Alkali metal-induced Si(1 1 1) 21×21 structure: The Na case

We report the first scanning tunneling microscopy (STM) and core-level photoemission (CL-PES) studies of a 21×21 structure induced by sub-monolayer Na deposition on the Si(1 1 1) 3×3-Ag surface. In the filled-state STM images, five bright protrusions in the 21×21 unit cell are located on the Ag trimers of the 3×3-Ag structure. The Si 2p core-level photoemission decomposition shows that the surface-shifted component originated from the Si-trimer atoms in the 3×3-Ag surface splits into two for the 21×21 structure, which are attributed to the Si-trimer atoms in the 21×21 unit cell which are affected and unaffected by the Na adsorption. These results show strong similarities between the Na-induced 21×21 superstructure and the noble metal-induced ones, indicating a common atomic structure and formation mechanism.