Initial stage of O2 adsorption on Ag(100): vacancies and local oxide formation

Abstract

The early stage of molecular oxygen adsorption on the Ag(100) surface was reexamined with scanning tunneling microscopy (STM) and density functional theory. Using an oxygen-functionalized STM tip, we demonstrated that the simple interpretation of the individual objects randomly distributed over the surface as chemisorbed oxygen atoms is incompatible with the cross-shaped structures observed in atomic-resolution images in the current work. In our model, each feature is an oxide-like cross formed by two intersecting O–Ag–O–Ag–O chains, with oxygen atoms in vacancies in the Ag(100) layer. Furthermore, we found numerous oxygen-free vacancies randomly diffusing over the surface under the STM probe even at 77 K.