Growth study of size-selected silver and silver-copper clusters by scanning transmission electron microscopy

Li, Cheng (2023). Growth study of size-selected silver and silver-copper clusters by scanning transmission electron microscopy. University of Birmingham. Ph.D.

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Abstract

The unique physicochemical properties of nanoscale clusters highly depend on their sizes, morphologies, and atomic structures, which can be tuned during the cluster formation process. The physical synthesis procedures, especially the gas-phase synthesis approaches, exhibit great advantages on the atomic-precision size control with the usage of mass filters. However, understanding the relationship between the morphologies and atomic structures of clusters and their preparation parameters is still a big challenge for these physical methods. Transmission Electron Microscopy (TEM), a powerful tool in material science, provides the possibility to visualize the nanoscale clusters, investigate their morphologies and atomic structures, and further establish the relationship between these physical characteristics of clusters and their preparation parameters.

This thesis presents investigations on growth of size-selected Ag and AgCu clusters by magnetron sputtering. A magnetron sputtering inert gas condensation mass-selected cluster source was used to produce size-selected Ag and AgCu clusters, and aberration-corrected Scanning Transmission Electron Microscopy with high-angle annular dark field (HAADF-STEM) was used for morphology and atomic structure identification. By utilizing this information, the growth process of Ag and AgCu clusters in the magnetron chamber was revealed and understood.

The results demonstrate the tunability of morphologies for size-selected Ag clusters using different deposition parameters. When Ag clusters are deposited with sizes smaller than the peak size observed in the mass spectrum, they exhibit a round shape. Conversely, larger Ag clusters exhibit an increased fraction of elongated morphologies as their size surpasses the peak size. With regard to the atomic structures of size-selected Ag clusters, despite the confirmation of Dh structure as the most stable form through e-beam irradiation and chip-heating experiments, the proportion of Ih structures predominates for size-selected Ag clusters within the size range of 1k to 14k. Additionally, the atomic structures of Ag8k clusters can be modified by varying the Ar and He gas flow rates. The increase in Ar gas flow rate promotes the cluster-cluster coalescence growth pathway, while the impact of increasing He gas flow rate on the growth pathway is negligible. Moreover, bimetallic core-shell Ag@Cu and Cu@Ag clusters were successfully prepared by adjusting the condensation lengths of two targets. In the case of prepared Ag@Cu clusters, the Cu atoms consistently arrange themselves according to the surface arrangement of the Ag core. However, it is common to observe a Cu-Ih or Cu-Dh core with an outer shell composed of Ag FCC domains in Cu@Ag core-shell clusters. This distinct atomic arrangement of the shell presents a novel approach for the controlled preparation of surface structures, offering potential insights into surface structure-controlled synthesis methods.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):