New structural model for Na6Si3 surface magic cluster on the Si(111)-7 × 7 surface

The adsorption of Na atoms on the Si(111)-7 × 7 surface is studied using first-principles calculations. Compared to the Triangle-Trimer model reported previously, we propose a more stable and robust Na6Si3 Hexagon-Trimer model which has six Na atoms with hexagon shape and three Si edge adatoms moving inward to form a trimer. The total energy of Hexagon-Trimer model is 0.252 eV lower than that of Triangle-Trimer model and 0.552 eV lower than that of Hexagon model. The simulated STM images of Hexagon-Trimer model are in good agreement with experimental STM observations. The most probable formation process of Hexagon-Trimer model is analyzed. The reaction is catalyzed by a Na atom and the energy barrier is reduced from 0.89 to 0.44 eV. These results have provided a complete picture for the formation mechanism of Na6Si3 surface magic clusters on the Si(111)-7 × 7 surface.