Scanning tunneling microscopy and X-ray photoelectron spectroscopy of silver deposited octanethiol self-assembled monolayers

Ag deposited self-assembled monolayers (SAMs) of octanethiol (CH3(CH2)7SH) have been studied using scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). At the initial stage of Ag deposition, monatomic height islands, ∼7 × 1011 cm−2 in density, grow at the SAMs/Au(1 1 1) interface and become larger as more Ag atoms are deposited up to a full monolayer coverage of Ag. The differences of the nucleation density and the growth property between Ag and Au islands can be attributed to the higher mobility of Ag atoms and the difference of the molecular packing on these islands. XPS analysis of this structure (SAMs/Ag monolayer/Au) shows that the Ag 3d5/2 binding energy is shifted −0.3 eV with respect to bulk Ag, the C 1s binding energy is ∼0.3 eV higher than that before Ag deposition, and the S 2p3/2 binding energy exhibits little shift before and after deposition. The origin of the shift can be explained by the change of the dipole at the interface and the electrical isolation of alkyl chains from the surroundings.