Unusual diffusivity and clustering of alkali metals on the Si(111)-7x7 surface

In this paper I review our recent experimental and theoretical works on the behaviors of submonolayer alkali metals (Li, Na, K, Cs) on the Si(111)-7x7 surface. The important aspects in the experimental preparation are discussed. While Li forms clusters even at the initial coverage, all other alkali metals (Na, K, Cs) form two-dimensional (2D) gases at room temperature and low coverage, due to their unusual high diffusivity on the surface. At higher coverage, cluster formation is observed, but with very different behaviors for Na, K, and Cs. Issues relative to the phase transition with increasing coverage are discussed: (1) the mechanism for the 2D gas formation; (2) the effect of 2D gas formation on STM observations; (3) The effect of structure on the charge transfer from alkali metals to the surface; (4) the exchange of alkali metal atoms with the Si adatoms during the cluster formation.