A STM study of the self-assembly phenomenon and mechanism of cobalt-C $_{60}$ clusters on Au(111) surfaces

Zhao, Haoyu (2023). A STM study of the self-assembly phenomenon and mechanism of cobalt-C $_{60}$ clusters on Au(111) surfaces. University of Birmingham. Ph.D.

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Abstract

In 2013, Self-assembled Au-C $_{60}$ magic number clusters on Au (111) surfaces were successfully manufactured by the NPRL laboratory of the University of Birmingham. This work has important significance for the surface self-assembly of carbon nanostructures. However, this work has two key issues that can’t be solved: the Au cluster is too stable to be characterized and can’t expect further structure evolution. So, in the further work, the similar cobalt-C $_{60}$ clusters were attempted to be prepared. However, the C $_{60}$ on Au (111) substrate show the phase separation although the interaction between cobalt and C60 is much stronger than gold and C $_{60}$ . In this thesis, a reasonable explanation for the formation mechanism of Cobalt-C $_{60}$ clusters is given based on the STM technique.

Due to the 14% lattice mismatch between cobalt and gold, the cobalt clusters on Au (111) are irregular and rugged. Therefore, although the cobalt atoms have a good affinity for C $_{60}$ , the cobalt clusters on the gold surface cannot form Cobalt-C $_{60}$ clusters, due to inefficient contact with C $_{60}$ . Only at high temperature, the thermal motion of cobalt atoms is enhanced. Cobalt atoms refine themselves to be in complete contact with C $_{60}$ molecules, thus adsorbing C $_{60}$ to form Cobalt-C $_{60}$ clusters. As cobalt clusters at high temperature will gradually sink into the gold surface, if the order of annealing and C $_{60}$ deposition is exchanged, the pre-annealed cobalt clusters will partially sink into the gold surface resulting in a lack of C $_{60}$ adsorption sites. So, the Cobalt-C $_{60}$ clusters will not be able to form. In a common heating treatment for both, cobalt cluster will be wrapped by C $_{60}$ molecules in advance to prevent it from sinking. Further experimental evidences suggest that the opening of the carbon cage and the formation of the cobalt-carbon bond may also have occurred at higher temperatures.