Heterostructures of C $_{60}$ /C $_{70}$ and rotation of C $_{70}$ -Au magic number clusters on Au(111) and highly oriented pyrolytic graphite

Wang, Yitao (2020). Heterostructures of C $_{60}$ /C $_{70}$ and rotation of C $_{70}$ -Au magic number clusters on Au(111) and highly oriented pyrolytic graphite. University of Birmingham. Ph.D.

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Abstract

In this thesis, the nucleation, growth and mixing of C $_{60}$ and C $_{70}$ molecules are investigated on Au(111) and HOPG. The properties of (C $_{70}$ ) $_{m-}$ (Au) $_n$ magic number clusters on Au(111) are also studied.

Similar to C $_{60}$ molecules, C $_{70}$ molecules form close-packed multilayers on Au(111), but with a different lattice constant. C $_{70}$ molecules within the molecular layers are able to rotate freely at room temperature. Several azimuthal orientations have been found for C $_{70}$ layers giving rise to the R30°, R0° and R14° domains. When C $_{70}$ molecules are deposited on top of a C $_{60}$ layer, C $_{70}$ molecules occupy the three-fold hollow site and form a two dimensional (2-D) lattice matching that of C $_{60}$ . When C $_{60}$ molecules are deposited on top of a C $_{70}$ layer, the C $_{60}$ molecules tend to form a close-packed layer and that forces the underlying layer of C $_{70}$ to stand up. At elevated temperatures, significant molecular diffusion takes place leading to mixing of C $_{60}$ and C $_{70}$ from different layers.

The stable magic number (C $_{70}$ ) $_{m-}$ (Au) $_n$ clusters on the Au(111) substrate are fabricated in ultrahigh vacuum. These clusters are observed displaying unpredictable rotational movements characterised by sudden change followed by the long interval of inactivity. During rotation, these clusters can maintain certain configurations associated with structural polymorphism. The time durations between two successive rotations of magic number clusters such as (C $_{70}$ ) $_{10-}$ (Au) $_{35}$ and (C $_{70}$ ) $_{12-}$ (Au) $_{49}$ can range from minutes to hours.