusters with icosahedral structure, while the slow condensation was favoured by clusters with FCC structure. In addition, the stability of Au887 clusters under the electron beam irradiation was investigated by following electron beam induced structural transformation in situ. The transformation sequence was found to be dominated by icosahedron to decahedron to FCC structure. The results provide insights to the understanding of cluster growth mechanism and dynamic behaviour at atomic scale, enabling the ability to control the three dimensional atomic structures of clusters.

[发布机构] University:University of Birmingham;Department:School of Physics and Astronomy

[关键词] Q Science;QC Physics