Identification of Tetramers in Silver Films Grown on the Si(001) Surface at Room Temperature.

We describe the atomic structure of the silver film grown on Si(001) at room temperature, as studied by low-temperature scanning tunneling microscopy and density functional theory. Experiment and theory agree on a film structure in which Ag tetramers are identified for the first time. Ag tetramers are found to be adsorbed exclusively at the trough between two Si rows, interacting with four adjacent Si dimers via covalent bonding. Consequently, the pi-bonds of the Si dimers underneath the silver film are eliminated.