Dynamics and nano-clustering of alkali metals on the Si(111)-(7x7) surface

Alkali metal (AM) adsorption on semiconductor surfaces is one of the classic problems in surface science, owing to the prototype nature of AM and the technological interest in metal-semiconductor interfaces. We report a new model for the adsorption and nano-clustering of AM on the Si(111)-(7x7) surface, by using scanning tunneling microscopy (STM) and first-principles calculations. At room temperature and low coverage, we find intriguing contrast modulation patterns instead of localized AM adsorbates. AM atoms are invisible to the STM tip at room temperature due to their fast motion, while the contrast modulation patterns are the result of an averaged charge transfer from the AM adsorbates to the Si substrate. Temperature-dependent experiments have revealed the anchoring of individual AM atoms on the surface at low temperature. At higher coverages, AM form an array of magic clusters. The structures of the magic clusters as well as the mechanism of the coverage-driven gas-to-cluster transition have been proposed based on the current experiments and first principles calculations. It is found that the occupation of the basic units (¡°basins¡+/-) by individual AM atoms and the interaction among them determine the dynamics of AM atoms on the Si(111)-(7x7) surface.