Dynamics and nano-clustering of alkali metals (Na, K) on the Si(1 1 1)-(7×7) surface

We report scanning tunneling microscopy and work function measurement studies on Na (K) adsorption on the Si(1 1 1)-(7×7) surface at room temperature and sub-monolayer coverage. A systematic comparison is reported on their dynamics and clustering behaviors to reveal the atomic size effect. We find that both Na and K form two-dimensional gases at room temperature and coverage<0.08 ML. K shows faster diffusion speed and stronger charge transfer to the Si substrate as compared to Na. At coverage >0.08 ML, both Na and K form magic clusters consisting of six Na(K) atoms. However, the clustering behaviors of Na and K are very different. In the K 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 Si adatoms.