Alkali metal adsorption on the Si(111)-(7×7) surface

A systematic comparison is reported on the dynamics and clustering behaviors of alkali metals (Li, Na, K, Cs), by using scanning tunneling microscopy, work-function measurements and first-principles calculations, to reveal their adsorption behavior differences. At room temperature, Li forms magic clusters whose number density increase linearly with the Li coverage, while both Na and K form two-dimensional(2D) gases at coverage < 0.08 ML, and magic clusters at coverage > 0.08 ML. K atoms diffuse faster in the 2D gas phase, with stronger charge transfer to the Si substrate than Na atoms. Cs behaves somewhat similarly, i.e., from 2D gas to clusters depending on the coverage, although the Cs clusters appears rather irregular. The clustering behaviors of Li, Na, K, and Cs are very different. In the K cluster case there is no intermixing between K and Si atoms, and the K clusters are very mobile inside the half(7×7) unit cell, whereas Na clusters are unmovable due to the site-exchange between Na atoms and the adatoms.