Immersion structures of monovalent metal adatoms on Ag/Si (111) 3×3: First-principles study

We present first-principles calculations for the adsorption of monovalent metal adatoms (noble and alkali metals) on the Ag/Si (111) 3×3 surface, a tunable two-dimensional electron gas system by electron-doping adatoms. The noble metals and Li immerse spontaneously into the topmost Ag layer with a significant amount of structural relaxation. The clustering of Ag adatoms is investigated, and the cluster with three-immersed-adsorbates is found to be the most stable one. The calculated atomic structures with one and three immersed Ag adatoms well reproduce the measured scanning tunneling microscopy features at low dopant coverages, called "star" and "propeller," respectively. The present results provide a unified view of the atomic structures of metal adatoms on the Ag/Si (111) 3×3 surface, which play an important role in the surface electronic states.