Adsorption and electronic structures of Na adatoms on the Ag/Si(111)-√3 × √3 surface

The stable structures of the Ag/Si(111)-√3 × √3 surface with Na adatoms are investigated by using the density-functional calculations. The Na adatoms prefer to make a heterogenous (Na+Ag) cluster structure with three Na adatoms. The Na cluster structure becomes the building block of the (Na+Ag)/Si(111)-√21 × √21 super structure. The simulated scanning tunneling microscope (STM) images of the cluster structure and the √21-(Na+Ag) surface are consistent with the experimental ones. The calculated band structure of the √21-(Na+Ag) surface is almost equal to that of the √21-(Ag+Ag) surface, which is consistent with a previous angle-resolved photoemission spectroscopy experiment.